Discovering The Animal Kingdom: A guide to the amazing world of animals 9781398817050, 1398817058

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Contents Introduction Part 1: Animal evolution The origins of Earth and the solar system A timeline of life on Earth How life began How evolution works The first animals on Earth Life’s other kingdoms The animal family tree Evolutionary milestones How we study evolution Still evolving! Part 2: Invertebrates Diversity of invertebrate life Sponges Comb jellies Cnidarians Life on coral reefs Echinoderms Molluscs—the slugs and snails Molluscs—the cephalopods Nematodes Arthropods—the most diverse invertebrates

Spiders, scorpions and their relatives Myriapods Crustaceans Insects—an introduction Insects—the plant eaters and scavengers Insects—the pollinators Insects—the predators Insects in ecology and economics Annelids Other invertebrates Part 3: Vertebrates The evolution of vertebrates Fish—a family tree Fish—sharks and rays Fish—bony fish Fish and other animals of the deepest oceans Amphibians—the swimmers and burrowers Amphibians—the climbers and jumpers Reptiles—a family tree The age of reptiles Snakes Lizards Turtles Crocodiles and alligators Birds—a family tree Extinct birds The feather Flight and flightlessness

Birds’ senses and intelligence Birds’ bills and feet Seabirds and their adaptations Predatory birds Songbirds and their songs Mammals—a family tree What makes a mammal? The egg layers The pouch bearers—marsupials The sharp-toothed scuttlers—insectivores A taste for ants—pangolins, aardvarks, anteaters and armadillos Sloths—a slow pace of life Giant Ice Age mammals The ubiquitous nibblers—mice, rats and voles Other rodents Rabbits and hares Mammals of the air—bats Madagascar’s unique mammals Monkeys The great apes Hoofed herbivores The grazers—grassland communities of hoofed mammals Elephants, manatees and hyraxes Carnivores—the “cat-likes” and the “dog-likes” Wolves, foxes and other wild dogs Domesticated mammals Bears Big (and small) cats Underwater hunters—otters, seals and sea lions

A farewell to land—cetaceans The great whales Part 4: Ecology and conservation What is ecology? How ecosystems work Relationships between different species Animal behaviour Threats facing animals today Extinction Conservation—principles Conservation—success stories The future for animals Further Reading Picture credits

// Introduction If we picture an animal, what comes to mind? In all likelihood, it is something with four limbs, a hairy skin that’s warm to the touch, and a face equipped with a pair of eyes, ears, a nose and a mouth. In terms of its internal anatomy, it’s probably something with an internal skeleton, the bones held together with muscle, and protecting at their centre a complicated mass of organs. Something just like us, in short. Yet we and our fellow mammals represent a tiny proportion of all of the world’s animal species. Even if we join forces with our fellow vertebrate animals, we are still enormously outnumbered by the invertebrates, and their diversity is both bizarre and breathtaking. The first animals evolved in the sea, and most of them are still there now. Just as green plants grow on land and use photosynthesis to capture the Sun’s energy, so the ocean is also a vast photosynthesizing factory, although most of its workers are not rooted to the ground but are free-drifting, miniscule planktonic life forms—the diatoms or phytoplankton. Animals are all ultimately sustained by the Earth’s photosynthesizers, because all animals have evolved to consume organic material—the bodies of other living things. In turn, animals are food for other animals, and the fundamental chemicals that form their bodies are recycled by the photosynthesizers when they die. Because eating is living, it is the evolution of different feeding methods that has driven the diversification of animal body shapes. In the sea live a vast number of animals that extract fragments of food from the open water. They range from non-moving animals such as

sponges, mussels and exquisite flower-like crinoids and corals, to enormous basking sharks and blue whales, which roam the entire planet’s undersea realm. On land, animals that consume living plants include herds of hoofed mammals but also swarms of locusts and armies of ravenous caterpillars. Our mental image of a predatory animal, making its living by hunting and killing other animals, is probably something fast-running, with keen eyes and powerful jaws— but this description matches wolf spiders as well as it does wolves, and describes a tiger beetle as well as it does a tiger. Animals form just one of the kingdoms of life on Earth. Plants and diatoms could live without animals, but the opposite is not the case. However, animals have evolved to be the most dynamic, diverse, adventurous and complex of all living things; it is not just because we are animals ourselves that we find the animal kingdom so endlessly fascinating, beautiful and awe-inspiring. Wherever we care to look on our planet, we will find animals. We will also notice that different kinds of animals, together with other living things, form distinct communities, with clear interrelationships. This interconnectedness of all living things on our planet is what we call ecology, and a particular community of life is called an ecosystem. Depending where you are looking, ecosystems may be huge and incredibly complex, or relatively simple. In a lush rainforest with hundreds of different species of plants growing in each square metre, the ecosystem is huge, its web of connections almost unfathomably complex. But head to an area of tundra in the high Arctic and you may find a much simpler ecosystem—a handful of lichens and grasses that are eaten by lemmings, and a few pairs of snowy owls that hunt those lemmings.

Great white sharks are found in most of the world’s oceans. Adults may weigh 2400 lb (1100 kg) at maturity.

African elephants form large family groups with a matriarch at the top— seen here in Botswana’s Okovango Delta.

This book introduces the animal kingdom in all its incredible variety. The tree of animal life, which has been growing through space and time for 900 million years, has produced countless branches and twigs. We know that our own species, for example, branched off from a lineage of apes, which itself came from a lineage of monkeys. Heading further back in time, there was a point where the “monkey branch” parted ways from the “lemur branch”, and further back still we find the single ancestor of all primates that have ever lived. Some branches have changed much more dramatically and produced far more offshoots than others over time, but we know that all animals alive today are equally “highly evolved”, because every

single living animal has an unbroken evolutionary lineage that can be traced all the way back to that single origin, 900 million years ago. Looking at evolutionary history is one way to categorize animals. Another is to look at an animal’s place in life’s ecology. What is its role in life—what does it depend on, and what depends on it? Bees need flowers for nectar, but flowers also need bees for pollination. Predators cannot live without prey, but predation also helps to keep a prey species strong, and thus better able to survive life’s other hazards. In this book, we look both at how the animal tree of live evolved and continues to evolve, and how different kinds of animals live today as individual parts of the whole interconnected web of life on Earth.

A Papilio demoleus butterfly emerging into its final stage of metamorphosis, its chrysalis still almost intact.

A host of goose barnacles exposed at low tide. Unlike other barnacles, they rely on the motion of the water to feed successfully.

ANIMAL EVOLUTION The wonderful diversity of animals that inhabit our planet today has come about through evolution. This near-miraculous but biologically inevitable process has shaped animals to thrive in every kind of natural environment, and continues to do so today.

A Wolf giant tortoise (Chelonoidis becki) on the rugged west slope of Wolf Volcano, Isabela Island, Galapagos. Many hybrids of mixed parentage with different shell shapes found on different islands in

the archipelago, thanks to early explorers moving the animals from island to island.

// The origins of Earth and the solar system The universe came into being about 13.75 billion years ago—we think. How did we conclude this? When humankind devised telescopes that could look beyond the Milky Way—our “home galaxy”—we discovered that many other galaxies existed. We also found that all of the galaxies we can see are moving away from each other—evidence that the universe is expanding as time passes. By looking at the rate of expansion, we can imagine that process in reverse, and work backwards to a time when all of the matter that today forms all of the stars and planets was condensed into a single point. In its first milliseconds of existence, the universe was extremely hot and expanding in all directions incredibly rapidly. This is sometimes described as “the big bang,” though some scientists say it would be more helpful to picture this process as “the big stretch”—a rapidly inflating balloon, rather than a chaotic explosion. Exactly how and why this happened is still mysterious, and these may be questions we’ll never be able to answer. But what happened—and still happens—to matter in the expanding and fast-cooling universe is (a little) easier to understand. Over time, the tiniest fundamental particles begin to coalesce into atoms, and atoms come together under the force of gravity to form clouds of gas. When a cloud of gas contracts

under gravitational pressure, it turns into a swirling disc. Its gravitational center forms a star. The remaining material circles (orbits) the star, and through gravity this material may coalesce into planets (and their moons). Our “home star” is the Sun. It is a vast sphere of gaseous plasma, generating enormous amounts of light and heat. It is orbited by eight planets, themselves orbited by a total of more than 200 moons (Saturn alone has at least 82 moons), and by countless smaller objects – planetoids, meteors, asteroids and comets. The planets nearest the Sun (Mercury, Venus, Earth and Mars) are small and rocky in nature, while the more distant bodies (Jupiter, Saturn, Uranus, and Neptune) are large and gaseous. The Earth orbits about 93 million miles (149.6 million km) out from the Sun. Its own gravity holds a cloud of gas—an atmosphere – around it. Without its atmosphere, Earth’s average surface temperature would be about -0.4ºF (-18ºC), but the atmosphere traps enough of the Sun’s heat to maintain an average surface temperature of 57.2ºF (14ºC).

At this point in our solar system’s lifespan, our planet sits comfortably within the habitable zone (shown by the blue ring), but as the Sun ages and grows, the zone will move away to the outer planets.

Life probably began in warm, shallow water, and this habitat is still extremely rich in life of all kinds.

This nebula, 900 light years from Earth, is a cloud of gaseous molecules, possibly the remnants of a large star that exploded (supernova). New galaxies form from large amounts of this kind of interstellar “dust.”

Does life exist in other worlds? Earth is the only planet in our solar system known to hold life, and scientists believe that this is because its temperature permits the existence of liquid water, on which every known living thing depends. However, there is no doubt at all that there are other solar systems – in our galaxy and beyond – that hold planets and moons that have liquid water and in many other ways will be just like Earth. Astronomers estimate that the Milky Way alone may contain as many as 40 billion such planets. So, the odds are very good that life exists elsewhere in the universe.

// A timeline of life on Earth The planet Earth formed about 4.54 billion years ago, making it about one-third the age of the universe itself. The newborn Earth was a hostile and volatile place. Its gravity was pulling in other smaller objects that were orbiting the Sun, and these collisions caused violent volcanic activity. The biggest collision of all, with a hypothesized Mars-sized object known as Theia, produced the cloud of debris that went on to form our planet’s only moon. Over time, Earth cooled down, and its molten rock surface condensed into a solid crust. Volcanic eruptions released various gases, which went towards forming Earth’s first atmosphere. This contained virtually no oxygen, but was made mostly of carbon dioxide, with some nitrogen, methane, ammonia and hydrogen, and water vapor. When the planet’s surface had cooled enough, the water vapor condensed and rain fell, forming rivers and oceans on top of the rocky crust. The first forms of life evolved in these waters. Some of these simple bacteria-like microorganisms were capable of photosynthesis – using carbon dioxide, water, and sunlight to produce glucose, a simple sugar that is a key energy source used by nearly all living things. The chemical reaction of photosynthesis releases oxygen as a by-product.

Thanks to these first photosynthesizers, oxygen accumulated in Earth’s atmosphere. The process of breaking down glucose to

release energy can be done with oxygen (aerobic respiration) or without oxygen (anaerobic respiration). Before photosynthesis, nearly all respiration was anaerobic. However, because aerobic respiration is a much more efficient process, the first simple organisms that could use oxygen for their respiration began to proliferate and to out-compete their anaerobic cousins. This was the trigger for an explosion of oxygen-consuming life on Earth, which led to the evolution of complex life forms.

// How life began One of the greatest mysteries that scientists grapple with is the origin of life—and even the very nature of life. Why can’t we bring a dead animal back to life, even though all of its bodily systems are still there? The “spark of life” that’s required to do this is elusive, and indeed seems to be almost mystical. How can something like this have appeared from non-living material? What we understand as “alive” does change, though, depending on what we are looking at. All of the animals familiar to us seem obviously alive—we can see them breathe, move, eat, have offspring, and eventually die. But can we consider an organ or a single cell in that animal’s body to be “alive” in the same way? And what about other organisms? It’s not as easy to see these same signs of life in a plant, a mushroom, or a bacterium— and what about a virus? Biologists today recognize several distinct characteristics of life that must be fulfilled for an entity to be regarded as a living thing. The most widely used list comprises the following seven traits: • They respond to their environment in some way; • They grow and change in some way; • They can reproduce themselves; • They have metabolic processes that build up and break down molecules (for example, respiration);

• They maintain a stable internal environment • They are made of one or more membrane-bound cells; • They pass their traits on to their offspring, through genetic inheritance. An animal, a plant, a mushroom and even a bacterium displays all seven characteristics. A virus, however, does not – for example, it neither grows nor does it have a cellular structure, so it does not fully qualify as a living thing. Looking at the structure of a virus can help us imagine what early life (or protolife) on Earth might have been like. The simplest types of virus consist of some RNA (ribonucleic acid, a molecule that can replicate itself, and tells a cell how to build proteins) in a protein coat. Is it possible that strands of RNA could have existed in a free state, replicating themselves and being “almost-alive?” Their building blocks – simple molecules made of carbon, hydrogen, oxygen, and nitrogen, are the nucleic acids. Laboratory studies show that it is possible to create nucleic acids by applying lots of energy to water that is rich in dissolved ammonia, methane and carbon dioxide, as would have existed in the first oceans on the planet. That energy could have come in the form of ultraviolet radiation emitted by the Sun. Many scientists agree that this “RNA world” scenario may well be how life first appeared. The simplest modern living things – the bacteria and the similar-looking archaebacteria or archaea – are known as prokaryotes. They are little more than a tangled ring of DNA (deoxyribonucleic acid, the double-stranded version of RNA) held inside a membrane formed of fats and proteins. Life forms with complex cell types (whether they are single-celled or

multicellular organisms) are known as eukaryotes. A eukaryote cell is much bigger than a typical prokaryote. Its DNA is held inside a separate membrane (forming the cell nucleus) and it has other structures (organelles) inside the cell, too, each with their own function. Eukaryotes probably first evolved when larger prokaryotes engulfed smaller ones. Mitochondria—structures found in eukaryotic cells that generate energy—are very similar to free-living bacteria, since they have their own DNA, which is quite distinct from the cell’s nuclear DNA. RNA strand

Virion (a single virus particle)

Animals are aware of the difference between life and death, and humans are not the only animals that show grief at the permanent loss of a companion.

Prokaryotic cell

Eukaryotic cell

// How evolution works Imagine that you are a fennec fox, living in the Sahara Desert. You are a deadly predator—of insects and perhaps the occasional lizard. To find and capture your prey, you need keen senses (those ears aren’t just there for decoration) and stealth. However, you might also be the hunted rather than the hunter. To escape your predators, you need alertness and speed. It will help you out a lot if you have excellent sandy-coloured camouflage, too, and top-notch kidneys to deal with desert life. Your big ears also help you keep cool. If you want to have babies, you have to be fit enough (in both senses of the word) to attract and keep a partner, and you have to be nurturing enough and a good enough provider to take great care of your offspring so that they survive to adulthood. If you have all those attributes in abundance, you will probably live a long life, and leave behind many descendants. It’s easy to see that an animal that functions very well in its environment will survive longer than one that doesn’t. It’s also easy to see that breeding success is a combination of living long enough to have lots of breeding opportunities and functioning very well in the specific ways involved with breeding. Those that are less well adapted have shorter lives and fewer offspring. This is the process of natural selection, or “survival of the fittest.”

The fennec fox has a sandy-colored coat to blend in with its desert habitat, and its large ears help it cool down as well as hear every tiny sound.

The idea of natural selection presupposes that all of the animals in a population are at least a little different to one another, in all kinds of ways – and indeed they are. Much of this variation is there from the start of life, and is down to genetic variation. Not only do animals inherit a different combination of

genes from their parents, but genes can also spontaneously change (mutate). With natural selection at work on genetic variation, each new generation is the offspring of a non-random sample of the generation before—the progeny of the “best.” Surely after many generations of this, though, you will just end up with “better” fennec foxes? Let’s try, then, a further thought experiment. Let’s take half of our Sahara-evolved fennecs and distribute them in other habitats. Some can go to a snowy mountain and some to a deep rainforest. We will leave them to it for a few hundred millennia, then return. What do we find? The traits that made these animals so well adapted for desert life are no longer advantageous, but other traits have been favored instead. For example, on the mountain, the foxes with the thickest and whitest fur and smaller ears would have survived best, and passed on these traits through the generations. In the forest, a thinner and darker coat is better, and extreme kidney efficiency has become redundant. Meanwhile, the fennecs that stayed in the desert have carried on being well adapted to desert life. We now have three very different and distinct lineages of fennecs—they differ in the way they look, their anatomy and their behavior—and if we reintroduced them they might not even recognize one another as close cousins.

Dark or melanistic morphs of the peppered moth outnumber pale morphs in environments where industrial pollution blackens walls and tree trunks.

Now, stretch out this process for many millions of years, and replace our deliberate meddling (which in reality would probably kill off our subjects before they had a chance to adapt) with slowpaced but very far-reaching changes to the entire planet’s environments. The scene is set for evolution to occur and, over our planet’s lifetime, produce the wonderful and diverse array of living things we see today.

Adaptive radiation: a single tanager species reached the Galapagos islands about 2.3 million years ago, and there, over many generations, it diversified into many different species (the Galapagos finches), with varied bill shapes to eat the different kinds of food available.

// The first animals on Earth The very earliest indisputable traces of life on our planet date back to 3.5 billion years ago—a billion years after the planet came into being. There are, however, much older fossils that may also indicate the presence of simple life—if so, then bacteria-like organisms already existed when Earth was less than half a billion years old. For more than half of its existence, Earth only held these very simple, bacteria-like life forms. The first complex single-celled organism appeared just 2 billion years ago, and multicellular life forms have been around for a little over 1.5 billion years. The earliest true animals? They are a mere 900 million or so years old. This is because while all animals are multicellular, not all multicellular organisms are animals. When we think of “an animal,” we probably picture a cat, a dog, a horse—something that’s just like us. But we and our fellow mammals make up a tiny fraction of the animal kingdom. Even if we add in all of the other vertebrates—the birds, reptiles, amphibians, and fish—we are still only looking at barely 5 percent of all animal life, and a very recently evolved 5 percent at that. The first kinds of animals on Earth resembled modern-day sea sponges and comb jellies. The former, branching out of the seabed, might have struck us as much more like plants, and the

latter, with their strangely amorphous, soft and translucent bodies, look more like drifting blobs of snot than living things; they don’t have distinct body parts like we do, and are akin to colonies of co-operating cells. However, the truth of their animal nature is revealed not in their outward form, but in the anatomy of those cells. If you have ever looked at plant and animal cells under the microscope, you will know that there are several distinct differences. One of the most obvious is that the plant cells have a rigid cell wall which often gives them a rather geometric shape, while animal cells only have a flexible cell membrane and tend to look round and soft. Fungal cells also have a cell wall. This is a key difference between animals and other multicellular life.

A comb jelly. This simple but successful creature is a modern representative of one of the most ancient animal lineages.

Plants are producers – making their own energy stores by using sunlight. This energy passes up the food chain, via plant-eating primary consumers such as zebras, and on to animal-eating secondary consumers such as lions.

Another important difference between animals and plants is how animals obtain their food. Plants can make glucose out of carbon dioxide and water, through the process of Sun-powered photosynthesis. Animals must consume other organic material. This means that animals cannot exist without other living things for them to eat. Both the early sponge-like animal and the early comb jelly-like animal extracted tiny bits of drifting organic matter (such as the fragmented remains of dead plants) from the

sea water and allowed these to pass through their bodies, just as modern sponges and comb jellies do today. Over time, animals evolved that could move freely and decisively, and actually attack and consume still-living plants and other animals.

Plants are producers – making their own energy stores by using sunlight. This energy passes up the food chain, via plant-eating primary consumers such as zebras, and on to animal-eating secondary consumers such as lions.

// Life’s other kingdoms Life on Earth is naturally divided into several groups, which branched off from the tree of life at various different times. The group of living things with complex cells—the eukaryotes—is itself further divided into several major groups, based on fundamental differences in their cell structures and how their bodies are organized. These differences date back to the earliest days of their evolution. The groups are usually called kingdoms. Over the history of biology, different biologists have described anything from two to six distinct kingdoms of eukaryote life—but they have always agreed that the animals form a kingdom of their own. The plants have also been consistently defined as a kingdom. Almost all plants can photosynthesize, but photosynthesis existed before plants did. The first photosynthesizers, as we saw on page 12, were simple, prokaryote organisms called cyanobacteria or blue-green algae. Modern plant cells contain tiny green objects called chloroplasts, which carry out the cell’s photosynthesis— these chloroplasts originated from free-living cyanobacteria that began to live inside other cells as much as 1 billion years ago.

As the first living things to harness the energy in sunlight for their own use, cyanobacteria play a crucial role in the story of life on Earth.

Free-living cyanobacteria still exist in abundance, and both they and the plants keep the planet alive and the air breathable by using photosynthesis to capture the Sun’s power and to release life-giving oxygen into the atmosphere while extracting lifestifling carbon dioxide from it. Today, plants cover much of Earth’s land surface, and they are also present in abundance in shallow seas all around the planet. As well as oxygen, they provide a food source and shelter for animals. Quite simply, without them, animal life could not exist. The fungi were formerly classified as a subgroup of plants, but studies of their cell structure show that they are not plant-like at all. In fact, they are closer cousins to the animals than to the plants. Like animals, they are heterotrophs, feeding on organic matter rather than building their own. This matter may be dead or living – some fungi attack plants and animals and cause disease. Fungi may be single-celled or multicellular. Those that are multicellular are mainly made up of thin strands called hyphae, which grow very extensively through soil. The largest single organism on our planet is a honey fungus in Oregon—its network of hyphae measures nearly 3.7 miles (6 km) across. Fungi also produce fruiting bodies – the familiar mushrooms and toadstools that release their reproductive spores into the air.

The classification of some single-celled eukaryotic organisms, such as this Paramecium, is still in flux.

Another kingdom that some biologists recognize is Protista or Protozoa. These are single-celled organisms that move about freely and lack cell walls. However, it this not a clearly defined group. Its members are not considered to be plants, fungi, or animals, but other than this, what they actually have in common with each other is not clear, and they may not all have evolved from the same common ancestor. One distinctive group within them, the chromists, is sometimes treated as a kingdom in its own right—Chromista—and this also includes some multicellular organisms that were formerly classified as plants. The singlecelled organism Paramecium, which is much used in biology classrooms, is an example of a chromist.

Animals, plants and fungi together make up the vast majority of living things on Earth from our human perspective, though all are greatly outnumbered by much simpler and tinier life-forms.

// The animal family tree We humans have always recognized that animals come in different types. We might once have thought that these types had persisted—distinct and unchanging—since the beginning of time. As soon as we understood that evolution happens in nature, though, and that over time one sort of living thing can diversify into several different ones, we knew that this way of looking at nature was wrong. Instead, life is a constantly growing tree, spreading and branching through space and time. The branch of life’s tree that we label “animals” first sprouted from life’s main “trunk” less than a billion years ago, and has since produced numerous new side branches, which themselves have branched even more. Some of the branches—big and small— have died over time, but others have thrived. We humans, although we like to picture ourselves as the most advanced entities that evolution has produced, make up just one little twig on this huge and wonderful tree.

This acorn worm represents a lineage of early deuterostomes. This group evolved more recently than the simpler protostomes (which form the majority of invertebrate life), and differs from them in the way the digestive tract develops in the embryo.

Animals such as sea anemones have radial symmetry, with the same body parts repeated all the way around a central point. If bisected anywhere through this central point, its two halves are mirror images. Most more advanced animals have bilateral symmetry, meaning that only a bisection through the midline would result in mirror-image halves.

This diagram shows the evolutionary pathways of the major animal phyla, and significant anatomical developments that occurred along the way.

// Evolutionary milestones Before animal life ever appeared, several important events took place in life’s history that paved the way for more complex life to evolve. The same is true during the course of animal evolution. Some of these great advancements or innovations have appeared independently, more than once, while others seem to have been one-time deals (at least for now—evolution, though, is ongoing). We’ll look at a few of them, in order of the time they appeared.

Specialized cells Simple animals like sponges are made up of cells that are all much the same and are not all that different to a single-celled organism. The emergence of more differentiated and specialized cell types led to animals developing bodily systems with particular functions. A wider range of specialized cell types eventually became organized into the full range of tissues, systems, and organs that we see in higher animals.

Anemones have stinging cells, to kill prey.

Mating is an anatomically complex business in damselflies.

Bilateral symmetry

Having a symmetrical body makes movement more efficient, and also lends itself to a more organized interior environment. The bilateral body shape, which first appeared in animals about 600 million years ago, allows for easier directional movement. Its emergence drove the evolution of a distinct “head end,” housing sensory and feeding equipment so the animal can tell where it’s going and eat what it finds there, and a “tail end,” so it can leave its waste behind as it goes.

A bilaterally symmetrical gull with its radially symmetrical starfish prey.

Sex Most animals reproduce sexually, combining male and female cells. Each reproductive cell, or gamete contains half of the parent’s DNA, and when united they form an animal with a unique mix of both parents’ genes. This form of reproduction is present in some of the simplest animals as well as many nonanimals. It provides more genetic variety in the animal population, and also introduces sexual preference and therefore

sexual selection. These add a new dimension to evolutionary change, because being a great survivor and being very attractive to the opposite sex are different ‘skills’ and don’t necessarily require the same traits.

The air-breathing mudskipper shows us how fish may have first taken to land.

Air breathing Life originated in the sea and most of it is still found there. However, once plants had begun to colonize land, they were joined by several different lineages of animals, with insect-like creatures leading the way, some 500 million years ago. Living on

land required new ways to breathe and new outer coverings to prevent the body from drying out, but it also offered a huge range of new opportunities.

Flight Self-powered flight has evolved four times, in four different animal groups—the insects, pterosaurs, birds, and bats. The anatomy and function of the four kinds of wings they evolved is very different. On top of these, many other groups have evolved the ability to glide. Traveling long distances through air allowed these animals to exploit an enormous number of new territories.

Bats are one of the most successful of all mammal groups.

Japanese macaques learn many things from each other, including the joys of a hot-springs bath.

Intelligence However you define intelligence, there’s no denying that the ability to learn, plan, visualize, create, and innovate is helpful to survival. Intelligent and social animals can pass on learned skills and information to others in their group. This is cultural evolution. It is a much faster process than “normal” evolution and has enabled humankind to become such a dominant and influential animal.

// How we study evolution Until we invent a way to travel through time, we can never be certain which animal was the very first to exist, what our world was like 4.4 billion years ago, or when the first early human uttered the first word. Studying the past, especially the prehistoric past, means putting together whatever evidence we can find, and the longer ago an event occurred, the more indirect and difficult to interpret that evidence tends to be. Most animals that die are either eaten by other animals, or their corpses are soon destroyed by other natural processes. Even bones wear away through erosion over time. However, under a few special circumstances, a corpse is preserved long enough to leave a permanent imprint behind. The classic case is an animal that dies in very still water, and its body is gradually covered in sediment. Over many millennia, the sediment’s lower layers are compressed, becoming rock. Trapped in the rock, the animal’s body decays slowly, but the space where it lay is preserved, often by being filled up with deposits of different types of minerals, carried in water that gradually seep into the rock. The result is a fossil. Fossils are rare, but throughout the world’s deposits of sedimentary rock we have found—and are still finding—many thousands of them. Through fossils we know of the existence of the great dinosaurs, plesiosaurs, and pterosaurs, of saber-toothed cats, and giant sloths, of the first fish that stood on their fins and

walked out of the sea, and of a whole pageant of anatomically bizarre marine invertebrates—entire ecosystems that lived and died long before our own species existed. Geology and chemistry provide ways of determining the age of rocks with considerable accuracy. Armed with this knowledge, studying the anatomy of different fossil animals from different eras and comparing them with living animals helps us to piece together how the tree of life has grown.

A beautifully detailed pterosaur fossil from China. Fossils like these help us infer how long-extinct animals looked in the flesh, and how they lived.

The DNA of humans and our closest cousins, chimpanzees, differs by about 1.2%. However, that amounts to about 35 million differences

in DNA base pairs, and there are also differences in how genes interact and are expressed.

Another, more recent, field of study that has helped us understand evolution is that of genetic sequencing. Every cell of an animal contains strands of DNA, called chromosomes. The DNA molecule is a set of instructions for making all of the proteins that go into building that animal’s body, and each section that codes for one single protein is called a gene. We now have the technology to extract DNA from a cell and map the full sequence of its genes – its genome. This can then be compared to DNA from a cell from a different animal. Many genes are shared by almost all animals, and animals that only went their separate ways on the evolutionary tree very recently have the vast majority of their genes in common. For example, two randomly selected humans share about 99.9 percent of their genes. The human genome shares more than 98 percent of its genes with the chimpanzee genome, but only about 92 percent with the house mouse genome. We know roughly how quickly genes mutate into new forms from generation to generation, and combining this knowledge with the differences between two animals’ genomes gives us an idea of how many years ago those two animals’ shared common ancestor was alive.

DNA contains four types of chemical “bases,” which pair up together to form the double helix shape. The bases are adenine (A) and thymine (T), which always pair together, and cytosine (C) and guanine (G), which always pair together. A single gene contains thousands of base pairs in a unique sequence, and each chromosome in a cell is made up of thousands of genes.

// Still evolving! Compared to the lifespan of Earth, we humans have existed for a mere eyeblink of time. In fact, if we condensed the 4.54 billion years since Earth was formed into a 24-hour day, then the human species only appeared in the last second of that day. It is little wonder that we struggle to get our heads around the huge timespans involved in the evolution of life on our world, and that we get the impression that, although the past was filled with the rises and falls of countless amazing creatures, life on Earth today is just staying the same. However, for as long as life exists, evolution will carry on. The rate of evolutionary change is almost always too slow to be that noticeable in our lifespans, but it varies from place to place—in the deepest oceans, for example, where conditions are very stable, there are animals such as the chambered nautilus that have barely changed over hundreds of millions of years. Conversely, in fast-changing environments, evolution can happen quickly, too. Such changes can create many new opportunities for ways to live, but they also bring great danger—animals that cannot adapt quickly just die out. Large-scale change to our landscape is constantly happening, because of processes such as tectonic plate movement and erosion. This kind of change is often very slow. For example, some mountains are gradually growing taller, as two tectonic plates squash together, while erosion is causing others to

gradually shrink. Natural change can also be fast. Volcanic eruptions can cause brand new islands to uplift out of the oceans, almost overnight, and a new island will quickly be colonized by living things. Charles Darwin established his theory of evolution after realizing that the array of different birds living on the relatively recently formed volcanic Galapagos Islands had descended from mainland birds that flew to the islands many millennia ago, and had changed and diversified through evolution as they adapted to their new home. Conversely, an eruption could also completely destroy an existing island, and wipe out all the animals that evolved there.

Urban and rural foxes are unlikely to “split” into two separate species any time soon, because they are not completely isolated from each other (some foxes’ territories include both town and country areas).

Mockingbirds on the Galapagos islands have evolved to drink blood and peck scabs from sealions—this food resource is not available to their mainland relatives.

Human activities today are changing the world’s habitats more quickly and more extensively than any geological process. We can see evolutionary responses to this. For instance, down in

London’s underground railway system, a new species of mosquito has evolved, with different genes and behavior to its aboveground ancestor. Up on the streets, the UK’s urban foxes are now different to rural foxes, including having shorter snouts—an adaptation brought about by to mainly scavenging rather than hunting. Death and extinction are part of the evolutionary process. Nearly all animal species that have ever evolved are already extinct—the average lifespan for a single species is only about a million years. However, far more species affected by modern human activity will become extinct than will adapt. The animal kingdom of Earth is in the midst of great change—it’s hard for us to see evolution happening, but we cannot help but see the extinctions going on all around us.

This diagram shows the cumulative number of extinctions of vertebrate animals in recent centuries, relative to the number that would be expected at the normal “background” rate of extinction.

INVERTEBRATES Traditionally, we divide animal life into ‘invertebrates’ and ‘vertebrates’, as if they were equal halves. However, vertebrates form just a small offshoot of the taxonomic tree. A glorious diversity of invertebrate life thrives everywhere on our planet, showing between them a full mastery of every imaginable way of life.

Millions of golden jellyfish migrate horizontally across Jellyfish Lake on a daily basis. The marine lake lies on Eil Malk island in Palau in the western Pacific, and is connected to the ocean through the many fissures and tunnels in an ancient limestone reef.

// Diversity of invertebrate life We humans often like to divide all of animal life into two fundamental groups: 1) vertebrates, or animals with backbones; 2) invertebrates, or animals that lack bones completely. This division reflects the fact that we, and most of the animals we know best, are vertebrates, so we assign them a great deal of importance. However, biologists divide the animal kingdom into about 35 fundamental groups, called phyla, and all but one of those phyla are different kinds of invertebrates. Even the phylum that does contain the vertebrates also contains some invertebrates. In fact, more than 95 per cent of all known animal species are invertebrates. Between them they live almost everywhere on Earth, from the deepest and coldest seas to the hottest deserts and highest mountains, and in tremendous profusion in the more hospitable habitats on our planet – shallow reefs and kelp ‘forests’ in tropical and temperate seas, tropical rainforests and deciduous woodlands, inland fresh waters, marshes, meadows and prairies. Some even thrive in the new and challenging habitats associated with human settlements – plantations, farmlands, gardens and even inside our buildings. Their ways of life are as diverse as their body structures, and some of them are keystone species in their environments, propping up entire

ecosystems through their roles as predators or prey and even actual builders of habitats.

This pie chart represents the species diversity of the larger animal phyla on Earth. As can be seen, the arthropods are by far the most diverse of all living phyla.

The nudibranchs or sea-slugs are riotously colourful inhabitants of seas worldwide.

Jellyfish belong to the phylum Cnidaria. They exhibit clear behavioural patterns despite not possessing conventional brains.

Relatively few groups of invertebrates dwell entirely on land, but the main group that does, the insects, outnumber land vertebrates by a huge factor.

We think of invertebrates as small animals, and indeed many of them can only be properly seen under a microscope. However, even these tiny life forms are far from insignificant. The innovation of a body skeleton, along with a different way of breathing and of maintaining internal water levels, is what enabled the vertebrates to become much larger in body size than any land invertebrates. However, in the deep seas some truly huge invertebrates exist, and if size is measured by biomass (the weight of all individuals combined), then invertebrates easily outweigh vertebrates in all environments. Among them are the world’s most agile aeronauts, the most long-lived survivors, the

bearers of the most deadly venoms, and the owners of the most advanced sensory systems on Earth. And, although some of them look quite alien to our vertebrate-biased eyes, their beauty can be breathtaking.

Various anatomical factors impose an upper limit on potential size of land-dwelling invertebrates. At 4.5 kg (9.9 lb), the coconut crab is the largest of them.

Brittlestars and sea anemones, of the phyla Echinodermata and Cnidaria respectively, both demonstrate radial symmetry, a trait common to several more ancient invertebrate groups.

// Sponges Today, your bathroom sponge is probably made of plastic, but if you have a ‘natural sea sponge’ then this is the remains of an actual living animal – members of the phylum Porifera. This word refers to the many pores and channels in the structure of a sponge, which (when the animal is alive) allow water to pass unimpeded through its entire body. Most sponges live in the sea, but there are also some freshwater species. Porifera cell types

Sponge anatomy

Sponges look more like plants than animals at first glance. They are rooted to the seabed or other static objects, and many of them grow in a stalked and sometimes branched form, each branch a hollow tube with a large central opening. Others are fat and round, such as giant barrel sponges. They are colourful, with bright yellows and purples particularly frequent, and can grow to a tremendous size. A broken-off piece of sponge can attach itself to the seabed and grow into a new individual, but sponges can also reproduce by spawning sperm and egg cells, which unite to form new embryos. Dying sponges can produce ‘survival pods’ called gemmules, which can lie dormant until conditions become suitable for growth. The body structure of a typical sponge is made of a matrix of collagen protein, mixed with a jelly-like substance. Its living cells

are inside, in layers on top of this matrix. They carry out the sponge’s basic functions, such as absorbing its food (scraps of organic matter in the water that pass through it) and generating the collagen that enables the sponge to grow. The ‘glass sponges’ have a different supporting structure – a web-like mesh of hard, glassy spicules, with the living cells in layers deep within this structure. These tough sponges, often found deep in very cold water, can live for more than 10,000 years.

Like plants on land, sponges on the seabed provide structural shelter for other marine animals.

Some bottlenose dolphins use a sponge to protect their snouts, as they root through loose material on the seabed in search of prey.

A sponge lacks the bodily systems that we find in most animals. Its cells are generally undifferentiated, but can change into one of several specialized types (and back again). You might think of it more like a colony of cooperating cells than a complete animal. However, it does influence its own internal environment to some degree. It can control the water flow through its body through valves in its pores, which let water in but not out, and by

beating its flagella (whip-like hairs present on some of the cells) to push the water along. Embryonic sponges can swim and crawl on the seabed before they settle. A few sponges have even adapted to become predators, capturing living prey on sticky threads or spines and then digesting it gradually. Some predatory sponges can immobilize small creatures that enter their channels by using water flow to blow up internal ‘balloons’ that prevent the prey’s escape. The fossil record shows that sponges existed at least 580 million years ago and perhaps as long as 760 million years ago, making them among the very first animals on Earth. Today, about 9,000 species of sponges have been described to science, but there are likely to be many more, living in less well-explored parts of the world’s seas.

Sponges are not as effective at drawing in water as some other filter-feeding animals, but their very open structure helps.

// Comb jellies Vying with the sponges for the title ‘earliest animals to evolve on Earth’ is the phylum Ctenophora – the comb jellies. These animals are similar, superficially, to the jellyfish: they have soft, transparent bodies and in some species possess a pair of long tentacles, and they swim or drift in the sea. However, the two groups are not closely related at all. The name ‘comb jelly’ relates to the rows of tiny hairs or cilia that a comb jelly uses to swim. Rather like a sponge, a comb jelly has a large central opening and a network of other channels (a ‘canal system’) running through the rest of its body, through which water can freely move. The main jelly-like mass of the body is covered by an internal and external layer of living cells. Cells on the body’s outer side grow large swimming cilia, in (usually) eight distinct rows. The outer side also has sensory cells, which communicate with other sensory cells inside the body to form a rudimentary nervous system. The cells on the inside have smaller cilia, which help to control water flow through the internal channels and main body cavity. There are also digestive cells, contractable muscle cells and germ cells (which can develop into egg or sperm cells). The two long tentacles that some of them possess have sticky tips that are used to catch prey, and can be pulled completely into the body. Those that lack these prey-catching tentacles capture their prey by sucking it into their large mouth openings, into a central chamber or ‘stomach’ that douses the prey in enzymes to break it

down. The resultant food and enzyme ‘soup’ then passes through the body channels where nutrients are absorbed by the digestive cells, and waste is expelled through a pair of anal pores. Ctenophore anatomy

The long, paired tentacles sported by some comb jellies are used to ensnare prey.

Only about 150–175 species of comb jellies are known to science at present. The most familiar are the double-tentacled cydippids. Most others have simple egg-shaped or oval bodies, but the cestids have flattened, elongated, ribbon-like bodies. They swim by rippling their long bodies in an undulating motion, and among them are the largest of all ctenophores – some species are

more than 1 m (3.3 ft) long. Most other ctenophores are much smaller – some just a few millimetres in length. These animals are still rather mysterious creatures, although swimmers in north Atlantic and north Pacific seas may encounter those of the genus Pleurobrachia, the ‘sea gooseberries’, in large numbers at certain times of the year. Brushing against them in the water can be unnerving, but is not dangerous, as no comb jellies possess the stinging cells that distinguish the true jellyfish.

The species Cestum veneris, or ‘Venus’s girdle’ is an unusual comb jelly with a ribbon-like rather than oval shape.

Which came first – the jelly or the sponge? Porifera and Ctenophora are the oldest distinct lineages of animals that still survive today. It was long supposed that the sponges must predate the comb jellies as their bodies are so much ‘simpler’, lacking any kind of nervous system or muscle tissue. Fossil evidence for comb jellies is very rare because of their soft, gelatinous bodies, but evidence from their genetic make-up suggests that their lineage may in fact be even older than that of the sponges. It is even possible that modern sponges actually evolved from more complex ancestors. It is always important to remember that evolutionary time does not always equate to increased complexity. If a simpler body form and lifestyle is better for survival, then this is what will develop over time.

The function of bioluminescence in comb jellies is not known with certainty, but may be an adaptation to discourage predators.

// Cnidarians The large phylum Cnidaria is home to a diverse range of sea animals. The 11,000 or so species share one distinctive and devastating trait: their bodies contain a special type of cell called a cnidocyte, which has one function – it stings. The cnidarians use their stings to kill their prey, and to defend themselves against attackers. They are mostly soft-bodied animals, but their cnidocytes make them far from helpless, and among their number are some of the most dangerous animals on Earth.

The Portuguese man o’ war has two distinct body parts – the floating, sail-topped pneumatophore or bladder, and the trailing mass of deadly tentacles which can reach 30 m (98 ft)in length.

The most familiar members of the group are the sea anemones and the jellyfish. The former are sessile (rooted to the spot) in their mature form, while the latter swim freely, but their bodies are otherwise rather similar structurally. Both are equipped with a mass of tentacles, which deliver multiple stings to any prey unlucky enough to walk or swim into them. The other cnidarians are the corals and the hydras. The former group are well known for the calcium carbonate skeletons some of them

secrete, which eventually form reefs. The latter are a very diverse group of marine and freshwater sessile and swimming animals, most very small in size, although the group also includes the remarkable colonial-living siphonophores, such as the Portuguese man o’ war. These animals are formed of a colony of seven different kinds of genetically distinct ‘zooids’, each with a different appearance and function.

There are about 50 species of box jellyfish known to science, a few of which are among the most deadly (to humans) animals on Earth.

Although many cnidarians become sessile eventually, they begin life in a freely swimming larva or planula form. They will then either attach to the seabed in a polyp form, or mature into a free-swimming medusa form. At least one species, the immortal jellyfish, develops as a colony of polyps, from which medusae bud off. These medusae are able to revert to polyps, and back to medusae again, an indefinite number of times, making them biologically immortal.

A clownfish’s ‘home anemone’ is a safe haven indeed – only the most foolish or reckless predator would pursue it into that deadly forest of tentacles.

Unstung hero Clownfish families famously shelter from danger within the tentacles of a favourite ‘home’ sea anemone. Often, the would-be predator is lured close enough to become lunch for the anemone instead. But why does the clownfish itself not get stung?

A cnidocyte contains a structure that resembles a spear on a coiled spring, sitting within a venom-filled capsule. When the cell is stimulated, by water movement or the presence of certain chemicals in the water, it ‘fires’ its spear into the victim and then squeezes out its venom. With many of these cells firing at once, even large prey can die very quickly. The clownfish, though, has a particularly thick mucous coat that protects it. Additionally, a clownfish may gradually ‘train’ an anemone to accept it. By making light contact, it receives a few stings that don’t do too much harm, but give it a dose of the anemone’s chemical antigens. Eventually, the fish’s chemical signature is so similar to the anemone’s own body that its cnidocytes stop reacting to the fish.

LIFE ON CORAL REEFS The coral reef, with its teeming, colourful life, is a key ingredient of any tropical paradise setting. However, reefs can also form in temperate and even polar waters, and some reef systems are in deep rather than shallow waters. The classic reef, though, develops in warm, shallow and clear water, and, like most other ecosystems, depends on sunlight and the power of photosynthesis for its energy. This harnessed energy is passed onwards through the food chain to sustain plant-eating and predatory animals alike. The photosynthesizers in a reef include marine plants, but also zooxanthellae, a group of single-celled, photosynthesizing organisms that can live inside the body tissues of other, larger organisms – especially reef-building corals. The zooxanthellae provide the coral with oxygen as well as energy in the form of a food supply. If environmental conditions (especially a rise in water temperature) cause the corals to expel their zooxanthellae, the result is coral ‘bleaching’ – the coral polyps lose their colour and then die. As we have seen, corals are cnidarians – soft-bodied animals with feeding tentacles. Like anemones, they are polyps (fixed to the spot rather than free-swimming). Those that form reefs live in dense colonies, and all individuals in a colony are genetically the same, because they reproduce through new animals ‘budding’ from existing ones, or by one polyp dividing into two. Their stings

are not usually as powerful as anemone stings, but they have another means of defence – they secrete a hard and stony physical shelter for themselves. This casing, made of calcium carbonate, is the hard structure of the reef. The coral polyps’ bodies are protected within it, and their feeding tentacles extrude out into the water to trap tiny prey and other food particles – most feed only at night when there is a lower risk of them being attacked by predators themselves.

Corals form a complex habitat which provides home and shelter to a wide array of other marine animals.

Locations of the world’s largest and most biodiverse coral reefs.

Ring-shaped coral atolls form around the mouth of a submerged and eroded volcano.

Reefs support a huge number of other animals. The corals themselves form a great range of shapes – some are branching, some are round and mound-like, and some form flat plates or fan shapes. They give the seabed a complex structure, full of nooks, crannies and other hiding places, so offer safe shelter for young fish and other vulnerable animals. They also calm down the incoming waves, and in many cases form a lagoon of relatively still, warm water between the open sea and the land. Many fish come to reefs and lagoons to spawn. The deep-water corals,

which survive without zooxanthellae, also provide habitat for all kinds of other animals, as deep underwater as 2,000 m (6,560 ft).

Some soft corals have a delicate, branching structure, strongly reminiscent of plant foliage.

The bizarre ‘wire corals’ form a long single strand that often assumes a coiled or spiraling shape.

When coral polyps die off, their calcium carbonate skeletons remain, and still provide a habitat for other animals. However, without the living coral animals, the reef will no longer grow, and the loss of the zooxanthellae creates a big hole in the food chain, which has a devastating knock-on effect on all the reef’s other living things.

// Echinoderms The word ‘echinoderm’ means ‘spiny skin’, and many of the animals in the phylum Echinodermata are conspicuously spiny or prickly. They include the sea urchins and sand dollars, the sea stars or starfish, the tube-shaped sea cucumbers, and the manyarmed crinoids (or ‘feather stars’) and brittle stars. All echinoderms live in the sea, many of them in the deep ocean, and there are about 7,000 known species. These animals have an unusual type of body symmetry called pentamerism, or five-fold symmetry – the body is divided into five equal parts around a central point. This is most obvious in a five-armed sea star, but is present in other echinoderms, too. Beachcombers in some parts of the world will often find the tests (hard outer shells) of dead sand dollars, and these are clearly and beautifully marked with a five-leaved pattern. However, echinoderm larvae show bilateral symmetry (their left and right sides are mirror images) and often have several swimming ‘arms’. The larvae are much more mobile than the adults and swim freely in their early lives, until they begin to metamorphose into their adult forms. Another feature that the echinoderms share is a ‘skeleton’ made of hard plates, below their outer ‘skin’. In some cases, such as the sea stars, these can move relative to one another with a lot of flexibility, but they can also ‘lock’ into place if necessary, making the body go suddenly rigid. This can help protect them

from predators. Their undersides have numerous little projections called tube feet, which in most types of echinoderm enable them to crawl along (though often they can only move very slowly). Echinoderms have great powers of regeneration. A starfish that loses an arm can often regrow it, and it is even possible in some cases for an entire new starfish to grow from one severed arm. This is because each arm contains the same digestive, reproductive and nerve tissues. Anatomy of a starfish

Hunting method

Echinoderms have a central mouth (which doubles up as the anus in some species). Some use their arms to capture prey, while some starfish actually turn their stomach inside out to engulf prey, after which they pull it back into their body. The crinoids wave their long arms in the water – the arms have sticky tube feet that trap prey, which the arms then pass towards the mouth. The sea urchins are vegetarian, grazing algae from rocks on the seabed. Their long, sharp spines are for self-defence and they can move them around in response to stimuli – if you touch one, all the spines near that area will angle themselves towards the spot you touched.

Sessile crinoids are also known as ‘sea lilies’, though most species are free-moving and can swim and crawl along the seabed.

The tests of the flat sea urchins of the order Clypeasteoida look like pretty coins, hence the group’s popular name of ‘sand dollars’.

Like a lot of marine invertebrates, echinoderms can reproduce both sexually and asexually, though the former is more common. In most species, there are separate males and females, which breed together by releasing their respective gametes. These intermingle as they are released into the water. In many species, there are distinct breeding seasons, when numerous individuals come together in large gatherings to release their eggs and sperm. In a few species, the parents take care of the fertilized eggs until they hatch – for example, some sea cucumbers carry their eggs around on their backs or bellies.

Each spine on a sea urchin is formed around a crystal of calcium carbonate, as are the plates that make up its test.

The urchin clingfish is a tropical reef species that lives in close association with long-spined sea urchins of the genus Diadem.

// Molluscs – the slugs and snails The molluscs make up one of the largest of all the animal phyla, and one of only a few that has truly conquered the land as well as the waters. Naturally, it is the slugs and snails in our gardens that we know the best (even if the keen gardeners among us might wish we didn’t), along with the few mostly sea-dwelling species that some of us like to eat, such as clams and whelks. In total, there are about 85,000 known species of molluscs, of which the vast majority are slugs and snails. Molluscs have soft bodies and may also have an obvious protective shell or pair of shells. Those with a single shell are the gastropods. Slugs are also included in this group, because they evolved from shell-bearing ancestors, and some still have a small internal shell. Two-shelled molluscs, such as mussels and cockles, are known as bivalves. The shell or shells, formed by secretion of calcium carbonate, provides protection for the animal’s soft body. They are also what allows land snails to live even in very dry areas, as the animal can withdraw into its shell and seal up its opening to protect itself from drying out. Land slugs, though, can only live in damp areas.

Chitons are also known as ‘coat of mail snails’. The wing-like plates of their shell come apart after the animal dies and are often found individually.

These animals have a variety of different diets, feeding methods and ways of getting around. Common limpets stay on their ‘home scar’ while the tide is out and they are exposed to the air. When the tide rises and submerges them, they begin to move through muscular ripples of their ‘foot’, and travel very slowly over the nearby rocks to feed, scraping algae off the rocks with their round, rasping mouths. They return to their scar as the tide

goes out. By contrast, scallops can swim in open water by fluttering their two shells, taking in water and ‘jetting’ it out for propulsion. Like other bivalves, they are filter feeders, extracting particles of food from the water. Anatomy of a snail

A giant clam may live for more than 100 years, and grow to a length of more than 1 m (3.3 ft).

Chitons and nudibranchs (a group of sea slugs) are strikingly colourful marine molluscs that live on the seabed and on reefs. Chitons have shells made of articulating plates, which provide protection but also allow them to curl up their bodies. Most species eat algae. The nudibranchs have soft bodies, often covered with sensitive tentacles. They are predators, and some can ‘steal’ the stinging cells of the hydras and anemones that they eat, using them for their own self-defence. Some other kinds of sea slugs are unique among animals in that they can actually photosynthesize – they eat algae and incorporate the algae’s photosynthesizing chloroplasts into their own body tissues. These molluscs are typically hermaphrodites and reproduce sexually. Each individual produces both male and female

gametes, and when pairs mate, each provides the other with sperm to fertilize its eggs. In addition to an advanced reproductive system, these animals also possess quite complicated systems of internal organs and have well-developed senses, as well as brain-like structures (ganglia).

Some slugs copulate in mid-air, entwined and dangling from a ‘rope’ of mucus. Both individuals evert their reproductive organs during

mating.

The nudibranch Costasiella kuroshimae is capable of photosynthesis, thanks to retained chloroplasts from the algae it eats. Its distinctive face has earned it the nickname ‘leaf sheep’.

// Molluscs – the cephalopods Most of the molluscs are rather slow and ponderous animals, but there is one distinctive group of them that is quite different. The cephalopods are not only fast and agile on their ‘feet’ and through the water, but they also demonstrate considerable intelligence – the larger species are widely considered to be the brightest of all invertebrates. They are the squids, octopuses, cuttlefish and nautiluses. There are about 800 species around the world, all of them sea animals. The nautiluses are the only ones with an external shell; the others have an internal shell (such as the ‘cuttlebone’ of a cuttlefish) or none at all. The name ‘cephalopod’ means ‘head-foot’, because of the often long and prominent tentacles or ‘arms’ surrounding the mouth, which in some octopuses function as walking legs. Cephalopods are predators. Some actively chase their prey, while others are ambush hunters that rely on camouflage, stealth and then a rapid strike to secure their victim. They catch their prey with their tentacles, which have sucker pads that provide grip – when they make contact, they contract to squeeze out water between the sucker and the object they touch, creating a vacuum. Each sucker is controlled individually by a ganglion of nerve cells, like a miniature brain. Once it has ensnared its prey, the cephalopod brings the victim to its mouth, in the centre of the

tentacles. The mouth has a hard ‘beak’ and the bite is venomous – very powerfully so in the case of the blue-ringed octopuses, which are among the most dangerous animals on Earth despite their tiny size. Anatomy of a squid

Internal anatomy

Most cephalopods have large, prominent eyes and a welldeveloped visual system. They use visual signals for communication as well as camouflage. Special cells in their skin (chromatophores) allow them to change colour, at remarkable speed and with great precision. A male cuttlefish displaying to a female can show bright colours on the side of his body that faces her, while showing a dull, camouflaged pattern on his other side. Some cephalopods can also display light – bioluminescence.

Blue-ringed octopuses are very small but extremely dangerous – if touched, they may give a venomous bite that can kill a person in minutes.

Octopuses are usually slow-moving and spend their time of the seabed. They hide from prey (and from danger) by squeezing themselves into nooks and crannies for shelter, or burrowing into the sandy seabed. Squids are more adapted for fast swimming, with fins along the sides of their bodies that allow slow propulsion. For fast movement, they use jet propulsion, taking water into their body cavities and forcefully squirting it out again. If they are darting away from a predator, the outward jet of water will probably be accompanied by a squirt of blackish ink from a gland near the anus – this creates a dark cloud that hides the fleeing squid.

Nautiluses are threatened due to over-hunting – their beautiful whorled shells are prized as ornaments.

Some cephalopods are tiny, but the giant squid and colossal squid are the world’s largest invertebrates. These deep-sea species are little known, but the latter, found in the Southern Ocean, is believed to reach weights in excess of 600 kg (94 st). Among the many extinct cephalopods known to science are the ammonites, shelled animals, similar in appearance to nautiluses, that died out at the same time as the dinosaurs. Their fossils are among the most abundant on Earth.

// Nematodes Despite their huge abundance and diversity on Earth, the nematodes or roundworms are rather little known by the average human. The closest many of us come to thinking about them is when we give worming treatments to our pet cats or dogs. Yet many nematodes are not parasites but free-living animals, and there is barely a square centimetre of the Earth’s surface – in the damp topsoil and underwater – that is not alive with them. The phylum Nematoda contains an estimated 40,000 species, and if you count up all the individual animals living on our planet, about 80 per cent of them are nematodes. It has been said that if all the solid matter making up the Earth’s topsoil were taken away except the nematodes, every detail of landscape would still be clearly described in the film formed by their massed bodies – even the whereabouts of other animals could be determined by the presence of their nematode parasites. It goes without saying that such abundant animals are of great ecological importance. Nematodes are very small, cylindrical animals, with most freeliving species measuring between 0.1 mm to 2.5 mm (0.004–0.1 in) long. Some of the parasitic species can be much larger, even exceeding 1 m (3.3 ft) long. They often have bristles or rings on the outside of the body, but they do not show the segmentation of the annelid worms (such as earthworms). Although they look cylindrical, their bodies actually have bilateral symmetry. Most of

them have separate male and female individuals and reproduce sexually – in some cases, the fertilized eggs develop and hatch inside the bodies of the females and the juvenile worms then consume their parent’s body.

The sensory system is rudimentary – their body bristles provide a sense of touch, and there are structures on the head end that function as organs of smell or taste, but most appear to

be completely blind. The digestive system is also quite simple, with no distinct stomach, just a straight tubular gut that absorbs nutrients through a layer of cells along its length. The mouth is adapted for sucking, and in many cases also has a sharp stylet, which can pierce holes in food and then be used as a sort of drinking straw. Some feed on smaller animals or even bacteria, ingesting them whole, while others attach themselves to living animals, plants or fungi to feed. Many of the parasitic species are host specific, and may need more than one kind of host to complete their life cycle. Some species of nematodes are used in gardens and in agriculture as biological control agents, to attack and destroy crop pests.

Nematodes are relatively simple organisms but their enormous global abundance demonstrates that this is no barrier to evolutionary success.

Humans host about 35 nematode species, which go by names such as hookworms, threadworms and whipworms. They are not usually dangerous but can cause unpleasant symptoms. The species Dirofilaria immitis is passed to mammal hosts via bites from its intermediate hosts, several kinds of mosquitoes. Once inside the mammal host, it lives in the right side of the heart and the lung arteries. The most common host is the domestic dog, in which infection with Dirofilaria immitis can be fatal if it is not treated in time.

The root-knot nematodes are parasitic on plant roots, causing the formation of swollen galls and draining the plant of nutrients. Some species can be serious crop pests.

Exhaustion is a symptom of infection with Dirofilaria immitis, or ‘heartworm’ in dogs. The infection is usually well established by the time symptoms appear, but preventative medications are widely available.

// Arthropods – the most diverse invertebrates So far, most of the animals we have met live in the water, or at least in a watery film, and move by swimming or wiggling along. The arthropods’ great innovation – the rigid but jointed legs that give them their name – enable them to move over solid objects in a whole new range of ways. They can walk, run, climb and leap. Not only that, but one group of arthropods has also evolved the ability to fly. It is little wonder that these are the most familiar and visible of all invertebrates. The arthropod body is built as a system of repeating segments. Each segment contains its own self-contained (though interconnected) set of equipment, including muscles, and a ganglion or nerve bundle. Each also has a pair of segmented appendages, each of which has a jointed ‘leg’ branch and a feathery ‘gill’ branch. In the simplest arthropods, the segments and appendages are all very much alike. The extinct trilobites, for example, were simple water-dwelling arthropods whose fossils show a simple structure of repeated segments that look much the same from head end to tail end. In more recently evolved arthropods, the body segments may be highly modified. In land insects, for example, the gill branches of the appendages are mostly lost, and the jointed ‘legs’ are tremendously varied in appearance – they may function as parts of the mouth, antennae

or reproductive anatomy, depending where they are on the animal’s body. Some segments have no appendages at all. The segments vary a great deal in size and shape and are organized into distinct regions.

The phylum Arthropoda contains the most species of any animal phylum. More than 80 per cent of all described animal species are arthropods, and there are probably in excess of 5 million different species on Earth. The earliest arthropod fossils date back a little more than 540 million years ago, during the Cambrian explosion – a period when many modern animal phyla first appeared on Earth. The Burgess Shale fossil beds in Canada, which are about 505 million years old, hold the remains of an amazing array of sea-dwelling arthropod life, many of them the long-lost representatives of lineages that left behind no descendants.

Trilobite fossils show very clearly the repeated, segmented shape and jointed appendages characteristic of arthropods.

The modern arthropods comprise several distinctive and familiar groups. There are: the myriapods – centipedes and millipedes; the crustaceans – lobsters, crabs and their relatives; the chelicerates – spiders, scorpions and similar animals; and the hexapods – the insects and a few other six-legged animals. All

four of these groups include water-dwelling species but also many (the majority of species, in three out of four cases) that are entirely land dwelling. The key to their successful colonization of land is the fact that their segmented bodies and appendages have a rigid outer covering or cuticle, allowing them to retain water in their bodies while still being able to move freely. It does also mean, however, that they need to moult their cuticles in order to grow larger.

Marella, or ‘lace crab’, was a beautiful arthropod whose fossils are abundant in the 505 million-year-old Burgess Shale fossil beds.

The way that land invertebrates obtain oxygen from the atmosphere places limits on their body size, but during the Carboniferous period (360–299 million years ago), the Earth’s atmosphere was much richer than it is today, and during this time there were some extremely large land arthropods, including a 2.5 m (8.2 ft)-long millipede and dragonfly-like insects with a 70 cm (27.5 in) wingspan.

The woodlice and their marine cousins the sea slaters are distinctive crustaceans with seven pairs of legs, and (hidden on their undersides) five pairs of gills.

// Spiders, scorpions and their relatives This grouping of arthropods, the chelicerates, are mostly land animals. There are about 80,000 described species of chelicerates, of which the vast majority are land-dwelling eightlegged arachnids – spiders, scorpions, ticks and mites. The marine chelicerates include sea spiders and horseshoe crabs. The spiders have long held humankind’s fascination and respect, because of their web-building skills and their venomous bite, which in a few species is powerful enough to kill an adult human. Spiders produce their silk from glands, and extrude it through modified appendages called spinnerets on their abdomens. Their webs may be flat sheets, with strong supporting threads and sticky, prey-holding cross-threads, or complex threedimensional tangles. Some spiders use silk to make a sticky ball that they throw at their prey. Some baby spiders create silk ‘parachutes’ that catch the air, enabling them to drift long distances and thus disperse into new habitats. Silk has qualities of toughness, light weight and elasticity that have yet to be matched by any artificial material. Spider anatomy (female)

The mites on this bumblebee’s back are ‘phoretic’; they use it for transport. They live harmlessly in bees’ nests but may hitch a ride to a flower, and board a new bee to travel to a new nest.

Virtually all spiders and all scorpions are predators, and both also produce venom, used to kill prey and also in self-defence, though spiders deliver theirs through a bite from their fangs and scorpions through a sting at the end of their abdomen. Both mainly prey on other invertebrates, especially insects. While many are ambush hunters, some (such as the jumping spiders) actively pursue prey.

Horseshoe crabs are regarded as ‘living fossils’, as modern species do not look very different to their 240 million-year-old fossil relatives.

The spiders and scorpions have well-developed senses, and both can show complex behaviours. Living solitary lives and being hunters makes the process of courtship and mating fraught with danger. Female spiders are usually much larger than males and will happily kill and eat would-be partners. Some males distract the females with a silk-wrapped gift of food – this keeps her busy, allowing him to mate and then escape. When male and female scorpions meet, they face one another and perform a courtship dance, before the male deposits a package of sperm on the ground, which the female picks up via an opening on her abdomen.

Baby scorpions have suckered feet with which they cling on to their mother’s carapace, for up to 25 days.

Ticks and mites are small and relatively simple-bodied chelicerates. Ticks are parasites, attaching themselves to the skins of mammals, birds, reptiles and amphibians, and feeding on blood. Most mites are not parasitic but live in soil or in animal nests, and eat dead organic matter. They can sometimes be seen hitching a ride on the backs of insects such as bumblebees and scavenging beetles, which will carry them to food sources (the bee to its nest, full of organic detritus, and the beetle to a dead animal). There are also some parasitic and some predatory species. The chelicerates also include the solifuges and harvestmen, both of which resemble spiders. The horseshoe crabs are ten-

legged, vaguely crab-like marine animals with a large round carapace. They have shown very little change in appearance over their evolutionary history. Sea spiders look like extremely spindly versions of land spiders. Their very high surface-area-to-volume ratio enables them to absorb oxygen efficiently from the water. Like their land cousins, the marine chelicerates are predatory.

Jumping spiders’ huge eyes are not just for spotting prey, but for watching each other’s often elaborate courtship dances.

// Myriapods The myriapods have more legs than any other animals on Earth, although their names do carry more than a hint of exaggeration – centipedes do not have 100 legs and millipedes do not have 1,000. Technically, the name ‘myriapod’ actually means ‘10,000 legs’, though ‘myriad’ is now more generally understood to mean simply ‘many’. Centipedes bear one pair of legs on each of their segments, which in some cases adds up to fewer than ten in total, while millipedes appear to have two legs on most of their segments – but this is because each apparent segment is actually two, fused together. The species Illacme plenipes has the largest number of legs – up to 750 when fully mature, on a body up to 3 cm (1.2 in) long. There are about 16,000 species of myriapods worldwide, of which the majority (12,000) are millipedes. They are nearly all land animals, and in fact the earliest land animal in the fossil record was a millipede. The group also includes the symphylids, which resemble tiny, translucent centipedes, and the pauropods, which look more like squat, wingless insects, except that they have too many legs. Centipedes typically have longer, more splayed legs than millipedes – the span of the legs can be considerably wider than the body in some cases – and have long antennae. They are fast runners and are predators, chasing prey and disabling it with a venomous bite, delivered by sharp-tipped forcipules – a pair of specialized appendages on the head that

work like pincers and are unique to centipedes. A bite from one of the larger centipedes (some are up to 30 cm/12 in long) can be extremely painful.

Giant millipedes are very striking to look at and have a docile nature and a lifespan of up to 10 years, making them popular among exotic pet-keepers.

Pill millipedes can roll up their bodies completely, protecting their vulnerable undersides – this trait is shared with the unrelated pill woodlice.

Millipedes are usually scavengers of decaying plant matter, or in a few cases they eat living plants and fungi, but only a handful of species are predators, and none has a venomous bite. Most have short legs and tubular bodies, and move slowly, making them rather less alarming than the rapidly scuttling centipedes. The largest species, Archispirostreptus gigas, can reach nearly 40 cm (16 in) – this African species can live for up to 7 years. Like many other millipedes, it will curl into a tight ball if threatened,

protecting its soft underbody, and it can also secrete a stinging fluid from pores in its cuticle. Myriapods reproduce sexually, sometimes showing elaborate courtship behaviour before they mate. In centipedes, there is no direct mating – the male deposits sperm and the female collects it. The female may lay up to 300 eggs after mating. She will then usually abandon them, but in a few species the mothers care for their young after the eggs have hatched. As the youngsters grow and moult, they gain more segments and therefore more legs, until they reach full adult size.

Centipedes of the group Scutigerimorpha have extremely long legs and can move at startling speeds.

Ambitious hunters The largest centipedes can kill small birds, mammals, reptiles and amphibians, as well as insects and other invertebrates – the South American species Scolopendra gigantea lives in caves where bats roost, and dangles from the cave roof to capture bats as they fly past. Most centipedes, though, are not specialized in their diets and will eat anything they can catch.

One look at the business end of a centipede, with its impressive venomous forcipules, reveals a seriously well-adapted predator.

// Crustaceans In contrast to most other arthropods, the crustaceans remain a group of water dwellers, although a few have adapted to live on land. Crabs and lobsters get about mainly on foot, and many crabs can live happily with prolonged exposure to the open air – a few are almost entirely terrestrial, though they usually have to live in damp, sheltered environments to prevent too much water loss from their bodies. Some other, smaller crustaceans are swimmers rather than walkers, and the same goes for the larval forms of crabs and lobsters. Tiny, swimming crustaceans of all kinds are a key component of the zooplankton that is so important in marine and freshwater ecosystems. There are about 67,000 species of crustaceans in the world. Many are tiny, almost microscopic, but the group also includes the hefty coconut crab, which, at more than 4 kg (8.8 lb), is the world’s biggest land-dwelling arthropod, and the Japanese spider crab with its almost 4 m (13.1 ft) leg span. Like other arthropods, crustaceans have segmented bodies and jointed appendages (which usually have a gill branch as well as a leg branch). The number of legs is variable – in crabs, lobsters and their close relatives though, there are ten true legs (hence the name ‘decapods’ for this group).

Daphnia are tiny crustaceans; they are known as ‘water fleas’ because of their erratic swimming style, powered by oversized antennae.

Male fiddler crabs guard a nesting burrow, and display to attract females to it by waving their overgrown ‘major claw’ in the air.

Most crustaceans are scavengers, feeding on both plant and animal matter. The strong claws of crabs and lobsters are there for self-defence purposes, though they may also be used in displays of dominance and in territorial battles with others of their kind. A few crustaceans are parasitic, such as the fish lice. (These are not related to land-dwelling lice, which are insects.) The barnacles begin life as free-swimming larvae but once they mature they become sessile (non-moving), settling on to a rock or other structure and secreting a sheltering shell of calcium carbonate around themselves.

Hermit crabs have soft, unarmoured and asymmetrical bodies which coil into their chosen shell.

When not actively seeking food, or a mate, crabs and lobsters tend to hide in sheltered crevices or bury themselves in a soft substrate. The hermit crabs find a portable shelter, usually the empty shell of a gastropod mollusc. This must be big enough for them to withdraw into completely, and as they grow they will need to upsize to larger accommodation. They also need to come out of their shells at least partially in order to mate. Crustaceans mainly reproduce sexually – for the sessile barnacles, this has meant the evolution of nature’s largest (relative to body size) male appendage, so that they can mate with neighbouring barnacles without any of them needing to move.

Spiny lobsters are noted for their long-distance seasonal migrations, in which they walk across the seabed in long lines, navigating by sensing the Earth’s magnetic field.

A long walk home For many crabs, mating happens en masse. Since their eggs are food for many other animals, releasing an overwhelming number of eggs all at once improves the chances of survival for a good proportion of them. Christmas Island red crabs, which live on land, migrate to favoured breeding sites on beaches once a year, stopping traffic on the island as they travel in vast groups. Males dig and defend burrows, in which females lay and guard their eggs. Spiny lobsters are also noted for migration, walking across the seabed in long single-file chains when the seasons dictate a move to new feeding grounds.

Christmas Island’s red crabs stop traffic every year when they migrate en masse to their breeding beaches.

// Insects – an introduction Wherever you are right now, the chances are good that you’re not far away from many insects. These mostly land-dwelling, mostly winged arthropods represent perhaps the most successful colonization of land by any animal group. More than a million different species are known to science, and at least that number again (possibly far more) have yet to be found and named by us. The first insects probably lived more than 420 million years ago, with definitive fossils of winged insects dating back more than 400 million years. They likely evolved from a lineage of crustaceans. Their diversity expanded enormously during the Carboniferous period (360–299 million years ago), when the first forests covered the Earth and a very high level of oxygen in the atmosphere made the land more hospitable to air-breathing animals. Modern insects are distinguished by having six legs, a pair of antennae, compound eyes (eyes made of numerous selfcontained light-sensing ‘units’ called ommatidia) and a segmented body, with the segments grouped into three distinct parts – head, thorax and abdomen. There are a few other arthropods with six legs that are not always considered to be true insects (these are the springtails, bristletails and coneheads). Insects form two major groups based on their life cycles. The hemimetabolous insects go through ‘incomplete metamorphosis’. Their immature form grows larger through regular moults, and in

the last moult its adult form (with mature reproductive organs and, in some cases, wings) emerges. In holometabolous insects, the metamorphosis is known as ‘complete’ and there is a pupa stage in between larva and mature, winged adult.

Holometabolous insects transform dramatically through their life cycle, while hemimetabolous insects generally just become larger, attaining wings and sexual maturity after their last moult.

This tree diagram shows the evolutionary pathway of the insects as their lineage split into several distinct groups (orders), and shows the approximate number of described species for each order.

// Insects – the plant eaters and scavengers The majority of insects on Earth are plant eaters, although the way that they eat plants varies depending on their mouth anatomy. The caterpillars of butterflies and moths have simple biting jaws and mainly eat leaves. Larger caterpillars nibble the softer parts of a leaf away completely, while tiny ‘leaf-mining’ caterpillars actually live between the upper and lower surfaces of the leaf and eat the tissues in between, leaving behind a transparent pathway that widens as they grow. The true bugs have mouthparts that are adapted into sharp-tipped sucking tubes, and only take a liquid diet. Many true bugs are plant eaters – aphids, for example, are herbivorous true bugs that feed by piercing plant stems and sucking up the sap. Some beetle larvae have strong enough jaws to chew through solid wood. Planteating insects may be generalists, or they may only live on one particular species of plant.

Termites and elephants are both important modifiers of the African savanna landscape, in their very different ways.

Plants don’t necessarily want to be eaten by insects or any other animals, and have evolved defence mechanisms, including storing toxic substances in their cells. This often discourages the generalist plant eaters, but you will often find one or a few insects that have adapted to eat a poisonous plant species, and even use its toxicity to their advantage. For example, the caterpillars of the cinnabar moth eat ragwort and store its toxins in their bodies so that they, too, are poisonous and bad-tasting. The poisons stay in their bodies as they mature into moths, and both caterpillar and moth have bright warning coloration. This helps to remind any predator that eats one cinnabar moth or caterpillar – and regrets the experience – not to do it again.

Many insects feed on whatever non-living organic matter they happen to find. These animals could be called ‘scavengers’, although that name tends to be associated more with those that eat animal remains. A more accurate name would be ‘detritivores’ – animals that feed on organic detritus, regardless of its origin. Insects such as these, which include many beetles, earwigs and cockroaches on land, and in the water the larvae of mayflies, stoneflies and caddisflies, are nature’s waste-disposal workers, recycling nutrients into forms that other living things can use.

Cockroaches are extremely unwelcome house guests for humans, although most species do little harm.

Several kinds of insect larvae create ‘leaf mines’ by eating the cells between the waxy cuticle on the leaf’s outer surface.

For humans, plant-eating insects can be highly problematic. Gardeners are constantly at war with aphids, and in some parts of the world a surge in the locust population can lead to a devastating famine. When we clear some land and plant a swathe of a particular crop that we want to harvest and consume, we create an all-you-can-eat buffet for any insect species that also likes to eat that plant, and those insects can proliferate to population densities that you would never observe in a natural environment with a diverse mixture of plants. For this reason, we have developed powerful pesticides, but these kill off ‘good’ as well as ‘bad’ insects and have a severe knock-on effect through the whole ecosystem. Today, we are aware of the importance of

less deadly ways to control crop pests, but overuse of pesticides is still a global problem for wildlife.

The spectacular true bug Phrictus quinquepartitus, or ‘dragonheaded bug’, is found in Central and South America and feeds on

tree sap.

// Insects – the pollinators Plant tissues are not always rich in proteins, which insects – like all animals – need to eat for their growth and development. This is one of the reasons why some plant-eating insects have such a voracious appetite. One part of a plant, though, is very protein rich – the pollen produced by flowers, which carries its male reproductive cells. Many insects have a taste for pollen, but in this case, their attentions can benefit the plant. Flowers need to spread their pollen to other flowers, as this is how they form their seeds and reproduce. More primitive plants rely on the wind to carry out this task, but insects can provide a much more accurate and efficient alternative. Thus, a co-operative relationship between flowers and pollinating insects has evolved. The relationship between a plant and its pollinator is, on the face of it, simple. Most flowering plants reproduce sexually and most flowers have both male and female reproductive equipment. The insect visits flower A and picks up pollen on its body from the stamens (male reproductive parts). This is transferred to the stigma (female reproductive parts) of flower B. The insect may eat some pollen, but flowers produce enough of it to go around. Many flowers also provide the insect with an extra food source – nectar. This sweet fluid is there solely to entice pollinators, and some (such as butterflies and moths) don’t want pollen at all, only nectar. Any insect that visits flowers, for whatever reason, is a potential pollinator. The ones that come to mind straight away

are the bees, but wasps, hoverflies, beetles, butterflies and moths are all also important pollinators.

The vital importance of bees (both wild species and the domestic honey bee) as pollinators has led to strong interest in their conservation.

Its extremely sugary diet allows the hummingbird hawkmoth to maintain one of the highest possible metabolic rates of any animal.

Pollinators are crucial parts of the ecosystem, as they allow plants to reproduce (which also provides food for the many animals that eat fruits and seeds). Many plants are ‘annuals’ – they live for only 1 year, and so would die out straight away if they did not successfully reproduce each year. Pollinators also improve the plants’ genetic diversity by allowing them to ‘mate’

with other plants that are not in their immediate vicinity. Not all key pollinators are insects – some plants are pollinated by hummingbirds and some by bats, for example – but there is no doubt that without pollinating insects, life on the land as we know it could not continue.

Not all pollinators are insects. Flowers that have co-evolved with hummingbird pollinators often have pendulous flowers that the birds can easily access in flight.

Secret conversations When we look carefully at the structure, colours and patterns of flowers, we start to notice many adaptations that have been guided through their evolution by their relationship with their pollinators. Under ultraviolet light, which is visible to insect eyes, flowers show bold and striking patterns, guiding insects to their nectaries – the part that secretes nectar. The flowers’ shapes allow insects to access their nectar while making contact with the pollen-bearing stamens, and their scents attract pollinators from some distance away. Plants can even respond in real time to the presence of pollinating insects: some flowers produce extra supplies of extra-sweet nectar in response to ‘hearing’ (through vibration of their petals) the buzz of bumblebees’ wings.

As a pollinating insect moves from one flower to the next, it unintentionally fertilizes the second with the pollen of the first. Some insects eat pollen, but their body hairs still pick up plenty more of the dusty grains for the flowers to use.

// Insects – the predators Hunters and killers come in all shapes and sizes, and some of the most accomplished and devastating of them can be found in the insect world. Insects from a wide range of different groups have evolved predatory habits and diverse hunting tactics that rival anything you might see in ‘higher’ animals. The battle between a plant and a herbivore is played out primarily over long timescales – the plant evolving increased resistance, and the herbivore evolving ways to overcome that resistance. Between a predator and its prey it is the same – evolution is at work on both of their species over time, but there is also a very obvious real-time, physical struggle between them as individuals, with the predator striving to make the kill and the prey actively trying to escape. Usually, the prey is another insect, but a few insects are large, fast and strong enough to take down small vertebrate animals. Some predators rely purely on speed and power to catch up with and bring down their victim. In the insect world, this includes the fast-flying dragonflies and fast-running tiger beetles. Others are ambush hunters, hiding and waiting for prey to come along, before making a high-speed attack. The mantises are impressive examples of the latter. They have extreme camouflage and great patience, sitting very still on a plant for hours on end as they wait for prey to venture within arms’ reach. The larva of the antlion actually builds its own trap – a conical pit dug in loose

sand. When an insect wanders into the pit, it begins to slide down and is seized by the huge-jawed antlion larva hiding at the bottom.

A stunning (and stunningly well-camouflaged) flower mantis lies in wait for a flower-loving insect to land in its embrace.

Damselflies are arch-predators, and the larger species have no qualms about hunting their own smaller relatives.

Most predatory insects have strong and sometimes barbed and spiky front legs, which they use to grab their victim. Dragonflies in their immature form have a remarkable alternative to this: a barbed labium (part of the mouth – you could think of it as a lower jaw or lower lip) that can be fired out to pierce and hold prey. Predatory insects rarely kill their prey before beginning to eat it – as long as they can hold it reasonably still, there is no need. Wasps, though, will use their stings to help subdue their prey. These insects hunt to feed their larvae rather than themselves. The social wasps and hornets will snip off the wings and legs of prey before carrying it back to their nest. Some solitary wasps will paralyze their victim with stings, and then carry it into a burrow, where they lay an egg on it. The paralyzed prey then provides a source of fresh, living meat for the larva when it hatches. Other solitary wasps simply lay eggs directly on a living host, such as a caterpillar, and the larvae develop inside the caterpillar’s body as it continues to live – it only dies when the wasp larvae mature and break out of its body to pupate. These solitary wasps are known as parasitoids – they live on a living host and eventually kill it. Those that feed from a host but do not necessarily kill or even seriously harm it are known as parasites. Mosquitoes drink blood from a host, while fleas are also blood drinkers but live long term on their host’s body. Most species of lice live on a host – many birds carry a sizeable population of feather-chewing lice, for example.

This brood of shieldbug eggs is under attack by parasitic wasps, which will lay their own eggs inside the shieldbug eggs.

INSECTS IN ECOLOGY AND ECONOMICS Given how abundant they are, living in almost every habitat on Earth, it’s no surprise that insects are enormously important to other life on Earth, including human life. If we look at our planet’s animal life by biomass – the collective weight of all living individuals – insects and other arthropods make up 42 per cent of it, even though the average insect weighs just 1–4 mg. The social insects have a particularly powerful effect on their local ecologies – especially the ants. Their nests can be enormous and in some cases are interlinked to form ‘cities’, the largest of which (on Hokkaido, Japan) holds 45,000 nests, an estimated 307 million ants, and covers about 670 acres of underground real estate. Ants are predators but also scavengers, and even ‘farmers’ – they protect and guard aphids from predators, and feed on the honeydew that the aphids secrete. The social bees form a huge and efficient pollinating workforce, while social wasps are both predators of plant-eating insects and pollinators of many kinds of flowers, thus having a very important positive effect on agriculture. Termite mounds in savanna areas actually change the landscape, providing ‘islands’ of high-quality soil in their vicinity that supports a different array of plant life (and associated animal life) than is found in the surrounding countryside.

Wholesale destruction of insect populations through crop-spraying a broad-spectrum insecticide can be ecologically disastrous.

Insects are also food for other animals. Well over half of all bird species are primarily insectivorous, as are a high proportion of reptiles and amphibians. The presence and distribution of insects is an accurate predictor of how many ‘higher’ animals will be present in an ecosystem, and a wholesale loss of insects will quickly translate to massive declines in vertebrate animals, up to and including the ‘top’ predators. In the 1960s and 1970s, when the deadly pesticide DDT was widely used in agriculture, one of the most dramatic consequences was a huge decline in birds of prey. This was not just because their prey species were declining, but also because the pesticide was passed on through the food chain, so literally an entire ecosystem was poisoned.

Joining together to form a living bridge, ants demonstrate how teamwork allows them to exploit opportunities that may be out of reach to other insects.

In eastern Africa, red-and-yellow barbets often nest inside termite mounds, using their strong bills to dig out a burrow.

The insects that eat our crops can have a devastating economic impact and even cause deadly food shortages, but other insects can come to our rescue. Biological pest control involves using the pest insects’ natural enemies against them, and this includes parasitoid wasps. Many abundant herbivorous insects have one or more host-specific parasitoids. In natural situations, some herbivorous insects’ populations show dramatic cyclical fluctuations – when their numbers ‘boom’, so do the numbers of their parasitoids, which brings their numbers back down again quickly. Using this natural process can be a very effective way to control a specific ‘pest’ insect, without the massive collateral damage of heavy pesticide use.

About half of all the world’s bird species are insectivorous, between them eating some 400 million tons of insects each year.

It is well known and appreciated today that our survival as a species depends on pollinators. About 80 per cent of the world’s flowering plants (including a high proportion of crop plants) are pollinated by animals, and the vast majority of these pollinators are insects. We cannot afford to ignore this – it is essential that we take care of our planet’s insect life.

// Annelids If you have spent much time looking at the natural world, you may not be that surprised to learn just how abundant our planet’s insect life is. Insects are, after all, quite noticeable despite their small size. Another very substantial chunk of the world’s biomass is made up by a much less visible group of invertebrate animals – the annelids. The annelids that you know the best are probably the earthworms. Long, slim, burrowing animals with segmented bodies, they are inhabitants of our soils and consume all manner of organic material, converting it into more soil; their digestive system extracts nutrients and then they excrete the indigestible parts of it in the form of a worm cast. In the process, they mix up and aerate the soil and make its minerals more easily accessible for plant roots. They are as important for the underground ecosystem as insects are on the surface, and they are also a vital food supply for many other species, including numerous burrowing vertebrate animals, such as moles.

The squiggles of sand left behind by lugworms indicate how much life is present, though out of sight, in estuarine mudflats.

Another group of annelids are the ragworms, which are scavengers or predators that live in the sea. They are named for the long, leg-like outgrowths on either side of their bodies, which make them look a bit like frayed pieces of cloth. The rest of the

annelids comprise a variety of types of worms, including the lugworm, which is abundant in beach mud and sand, leaving its distinctive casts as squiggly coils on the surface. This animal is important prey for shoreline birds, and is also collected in large quantities by fishermen, who use it as bait.

Earthworms are very much a gardener’s friend, helping to aerate soil and recycle nutrients.

More than 22,000 species of annelids are known to science. They all have segmented bodies, the segments often very obvious. Each segment has much the same internal anatomy and the same set of organs. The outer surface is covered by a cuticle that helps protect against water loss through evaporation, but is still somewhat stretchy, and in many cases is also bristly, giving extra traction on the ground as the animal moves. Under the cuticle is a layer of circular muscle that allows the animal to move along and wiggle its body with great flexibility, meaning it can squeeze into small spaces. Leeches are particularly flexible and their entire body shape can change dramatically, from long and thin to almost spherical. Most annelids have only a rudimentary sense of sight, but are very sensitive to smells and touch.

It may come as a surprise to learn that leeches are still used medicinally today, mainly to improve blood circulation after

surgery such as skin grafts.

// Other invertebrates We have taken a look at the Earth’s best-known groups of invertebrate animals, but there are a number of others that are much less familiar to us. One example is the phylum Tardigrada —a group of animals that we know better through science fiction than fact, even though they are impressively abundant and live alongside us. The name “tardigrade” means “slow stepper.” These tiny, squat-bodied animals, with eight clawed feet, are also known as “water bears,” and inhabit the watery films that exist in clumps of moss and other similar habitats. They are notable for being able to survive in a variety of extreme conditions. They can be revived after many years of complete dehydration, and some can survive temperatures close to absolute zero or as high as 302ºF (150ºC). Interest in these ‘extremophile’ tendencies led to their being taken into low Earth orbit by a Russian space mission in 2007, to see if they could survive exposure to the vacuum of space—they could.

Bizarrely enough, sea squirts are among the closest cousins to humans out of all the invertebrates.

Many of the other animal phyla are marine dwelling, and are worm-like in form. Among them are the scavenging jaw worms (phylum Gnathostomulida) and the predatory arrow worms (phylum Chaetognatha). Another group of worm-like animals are those in the phylum Platyhelminthes. Some of these worms are free-living, such as the soil-dwelling flatworms, while the tapeworms are internal parasites of various different kinds of vertebrate animals (including humans).

Despite their lack of recognizable facial features, tardigrades are still rather appealing to humans, and studying their extremophile adaptations may help us improve our own technologies.

The rotifers (phylum Rotifera), which used to go by the charming name of “wheel animalcules,” are tiny animals (up to 0.08 in/2 mm long) that live in fresh water and have round, wheel-like mouths with which they eat dead bacteria and other

tiny particles of organic material. You might see them swimming about in a drop of pond water on a microscope slide. The Bryozoa is another phylum of very small aquatic animals. These do not swim but live in static colonies, within a shared “skeleton” of calcium carbonate, which they secrete around themselves, rather like corals. The members of the phylum Brachiopoda secrete an individual shelly shelter for themselves. This has two halves and looks very similar to the double shell of a bivalve mollusc, but the animal inside has very different anatomy.

Although they are very small, with only about 1,000 cells in their whole bodies, rotifers have brains and nervous systems, and up to five simple eyes.

The phylum Chordata is home to all of the vertebrate animals, but also to some invertebrates, including the tunicates or sea squirts. These tube-shaped animals attach themselves permanently to rocks and are rather plant-like in appearance—

some even have plantlike names, such as the sea tulip and the bluebell tunicate. They are filter feeders, sucking water in and absorbing nutrients from it, and they form colonies that grow by budding off of new (but cloned) individuals. Despite their appearance and lifestyle, they are much closer cousins to us than any of the other animals we have looked at in this section of the book.

Tapeworms have rings of hooks on their heads to hold them in place inside their mammal host’s intestine.

VERTEBRATES Vertebrate animals live all over the world, in the seas and on the land. They cannot compete with the invertebrates in terms of number of species, or individuals, but in terms of diversity of body form, and thus adaptability, they are truly exceptional. This is the group to which we humans belong, along with almost all the animal species that we know and love the most.

Playing together helps these red fox cubs practise their hunting behavior.

// The evolution of vertebrates We and all other vertebrates belong to the phylum Chordata, but not all chordates are vertebrates. As we saw at the end of the last section, the tunicates are our chordate cousins, even though they look nothing like us or any other vertebrates. What is it, then, that we have in common with a sea squirt? We can understand our relationship a little better if we look at the tunicate in its first, larval stage of life. As a freely swimming organism, it still doesn’t look much like us, but it does look like another familiar vertebrate—a tadpole. It doesn’t have bones, but it does have a flexible cartilage-like rod called a notochord, and sitting above that is a dorsal nerve cord. These two structures run from front to back inside its body, above its internal organs, and they are both features that are unique to chordates. They are present in vertebrate embryos, and are the precursors to the spine and spinal cord respectively. Chordate structure

The earliest chordates are believed to have evolved about 540 million years ago. Unlike some other animals, such as molluscs and crustaceans, their bodies lacked any hard parts that would fossilize well, so their fossil record is not particularly good. The first true vertebrate, with a bony spine, appeared some 10 million years later—if we saw this animal today, we would have little trouble recognizing it as a fish.

Lancelets have a much simpler body form than typical fish, with no jaw, no fins and no tail.

Today, a handful of other non-bony chordates exist, besides the tunicates. These are the lancelets, and while they do resemble fish they are classified separately. Their slim silvery bodies do not contain any bones, and their mouths are toothless. They suck in water and filter out bits of organic material in their throats, pumping out the excess water through pharyngeal or gill slits. The pharyngeal slit is another anatomical feature that occurs only in chordates, and these slits can be seen in the embryos of humans and other vertebrates, though in air-breathing vertebrates they become remodeled into other structures as the embryo develops.

The mouth of a parasitic lamprey looks like something from a science-fiction or even horror movie—a suction cup full of barbed, flesh-tearing keratin “teeth.”

The first true vertebrates were jawless fishes. There are still jawless fishes alive today—the lampreys and the hagfish. Lampreys have simple tubular bodies without independently moveable fins, and have fixed, sucking mouths that contain

circles of hard tooth-like structures. They have a series of protective supporting rings around their nerve cord, but these are made of cartilage rather than bone—cartilage rings like these were destined to become bony vertebrae in the fish that evolved from ancestral, lamprey-like animals. Today, some lampreys feed by attaching themselves by the mouth to larger fish, and feeding on their blood, but others are non-parasitic and eat by filter feeding. The hagfish are also jawless and finless, and resemble modern eels in their body shape. They have very loose skin and can quickly generate large volumes of skin mucus—these traits help them escape from predators. After making their escape, they tie themselves in a knot and then pass the knot down their body length, which squeezes away the excess mucus. They are the most primitive animals to possess skulls; as with their spines, their skulls are made of cartilage rather than bone.

A larval tunicate resembles a newly hatched fish fry, and is quite different from its sessile adult form.

// Fish—a family tree In the world today, there are nearly 35,000 species of fish known to science. This makes them more diverse than any other vertebrate group, although in terms of their basic body form, they are generally not all that diverse—certainly when compared to mammals, for example. All fish live and breathe in water, though a handful can breathe air, too, and wherever there is liquid water on the planet, you will find fish. The three principle lineages of fish are the sharks and rays, which are distinguished by having a cartilaginous rather than bony skeleton, and two groups of bony fish. These two are the ray-finned fishes, whose thin, web-like fins are attached to the body skeleton via several tiny bones, and the lobe-finned fishes, whose fleshy fins are joined to the body by a single large bone (and appear to have an attaching “stalk”). More than 98 percent of all modern fishes are ray-finned, but all of our planet’s other vertebrate animals evolved from lobe-finned fishes. Nearly all ray-finned fishes fall into a group known as the teleosts, which share the trait of being able to push their jaws forwards and pull them in again. This adaptation enables them to quickly suck food into their mouths. The few ray-finned fishes that are not teleosts include the sturgeons, which have rather shark-like mouths. Sturgeons also have mostly cartilaginous skeletons, but actually evolved from more bony ancestors.

A Russian sturgeon. These venerable fish are found in Azerbaijan, Bulgaria, Georgia, Iran, Kazakhstan, Romania, Russia, Turkey, Turkmenistan, and Ukraine, but today are critically endangered because of over-fishing.

Another group of ray-finned fishes which are not teleosts are the bichirs. These unusual fish have cartilaginous bodies and simple jaws, but they have a much more pioneering adaptation as well. Most bony fish have a gas-filled swim bladder that helps them control their buoyancy. In bichirs, the swim bladder is modified and can function as air-breathing lungs, enabling a

bichir to survive out of water for a considerable time. Studies show that bichirs that are kept mostly on land will develop other traits suited to this un-fishlike lifestyle over just a few generations, including a stronger skeleton that can more easily bear their weight out of the water, and a more flexible neck. While we know that land vertebrates evolved from the lobefinned fishes, the bichirs show another way that fish might adapt over time to terrestrial life.

No such thing as a fish? The tree above illustrates a point that is difficult to put into words. There is no such thing as a fish. Alternatively, all vertebrates: cats, dogs, birds, frogs, humans and the rest, are fish. Vertebrates evolved from one of three distinct lineages of fish. This means that you cannot make an accurate evolutionary grouping that includes all of the fish without also including all of the other vertebrates. It would be like saying that your blood family is made up of your mother and father, yourself and all of your children, your brother and all of his children, but not your sister or her children. In reality, we all know what we think of as a fish, and it remains traditional to consider the fishes as one of the five distinct groupings of vertebrates, alongside amphibians, reptiles, birds, and mammals—although reptiles have a similar classification problem to the fish!

// Fish—sharks and rays Only vertebrate animals have bones, but another type of connective tissue, cartilage, is found in many animal groups. It is a very strong and tough material, as evidenced by the fact that the largest fish in the world—the whale shark and the basking shark —have cartilaginous skeletons, as do all other sharks and related species. This group of fishes form the class known as Chondrichthyes. The class has two subdivisions, of which the largest by far is Elasmobranchii, the sharks, rays, skates, and sawfish—about 1,150 species in all. The other is Holocephali—the ghost sharks or chimeras. These were once very diverse, but there are only about 50 species of these rather mysterious deep-water fishes alive today. The sharks include some top-tier undersea predators—fast swimmers with exquisitely sensitive senses of smell to locate prey, and rows of seriously fearsome teeth with which to dispatch and consume it. However, the largest of the sharks are filter feeders that only eat plankton, and there are many very small shark species as well. The rays are very distinctive with their compressed bodies and expanded pectoral fins that beat like wings, giving them an exceptionally graceful (if not particularly fast) swimming style. While sharks are mostly very active, many rays are seabed foragers or ambush predators. The stingrays have a venomous stinging barb on the top of their tail, which is deployed mainly in

self-defense, while electric rays attack their prey with an electrical discharge, produced by a pair of nerve-dense organs on their undersides.

The Greenland shark is a very large, rather sluggish Arctic species, which may live more than 300 years. Its slow swimming speed indicates that it is a scavenger and ambush hunter, rather than a chaser of prey.

Sawfish are superficially shark-like, but their pectoral fins are flattened and wide, like a ray’s. Their most distinctive trait is a long, flattened rostrum or nose, with sharp teeth on either side. The teeth can inflict nasty wounds when the saw is used in selfdefense, but its primary purpose lies in finding prey—it is full of sensory structures that can detect the electric fields generated by fish as they swim. The sawsharks are true sharks with a similar tooth-lined rostrum, which they do use to attack prey.

Spotted eagle rays are widespread in tropical seas. Like some other rays, this species has venom-bearing stingers at its tail base, which are used primarily for self-defense.

One of the key differences between Chondrichthyes and the “bony fish” is that Chondrichthyes produce a small number of

large eggs, representing higher investment into each individual offspring. Bony fish produce large numbers of very small eggs, and only a tiny proportion of them will survive long enough to hatch. Sharks and rays also produce their eggs within egg cases— the leathery, squarish “mermaids’ purses” you may find washed up on the beach each held a single embryo. Some of them give birth to live young rather than producing eggs. Some sharks’ offspring develop incredibly slowly within their mother’s uterus before they are born—the frilled shark’s pregnancy may last for more than three years.

Some sawfish species can grow to more than 23 ft (7 m) in length, with the “saw” making up more than a third of the total length.

Speedskin Shark skin is not scaly in appearance, but does have a covering of very small, flat, sharp-edged scales called dermal denticles. The shape and backward-pointing arrangement of these scales helps reduce drag as the shark swims, improving its speed and efficiency. Artificial fabrics that mimic this structure are used in the design of the swimming costumes worn by many elite human swimmers.

// Fish—bony fish More than half of all the animals in the phylum Chordates belong to the group Osteichthyes—the bony fish. They are present throughout the world’s oceans, lakes, rivers and streams and are of vital importance in underwater ecosystems, but they are also prey for (and sometimes predators of) many land-dwelling animals. These fish were the first animals to evolve a skeleton of hard bone and from them all of the other vertebrates evolved. Specifically, the lobe-finned fishes are ancestral to other vertebrates. They make up a very small proportion of all modern fishes (less than 1 percent); the overwhelming majority are rayfinned fishes. The typical ray-finned fish has a streamlined, vertically flattened body, which it moves in an undulating, side-to-side motion when it swims. Propulsion and stability are provided by the dorsal fin or fins on its back and the pectoral and pelvic fins on the underside, as well as the tail or caudal fin. It extracts dissolved oxygen from the water and secretes carbon dioxide via its gills. Some of these gases are also held in the swim bladder, a large muscular organ that the fish uses to control its buoyancy. Fish anatomy (female)

Most fish are predators. How they find and capture their prey varies greatly. Some excel at the high-speed chase but many more use stealth or trickery. Archerfish attack insects by spitting a jet of water at them to knock them into the water. Frogfish have the ability to enormously expand the volume of their mouths in a fraction of a second, sucking in their prey. “Cleaner fish,” which include various types of wrasse and others, feed on the external parasites and dead skin of other fish and also marine turtles and cetaceans.

A pinch of salt Coping with changes in salinity is a challenge for fish, as it is for other water-dwelling animals. For some fish, such as certain eels, trout, and salmon, which move between oceans and fresh water at different points in their life cycle, these changes are extreme. If a water-permeable barrier (such as a cell membrane) separates fresh water from salty water, the water molecules will naturally move from the latter to the former (a process known as osmosis). So, to retain a constant internal environment, fishes need active processes to get rid of excess salt or excess water. They may at some times need to gulp down large amounts of water, and at other times actively pump salts out of their bodies via their gills.

Moray eels have a set of pharyngeal jaws behind their “real” jaws. The pharyngeal jaws push forward into the mouth to grasp food and then pull back into the throat.

Bony fish are highly prolific. Females release large quantities of unfertilized eggs and the male produces sperm or milt, which fertilizes them in the water. There may be prolonged courtship between the pair before this happens. In some species, such as sticklebacks, males attract females to a nest they have built. Once the female has laid her eggs inside and the male has fertilized them, he then chases her away and guards the eggs until they hatch. Newly hatched fish (often known as fry) are, like fish eggs,

extremely vulnerable and many are eaten by predators. Fish improve the odds of producing more young by placing their eggs in naturally sheltered “nurseries” such as kelp forests and mangrove swamps rather than in open water.

The rays of a lionfish’s elaborate fins carry a venomous sting—its bold coloration warns would-be predators to keep their distance.

Seahorses are highly unusual in that the male gestates the young after the female deposits her eggs inside a special uterus-like pouch on his underside.

// Fish and other animals of the deepest oceans Most of the Earth’s surface is ocean, and most of that ocean is more than 3,280 ft (1,000 m) deep. The oceanic layer that spans 3,280–13,125 ft (1,000–4,000 m) depth is known as the bathypelagic or “midnight” zone, and below that is the abyssal or abyssopelagic zone. These deep zones form a distinct and ecologically fascinating habitat. Little to no light from the Sun can penetrate here, which means no plant life can exist and the water temperature is around 35.6–37.4ºF (2–3°C). Water pressure increases dramatically, reaching more than 2,000 PSI by 4,920 ft (1,500 m) down and more than 4,400 PSI by 9,840 ft (3,000 m), compared to 37 PSI at 164 ft (50 m) down. Levels of dissolved oxygen in the water become very low with increased depth. This is a highly hostile environment to most forms of life, but a few animals have adapted to survive and thrive in the dark, cold and crushing abyss. Deep-sea animals tend to be slow growing and long lived. They also tend to be quite bizarre in appearance. Some have huge eyes that are extremely sensitive to the blue light that penetrates deeper than other light wavelengths, while others have almost no visual sense at all. Some generate their own blue light source through a biochemical process—this is known as bioluminescence. Anglerfish, for example, carry a light on an

extended dorsal fin ray, which acts as a lure, attracting other fish within attacking range. They, like other deep-water fish, have disproportionately huge mouths and stretchy stomachs, so that they can eat very large prey—with life so sparse down here, any feeding opportunity must be taken. Many deep-sea fish have lost their swim bladders, which makes them less susceptible to being crushed by high water pressure.

That sinking feeling On land, and in the sunlit sea, photosynthesizing plants and phytoplankton convert the energy of sunlight into a food store for themselves. This provides the basis of a “trophic pyramid”—animals eat the plants, other animals eat those animals, and so on. In the deep ocean zones, there is no sunlight and so no photosynthesis, but because the world ocean is a continuous body of water, nutrients can move through all parts of it. Some of the remains of animals and plants that die on the surface or in shallower waters eventually end up sinking to the depths, and these organic fragments feed deep-sea life.

Female anglerfish are much larger than males. In some species, the tiny male attaches to the female’s body and lives parasitically, ready to fertilize her eggs when she spawns.

Many kinds of deep-sea animals, including fish but also invertebrates such as this brittlestar, use bioluminescence as a way of communicating or to attract prey.

Depths of hell In most deep-sea areas, the seabed lies at around 19,685 ft (6,000 m), but some oceanic trenches are much deeper than this. The deepest of all are more than 32,800 ft (10,000 m) down. Below 19,685 ft (6,000 m) is the “hadal zone,” named after Hades, the Ancient Greeks’ god of the underworld. Worldwide there are 46 separate hadal habitats supporting a sparse community of animal life. The water pressure goes beyond that survivable for bony fish below about 27,890 ft (8,500 m), but a few invertebrate animals can survive deeper down. The primary source of energy down here comes from sunken organic matter, and bacteria that can live on chemicals that seep from deep underwater vents.

The pacific blackdragon has a very high concentration of melanin in its skin for camouflage. Its skin can absorb 99.95% of visible light.

// Amphibians—the swimmers and burrowers All of the non-fish vertebrates form a single grouping called Tetrapoda—”four-footed.” Their four limbs originated as the pectoral and pelvic fins of the lobe-finned fish that were their ancestors, and the first true amphibians appeared on Earth about 300 million years ago. Adaptation to life on land included modification of these fins into weight-bearing legs, and the evolution of lungs (which have the same physiological origin as the swim bladders found in modern fishes). The most fish-like tetrapods, the amphibians, are still limbless, gilled and fish-like in their early larval (or tadpole) life stage, and many amphibians are still semi-aquatic as adults, too, or at least need a damp environment to survive. Nearly all species are entirely predatory as adults, though may eat vegetation as larvae. There are about 8,000 species of amphibians, of which the large majority (almost 90 percent) are frogs and toads (see pages 86–87). The remainder are the salamanders and newts, which are long-tailed animals with four small legs, and caecilians, which have lost their legs and resemble worms or smooth-skinned snakes.

Caecilians resemble earthworms, but a closer examination reveals the miniscule eyes, plus a mouth full of needle-sharp teeth.

The giant salamanders are the largest of all amphibians. All species are threatened with extinction, though they will breed readily in captivity.

Salamanders and newts look rather like lizards, but have soft, moist skin without scales. Some species have toxic skin, and are brightly colorful; this is warning coloration to discourage predators. Breeding-condition male newts develop dorsal and tail crests as well as colorful spotted patterns on their bellies, which they exhibit in a wiggling aquatic dance when trying to attract a female. The female expels her eggs into the water and the male

then fertilizes them. The embryos develop within the egg’s protective jelly coating, and emerge as slim, gilled larvae that eventually grow front and hind legs and develop functional lungs. Most of these amphibians spend their adult life on dry land, but they have to seek out damp sheltered places to avoid dehydration. Although they have lungs, they can also take in oxygen through their skins.

Newts spend much of their time in the water during the breeding season, when males complete to display their crests and colorful bellies to females.

Neoteny The axolotl, a Mexican salamander and also a popular pet, is a famous example of the phenomenon of neoteny—it reproduces in its gilled larval stage, and in natural conditions will never metamorphose into an air-breathing adult, though metamorphosis can be chemically induced in captivity. Many other salamanders are also neotenous, either some or all of the time; among them are the sirens, which live in water all their lives and develop tiny front legs but no hind legs.

Axolotls are popular pets and have been bred in an array of different color forms, including albino (lacking all pigment).

Caecilians live on land, or rather in it—they burrow in damp soil, and prey on other soil-dwelling organisms. Their worm-like bodies are blunt at both ends and proportionately fairly thick; their skulls are sturdy to push through the earth. They have tiny eyes and small, sensitive tentacles close to their mouths. Unlike most amphibians, their eggs are fertilized internally, and in most cases the female gives birth to living larvae rather than laying eggs; in a few cases, she gives birth to fully metamorphosed, miniature adults.

Fire salamanders defend themselves with toxic skin secretions, and signal this to predators with memorably bright colors.

// Amphibians—the climbers and jumpers The amphibian order Anura comprises all of the world’s frogs and toads. As adults, these amphibians have no tails, and most have long, powerful legs (especially the hind legs), though they tend to assume a very compact resting posture with the legs folded up neatly against their bodies. They mostly begin life in the water as tadpoles, swimming by beating their long, crested tails, but as they develop and their limbs appear, the tail gradually retracts. They have large eyes and blunt snouts, soft skin that often bears striking markings, and many communicate through sound. There are more than 7,000 species of frogs worldwide. The term “toad” is not biologically precise but tends to refer to frogs that are more comfortable on land, more inclined to crawl than to hop, and that have bumpy, warty skin.

A male midwife toad can carry the egg clutches from up to three females at the same time.

Many frogs and toads live their adult lives well away from water, but need to return to it in order to mate. Common toads in Europe migrate back to the pond where they were born, and on the way a male will often climb on the back of a larger female, holding her in a firm clasp known as amplexus. This not only saves him the walk but also helps to ensure that he will be the one to fertilize her eggs when she lays them into the water. In some other species, males are territorial, staking out a suitable egglaying spot and advertising it with loud chirping, bleeping or croaking calls.

Many unrelated lineages of frogs have adapted to live in trees. Male red-eyed treefrogs vibrate the leaves they are sitting on to fend off rival males in the mating season.

Some frogs show highly unusual parental care behaviors. Male midwife toads carry the fertilized eggs in strings wrapped around their legs, and move into water when the eggs are ready to hatch. The female Surinam toad carries her fertilized eggs on her broad back—over time, the eggs sink deeply into her soft skin, and when they hatch, the small tadpoles live and develop inside the resultant pockets. Females of the recently extinct gastricbrooding frogs swallowed their fertilized eggs, later regurgitating fully developed froglets.

Each individual Ranitomeya amazonica frog has a unique pattern of spots and stripes.

A deadly mouthful Most frogs are small animals and, with their soft bodies, make easy prey for hunters both in the water and on land. Their defenses include camouflage and the ability to leap away from danger (some species can leap a distance more than 40 times their own body length), and also poison. The poison dart frogs (right), of the family Dendrobatidae, live in rainforests in Central and South America, and are famed for their powerful and bad-tasting skin toxins, which they obtain via the prey they eat. Some species carry enough poison to kill off ten or more humans—their common name comes from the traditional use of their skin secretions to coat the points of arrows. They are brightly and distinctively colored, ensuring that any predator that attempts to eat one will remember not to try it a second time. The cane toad, a large South American frog introduced to Australia to control crop pests, has proved disastrous for native predatory mammals as they attempt to kill it and die from ingesting its potent skin poisons.

When Surinam toads are spawning, the pair moves vigorously in the water, the male’s movements causing the fertilized eggs to become embedded in the female’s back.

Once the limbs are fully grown and the tail is shrinking away, a young froglet begins to spend more time on dry land.

// Reptiles—a family tree With the advent of the reptiles, the tetrapod lineage could finally bid farewell to aquatic life. Reptiles possess a thick and scaly skin that protects them from water loss, and their eggs also have a toughened shell that protects them from dehydration. This means that they can hatch without being immersed in water. Mammals and birds both descend from different now-extinct lineages that would today be classed as reptiles, and so the term “reptiles,” like “fish,” is taxonomically imprecise (see page 77). The name for the grouping that comprises all reptiles, birds and mammals is Amniota, after the structure of their eggs. Modern reptiles fall into several distinct groupings—the turtles and tortoises; the crocodiles; the snakes; and the lizards— each of which we shall look at in more detail on the following pages. As a group, the reptiles show great variation in shape, size, and way of life. The vast majority of them are predators and many are ambush hunters, using camouflage to hide from prey and a sudden burst of speed to launch their attack. Some reptiles have returned to an aquatic life, but all still need to come on to land in order to lay their eggs. Unlike birds and mammals, reptiles lack sophisticated mechanisms for regulating their own internal temperature. This means that when the air temperature is low, their ability to move becomes limited. Many reptiles enjoy basking in the sun to warm

up, and those that live in temperate regions usually hibernate through the coldest months.

Chameleons’ color-changing skills are useful for camouflage and for temperature control, but also as a way of communicating with other chameleons.

The essential egg Through evolutionary history, certain events stand out as milestones, such as the advent of multicellular life and the colonization of land by the arthropods. Another is the appearance of the amniotic egg. Unlike the eggs of amphibians and fish, in which the embryo is covered in a protective layer of jelly, the “amniote eggs” of reptiles, mammals and birds have a system of protective membranes, and the hatchling that emerges is a miniature version of the adult, rather than a larval form that will undergo dramatic changes on its way to adulthood. Being able to breed on land allowed the earliest members of the group Amniota to spread into new habitats. Almost all modern mammals and some reptiles carry their young inside their bodies in the form of a pregnancy, but they all evolved from egg-laying ancestors.

Crocodile eggs hatch after 10 to 12 weeks of incubation. Their sex is determined by the temperature at which the eggs are incubated, with cooler temperatures resulting in females.

The evolutionary history of the amniotes shows that there is no natural grouping for reptiles that does not also include birds.

Ancient oddity The tuatara, a reptile native to New Zealand, looks like a fairly standard lizard, but has a ridge of crocodile-like spines along its back. Its brain structure and its gait are more similar to those of a salamander than a lizard, and its skeleton has some distinctly fish-like characteristics. In fact, this peculiar animal is not a true lizard, but an early evolutionary offshoot of the lineage that was later to give rise to modern snakes and lizards. It is sometimes described as a “living fossil,” as it is the sole descendant of a group of reptiles that were common and diverse 240 million years ago, and its anatomy is still very similar to that of its longextinct relatives.

Its lizard-like appearance belies the tuatara’s true status as a “living fossil,” its lineage ancestral to both snakes and lizards.

THE AGE OF REPTILES Reptiles’ ability to live away from water enabled them to colonize and dominate the land. Their bodies, supported by a strong internal skeleton, and supplied with oxygen via lungs rather than passively through spiracles, could grow much larger than those of the land arthropods. We know from fossil remains how diverse reptiles became, and how huge some of them grew. The dinosaurs, a grouping that we now know includes all modern birds, are the first that come to mind, but other remarkable reptile lineages emerged, too—the winged pterosaurs, the marine plesiosaurs, pliosaurs, and ichthyosaurs, and the synapsid reptiles that eventually gave rise to the mammals. Dinosaurs occupied all ecological roles during their reign on Earth—among them were predators, scavengers, browsers, grazers and adaptable omnivores. Many later dinosaurs were feathered, capable of regulating their own body temperatures, incubated their eggs, and appeared to show advanced social behavior and high levels of parental care. They died out during the mass extinction event at the end of the Cretaceous period, 66 million years ago—only the birds survived. Besides the birds, the dinosaurs’ closest living relatives are the crocodiles; the ancestors of mammals and of modern turtles, snakes, and lizards branched off from the reptile family tree at much earlier points.

This famous fossil plesiosaur was found in 1823 by self-taught palaeontologist Mary Anning, in Dorset, England.

The “age of dinosaurs” might be better named the “age of reptiles,” as many of the famous long-extinct giant reptiles were not dinosaurs at all.

The pterosaurs are one of four groups of animals to have independently evolved the power of flight (the others are winged insects, birds, and bats). Their wings were sheets of thin

muscular tissue (patagia), well adapted for powerful and active flight rather than merely gliding. The patagia were supported on a hugely elongated fourth finger and connected to the hind feet. The largest species, Quetzalcoatlus northropi, had a wider wingspan than any modern bird, but all pterosaurs had very slender, lightweight bodies. Plesiosaurs were elegant predatory marine reptiles with fins for limbs, and long necks and tails—the mythical Loch Ness Monster is usually depicted as a plesiosaur-like animal. The related pliosaurs were also finned swimmers but were stouter with longer jaws, somewhat resembling crocodiles, and were more active predators, while the sleek, beaked ichthyosaurs were very fish-like in body form and could be considered the reptile equivalent of dolphins.

The ichthyosaurs had a remarkably fish-like body shape, despite having evolved from land-dwelling, four-legged ancestors.

Temporal anomaly We tend to think of the “age of dinosaurs” as a short moment in evolutionary history, and of dinosaurs as “evolutionary failures,” but in fact they lived and thrived on Earth for well over 150 million years—modern humans have existed for just 1/600th of that time. However, the average lifespan of any individual species is only about 1 million years, so many dinosaur species evolved and died out over that timescale. Our fanciful depictions of well-known dinosaurs interacting together rarely reflect reality—for example, nearly 100 million years passed between the last Stegosaurus and the first Tyrannosaurus.

Dramatic scenes like this never existed in nature, as Stegosaurus was extinct long before Tyrannosaurus had evolved.

The bones of Tyrannosaurus rex revealed that it had a rather horizontal stance in life, balanced by its long heavy tail.

// Snakes Although they are tetrapods by descent, the snakes have dispensed with their four limbs altogether, and get about (sometimes very rapidly) through sinuous movements of their long bodies. A few still retain a vestigial pelvic girdle, but most species have only spine and ribs, extending the full lengths of their bodies. Their internal organs are also arranged differently to those of other tetrapods, to fit everything into such a long and narrow internal space. For example, their kidneys sit one in front of the other rather than side by side, and they have only one functional lung. There are about 3,600 species of snakes in the world.

A rattlesnake’s rattle warns that it is ready to strike. It would rather scare off a threat than strike, and save its venom for prey.

The scarlet kingsnake (left) is non-venomous, but has evolved a striking and usefully protective resemblance to the eastern coral snake (below), which does have venom.

Through a combination of a slim body, powerful musculature, extreme flexibility, and (in some cases) a grippy outer covering of overlapping scales, snakes can climb, burrow, and squeeze themselves into tiny spaces. In general, they are stealthy stalkers or ambush hunters, using cover or camouflage to get close to prey without being seen. As long as their bodies are warm enough, they can uncoil and strike at tremendous speed. Some use venom to quickly immobilize their prey, while others use constriction, coiling around their victim and squeezing. Others simply hang on until the prey weakens. Their flexible jaws can open very wide, enabling them to swallow prey much broader-bodied than

themselves, and one large meal can sustain them for weeks or even months.

The scarlet kingsnake (left) is non-venomous, but has evolved a striking and usefully protective resemblance to the eastern coral snake (below), which does have venom.

Because they can swallow such large prey, many snakes will hunt very infrequently, and spend most of their time inactive.

The world’s largest snake is the green anaconda of South America, which can top 495 lb (225 kg). It is closely followed by a range of python species—all of these heavyweight snakes kill their prey through constriction and can take quite large mammalian prey (including humans on occasion). The reticulated python of south Asia is lighter but potentially longer than the green anaconda, with the longest verified individuals approaching 23 ft (7 m) in length. There are many more snakes that are just a few centimetres long when fully grown—these prey on insects, but some do have venom strong enough to kill much larger animals. Most snakes lay eggs and hide them in soft ground, but few show any parental care. Snakes hatch fully formed and, in the case of venomous species, fully capable of evenomating prey.

They tend to grow slowly, especially in colder climates where they must be inactive for much of the year, and some species can live for several decades.

Intoxicating About 600 species of snakes have a venomous bite. They use this primarily to kill prey but also in self-defense. Snake venom is a modified form of saliva, and is injected into the victim via grooves or hollow channels in the fangs. In most cases it affects the blood (hemotoxic), causing haemorrhage or clotting, or the nervous system (neurotoxic), causing paralysis, but other effects can occur, too. The victim is usually incapacitated very rappidly though may not die quickly at all. Only a few snakes are seriously dangerous to humans, and are only likely to attack if threatened or roughly handled. Treatment involves prompt administration of the correct antivenin. We make antivenins by giving small doses of venom to lab animals and then harvesting the antibodies that their systems produce in response.

Brown tree snakes were accidentally introduced to the Pacific island of Guam in the 1940s, and have wreaked havoc on native birds and the wider ecosystem there.

// Lizards The snakes and the lizards are close cousins—together they form the order Squamata—the scaly-skinned reptiles, with snakes arising from an existing lizard lineage. If asked to explain how snakes and lizards differ, we would probably point to their legs (or lack of legs), but there are several lineages of lizards that have also lost their legs. Most legless lizards can be distinguished easily from snakes by other features—for example, they possess eyelids and ear openings, which snakes do not, and many can shed and regrow their tails (a process known as autotomy). Most lizards do have well-developed front and hind legs, which are positioned on their body sides, producing a “sprawling” gait, rather than the more efficient upright gait seen in mammals (and also in dinosaurs). Most lizards are small, fast-moving animals that hunt insects and other invertebrates. A few species are herbivorous or omnivorous. The monitor lizards are the largest species, and the biggest of all of them, the Komodo dragon, is a formidable beast that can weigh more than 154 lb (70 kg) and measure more than 9.8 ft (3 m) long. As with snakes, most lizards are found in tropical and semi-tropical areas. In colder climates, they spend much time in a state of inactivity and will hibernate for many months. A very small number of lizard species possess venom.

The largest lizards are the monitors. Water monitors are semiaquatic but are highly adaptable, and are also formidable predators.

There are more than 6,000 species of lizards worldwide, making them the most species-rich group of reptiles by far. Among them are the colorful long-tailed anoles, the large-eyed, wall-climbing geckos with their adhesive toe pads (these bear a mass of microscopic hairs, which provide grip even on apparently smooth surfaces), the small-legged, burrowing skinks, and the superbly camouflaged, slow-climbing chameleons. Famous for their color-changing skills, chameleons can move different pigments to different parts of their skin. Their skin also contains a layer that holds a lattice of crystals, which can be altered in shape to absorb or reflect different wavelengths of light.

Many of the anoles have dewlaps on their throats, which tend to be larger in males and contrastingly colored. They are used for signalling in territorial displays.

Ladies only Most lizards breed in the conventional vertebrate manner, with a male and female getting together and the former fertilizing the latter’s eggs. However, some whiptail lizard species of the New World genus Cnemidophorus have evolved to only reproduce through parthenogenesis. In these species, no males exist and females produce genetic clones of themselves. A population of clones is clearly more vulnerable to an outbreak of disease than a more diverse population, but an advantage of this method of reproduction is that only one female is needed to establish an entire new population.

A gecko’s foot. The toe pads are wide and fissured, with microscopic hairs to help provide grip, enabling them to scamper up vertical walls with ease.

Amphisbaenians Among the strangest of all reptiles are the amphisbaenians or worm lizards, found mainly in Africa and South America, which are usually considered a separate group from the “true” lizards. These small burrowing animals, with their long, limbless bodies, ringed appearance, and almost invisibly small eyes, look similar to earthworms and are similarly adept at digging their way through soft earth. However, they have fully functional mouths and jaws with surprisingly large and sharp teeth, and readily tackle quite large prey.

Some species of skinks have tiny, barely functional limbs, and may be mistaken for snakes at first glance; some others are completely limbless.

// Turtles The unique protective shell makes any turtle immediately recognizable. These reptiles form the order Testudines, and there are about 300 species of them worldwide. In UK English it is usual to call all land-dwelling species “tortoises,” freshwater species “terrapins,” and marine species “turtles,” but none of these terms are taxonomically precise, and in US English it is usual to use “turtle” for all Testundines. We often imagine a turtle living inside its carapace (upper shell) and plastron (belly plate), as a snail lives in its shell, but the shell of a turtle is very much integral to its body. The carapace in particular is made of bony or cartilaginous plates and fully incorporates the animal’s spine and ribs. Turtles have long (sometimes very long) necks but many can fully retract their heads into their shells for protection. They often have long tails and large, sturdy limbs (modified into swimming flippers in the sea turtles). They lack teeth, but their jaws are beak-like and strong, with sharp and sometimes serrated edges that serve instead of teeth.

Sea turtle skeleton The bones, shell, and plastron of a turtle form a rigid, protective box.

Land turtles are primarily herbivorous and slow-moving, and the larger species are famously long lived, with lifespans far exceeding those of any other land vertebrates—there are several verified records of them surviving well beyond the age of 200. These animals, the giant tortoises, occur on the Seychelles and the Galapagos Islands, and several species are known, most of which are endangered now. Some of the smaller tortoise species can live more than 100 years. The largest of all turtles is the marine-dwelling leatherback turtle, which can be more than 6.6 ft (2 m) long and weigh more than 1666 lb (700 kg). Sea turtles do not appear to be as long lived as their land-dwelling relatives, but are known to live for several decades.

Unlike their smaller relatives, giant tortoises cannot fully withdraw their heads and limbs into their shells. Their size and predator-free habitats make this defensive move unnecessary.

The smaller turtles have long been kept as pets, their popularity surging after the advent of the children’s toy and entertainment franchise Teenage Mutant Ninja Turtles in the late 1980s. Many ended up being released into the wild after their young owners grew bored with them, leading to populations of non-native turtle species becoming established in various areas. The red-eared slider, a water turtle and a particularly popular pet, is now found in many areas well beyond its natural southern

US and Mexican range, and is considered one of the world’s most invasive animal species.

Red-eared sliders, native to the Gulf of Mexico, need to spend much time sunbathing to maintain their body temperature.

Race for the sea Flying underwater with slow beats of its long, wing-like forefins, a sea turtle possesses a grace and elegance that few other Testundines can claim. Sea turtles are as well adapted to their underwater life as any seal or penguin, but just like these animals, they still need to come to land to reproduce. Females return to land, sometimes to the very beach where they were born, and dig a burrow in the sand in which they lay their soft-shelled eggs. A couple of months later, the eggs hatch and the young turtles burrow to the surface and hurry to the sea, dodging the attentions of predators on the way. All seven species are threatened with extinction, and conservation efforts to help them include stationing human volunteers to guard the nesting beaches and escort the hatchlings safely to the sea.

Baby sea turtles use the light shining on the sea as a navigational guide, and can become disorientated and head the wrong way in areas with strong artificial light.

// Crocodiles and alligators The final group of reptiles, the order Crocodilia, comprises fewer than 30 species of large, powerful water-dwelling animals with long tails and long snouts, and thick, scaly skin, sometimes with rows of spikes running the full length of the body and tail. Their back-curved teeth are adapted to grip prey rather than to break it up—large prey is torn up into pieces by the crocodile rolling its body rapidly while holding the prey in its teeth. Many species have tremendous bite force. Some crocodilians are significant predators of land vertebrates, while others feed mainly on fish. Although crocodilians primarily live and hunt in the water, they can move at speed on land, and can switch between a sprawling and a more erect gait. However, they spend most of their time motionless, either basking in the sun or resting in the water, where their heart rate may drop down to just two beats a minute. A slow metabolic rate is typical of crocodilians—they do not need to eat very often, and some can spend as long as two hours underwater on one breath.

The gharial, a south Asian crocodilian now critically endangered in the wild, has a distinctively long, slim snout and is a specialist fisheater.

Female crocodilians bury their eggs in a hole or a nesting mound of soft earth, and (unusually for a reptile) take care of both the eggs and the newly hatched babies. Mothers can communicate with babies, and the babies with each other, with various vocalizations. This occurs even before the eggs have hatched, and helps the babies hatch at the same time and then stay close together, keeping them safer from predators. The world’s smallest crocodilian is Cuvier’s dwarf caiman, males of which can reach 5.2 ft (1.6 m). The male saltwater crocodile, the world’s largest species, grows up to 19.7 ft (6 m) long and can weigh well over 1.1 tons (1,000 kg). Another large species is the gharial, which can reach the same length as a “saltie,” but weighs much less (rarely topping 330 lb/150 kg)—a

significant proportion of its body length is due to its unusually long and slender snout, the perfect tool for catching fish.

Young American alligators remain with their mothers for at least a year and sometimes longer, before she begins to drive them away from her territory.

Sex determination Crocodilian eggs need warmth to develop, and the mother can regulate this by adding to or reducing the amount of earth that surrounds them. Incubation temperature influences the sexes of the hatchlings, with higher temperatures (above 89.6ºF/32ºC) producing more male offspring, and temperatures below 87.8ºF (31ºC) producing more females. In most other vertebrates, sex is determined by chromosomes inherited from the parents, but temperature-dependent sex determination is known in several reptile groups.

Crocodilians are unusual among reptiles in providing a high level of parental care for their young.

A few crocodilians are capable of attacking and killing humans, with the saltwater crocodile the most dangerous species. However, humans are far more dangerous to crocodilians than

vice versa—many species have been overhunted for their scaly skins, which are used to make decorative objects, and for their meat. Some large predatory mammals are also dangerous to crocodilians—in South America, large male jaguars may attack even fully grown caimans as large as themselves, relying on surprise and an accurate and deadly bite to the head to subdue this potentially deadly prey.

Black caimans are almost at the top of the food chain in Amazonia, but some jaguars specialize in hunting this dangerous prey.

// Birds—a family tree A couple of hundred million years ago, dinosaurs ruled the Earth, and today dinosaurs rule the sky. The first true birds emerged from a lineage of dinosaurs, well before the mass extinction that killed off their dinosaur relatives. However, some of the traits that we think of as uniquely bird-like, such as having a body covering of feathers and using their body warmth to incubate their eggs, were present in other non-avian dinosaurs, too. The theropod dinosaurs, which gave rise to the birds, included the mighty Tyrannosaurus rex but also many much smaller, lighter animals. They had the ability to raise and regulate their own body temperatures, assisted by the heat-trapping powers of body feathering, and they moved on two legs and developed longer feathers on their forelimbs that helped them to leap further and eventually glide. Archaeopteryx, an early bird whose fossils are about 150 million years old, had full-sized wings and was fully capable of self-powered flight, but retained various traits that modern birds have lost, including claws on the wings, a long bony tail and teeth.

The hoatzin is the only bird in the world to have functional claws on its forelimbs—a hangover from dinosaur ancestors. Young hoaztins use their wing-claws to help them climb among tree branches, but by adulthood the claws have disappeared.

Many of the adaptations we see in modern birds are weightsaving—the lighter the body the better when it comes to energyefficient flight. Their skeletons are proportionately smaller, with several standard tetrapod bones reduced or absent, and air spaces in some of the larger bones. The wings and tail are mainly formed from large, flexible and strong feathers rather than bone, and a lightweight keratin bill replaces a set of heavy teeth. The

ability to incubate eggs with body warmth means that birds can nest almost anywhere, rather than needing access to soft earth as most reptiles do, and this along with flight has enabled them to reach and colonize virtually every corner of land on Earth. There are about 10,000 species of birds today, which scientists group into about 28 orders. Although they show high species diversity, their variation in body form is rather limited compared to mammals, amphibians and reptiles. All birds have two wings and two legs, a feathered body and a bill, and all are instantly identifiable as birds. Yet between them they exploit a huge range of habitat types and exhibit a great variety of lifestyles.

This tree diagram shows the evolutionary relationships between some of the larger orders of birds. Convergent evolution is prominent throughout the bird world—for example, we can see here that hawks and falcons are not closely related to each other, despite their strong similarity in appearance and lifestyle.

Convergent evolution Throughout the animal kingdom, we see examples of species that look and behave alike but are not closely related at all. Comb jellies are similar to jellyfish; worm lizards and caecilians look like earthworms; and dolphins resemble fish. This is convergent evolution—a similar way of life drives the evolution of similar body forms. There are many examples within the bird world. Long-bodied birds that swim and dive on water include ducks, but also grebes, gallinules and auks. Fast-flying, wide-mouthed birds that catch their insect prey in flight include the swallows and martins but also the unrelated swifts and nightjars. Uncovering the true evolutionary pathways of these species involves study of anatomy and, increasingly, DNA, and surprises are still being uncovered every year.

Gulls and petrels are both seabirds and have superficial similarities despite having no close relationship. However, these two groups provide an incredible example of convergent evolution at the far ends of their global distributions. The snow petrel (top) and the ivory gull (below) breed in Antarctica and the high Arctic respectively, and are uncannily alike thanks to their adaptations to their hostile environment.

EXTINCT BIRDS Birds as a group are highly successful—they alone of all the dinosaurs survived the mass extinction that occurred 66 million years ago, and they then fully exploited the opportunity this provided to spread and diversify around the world. However, along the way many species and entire families did fall by the wayside, among them some of the most well-known of all extinct animals. The dodo is near synonymous with extinction. This is exaggerated by its popular representations, which show it as the antithesis of everything we admire about birds. It is flightless, big and chubby, oddly proportioned, and “ugly.” No wonder it died out... However, in reality, this peculiar pigeon was as well adapted to its environment as any animal. It evolved from flying ancestors on the relatively young volcanic island of Mauritius, where no predatory mammals existed. Being able to fly was no longer a “must-have” for survival, and evolution instead favored larger, heavier-bodied birds, as a big body is more energetically efficient. Dodos were not naturally chubby, though, and nor were they excessively slow-moving (the “fat and waddling” image was based on illustrations made of an overfed, captive bird). They were, however, ill-equipped to cope with the arrival of hungry humans and non-native predatory mammals such as rats on their island paradise. Humans first visited Mauritius in the early 15th

century, and the last wild dodo was observed in the mid-17th century.

A mounted specimen of ivory-billed woodpecker. This species, the world’s largest woodpecker, was last seen alive in Louisiana in 1944, though rumors persist that it still survives undetected.

Island extinctions The power of flight makes birds much more likely to reach and colonize remote islands than other vertebrates. However, those that form resident populations on predatorfree islands are inclined to lose this power over many generations. If you don’t need to migrate and you don’t need to take to the air to escape land predators, flight becomes an energetically expensive luxury. For ground dwellers, it is much better for survival to be proportionately heavier and so more resistant to starvation, and to not have to invest so much of the protein you eat into growing a full set of long, strong flight feathers each year. These birds are, in short, under high selective pressure to become weaker fliers or indeed completely flightless. This predictable evolutionary pathway led to equally predictable consequences once humans, along with the cats, rats, pigs, and dogs that they brought with them, began to reach and colonize these same islands. With no evolved defense against land predators, and only a very small geographic range and population, many of the birds became extinct very quickly. For instance, it took less than a year for a few feral cats to completely eradicate the Stephens Island wren, the world’s smallest known flightless bird until its extinction in 1894–5.

Artist’s impression of Gastrornis or “terror bird,” a widespread genus of large and powerful birds, some of which stood taller than a human, though they were probably herbivorous.

Moa-nalo were the dominant large herbivores in the Hawaiian islands until their extinction soon after humans (and their livestock) settled the islands in the 18th century.

A rich bird fauna evolved on New Zealand, as almost no land mammals ever occurred on the islands. Among them were the huge, flightless moas, which fed on plants, lived in herds, and behaved much like deer or antelopes, and the Haast’s eagle, the largest ever bird of prey, which hunted them. The first human colonists of New Zealand had wiped out both moas and eagles by the end of the 15th century—the last moa to go was the upland moa, a smallish species that lived in the relatively inaccessible highlands of South Island. A similar fate befell the moa-nalo, a lineage of giant ducks native to Hawaii.

Once the most abundant bird in North America, the passenger pigeon was completely wiped out by overhunting in just a couple of centuries, the last bird dying in Cincinnati Zoo in 1914.

This map highlights regions where the most bird extinctions in recent times have taken place. In general, island species are much more vulnerable than mainland species.

// The feather Birds are the only modern animals that possess feathers—a trait they inherited from their dinosaur ancestors. Feathers are modified versions of the scales that grow on the bodies of reptiles such as crocodiles. Both feathers and scales are made from a lightweight protein called keratin. The first feathers may have done little more than help keep their wearers’ bodies warm, allowing them to become capable of self-thermoregulation. However, evolution has since modified feathers into a great variety of shapes, with different (and often multiple) functions. A typical feather consists of a central shaft or rachis, which is rooted in the skin. Its side branches or barbs have tiny side branches of their own—the barbules. In larger body or “contour” feathers, the barbules of one barb “hook” on to those of the ones above and below, creating a continuous but flexible surface. In down feathers (and at the bases of contour feathers) the barbules have no “hooks” and the barbs are not attached to each other, but are soft and fluffy. Downy feathers create air spaces, and this air is trapped against the bird’s skin by the main “shell” of the plumage, formed by the interlocking barbs of the body or contour feathers. Each body feather can be moved by an individual muscle, which means that the entire body plumage can be “fluffed up” or “sleeked down” as required, to hold or release body heat.

Feather structure

Light direction affects the shade and intensity of color of iridescent feathers, producing a dazzling, shimmering effect.

Feather keratin can incorporate color pigments, and the microstructure of feathers can also selectively reflect certain wavelengths of light. These two factors give birds their wonderful array of color and pattern, from the incredibly detailed camouflage of owls, formed by melanin pigment in various concentrations, to the dazzling iridescent blues, greens and violets of hummingbirds, produced by structural color. Some birds also boast special, unusually shaped ornamental feathers

that are used in courtship displays—most famously the hugely elongated upper tail coverts of the peacock, displayed as a raised and quivering fan. The long flight feathers of the wings and tail provide an extremely lightweight, air-resistant surface that keeps the bird airborne and allows for fine control of movement in the air. The individual flight feathers of the wing have an aerofoil shape with the rachis positioned close to the leading edge. This helps provide lift as the feather moves forwards through the air.

Fluffing up the feathers holds a layer of air near the skin, which quickly warms up, helping birds like this American robin stay warm in winter.

It’s a safe bet that unusually elongated feathers, such as the head plumes of the King of Saxony bird-of-paradise, are used in courtship and territorial displays.

Molt Most birds replace their entire plumage once a year, after the trials of the breeding season have concluded. The process usually happens quite gradually, so the bird is not left completely flightless (or bald)—in some large birds it may take many months, but most small birds complete their molt in just a few weeks. Molt is also an opportunity for a costume change, often to a less colorful non-breeding plumage, and the bird will then undergo an extra, partial molt (of body feathers but not flight feathers) to its more colorful breeding plumage prior to the breeding season. Most feathers will be in situ for at least a few months, so birds devote much of their time to caring for their plumage, through preening and various forms of bathing.

// Flight and flightlessness Being able to fly opens up a great many evolutionary opportunities. Through flight, birds have successfully colonized nearly every area of land on Earth. Even birds that are highly sedentary and live in the same small territory all their lives benefit from being able to fly, to access food and safe nesting and roosting sites that are high above ground level, and to move quickly and efficiently from place to place. Successful powered flight requires using thrust (forward motion) to overcome drag (slowing down through air resistance), and lift (upward motion) to overcome weight (falling through the action of gravity). Birds generate thrust through wingbeats and sometimes a running start, and lift is produced through wingbeats, too, but also passively through the aerofoil shape of the wings and the flight feathers. Birds also make use of natural air conditions to gain lift and thrust, saving themselves some energy. This includes soaring on thermals (rising bodies of warm air) and, for low-flying seabirds, using the uplift generated between wave crests—this is known as dynamic soaring.

Soaring birds of prey are not usually very social but they will often “share” the same thermal when needing to gain height.

A lightweight body and long, broad wings gives the barn owl a low “wing loading.” This bird also flies into the wind for lift, making its slow hunting flight more efficient.

Most flightless birds are descended from flying ancestors. Many are species that have evolved on predator-free islands, as discussed on page 102. Others are adapted to extreme environments where predators are of little concern, or they are extremely good at hiding, or in some cases they are too large and intimidating to have much to fear from predators. The ratites— ostriches, rheas, emus, cassowaries, and kiwis—fit this last category, except for the kiwis, which evolved on mammal-free New Zealand. Some seabirds are flightless, most famously the penguins—they nest on mostly predator-free shores and islands and the sacrifice of flight has enabled them to be much more efficient swimmers and divers. In the northern hemisphere, the auks have followed a similar evolutionary pathway to the penguins—they use their short, flipper-like wings to “fly” deep underwater and have dense, highly insulating plumage, but they are still able to fly to reach predator-free nesting sites well above sea level.

Long, narrow, parallel-edged wings, suited to active soaring: for example, albatross.

Short, rounded wings, suited to a fast take-off and agile maneuvers: for example, wren.

Broad wings with “fingered” tips, suited to passive soaring on thermals: for example, eagle.

Long, broad-based, pointed wings, suited to sustained speed: for example, falcon.

Key Primary feathers Secondary feathers Primary coverts

Secondary coverts Alula Marginal coverts Scapulars

Wing shapes and flight style If you look at a few different birds in flight you will notice some variation in wing shape. This reflects their evolutionary relationships, but also the way they fly. Falcons and swallows have broad-based, long wings that taper to a point, giving a fast and powerful flight, driven with rapid flickering wingbeats. The even longer wings of albatrosses and related seabirds are narrow and parallel-edged, adapted to energyefficient dynamic soaring. Songbirds tend to have short and rounded wings—their flight is quite costly in energy terms but is agile. Soaring birds of prey have longer but roundtipped wings, and capture extra lift with fanned-out flight feathers—this very low-energy flight style allows them to stay airborne, with the help of thermals, with minimal effort for hours on end. The ratio of wing surface area to body weight determines how much energy a bird must put into flight— large-winged, light-bodied birds like barn owls and harriers have very low wing loading and can sustain flapping flight for long spells, enabling them to hunt actively close to ground level and find their prey by sound.

Hummingbirds’ flight is energetically costly, but, fueled by a highsugar diet, they can beat their wings up to 80 times a second when hovering and can even fly backward.

In its stooping dive, a peregrine falcon may accelerate to a speed of 200 mph (322 kph). It has fleshy “baffles” in its nostrils to cope with the air pressure when diving.

// Birds’ senses and intelligence All living things comprehend the world through their senses, but their individual sensory experiences—even of the same environment—may be completely different. With their wonderful colors and extraordinary songs, it’s obvious that birds live in a perceptual world dominated by sights and sounds. This makes them more similar to us than most of our fellow mammals, whose worlds are shaped far more by their sense of smell. Birds of prey are famously keen-eyed, able to spot moving prey from a great distance away. Their eyes do not magnify but they do resolve much more fine detail than ours do. Birds’ color vision is also superior to ours. Their retinas contain more different kinds of color-sensing cells than ours, and some birds can also see reflected ultraviolet light, making their plumage appear even more vivid to each other. Seeing great detail and intensity in color helps with foraging but also has a role in communication. Bright colors are an “honest signal” of good health, so the most colorful male birds appeal more to females, and their beauty discourages rival males from challenging them. Nocturnal birds have large retinas packed with contrastsensing “rod” cells (rather than color-sensing “cone” cells), so can see clearly even in minimal light. In some, a reflective membrane behind the retina (a tapetum lucidum) bounces the light back so

the retina has a second chance to pick it up. The size and shape of owls’ eyes makes them less mobile in their sockets, so they compensate with extremely flexible necks and rapid head movements. Most birds have eyes positioned on the sides of their heads, giving a very wide field of view. Predators tend to have front-facing eyes, which give a more limited visual field but better depth perception in the zone where the views of the eyes overlap, allowing for accurate targeting of prey.

Eye position determines the shape and size of a bird’s visual field.

The ear opening of an owl, situated at the edge of its soundchanneling facial disc of stiff feathers.

Sound, scent, taste, touch, and more Many birds communicate by sound. Some are expert mimics, listening to all manner of sounds in their environment and replicating it in their songs. Low-flying predators use sound to find prey. Owls are particularly keen listeners, their facial disks acting as parabolic reflectors to direct sound into their ear openings. Scavenging birds, both on land and at sea, have a keen sense of smell and can track down carcasses from several miles away. The sense of taste is generally not well developed but there are exceptions, for example in fruit eaters and nectar feeders. Many birds have a good sense of touch, via tiny hair-like feathers called filoplumes, which are especially concentrated on the face. Some, if not all, birds can sense the Earth’s magnetic field – an ability that is particularly pronounced in long-distance migrants, which in their first year of life must navigate their challenging journeys with no prior experience. There is great variation in avian intelligence. Species that feed opportunistically and sometimes hunt prey tend to be the brightest, with the crows widely acknowledged as the most intelligent of all. They have been shown not only to use tools to complete a task, but also to use tools to make other tools. They can recognize different human faces with ease, and some can recognize themselves in a mirror, showing a “concept of self” that is very rare among animals. Parrots are also very bright, with advanced social systems and a sharp memory that enables them

to recall when the fruit of each individual tree in their territory will ripen.

Parrots are noted for high intelligence. The kea, a playful and curious New Zealand species, will attack parked cars and remove windscreen wipers and rubber seals, apparently out of curiosity.

Kiwis find their food mainly by scent. Their unique nostril position, at the tip of the long bill, facilitates this.

// Birds’ bills and feet Birds made certain evolutionary sacrifices at they became better adapted to the aerial life, compared to other tetrapods. Their forelimbs, modified into wings, afford flight and, in a few cases, the ability to swim underwater, but are no good for walking or climbing, and certainly no good for handling objects. Their lack of teeth also means they face extra challenges when dealing with food. For these reasons, the avian body plan shows perhaps its greatest diversity when it comes to morphology of the bill and the feet, the main “tools” that birds use to physically interact with their world. Birds have an elongated rostrum (front of the skull) and mandible (lower jaw), forming the upper and lower parts of the bill. These bony parts are covered with a light but hard keratin sheath, which often bears bright colors. The keratin grows and wears down constantly. In most cases, the tips of the rostrum and mandible meet, but in some species (for example in birds of prey) the rostrum has a down-curved tip that overhangs the mandible, forming a hook that can be used to grip and tear.

The sword-billed hummingbird is adapted to take nectar from particular flower types that have very long corollas.

Those species that eat large, solid food items such as seeds, nuts and some fruits tend to have thick-based bills and strong jaw muscles to crush their food before swallowing. Parrots’ bills are like this, but also have a hooked tip, which they use as an extra “foot” when climbing among branches. By contrast, long, slender bills are useful for taking soft food that is only accessible by probing into crevices—shorebirds that feed in mud and hummingbirds that feed from flowers both exhibit this bill shape.

Flamingos attain their pink coloration from the pigments in the many small aquatic crustaceans that they filter out of the water.

Most birds have three forward-pointing toes and one backward. Those that run on the ground tend to have a reduced hind toe and broad and strong fore toes—in the fastest-running bird, the ostrich, there are just two large fore toes and no hind

toe. However, some smaller ground runners have a full-sized and long-clawed hind toe, which “grabs” on to the grass or earth as the bird runs, providing more stability.

With two toes pointing forward and two backward, toucans are skilled at climbing among tree branches as they search for fruit to eat.

Perchers and climbers have slimmer toes, long claws, and a well-developed hind toe; in some, such as woodpeckers and parrots, the outermost fore toe is rotated to point backwards,

giving better grip. Wading birds such as herons and sandpipers have long legs and long, slim toes. Birds of prey have strong, sharp, and curved claws for holding prey. Swimming birds mostly have webbed feet with skin membranes between the fore toes, forming a paddle. Some have fleshy lobes rather than full webbing, and in a few the hind toe is connected to the fore toes by webbing.

Unusual bill adaptations The oddest avian bills usually point to a highly specialized diet and/or feeding method. Crossbills are finches with crossed-over bill tips—tools they use to winkle the seeds out of pine cones. They grip the cones in their strong feet as they work the scales apart. Sawbill ducks have serrated edges to their bills, resembling miniature pointed teeth—these help them hang on to the slippery fish they eat. Flamingos’ bills have an almost right-angled downward bend with a thicker mandible than rostrum, and the birds feed by dipping their heads in the water upside down and filtering out tiny invertebrate prey through hairy structures (lamellae) that line the bill edges.

Grebes are expert swimmers and divers, with broad lobes on each toe rather than webbing connecting them.

// Seabirds and their adaptations Many different lineages of birds have become adapted to feed in, on or over the world’s seas. However, no bird can spend every moment of its life at sea—it must still come to land to breed. True seabirds get all their food from the marine environment and need nothing more from the land than a safe, dry space of ground big enough for a nest. This means that they often form very dense colonies on rough and rugged coasts and islands that are otherwise quite inhospitable. These colonies are the basis of entire ecosystems in some of the world’s more remote and hostile landscapes.

When feeding, skimmers open their bills and dip their elongated lower mandibles in the water in flight, ready to snap down on a prey item.

Food from the sea ranges from deep-dwelling fish and cephalopods to the floating corpses of the great sea mammals. Seabirds that dive for food include penguins and auks, which swim on the surface and “fly” deep underwater, and gannets and terns, graceful long-winged fliers that drop headfirst from the air to make a shorter, gravity-powered dive. Cormorants and shags are foot-propelled divers—their feet are totipalmate (with webbing linking the well-developed hind toe to the fore toes) and provide a very large “paddle.” They also lack plumage waterproofing, so are less naturally buoyant and expend less energy swimming underwater at shallow depths, unlike penguins and auks, which must dive deep to overcome their own buoyancy.

Young albatrosses may take five months or more to reach fledging age. The parental investment that goes into each chick is immense.

Seabirds that feed mainly at the surface include gulls (which are generalist feeders and also forage on land) and the smaller “tubenoses”—the petrels and their relatives, which have a highly developed sense of smell to locate floating food. Larger tubenoses such as albatrosses can also dive for food, and the auk-like diving petrels are specialists at finding prey underwater.

Guillemots or murres can nest on the narrowest of cliff ledges, safe from predators (though the occasional egg will fall off).

Some seabirds are highly aerial. Skimmers and tropicbirds take their food in flight and spend little time on the sea surface. Frigatebirds do not swim at all. They are kleptoparasitic—they tend to steal prey from other seabirds rather than finding their own. The skuas, relatives of the gulls, also practice food theft but supplement this by scavenging on the shoreline and sometimes killing and eating other birds.

Gannets and boobies’ nostrils open inside their mouths, so water does not get inside when they are plunge-diving for fish.

Many seabirds are exceptionally long lived, with albatrosses known to be able to reach their 60s at least, and even tiny storm petrels can reach their 30s. This long lifespan helps offset the fact that they have a slow reproductive rate, many producing just one chick in a season, and some will not breed every year. They also tend to form strong and lasting pair bonds, with one parent staying at the nest while the other forages for the family, sometimes for days on end.

An Arctic skua stealing a fish from an Arctic tern. The skua chases its victim until the exhausted victim drops the fish.

Salty solutions The vertebrate kidney is an efficient system for filtering excess salts from the blood, but its capabilities are not infinite and most land vertebrates need fresh water to drink. This is in short supply for seabirds, and would necessitate regular return trips to the land, seriously curtailing their freedom of movement. However, seabirds are able to drink seawater, by using their salt glands. These structures are in the head and actively pump salts out of the blood as it passes through them. The resultant highly concentrated salty solution is excreted via the mouth or nostrils—you may notice a gull or other seabird “sneezing” out a spray of droplets from time to time. Thanks to salt glands, a young albatross can spend six years at sea, with no need to visit land until it is ready to find a mate.

// Predatory birds Many birds eat other animals, primarily insects, and quite a number are fish eaters, but relatively few regularly prey on other land vertebrates. However, a few lineages are specialized hunters of this particularly challenging prey—in particular, the hawks and their relatives, the owls, and the falcons. These three groups are not closely related to each other but show a range of similar adaptations—most noticeably their hooked bills and powerful talons. For a bird, hunting mammals and other birds is a particularly challenging lifestyle. The prey is often strong, intelligent, and very keen not to be caught. Birds of prey therefore need to possess intelligence and strength of their own, and use strategy, stealth, and care to make the capture while avoiding injury themselves. Hunting may be active, the predator searching on the wing and using its keen vision (and, in some cases, hearing) to detect prey, or passive—lurking in a hidden spot and waiting for prey to wander within striking range. Most owls use the latter tactic, while hawks and falcons tend to combine both methods, depending on conditions. Falcons are mostly bird hunters and are adapted to fast dives and sustained chases in the open air, eagles often dive on prey on the ground from a great height, while hawks are agile fliers that use natural cover and obstacles to get close to prey before a swift final strike.

Eurasian sparrowhawks hunt other birds, using a powerful squeeze of the talons to restrain and incapacitate the victim.

Specialist hunters Some predatory birds have an unusual or limited diet. For instance, honey buzzards attack wasp nests, digging them up and ripping into them with their long, hooked bills. They have thickened facial skin to protect them from stings. The snail kite also has a very long bill hook, which it uses to extract snails from their shells. The Eleonora’s falcon, a bird hunter that lives on Mediterranean islands, times its breeding season for the autumn so that it can feed its chicks on the many songbirds that are migrating south through the region at that time. The short-eared owl preys preferentially on just one species of grassland vole (in Europe, this is the short-tailed vole) and will undertake great migrations and nomadic wanderings to track fluctuating vole populations. In a good year, it will produce very large broods to exploit the bounty. Most other owls have a very different life plan—they hold a small territory, year-round, and eat any and every living thing that they can catch.

A snail kite, showing the long, fine and strongly curved claws and bill it uses to get at its favorite food.

Most birds of prey are unsociable, but the burrowing owl lives in family groups, which work together to guard their underground nests.

Birds of prey usually catch their victim with their feet, which are strong with long, curved, and sharp-tipped talons. Depending on the force of the strike and the power of the squeezing foot grip,

this may kill the prey, or a fatal bite may be administered. Falcons’ bills have a notch called the tomial tooth, which is adapted for this purpose. However, not all birds of prey are able to kill their victims quickly and have to eat them alive, using their feet to restrain the prey as best they can. They will often close their eyes for protection when biting at living prey, instead using touch to sense what they are doing, via fine filoplume feathers around the bill.

Many birds will mob and harass birds of prey, to try to drive them away, though this means they risk being caught themselves.

// Songbirds and their songs One of the great joys of a woodland walk is the chorus of birdsong that you can enjoy, especially at the start and end of the day. Almost all birds have a range of vocalizations, but it is those in the order Passeriformes, the passerines or “songbirds” that have the most celebrated voices. Songs and calls provide a non-visual means of communication and so are particularly useful in “cluttered” environments such as woodland, where it’s difficult for birds to see one another. Birds call to let others know of their presence, or they may serve as warnings, a plea to be fed or simply as a way for members of a flock to stay in contact. Song is a specific type of vocalization that birds make to advertize their presence in their territory, to warn off same-sex rivals and sometimes (if they are unpaired) to attract a partner. It is performed mainly (but not exclusively) by male birds. Songs tend to be longer and more elaborate than calls. Each species’ song is unique—in some cases the songs are very beautiful arrangements of fluting notes, in others they are bizarre mechanical or yodeling sounds. Many accomplished songsters incorporate mimicry of other birds and other local environmental sounds into their songs. The migratory marsh warbler includes mimicry of more than 70 other bird species in its song—it learns some of these sounds in its breeding

grounds in Eurasia, and the rest during its winters in southeast Africa. Bird breathing cycle

The syrinx Birds produce sound not through their larynx, like us, but via a special structure located where their trachea splits into the two bronchi that enter its lungs. This organ, unique to birds, is the syrinx. It consists of rings of cartilage that reverberate to produce sound, and because of the circular, two-stage process by which birds breathe, with air entering the body’s air sacs and then the lungs (see diagram opposite), sound can be produced continuously through the breathing cycle.

Although they are not songbirds, snipes still produce a territorial “song” by flying at high speed with their tails fanned. The wind through the outer tail feathers makes a vibrating bleating sound.

Mockingbirds are noted for their skill at mimicking other birds’ songs, as well as for their own melodic compositions.

The order Passeriformes is the largest by far; with about 6,500 species, the passerines outnumber all other bird orders put together. They evolved more recently than other bird orders, and include some very familiar species, including the world’s most abundant wild bird (the red-billed quelea of Africa, with a population of up to 1.5 billion individuals) and the most intelligent of birds (the larger crows, especially the ravens). Most passerines are small and the majority are insectivores, but many

others take fruit and seeds, some eat nectar, and the shrikes and their relatives regularly hunt vertebrate prey.

Common starlings form vast pre-roosting flocks or murmurations, which perform dramatic aerobatics on winter afternoons.

It is not only the songbirds that use sound to proclaim their territories, and some birds make a territorial “song” without using their voices. Woodpeckers use drumming—a burst of rapid taps on a resonant dead branch. The “song” of snipes is also known as “drumming,” but their strange bleating sound is made by the air rushing between their modified outer tail feathers as

they perform a high-speed display flight. The kakapo, a large flightless parrot from New Zealand, enhances its voice with an environmental trick: it digs out a bowl-shaped depression and stands in it to call, thus amplifying its booming notes.

The most recently evolved birds are the nine-oscined passerines from the Americas, which include spectacular species like this greenheaded tanager.

// Mammals—a family tree Mammals—warm-blooded hairy tetrapods that nourish their young with milk produced by mammary glands—make up a tiny fraction of all known animals. However, to humans, the importance of mammals vastly outweighs their lack of diversity, because the animals most important to us in our daily lives are mammals—and we are mammals ourselves. In fact, to many people from a variety of cultures, the word for “animal” is used only in reference to mammals. The 6,400 or so species of modern mammals evolved from a lineage of animals known as the synapsids. The first true mammals emerged from this lineage a little over 210 million years ago, long before the extinction of the dinosaurs. Until the end of the Cretaceous period, 55 million years ago, only smallbodied mammals existed, and the extinction event that killed off the dinosaurs destroyed several mammal lineages, too. However, for the lineage that survived, there were now opportunities to evolve larger body sizes, and to diversify into new habitats and ecological roles. The group diversified enormously over a short space of time, with many modern mammal groups emerging in the first few million years after the event.

Surprisingly enough, the West Indian manatee is much more closely related to elephants and aardvarks than to seals or whales.

Today, about 21 mammal orders exist, and between them they show a stunning variety of body types. The most extreme examples are the bats, with forelimbs modified into wings, and the cetaceans—the whales and dolphins—with their fully aquatic lives and fish-like bodies. Some other well-known mammalian orders include Carnivora (predators, including cats, dogs, bear, and seals), Rodentia (gnawing mammals such as rats, mice,

beavers, porcupines, and squirrels, the most diverse of the orders with more than 2,000 species), and Artiodactyla, the even-toed ungulates (deer, antelopes, pigs, and relatives). Mammal taxonomy is still in a state of flux. Among recent discoveries is the recognition that the cetaceans evolved from artiodactyl ancestors, meaning that they should be grouped within Artiodactyla rather than as a distinct order (Cetacea) of their own. It has also been found, through DNA study, that several very dissimilar-looking orders of African mammals, including elephants, manatees, aardvarks, and sengis, are each other’s closest evolutionary relatives, forming a distinct lineage (Afrotheria).

This tree diagram shows the evolutionary pathways of the main groupings of mammals.

Our own order, Primates, is a sister group to the rodents and the rabbits—our lineage parted ways with theirs about 91 million years ago. The earliest significant split in the mammal family tree took place about 66 million years ago or earlier, when placental mammals first appeared. Today, placental mammals make up the majority of species, outnumbering the marsupials (which give birth to embryonic babies that they carry in a fleshy pouch). Only two lineages of egg-laying mammals have survived to modern times (see pages 122–3).

WHAT MAKES A MAMMAL? Mammals, together with their extinct immediate ancestors, are known as synapsids. The first synapsids were formerly classed as reptiles, and certainly looked quite reptilian, but they were evolutionarily distinct from the other emerging group of amniotes, the sauropsids, which gave rise to modern birds and reptiles. They are therefore more often described as “protomammal,” rather than the older and less accurate term “mammal-like reptiles.” The split between Sauropsida and Synapsida occurred somewhere around 330–300 million years ago. We think of mammals as distinct from reptiles by their hair, warm blood and mammary glands, but other more fundamental differences appeared first. The structure of the legs and ribcage is a key difference. Synapsids developed longer legs and shorter, lighter tails, and their leg position changed, becoming vertical under the body rather than sprawled out to the sides. Changes were underway inside the body, too. Synapsids developed a sheet of muscle—the diaphragm—under the ribcage, which moves forward and back when the animal runs, helping to push air in and out of the lungs. This, as well as having large and stretchy lungs, helps a running mammal to increase its respiration rate, which is essential when the other muscles are working hard.

Dimetrodon, which lived 295–272 million years ago, was a protomammal. Its large dorsal sail is thought to have been used in courtship or territorial display.

The synapsid skull is consistently different from the sauropsid skull, in terms of the number and position of the openings behind the eye socket—the temporal fenestrae. All synapsids have one temporal fenestra, set low on the skull. Most sauropsids—extinct and modern—have two, one positioned high and one low, while a few have one high opening, or none at all. These openings are associated with the arrangement of the jaw muscles; both lineages evolved species with strong and flexible jaws, but their skulls retain this structural difference.

Mammals and reptiles differ in their leg position, which affects their gait.

Mammary glands are modified skin glands, although their evolutionary pathway is not known for certain. All mammals possess them, and females nourish their young on the milk that the glands secrete. However, mammals are not the only animals that feed their offspring on a glandular secretion—some insects do it too, and some birds secrete “crop milk” in the crop, part of the digestive tract, which they regurgitate for their young.

Hair today The skins of most reptiles, all birds, and most mammals have scales, feathers and hair respectively. All of these structures are made of keratin and grow out of the skin. As we saw on pages 104–5, feathers are remarkable and complex structures that provide a way to trap heat and display color but also permit flight. No doubt mammals would have benefited greatly from feathers, too, but feathers only arose in the birds’ lineage, and mammals make do with the rather simpler alternative of hair. An individual hair grows out of a skin follicle and has a tiny muscle that can alter its angle relative to the skin. Mammals can therefore fluff up their fur, to trap more heat or to increase their apparent size (a good way of intimidating rivals or would-be predators). In many land mammals, each hair has alternating bands of darker and lighter color (a coloration known as “agouti”), a form of camouflage that gives the animal a more shimmering, insubstantial look than if it had solid color.

Mammal fur is an effective insulator for a warm-blooded animal, and also provides a canvas for color and pattern, to provide camouflage or ways to communicate.

Because female mammals produce milk, it is common for them to be single parents—a second parent isn’t needed for food provision. Meerkats and some other mongooses do live in family groups, but this is more for safety than providing for the babies.

// The egg layers— monotremes Like birds and most reptiles, mammals were all egg layers at an earlier stage in their evolution. Today, though, only a handful of egg-laying species or monotremes survive—the platypus, and four species of echidnas. They are found in Australia and New Guinea, the same areas where the more recently evolved marsupials also occur. It is likely that the marsupials’ more flexible reproductive methods enabled them to out-compete and eventually replace monotremes in most habitats. The platypus, a well-known and quite bizarre mammal native to eastern Australia, is well adapted to its aquatic lifestyle. It swims and dives with the aid of webbed paws and a beaver-like tail, and forages for worms, crayfish and other prey in the muddy beds of rivers using its large, leathery “bill.” The bill is highly sensitive to touch and also has specialized sensory organs for detecting the electrical fields generated by moving prey. Echidnas are stocky, cat-sized land mammals with spine-covered skins and long, slender snouts. They are shy, nocturnal woodland animals that eat insects—the short-beaked echidna (the only species found in Australia) is a specialist feeder on ants and termites. Like the platypus, echidnas have strong forelimbs and dig burrows for shelter and for their nests.

The platypus looks like a strange mixture of different animal parts stuck together, but at the same time it is beautifully adapted to its habitat and way of life.

Mammalian venom The platypus is often described as a mishmash of body parts from other animals, with its duck’s bill, beaver’s tail, and otter’s paws. It also has a trait more usually associated with snakes—it produces venom. Only male platypuses have venom, which strongly suggests that its function is linked with male vs male competition. The venom gland, and the spur that delivers it, are located low on the leg. There are a few other venomous mammals—some shrews and slow lorises (small lemur-like primates) produce toxic saliva and have a venomous bite.

Monotremes show many differences from other animals besides egg laying. Like birds and reptiles, they have a single opening—a cloaca—for defecation, urination, and reproductive function. The way the female’s ova or egg cells divide after fertilization is like that of birds and reptiles, rather than other

mammals. They also show differences in jaw anatomy; their mammary glands lack teats but seep milk through pores (and their milk is strongly antibacterial, which compensates for this less hygienic method of delivery); and they have an exceptionally slow metabolism (though this last trait may be a more recent adaptation, rather than a trait retained from their ancestors).

A spiny coat for protection has arisen in several unrelated animal groups. The short-beaked echidna is the spiniest of all echidna species.

Male monotremes may compete aggressively for the chance to mate, but the females care for the young alone. The eggs spend a

long period developing inside the female’s reproductive tract— longer than the time it takes them to hatch once laid in the nest burrow. The female keeps the round, leathery eggs warm with her body—in the case of echidnas, inside a skin pouch. The hatchling young, known as “puggles,” depend on their mother for several weeks. Three of the four echidna species occur in New Guinea and two of them are classed as Critically Endangered; the other as Vulnerable. The short-beaked echidna is found in New Guinea, too, but also throughout Australia. It is the only monotreme not in need of immediate conservation action; the platypus is classed as Near Threatened because of multiple threats to its riverine habitats, and is being bred in captivity to boost its total population and provide stock for future reintroductions.

The western long-beaked echidna is a critically endangered monotreme, found in mountainous parts of New Guinea.

// The pouch bearers— marsupials Most modern mammals retain their babies in the uterus for a long period before birth, nourishing them via a placenta as they develop. Marsupials give birth to live young, too, but their gestation periods are short and the newborn young are barely more than embryos. These tiny blobs of life, known in all species as “joeys,” crawl into their mother’s pouch, which in most cases is fleshy, sticky, and moist. Once inside, the baby latches on to a teat and stays put—in some cases for weeks—as it grows. This method of reproduction gives the female marsupial a freedom of movement that the monotremes lack.

Two male red kangaroos fighting. The largest of the kangaroo species, red males can weigh as much as 200lb (90 kg) when fully grown.

Marsupials once occurred worldwide but today they are only found in Australia, New Guinea, and the Americas, and are best known in Australia, which has few other native land mammals. Here, a wonderful diversity of more than 300 species of marsupials can be found, occupying the same ecological niches that placental mammals occupy elsewhere in the world. There are ground-dwelling, fast-moving grazers (the kangaroos and wallabies); fierce predators (quolls and the Tasmanian devil); worm-eating tunnellers (marsupial moles); ant eaters (numbats); climbing leaf eaters, fruit eaters and nectar feeders (the koala and various possums, including gliding species); ground-dwelling omnivores (bandicoots), and more. The largest predatory

marsupial of modern times, the thylacine, or Tasmanian tiger, was a dog-like animal that went extinct in the early 20th century (though reports of sightings continue today), and more ancient extinctions include marsupials that resembled lions and tapirs.

Pouch adaptations Some pouches face forward and some backward. The former type means a longer journey for the joeys, but are easier for the female to keep clean. Backward-facing pouches are the norm in burrowing species, so the joeys are not subjected to showers of flying earth when the mother is digging. It is quite common for marsupial mothers to produce more joeys than she has teats in her pouch. The Tasmanian devil, for example, may produce 40 or more embryonic young, but she has only four teats. The joeys therefore fight it out within her pouch in an extreme form of sibling rivalry. Kangaroos also have four teats but typically have just one joey at a time. Once this baby has left the pouch, it may continue to suckle now and then from its preferred nipple (which has grown in size) while a newborn sibling is attached to one of the other teats.

A wombat joey sleeping in its mother's backward-facing pouch.

Rabbits are invasive and unpopular in Australia, so chocolate Easter bunnies are sometimes replaced by a big-eared native species, the greater bilby.

Placental mammals have outcompeted marsupials over much of the world, but Australia’s geographic separation from the other continents predates their emergence. There are indigenous placental mammals on the continent today—a range of bats and rodents—but their ancestors reached Australia later, via the air and “rafting” on floating vegetation respectively. Australia also has a range of other mammals that were introduced by humans; these include rabbits, foxes, and others that have proved immensely damaging to the native fauna.

The Virginia opossum is noted for “playing possum”—very effectively pretending to be dead, to deter predators (it even releases a fluid that smells of decaying flesh).

The marsupials of the Americas are much less diverse than those of Australia; they comprise about 100 species of opossums, most of them mouse-like forest dwellers, and about seven shrewopossums. The best known by far, and the only species to reach North America, is the Virginia opossum. This cat-sized, omnivorous animal bucks the trend of marsupials failing to compete with placental mammals, as it is widespread, adaptable, and highly successful, even thriving in urban areas.

The eastern quoll (Dasyurus viverrinus), is a medium-sized marsupial with a distictive spotted coat. It is endemic to Tasmania.

The sugar glider is the best known of the six species of “gliding possum.” It uses its gliding membrane, or patagia, to move from tree to tree.

// The sharp-toothed scuttlers—insectivores

As with many desert-dwelling mammals, the long-eared hedgehog’s large external ears help it to lose excess body heat.

The mammalian order Eulipotyphla (which translates, unflatteringly, as “truly fat and blind”) is home to about 450 species of mostly small or very small ground-dwelling and insecteating mammals. They are characterized by a very keen sense of smell (making up for poor eyesight), short legs, and sharp teeth for grabbing and crunching up their invertebrate prey. The group includes shrews, hedgehogs, and moles, among others. It was formerly a larger grouping called “Insectivora” and included

tenrecs, golden moles, and otter shrews. However, DNA analysis has shown that these African and Madagascan species are not related to the other insectivores.

Gardeners are not fond of moles, because of the heaps of earth spoil (molehills) they leave across otherwise pristine lawns.

Shrews make up the majority of species in this order. They are short-lived, constantly hungry, and fearless little mammals—the Etruscan shrew, weighing less than 0.07 oz (2 g), is (together with Kitti’s hog-nosed bat) the world’s smallest mammal, but like other shrews it will attack prey as big as itself. Shrews mostly live in well-vegetated habitats and are solitary and territorial. A few species have venomous saliva, used to disable prey and for selfdefense. The two species of African hero shrews have remarkably overdeveloped and robust spinal columns, which may allow them to use their bodies as levers to turn over rocks and logs and access prey beneath.

A shrew family on the move stick together by holding on to each other with their teeth, and forming a “caravan.”

Shrews occur on most continents, but hedgehogs and moles are native to the Old World only. They have spines for protection —when under attack, they curl up to form a solid ball of outwardpointing prickles, which few predators can deal with. Moles are adapted to an entirely underground way of life, and have extremely soft, dense fur to make their bodies more slippery, as well as very large, powerful forefeet for digging their network of tunnels and chambers. Like most burrowing mammals, they have

very short tails. The two species of desmans are close relatives of moles, but are adapted for swimming rather than tunneling, and have long tails. The moonrats or gymnures, found in east Asia, are close relatives of hedgehogs but have no spines, and look more like oversized shrews.

The miniscule Etruscan shrew is no less a fierce and fearless hunter than any other member of its family.

The final species in the order Eulipotyphla are the two solenodons. One of these is found only on the island of Hispaniola, while the other is endemic to Cuba. Both are classed as Endangered, and have a number of extinct relatives from other Caribbean islands. Solenodons resemble rat-sized shrews with long tails, rather large ears, and very long noses. Both species are venomous, purportedly capable of killing cats and dogs with their bite.

Shrew-moles and mole-shrews The shrew family, Soricidae, contains about half a dozen species that are remarkably mole-like in appearance and behavior. They have stout snouts, tiny eyes and ears, short tails and dense mole-like fur. Within the mole family, Talpidae, there are a few species of moles that look as much like shrews as any true shrew, and lead shrew-like lives above ground, rather than being subterranean burrowers. These two cases are prime examples of convergent evolution, where unrelated species occupying similar ecological niches have evolved similar appearances. By outward appearances both would certainly be placed in the “wrong” family, but their tooth and jaw anatomy reveals their true identity, as of course do their genes.

// A taste for ants— pangolins, aardvarks, anteaters, and armadillos Ants are so very abundant on this planet that any animal that specializes in eating them should never go hungry. There is even a special word for an animal that feeds exclusively on ants: myrmecophagous. Around the world, several unrelated mammal groups have evolved to be eaters of ants, and they share certain traits that are adaptive to this way of life—though in other respects they are very different.

When threatened by predators, pangolins will curl up to hide their faces and undersides, and rely on their tough scales for protection.

The two most prominent traits of ant-eating mammals are powerful forefeet for digging out the nests, and a very long, fastflicking tongue with sticky saliva, for lapping up large numbers of ants at high speed. They also have a keen sense of smell to track down ant nests, and often sealable nostrils and thickened skin to protect themselves from flying debris as they dig, as well as from the insects themselves. These mammals lack front teeth, and anteaters and pangolins have no teeth at all. Instead, their food is broken down through the muscular action of their digestive tract, assisted by powerful enzymes and specialized gut bacteria. Aardvarks, armadillos (of which only some species are ant specialists), and some pangolins are ground dwellers and burrow diggers. The giant anteater is terrestrial, too (though not a burrower), while other anteaters and pangolins mainly forage in trees. Many ant-eating mammals also feed on termites, which live in large and often well-defended nests just as ants do.

Like other anteaters, the southern tamandua has powerful claws which it uses to defend itself as well as to climb and dig.

The aardvark, whose name means “earth pig” in Afrikaans, is part of the large grouping of related African native species known as Afrotheria. These mammals, which share a common ancestor, include elephants, hyraxes, manatees, the tenrecs of Madagascar, the sengis or elephant shrews, and the otter shrews. The anteaters and the armadillos, found in the New World, are related to sloths, while pangolins are found in Asia and Africa, and have no close living relatives; they descend from an early offshoot of the lineage that gave rise to the carnivores. A couple of

unrelated true carnivores—the aardwolf and the sloth bear—have also adapted to the myrmecophagous life. Many of these mammals are threatened in the wild. Pangolins in particular are under huge pressure due to illegal hunting— their meat and their pinecone-like body scales are prized in folk medicine, especially in China. The Chinese pangolin is Critically Endangered, and pangolins from other parts of the world are increasingly being imported illegally into the country to satisfy the demand. In 2020, China raised its protection status for all pangolin species to the highest possible level.

Armadillos have many unusual traits, including almost invariably having litters of genetically identical quadruplets.

Hunters and hunted Ants are predators, and through sheer weight of numbers they are formidable predators, too. They are also well protected, with some species not only able to give a powerful bite but also to sting and squirt acid at their attackers. Their nests are extremely well defended and often very well concealed. It is therefore no surprise that ant-eating mammals show such marked adaptations. These come at the expense of other potentially advantageous traits – many anteating mammals are slow-moving and also rather slowwitted.

From the front, an aardvark’s snout reveals why it has a name meaning “earth pig.” It has more scent-sensing olfactory bulbs in its nose than any other mammal.

// Sloths—a slow pace of life About 59 million years ago, the mammalian lineage Xenarthra emerged in South America. This group of mammals became one of the most diverse and successful mammal groups in the New World, giving rise to anteaters, armadillos, the similar but larger pampatheres (now extinct), and sloths. The sloths, forming the suborder Folivora (“leaf eaters”), included some very large and imposing animals in the past. The species Eremotherium eomigrans, a ground sloth, was larger than a male African elephant, and its skeletal remains reveal that it was extremely robust and powerful.

Sloths will supplement their leaf diet by eating the protein-rich algae that grows on their fur. The fur is also home to sloth moths, which lay their eggs on sloth dung, and their presence encourages more algae to grow as the moths carry nutrients to the fur. The three species thus all benefit each other.

With hooks for hands and feet, a sloth can hang securely upside down with no muscular exertion to speak of.

Today, all that remains of Folivora are the tree sloths, six species of medium-sized, tree-dwelling, leaf-eating mammals that live in tropical rainforest in South America and Central America. The best known and most widespread is the brownthroated three-toed sloth, while the rarest is the pygmy threetoed sloth, with fewer than 100 individuals left, confined to a single island off the coast of Panama. Sloths are very distinctive with their blunt faces, long arms, and tremendously elongated,

curved claws, allowing them to hang and climb upside down among tree branches with little effort. They can also swim strongly.

Sloths are very ill suited to movement on the ground and only descend in an emergency or for their weekly toileting.

Folivory Leaves are far from being highly nutritious, unlike fruits, which are often adapted to be eaten by animals, who then disperse the seeds in their droppings. To appeal to the seed dispersers, many fruits are soft, readily digestible and delicious. By contrast, plants “want” to keep their leaves, as these are their photosynthesis factories. Therefore, many leaves have evolved to be structurally tough as well as unpleasant tasting or even poisonous, and animals have to eat them in large quantities to extract enough nutrients from them. Leaf-eating mammals typically have large and complex digestive tracts, populated by a sizeable community of bacteria that works alongside the mammal’s own digestive enzymes to break down the leaf tissues. It may take a sloth several weeks to completely digest a stomach full of leaves, and the contents of a full stomach may weigh even more than the animal itself. Every week or so, a sloth climbs down from its tree to dig a latrine on the forest floor, in which it urinates and defecates. It then returns to its “home” tree. Only when in search of a mate does it venture further afield.

Most mammals can swim, and even sloths will take to the water— especially males in search of mating opportunities.

Sloths are well known for their slow, deliberate movements. They have slow body processes as well, including some of the lowest metabolic rates of any mammals, and spend much of their time hanging completely motionless (though not necessarily asleep). Moving slowly helps them to evade detection by predators such as large eagles, which are alert to movement among the branches. Sloths also have an unusual form of camouflage: algae colonizes their fur, giving them a green tinge. This algae is in turn the basis for a miniature ecosystem, providing food for various invertebrates (as does the sloth’s own hair and blood). Various moth species live as adults in sloth fur,

the females emerging to lay their eggs on the sloth’s droppings when it descends from the trees for its weekly toilet trip. Even more surprisingly, the algae is a food source for the sloth itself, providing nutrients that are lacking in its usual leafy diet.

Linnaeus’s two-toed sloth is larger than the familar three-toed sloth, and occurs across a broad swathe of northern South America.

// GIANT ICE AGE MAMMALS As we saw on page 130, giant ground sloths were once widespread in the New World. Several other modern mammal groups have long-dead relatives of impressive size. These mammals were most prominent during the Earth’s most recent Ice Age, from 125,000 to 14,500 years ago. Many were very widespread in the northern hemisphere, because lower sea levels (as a result of more extensive glaciation, keeping more of the planet’s water in a frozen state) meant that land bridges linked up the New and Old Worlds in the north. Climate change played a part in these Ice Age animals’ extinction, but so too did the growing population of Homo sapiens, for whom the mighty mammals were both a danger to be eliminated and a bountiful resource to be harvested.

Giant ground sloths, some species of which were more than 9.8 ft (3 m) long, were widespread in the Ice Age and heavily hunted by people.

Colder climates favor the evolution of increased body size, as maintaining heat is more energy-efficient, with a lower surfacearea-to-volume ratio. Perhaps the best known of the Ice Age megafauna are the mammoths and mastodons, although their widespread lineages first appeared well before the Ice Age and survived until just 4,000 years ago. Relatives of the modern elephants, the cold-climate mammoths were notable for their dense, furry coats and extremely large tusks, as well as small ears

for heat retention, as opposed to the large ears of today’s elephants, which are adapted for efficient heat loss. Alongside these elephantids were several species of large rhinoceroses with a range of cold-climate adaptations. Among them was Elasmotherium sibiricum, which was mammoth-sized with a single huge, thick-based horn. (Even this animal, the “Siberian unicorn,” would have been dwarfed by the more ancient longnecked, hornless rhinos of the genus Paraceratherium, some of which stood more than 16.4 ft/5 m tall at the shoulder.)

Island giants A cold climate isn’t the only environmental factor that encourages the evolution of large body size. Around the world, there are examples of the phenomenon of “island gigantism”—the tendency for species confined to isolated, predator-free islands to become larger bodied than their mainland counterparts. For example, a 26 lb (12 kg) lagomorph (related to rabbits and hares) once lived on Menorca, while in the Caribbean a range of very large rodents evolved, such as the blunt-toothed giant hutia, a chinchilla-like animal that weighed more than 110 lb (50 kg). This gigantism is mainly observed in island lineages of smaller animals that no longer need to hide from predators. Larger animals tend to experience the reverse phenomenon, island dwarfism, as an adaptation to more limited food supplies. Miniature island forms of elephants, mammoths, hippos, and ground sloths have all been documented. Most of these island giants and dwarfs are now extinct, thanks to human activity.

The array of preserved mammoths and mastodons found in the northern hemisphere show how successful this lineage once was.

The bigger predators living at the same time included a range of large and robust cats, such as the famous saber-toothed cats, the cave lions and the scimitar-toothed cats. The Eurasian cave lion was similar in size to today’s African lions, but the American lion was about 25 percent larger. The dire wolf, present in North America, was larger even than today’s biggest wolves, while the giant short-faced bear, at more than 2640 lb (1,200 kg), was possibly the largest ever land carnivore.

The red deer found on the island of Corsica are significantly smaller than their mainland equivalents, an example of island dwarfism.

// The ubiquitous nibblers— mice, rats, and voles The most abundant mammals on Earth, in terms of both species diversity and overall number, are the rodents. This is a diverse group of more than 2,200 species in more than 30 families. They all have a pair of long and constantly growing incisors (front teeth) on both top and bottom jaws, which are worn down by the hard foods they eat (and, in some cases, by chewing through wood and other hard materials when making their nests). The largest family of rodents is Muridae—the true or Old World rats and mice and their close relatives, with about 700 species. The family Cricetidae (voles, lemmings, hamsters, and New World rats) is closely related to Muridae. Rodents are small or very small, furry mammals that are either herbivorous or omnivorous. They occur in all kinds of habitats, with some being strict specialists and others highly adaptable and opportunistic. They tend to be quite furtive and elusive, nesting underground or deep within dense ground vegetation. They are short-lived and extremely prolific— population “booms” in response to favorable conditions are commonplace. Some mice and rats are highly social and form large communities with complex social hierarchies. Some hamsters, by contrast, are solitary, fiercely territorial, and have a large home range, from where they collect a large store of food

that will sustain them in their burrows over the winter, which they spend in a state of semi-hibernation. For their size, many of these rodents are very intelligent and quick to learn. The abundance of some of these species makes them extremely important in ecological terms. Meadow-dwelling voles are prey to a wide range of other animals, from owls, crows, shrikes, and falcons to weasels, cats, possums, mongooses, and snakes. Even animals that rarely take vertebrate prey, such as frogs, hedgehogs, pheasants, and ducks, will catch and kill mice if they have the chance. In environments with low species diversity, this disproportionate importance is even more pronounced. For instance, in the high Arctic, lemmings are classed as keystone species because so many predators depend on them almost completely. In years of low lemming abundance, snowy owls, skuas, and Arctic foxes prey instead on Arctic-nesting shorebirds, and the entire ecological balance of the region is shifted.

The European hamster is the largest of the world’s hamster species. It gathers food stores to see it through winter, amassing more than 22 lb (10kg) of food in some cases.

Global danger Brown and black rats and house mice are very comfortable in human environments, and this includes our vehicles. Boats on long exploratory voyages proved particularly fine homes for them, being made of gnawable wood and stocked with plenty of food. We have transported rats and mice all around the world in this manner without intending to, and they have caused untold harm to some of the planet’s most fragile ecosystems as a result. More than half of the world’s animal extinctions in the last 500 years were caused, at least in part, by the arrival of non-native mammals, with rats being the worst offenders. Island-nesting seabirds have been very hard-hit by this problem, although their numbers often recover quickly if the invasive rodents are eradicated.

Brown rats and black rats are both skilled climbers. Anti-rat baffles on mooring ropes help prevent them from getting on to boats, the historic route by which they spread worldwide.

A few species, most notably the brown rat, black rat and house mouse, are commensal with humankind—they live alongside us, often within our dwellings, make nests in the walls of buildings and plunder food stores. However, we have domesticated both

brown rats and house mice, and they are extremely important to us as laboratory animals. They, and some other small rodents, are also very popular pets.

The tiny harvest mouse has a monkey-like prehensile tail, helping it to climb among the long grasses where it lives.

// Other rodents Between them, the rodents pursue many different ways of life. Dormice and tree squirrels spend most of their time high in the branches, where they find food and build their nests, while the mole-rats live out their days underground. The mara of South America is a long-legged, hare-like ground grazer, while beavers, muskrats, and coypus are semi-aquatic. The desert-dwelling jerboas have a bipedal, kangaroo-like stance and get about by hopping, while the anomalures and flying squirrels can traverse the airspace between trees by gliding, held aloft by the skin membranes (patagia) that stretch between their front and hind feet. The world’s largest rodent is the capybara, a stocky animal that can weigh more than 132lb (60 kg). It lives in groups in marshy, grassy areas and spends much time in the water. Its size puts it beyond the reach of most predators. The porcupines are also large for rodents, and well protected with their array of sharp-tipped quills. The most impressive species is the crested porcupine of Africa, whose backward-pointing black-and-white quills are up to 14 in (35 cm) long. When threatened, it charges backward to try to hit the attacker with its quills.

The naked mole-rat’s eusocial living arrangement is almost unique among vertebrate animals. They live in cooperative groups where usually one female and several males are reproductively active. Non-breeding individuals care for the young and protect and provide for the group

The naked mole-rat, found in east Africa, is a bizarre animal with a hairless skin, very slow metabolism (and a potential lifespan of more than 30 years), enormous teeth that it uses for digging, and a social system similar to that of colonial ants and bees. The single breeding female or queen presides over a colony comprising up to three breeding males, and sometimes more than 100 sterile workers. The workers have many roles: they collect food, guard against intruders, help to tend the queen’s pups, and maintain the colony’s network of tunnels.

This diagram shows the evolutionary pathways of the main rodent families.

Chipmunks are ground-dwelling squirrels, using burrows both for nesting and for stashing a store of food.

Several other rodents live colonially in underground burrows, including the prairie dogs—ground squirrels found in the plains of North America. In their “towns,” burrows are not shared universally but families do live in close proximity, and all citizens of the “town” take action when they hear one individual’s alarm call, warning that a predator is around. The alarm calls differ depending on the type of predator and its behavior, and the response varies accordingly. Between them, rodents inhabit almost all of the Earth’s land masses and almost every kind of habitat. Many are intelligent and adaptable, and ecologically important—as prey but also as predators, seed dispersers, hosts for parasites, and vectors of disease, and as active modifiers of their physical landscape.

Dormice are relatives of squirrels rather than true mice, and several species have squirrel-like fluffy tails.

Waterway management Beavers are very large, river-dwelling rodents, and are well known for their ability to modify river flow through the construction of dams. With their powerful incisors, they can fell small trees quite easily, and use these trees not only to build their nests (lodges) but also to control the flow of “their” rivers. Their log dams create pools and new channels, and their activities have been shown to greatly increase biodiversity along the courses of rivers and streams, as well as reducing the likelihood of floods. In the UK, the Eurasian beaver was eradicated in the 16th century, but has been reintroduced in some areas in the 21st century. Its presence is already having significant positive effects on its environment.

Beavers create a more varied habitat along the course of a river, providing habitats for a range of other wildlife.

Porcupines are slower-moving rodents that are armed with sharp quills for self-defense.

// Rabbits and hares The order Lagomorpha—home to rabbits, hares, and the pikas—is a sister group to Rodentia. Together, Rodentia and Lagomorpha form a superorder known as Glires. Like rodents, the lagomorphs have constantly growing incisor teeth, which are worn down by the food they eat. However, the lagomorphs have different numbers and arrangement of teeth. These mammals are strictly herbivorous, and most of them are grazers. Grass is a tough food to digest. Large hoofed mammals deal with it by having complex, multichambered stomachs, where fermentation of the grass cellulose takes place, and they also “chew the cud”—regurgitating food for further chewing after it has been somewhat softened. Lagomorphs handle this problem in a different way. They produce soft droppings called cecotropes, which they eat—thus extracting more nutrients on a second trip through their digestive tracts.

Several species of Arctic-dwelling hares develop a white coat for camouflage in winter.

Mountain singers Pikas look a lot like chubby ground squirrels, with their relatively short legs and round ears, but are in fact cousins of rabbits and hares. Many species live at high altitudes, with the long-eared pika of the Himalayas occurring up to 19,685 ft (6,000 m) above sea level. Pairs of pikas defend a territory together, and live in burrows, which they provision with stockpiles of dried vegetation to see them through the worst weather. Unlike other lagomorphs, they are highly vocal, with a wide repertoire of whistling calls with different functions, from attracting a mate to warning of approaching predators.

A pika appears to be carrying a bouquet—but this is food rather than a romantic gift, to be stored for later consumption.

Rabbits and hares have long ears and long hind legs, to help them detect and then escape from predators. Rabbits have burrows in which to hide, and they also make underground nesting chambers in which they bear their blind, hairless young. Hares, however, both rest and have their babies out in the open, so they rely on camouflage, keen senses, and an incredible turn of speed to keep them safe. Young hares are born furry and openeyed, able to hide and also to run from an early age. Most rabbits live in colonies (“warrens”) and have advanced social systems. Hares will feed in groups but are generally more solitary.

America’s open spaces are home to many species of jackrabbits. These animals’ huge ears help them to lose heat, as well as hear

danger.

These grazing animals were once very widespread and successful, but the rise of the hoofed grazers saw their diversity and abundance fall. They are also hunted by humans, as well as a great variety of wild predators. Several species of hares live in Arctic regions and develop a white coat in winter for camouflage in snowy surroundings, but climate change is causing their molt patterns to fall out of sync with seasonal snowfall, leaving them more at risk to predators. There are about 90 species of lagomorphs living today, which are divided quite evenly between rabbits, hares, and pikas. The American jackrabbits, notable for their incredibly long and broad ears, are in fact hares. Lagomorphs occur almost worldwide— there are even a few forest species of rabbits, such as the striped rabbits of east Asia. The European rabbit has been introduced (as hunting quarry) to many other parts of the world, and is seriously invasive in Australia. It has also been domesticated and developed through captive breeding into many distinctive forms, including types with long silky hair and drooping ears.

Kept for its fur and meat but also as a pet, the domestic rabbit today occurs in a range of shapes, sizes, and colors.

// Mammals of the air—bats The skeleton of a bat looks surprisingly human-like—if humans had hands many times larger than their bodies. Bats are the only mammals to have evolved true, powered flight, and they achieve this through forelimbs modified into wings. The fingers are hugely elongated, and connected by a membrane that also links up with the hind feet. These feet are very small. The long, strong claws allow the bat to climb and cling, but its movement on land is generally very limited. There are more than 1,300 species of bats in the world— together they form the order Chiroptera. They have no close relatives among extant mammals; despite resembling, in some ways, the rodents and also the shrews and moles, they actually belong to the same lineage as the carnivores and the hoofed mammals.

Plants that are pollinated by bats tend to have upward-pointing flowers with robust stems, and release scent at night.

Most bats are hunters, feeding on flying insects. However, the group also includes nectar feeders, and the fruit bats, most of which are much larger than insect-eating bats. The nectar eaters are vital pollinators for many wild and cultivated plant species, while the fruit eaters play a crucial role in ecosystems by dispersing the seeds of trees and shrubs. A handful of species have different specializations, such as the fishing bats, which use their feet to seize fish from the water, and the vampire bats, which drink blood from vertebrate animals. Nearly all bats are strictly nocturnal, which helps them to avoid competition for food and airspace with birds.

A roost or “camp” of grey-headed flying foxes, a declining fruit bat species native to Australia.

Most bats breed once a year and have only one pup at a time, which the female carries on her body while she forages. For such small mammals (many weigh 0.35 oz/10 g or less) they are long lived, able to reach their 30s or even 40s (by contrast, the average mouse is unlikely to make it to the age of two). They tend to form communal nursery roosts in the breeding season, and in colder climates they hibernate through winter (though a few species are

migratory). Bats can be found almost everywhere in the world, though their diversity is greatest by far in tropical regions. The power of flight has allowed them to colonize many remote islands, including New Zealand, where the only native mammals present are two species of bats.

Bat caves can be popular eco-tourism attractions, though must be managed carefully as bats are highly sensitive to disturbance.

A map of sound Most insect-eating bats have large and anatomically complex ears, and often have elaborate folded skin on their faces. These are the tools they use to navigate by night. As they fly, they call constantly and listen for echoes as the sounds they make are reflected back by any objects in their path. The nose folds help them to channel and direct their calls more precisely. Through this echolocation, bats can avoid obstacles in the open and within their crowded roosts. They can also detect prey and tell the type, size, and speed of the prey, and use this information to decide whether, and how, to tackle it. Fruit bats have no, or only rudimentary, echolocation. They have keen eyesight and noses, and find their way around, and their food, through sight and scent, experience, and memory.

Horseshoe bats have complex fleshy growths on their faces, which help them channel their echolocation calls.

The largest bat genus, Myotis, comprises more than 100 small, insectivorous species, which generally lack facial ornamentations.

MADAGASCAR’S UNIQUE MAMMALS Madagascar, a forested island lying 250 miles (400 km) off east Africa, is the world’s fourth-largest island. Although it lies close to Africa, it was originally connected to what is now the Indian subcontinent, separating some 88 million years ago (well before many modern plant and animal lineages had evolved). This long period of isolation has resulted in a very distinctive flora and fauna—about 90 percent of all the plant and animal species on Madagascar are endemic (they don’t occur anywhere else). Leaving aside bats and marine species, all of its native mammal species are endemic. Their ancestors are thought to have colonized Madagascar through “rafting” events—they were carried on floating mats of vegetation from Africa. The best-known Madagascan mammals are the lemurs. These beautiful and charismatic creatures are primates. On mainland Africa and Asia, a few species of lemur-like primates exist, but they are greatly outnumbered by their more recently evolved cousins, the monkeys. No monkey species ever colonized Madagascar, so lemurs have become highly diverse, with more than 105 species. They are very variable in size, and some larger species have bold and bright fur colors and patterns. Most are adapted for climbing, and spend their time foraging in the treetops. The sifakas get about on the ground on two legs,

dramatically leaping along with their arms raised. Ring-tailed lemurs, the most familiar species with their masks and long, banded tails, live in female-led social groups and mainly forage on the ground.

A ring-tailed lemur with her baby. As with most primates, the babies are carried by (or cling to) their mothers.

Madagascar is also home to a unique family of carnivores, Eupleridae. These are mongoose-like or cat-like hunters, the largest of which is the graceful, long-bodied fossa, a skilled climber and lemur hunter of great agility. Fossas weigh up to 18 lb (8.5 kg)—a little less than the largest of the lemurs, the indri. Madagascar did formerly have a range of much larger mammals, along with the elephant birds, the heaviest birds ever to have existed, but the arrival of humans 2,000 years ago quickly led to their demise.

With its elongated fourth fingers (used to probe tree holes for insect larvae) and striking facial expression, the rare aye-aye is uniquely adapted to its nocturnal life in the forest canopy.

Lemur-like The term “prosimian” was formerly used to describe all primates that were not apes or monkeys, although it has now been discovered that these animals are not all closely related to each other. The category included the lemurs, and also a range of other species from Asia and Africa: the lorises, tarsiers, galagos, and angwantibos (collectively, the lorisoids). These animals are mainly tree dwelling and nocturnal, and are very distinctive in appearance with beautiful markings, soft fur and very large eyes. They have pointed snouts, unlike the flatter faces of monkeys, and tend to eat insects, whereas most monkeys are mainly herbivorous. The tarsiers are closely related to monkeys, but the others form a separate lineage—Lemuriformes—within the order Primates.

The fossa lives in forests over most of Madagascar, and hunts lemurs as well as almost all other land vertebrates found on the island.

The tenrecs are also unique to Madagascar. These mammals are related to the golden moles and otter shrews of Africa, and like them are primarily ground-dwelling insect eaters. Most tenrecs resemble shrews, but others have hedgehog-like spines, and one, the web-footed tenrec, is semi-aquatic, like the otter shrews.

The bizarre-looking lowland streaked tenrec communicates by rustling its quills to create sound.

// Monkeys Agile and graceful, monkeys (of most species) live in the trees in mainly tropical and subtropical habitats. Often highly social, vocal, intelligent, and strikingly colored and patterned, they are like the mammalian equivalents of the parrots. There are about 270 species worldwide, fairly evenly spilt between the two main lineages—the Old World monkeys of the parvorder Catarrhini, and the New World monkeys, which form the parvorder Platyrrhini. The hands and feet of monkeys have short claws (or nails) but long digits, for wrapping around tree boughs. Some New World monkeys have prehensile tails, which can coil strongly around branches and effectively provide an extra climbing limb. Their faces are usually quite blunt and flattened, with forward-facing eyes—this makes them better at judging distance than mammals with side-mounted eyes, which is vital when so much of their time is spent making prodigious leaps from one tree to another. They also have good color vision, unlike many other mammals, which is helpful when assessing the ripeness of fruits.

Colobus monkeys at play. Interactions like this are very important for establishing and maintaining social bonds.

Monkeys are mainly herbivorous, feeding on the fruits and sometimes foliage of the trees in which they live. The smaller New World monkeys also include a high proportion of insects in their diets. Those that have adapted to a more terrestrial existence, such as the baboons, have a more generalized diet. Life on the ground is more hazardous, so these monkeys tend to be bigger and more powerful in body size, and to have tighter social organization, with the ability to communicate in detail about any dangers around and to act co-operatively to drive away predators.

This diagram shows the evolutionary tree of the primates, including monkeys, great apes, and lemurs.

As a group, monkeys show a great variety of mating systems. In some cases, males are much larger than females and compete between themselves for dominance, the top male being the only one that gets to mate. In some of the South American marmosets and tamarins, a polyandrous system is common, whereby a female has two male partners who care for her twin babies between them. Baby monkeys habitually cling to a parent’s chest while they are small, and once they become larger and stronger

they switch to riding on an adult’s back. In monkeys that form large social groups, females will often share care of the babies in the group between them.

With its impressive mustache, the emperor tamarin is one of the most striking New World monkeys.

Gibbons The apes, humans included, form a distinct lineage within the Old World monkeys. There are two families of apes living today: the great apes (family Hominidae) and the gibbons (family Hylobatidae). Gibbons are small apes that occur in warm parts of Asia, including on some island groups. Like other apes, they lack tails, and have well-developed forelimbs with very flexible shoulder joints. Gibbons also have unusually flexible wrist joints, and their specialized forelimb anatomy enables them to get around by arm swinging, or brachiating. Their hands are very elongated but the thumbs are small—by using the hand as a hook rather than gripping, they can transition from branch to branch very quickly. There are 17 species of gibbons—they live in forested regions and are notably social, intelligent, and long lived.

Although they are incredibly skilled and athletic, gibbons sometimes miss their grip and fall. One in three wild adult gibbons have healed fractures, despite their unusually robust bones.

It is riskier to live on the ground in the open than in the trees, so baboons live in large groups and are always vigilant.

// The great apes Great apes are the biggest of all the primates—sizeable enough to have little to fear from most predators. If we leave humans aside for the moment, the other great apes share several traits—in particular, their habitat, which is deep forest close to the equator, in Africa (chimps and gorillas) or Asia (orangutans). The great apes are mostly herbivorous, and while they remain skilled climbers and are very comfortable in the trees (the orangutans especially), their size keeps them safe from most predators, which means they are quite at ease spending time on the ground.

Although they are more solitary than other great apes, orangutans have lasting family ties, as youngsters feed from their mothers for up to eight years.

Friends and relations There are four genera of great apes living today: Pongo, the orangutans; Gorilla, the gorillas; Pan, the chimpanzees; and Homo—humans. Until relatively recently, there were only considered to be one living species of each, but studies of DNA and other features have resulted in some changes in the non-human great apes. Today, we recognize three species of orangutan (Bornean, Sumatran, and Tapanuli), two of gorillas (western and eastern), and two of chimps (common chimp and bonobo). Our own species, Homo sapiens, is the only human species alive now, but we have found fossil evidence of many other, now-extinct human species. Many of us carry some genes that indicate that early Homo sapiens interbred with other Homo species living at the time, including Homo neanderthalensis (“Neanderthal Man”).

Gorillas and other wild great apes are highly vulnerable to diseases carried by humans, so it is very important to minimize the risk of transmission.

The great apes are widely acknowledged to be highly intelligent. They use both facial expressions and a range of vocalizations to communicate with others of their species. The chimpanzees, our closest living relatives, show complex and very dynamic social organization, and exhibit a great deal of behaviors that we class as intelligent, including tool use and the ability to form organized hunting parties to capture monkeys. These behaviors are passed through the group and down the generations by learning, and a useful new behavior will quickly

“catch on”—an example of cultural transmission. In captivity, chimps and gorillas have both been trained to use sign language and show the ability to express some complex and abstract ideas. All of the great apes are long lived and their young are slow to mature, relying on their mothers for direct care for their first eight or more years, and often retaining a close relationship for life.

A young common chimpanzee watches an adult using a twig to extract termites from a mound. Tool use and cultural transmission of ideas are signs of advanced intelligence.

The early ancestors of humans adapted to life in more open habitats, for which a bipedal rather than a four-footed gait was quickly advantageous; other apes can walk on their feet for short periods, but only humans show the anatomical adaptations necessary to do so full-time and with ease. As with the baboons,

early humans would have been under more pressure than their forest-dwelling relatives to defend themselves from danger, and would also have needed to exploit a wider range of food types. These factors pushed human evolution toward increased intelligence and sociality. Today, our species dominates the entire planet, to the detriment of many other living things—including our closest cousins, the other great apes. All species of orangutans and gorillas are classed as Critically Endangered, while both of the chimp species are Endangered.

Bonobos live in family groups and show much more gentle interactions compared to common chimpanzees.

// Hoofed herbivores We now move from the primates, whose forefeet are modified into extremely sensitive and dextrous hands, to the ungulates, whose evolution has taken their feet along a very different path. Yet the hoof is as much a game-changing evolutionary innovation as the hand, and today the hoofed mammals (forming the bulk of the taxonomic grouping Ungulata) are the dominant grounddwelling large herbivores on our planet. There are at least 250 extant species of hoofed mammals and perhaps as many as 450, depending on taxonomy. The hoofed mammals can be divided into two main groupings, known as the “odd-toed ungulates,” Perissodactyla, and the “even-toed ungulates,” Artiodactyla. The former grouping, comprising the horses, tapirs and rhinoceroses, is much smaller; most hoofed mammals are artiodactyls. They include deer, antelopes, cows, camels, hippopotamuses, giraffes, pigs, sheep and goats. Perissodactyls walk primarily on their middle toes— the other four toes are reduced in size. In the case of horses, they have disappeared altogether and the middle toe is greatly enlarged, with a full covering of hardened hoof material. Artiodactyls bear their weight on enlarged third and fourth toes, together forming a “cloven hoof”, with the other toes reduced or absent.

In summer, reindeers’ hooves become wider and more spongy for better traction on boggy ground. In winter, they harden up, for grip on ice.

A hoof is, in essence, an enlarged and thickened claw. Hoof material continues to grow through the animal’s life, as it is worn down through everyday activity. Its durability enables hoofed mammals to run hard and for long spells on firm ground, such as dry plains or rocky terrain, while some species have very broad hooves that help them keep their footing on treacherously soft ground. Hooves are not as good for some of the other things that non-hoofed mammals use their paws for, such as handling food or grooming the coat, but some ungulates, such as pigs, use their hooves to dig into soft ground to find food. Furthermore, a lack of mechanical grip has proved no obstacle to species such as the ibex and the klipspringer, which move with amazing surefootedness along the steepest and most precipitous rocky slopes.

The closest living land relatives of the whales, hippopotamuses feed on land but spend most of their time relaxing in the water.

A surprising legacy Not all early ungulates were herbivores, and nor are all modern ungulates, either. There is one lineage within Ungulata that seems completely out of place, but studies of fossils and DNA have both confirmed the truth—that the cetaceans (whales and dolphins) emerged from a lineage of artiodactyls. The hippopotamuses are the cetaceans’ closest surviving cousins, and the four-legged ancestors of cetaceans may have resembled modern hippos. However, fossil evidence suggests that the lineages of hippos and cetaceans diverged before their shared ancestors began to adopt an aquatic lifestyle, and the earliest cetaceans were in fact small, fully land-dwelling carnivorous ungulates.

Hooves may not look like good climbing tools, but some hoofed mammals, such as the klipspringer, are incredibly surefooted on precipitous terrain.

Hoofed ungulates are almost exclusively plant eating. Some graze on the world’s grassy plains, while others browse from bushes or trees. They are hunted by large carnivores of various kinds, and so tend to be alert and equipped with keen senses— sensitive noses, side-mounted eyes giving them a good all-round view, and large and mobile ears that are constantly swivelling and twitching. Those that live in open countryside are often very social, benefiting from group vigilance, while forest-dwelling species are more likely to be solitary and to rely on camouflage and unobtrusive habits.

Wild pigs enjoy a watery wallow just as much as their domesticated cousins, as these red river hogs demonstrate.

// The grazers—grassland communities of hoofed mammals On a worldwide scale, the largest concentrations of land animals can be found in tropical rainforests. However, forests hold relatively few large animals—most of that abundance and diversity comes in the form of insects and small vertebrates, which are often hard to find, thanks to camouflage or other concealment skills, along with the fact that most of them live in the canopy, many yards above our heads. The rainforest sounds full of life, but seeing that life can be frustrating.

North America’s bison population suffered lengthy persecution at human hands, which nearly wiped them out completely.

For this reason, when we think of a terrestrial environment that’s full of animal life, our minds often head straight to the savannas of eastern and southern Africa. Here are some of the world’s most impressive large land mammals—the biggest (African bush elephant), tallest (giraffe), and several large predators that live in social groups: lions, spotted hyenas, and African wild dogs. Yet it is the huge herds of grazing mammals that create the most memorable, elemental spectacle. More than 1.5 million blue wildebeest migrate each year between the Maasai Mara and the Serengeti, following the seasonal rains and the new grass growth that the rains bring. They are accompanied by smaller but still impressive numbers of other antelopes: common eland, Grant’s and Thomson’s gazelles and impalas. The female wildebeests all bear their calves in the same short time window— many are killed by predators, but their sheer numbers mean that a good proportion make it to a more survivable age.

Wildebeest dominate the great savanna migrations of East Africa, following the fresh grass growth brought by the rains.

Africa’s savannas are also home to kudus and hartebeests, topi and springbok, black and white rhinos, African buffaloes, rhinoceroses and zebras. They may seem to be in competition, but each has a slightly different dietary preference and a different way of feeding—wildebeests, for example, eat new growth by preference and crop it very efficiently, while hartebeests feed more slowly but are less fussy about what kind of grass they eat.

Some of the other ungulates are browsers by preference, taking tree and bush foliage. Giraffes can reach the highest growth, while gerenuk antelopes, which are also long necked, extend their reach by standing on their hind legs.

Some antelopes mainly graze, while others, like these gerenuks, are browers, preferring to eat taller vegetation.

The world’s other expanses of grassland have their own large grazers. North America’s prairies have American bison and pronghorn antelopes, while the steppes of Eurasia are home to saiga antelopes, European bison, and onagers (a wild relative of the donkey). Arctic tundra supports reindeer (known as caribou in North America) and musk ox. These habitats have been heavily exploited by humans over the last few centuries, being converted for cultivation and livestock rearing, and many grassland animals are now threatened with extinction. Today, 60 percent of mammal life on Earth by weight is made up of livestock, and 36 percent human beings, with just 4 percent of that biomass made up of wild mammals. It’s a sobering statistic at any time, but never more so than when looking out over those vast herds on the savanna.

The endangered saiga antelope’s giant nose helps warm up the chilly air it breathes on the central Asian steppes.

// Elephants, manatees and hyraxes Planet Earth today does not play host to the array of huge animals that existed in the past (though it’s another story undersea). However, there are still a number of impressively huge mammals living, and the greatest of them all are the elephants. We think of these huge herbivores as dwelling alongside other big plant eaters, such as rhinos, hippos, and buffaloes, but their evolutionary origins are very different to those of the hoofed mammals. The order Proboscidea shows enormous diversity in the fossil record—from mammoths and mastodons to miniaturized pygmy elephants living on isolated islands. Today, though, only three species survive. Up until 2010, there were considered to be just two: the African and Asian elephants. They arose from two lineages that parted ways more than 4 million years ago—the mammoths were cousins to the Asian elephant rather than the African elephant. However, recent DNA studies indicated that the African elephant actually comprised two species—the larger, more widespread savanna-dwelling African bush elephant, and the African forest elephant of West Africa.

Asian elephants’ trunks have one “finger” at the tip (as shown here), while African elephants have a second “finger” on the bottom of the trunk tip.

Elephants have an array of striking features: their bare skin, thickly padded hoofless feet, oversized ears, an enormously elongated pair of upper incisor teeth (tusks) that are used as powerful lifting equipment as well as in self-defense, and most of all the trunk, formed by the nose and upper lip. The trunk is highly sensitive, strong and flexible, and the elephant uses it for a wide range of tasks—from carefully handling small objects to hosing itself down with water and tearing twigs and foliage from a tree. These animals are also noted for their longevity and

intelligence, and the profoundly deep and lasting nature of their family bonds. Asian elephants are domesticated and used as beasts of burden in many parts of their southern Asian range, but this is controversial—it is very difficult to keep and manage elephants in captivity in a manner that meets their social needs. African elephants have never been domesticated.

An elephant’s trunk has many uses, including providing a way to drink without having to bend down.

The closest surviving relatives to the elephants today are two very different groups of mammals. The sirenians, comprising the dugong and three species of manatees, are large, aquatic herbivores which live on calm coasts and in river systems. They have no hindlimbs, and flattened, whale-like tails, and use their large, pliant lips to gather in the underwater vegetation that they eat. The nickname “sea cow” describes their rather slow movement and placid nature. Like elephants, manatees are very long lived, and constantly grow new teeth, which erupt in the backs of their mouths and move forward to replace worn-down teeth.

Manatees are slow-moving and curious by nature, leading many to suffer injuries from contact with propeller-driven boats.

The hyraxes are five species of small, furry animals that, at a glance, look like they belong to the rodent order. However, the shape of their teeth and the position of their teats reveal their true lineage. The rock hyrax or dassie is a common animal in parts of Africa and Arabia. It is a highly vocal animal, with calls and “song” that show marked regional variation, and it lives under a complex and notably egalitarian social structure.

A rock hyrax displays its large incisors, the same teeth which are elongated into tusks in elephants.

// Carnivores—the “catlikes” and “dog-likes” The large order Carnivora is home to most of the mammals that habitually hunt and kill other mammals, and other vertebrates in general. Some smaller carnivores, though, feed mainly on invertebrates, such as the ant-eating aardwolf. Others have adapted partly or, in the case of the giant panda, completely to a vegetarian diet. However, even the giant panda, which feeds on bamboo and virtually nothing else, reveals its predatory ancestry through the shape of its teeth. Carnivores can be found everywhere in the world and in the seas as well—they include some of our most charismatic and beloved species. Some are fast runners and others skilled climbers; some show strength and courage far beyond what you might expect for animals of their size, and many demonstrate amazing stealth, patience, and tactical intelligence to outwit their prey.

This diagram shows the evolutionary pathways of the carnivores.

Genets are sleek, agile tree-climbers, with cat-like curiosity in their surroundings.

There are two main lineages of carnivores: Feliformia (“catlike”) and Caniformia (“dog-like”). Naturally enough, the cat family (Felidae) are feliforms and the dog family (Canidae) are caniforms. All other carnivore families fall into one of these two groups. Feliformia and Caniformia differ in certain anatomical ways, particularly in the structure of their skulls—the part of the skull enclosing the inner ear is always fully divided into two chambers in feliforms, but is single-chambered or only partially divided in caniforms. The other distinctions apply in most but not all cases. The two groups generally differ in the shape of their snouts, and

the shape and number of their teeth. Most feliforms are short nosed with larger canine teeth and carnassials (enlarged premolars, adapted to tearing), so for their size they generally have stronger and more effective bites and are more predatory. Caniforms, with their longer, narrower snouts and less powerful bites, tend to be more generalist feeders. Most feliforms are digitigrade (walking on their toes) while most caniforms are plantigrade (walking on their flat feet), and most feliforms can retract their claws while most caniforms cannot.

In North America, raccoons are well known for breaking into trash cans to scavenge for scraps, and are even nicknamed “trash pandas” because of this behavior.

Besides the cats, Feliformia also includes the civets, linsangs and genets (families Viverridae, Nandiniidae, and Prionodontidae), the hyenas (family Hyaenidae), the mongooses (family Herpestidae), and the carnivores of Madagascar (family Eupleridae)—in total, about 114 species. In Caniformia, besides the dogs, are the weasels and their relatives (family Mustelidae),

the bears (family Ursidae), the raccoons and their relatives (family Procyonidae), the red panda (family Ailuridae), the skunks and stink badgers (family Mephitidae), and—probably— the seals, sea lions, and walrus (families Phocidae, Otariidae, and Odobenidae). There are about 165 species of caniform carnivores, though sometimes the 33 species of seals and sea lions are separated as a different order (Pinnipedia).

The beautiful ringtail or ring-tailed cat is related to the raccoons but is a considerably shyer animal.

Carnivores can seem incredibly imposing, at the “top of the food chain,” with no natural predators of their own. Yet all

carnivores can only survive if there are good numbers of their prey around. The presence of the top-tier carnivores in a habitat indicates a healthy ecosystem, functioning as it should.

Members of the family Mustelidae are noted for their fierce and fearless natures. The wolverine is the largest and arguably most formidable of them.

A small and mainly insectivorous animal, the aardwolf is the smallest member of the hyena family.

// Wolves, foxes, and other wild dogs The wolf or grey wolf is one of the world’s most revered (though also feared) wild carnivores, and it is also the ancestor of all domestic dogs, in their fabulous variety. It belongs to the “true dogs” lineage (Canini) within the family Canidae—this lineage is sister to the “true foxes” (Vulpini). In all, Canidae comprises about 34 species, of which 23 or so are true dogs (though, confusingly, some of them have “fox” rather than “dog” in their names). They can be found worldwide, though the dingo of Australia is descended from very early domesticated dogs, brought to the country by seafarers from Asia.

Arctic foxes have short snouts and small ears to conserve heat in their chilly environment.

The wolf ranges across most of the northern hemisphere, living in co-operative social groups. Working in teams allows it to bring down very large mammals such as moose, bison, and musk ox. Social living and co-operative hunting is a feature of some of the other true dogs as well, such as the beautiful African wild dog, which hunts large antelopes, and the dhole of southern Asia, which hunts deer in large packs. The pack-hunting dogs may pursue their prey over many hours and extremely long distances, exhausting it before moving in for the kill. The more solitary dogs, such as the coyote and the various species of jackals, are more opportunistic, and a large proportion of their diet is scavenged.

The smallest of the canids, the fennec fox is widespread in arid and desert regions in northern Africa.

Other true dogs include several South American species, including the short-legged, almost pig-like bush dog, a packliving species with a very wide distribution; the comically lanky and leggy maned wolf, a shy and solitary omnivore; and the

culpeo, found on the western side of South America. Culpeos were domesticated by the Yaghan people of Tierra del Fuego, giving rise to the Fuegian dog, but this died out early in the 20th century.

Wild wolves can exhibit a variety of coat colors, even within the same family group.

The foxes mostly live alone or in pairs, though the bat-eared fox, which has an almost entirely insectivorous diet, often lives

long term in family groups. This species has extraordinarily large ears, a trait it shares with the smallest of all canids, the fennec fox. Both of these species are African. The northern hemisphere has relatively few fox species but includes the extremely widespread red fox, and the Arctic fox. This latter species is one of a few Arctic-living mammals that undergoes a complete seasonal change of coat color, becoming pure white in winter.

Humans’ worst enemy? Canids of all kinds sometimes come into conflict with humans, as they can attack livestock. The wolf and the African hunting dog have both suffered extremely heavy persecution, the latter species now classed as Endangered, with a total population of fewer than 7,000 individuals split into nearly 40 separate subpopulations, allowing little opportunity for genetic mixing. Wolves have been eradicated in several European countries, though more tolerant modern attitudes have seen them return, either naturally or through reintroduction.

Raccoon dogs feature heavily in Japanese folklore, where they are known as tanukis.

DOMESTICATED MAMMALS Given the huge number of wild species on our planet today, the number that have been fully domesticated is extremely small. However, selective breeding over many generations, to develop and combine specific traits, enables us to develop distinct forms of any given species to suit our needs or tastes. The domestic dog, for instance, descended from wild wolves, and having lived in association with humans for at least 14,000 years now exists in more than 200 distinct breeds, which show far more variation in size and shape than is seen across all wild species of dogs. The wolf and the African wild cat are, respectively, the ancestral species of our two most popular domestic companion animals, the domestic dog and domestic cat. The former was probably originally domesticated to assist with active hunting and for defense against other large predators, but today dogs are used for many other purposes. It is theorized that cats effectively domesticated themselves, being attracted by rodents that fed on humans’ food stores, and being encouraged by humans as pest controllers. Rabbits have long been kept and bred as a source of meat and fur, but are also now popular pets.

The wild house mouse may be regarded as a pest, but in its domesticated form it is extensively used in medical research.

Marks of domestication Compared to their wild ancestors, domestic animals tend to be smaller, with smaller brains, shorter jaws with smaller teeth, more placid temperaments, and show certain anatomical quirks, such as drooping rather than erect ears, curled tails, and white patterning on the coat. These traits may arise spontaneously in both wild and domestic breeding populations, but are retained in domestic populations because of deliberate selective breeding. However, there may be more to this. An experimental domestication experiment on foxes, carried out in Russia in the late 20th century, used a single trait to select individuals for each successive breeding generation: a docile personality. A few generations in, the foxes had become friendly and dog-like, but were also beginning to show all of the physical traits described above. This suggests an enduring relationship between the genes controlling this set of traits in mammals—now known as “domestication syndrome.”

A tame pet fox, with a gray and white coat that is not seen in wild foxes. Its pattern looks strikingly like that of certain domesticated cat and dog breeds.

Humans have also domesticated a variety of hoofed mammals, which we use for meat, as beasts of burden, and in some cases also for milk, and hair or wool. Domestic cattle today have been found to carry genes from several closely related wild cow species, particularly the aurochs that formerly ranged through Eurasia and North Africa but are now extinct. Both species of camels (the one-humped dromedary and two-humped Bactrian) have been domesticated, while domestic sheep, goats, and pigs descend from (respectively) the mouflon, bezoar ibex,

and wild boar, all of which are native to Eurasia. The larger hoofed mammals are potentially dangerous, so breeding selectively for a docile temperament is very important.

Cross-breeding domestic cats with Asian leopard cats produced the Bengal, a stunning domestic cat breed (albeit with a more assertive character than the average pet moggy).

The tarpan, ancestor of domestic horses, is extinct in the wild, although there are populations of wild-living feral horses in many parts of the world. Domestic donkeys descend from the African wild ass. Donkeys and horses today are widely used as riding animals and as beasts of burden—both have played a key role in

the expansion of human populations, and improving efficiency in the ways we work the land. In modern times, two much smaller mammals—the domestic rat (descended from the brown rat) and domestic mouse (descended from the house mouse)—are of great importance in scientific research, especially in medical fields.

Many cultures have their own domestic beasts of burden—large hoofed mammals which can carry a human rider, a pack of supplies, or both.

// Bears Many human children grew up with a toy bear to cuddle, but real bears are the biggest and potentially most dangerous of all land carnivores, and must be treated with respect. These hefty carnivores are mainly omnivorous, though the polar bear eats little but meat, and the spectacled bear’s diet comprises more than 90 percent plant material. The world’s eight bear species are widely distributed in Eurasia and North and South America, though are absent from Africa. Bears are powerfully built, with huge limbs, and have very short tails. They appear rather slow and lumbering but can run at high speed when necessary, and four species are strong climbers, spending much of their lives foraging in trees. They are intelligent and opportunistic, and thanks to their size and power they have few or no natural predators. The sloth bear, found mainly in India, is mostly insectivorous and herbivorous, with a mouth adapted for consuming large quantities of ants and termites, but it is still a formidable animal—even tigers will rarely risk attacking one.

In any discussion on climate change, polar bears are likely to be mentioned, because of their dependence on Arctic sea ice.

The polar bear is the world’s largest living land carnivore, and is found in the high Arctic where it is adapted to prey mainly on seals, which it ambushes on sea ice. Its dependence on this food source and habitat, and ability to swim long distances, makes it almost more of a sea animal than a land animal, though it does move inland to solid ground when it has cubs. This bear is active year-round, but some other northern-hemisphere bears hibernate through winter, after first building very large fat deposits to keep them alive through the long sleep. The large subspecies of brown bear inhabiting northern North America (known as the “grizzly bear”) hibernates for up to seven months in a year, and may almost double its body weight to more than 660 lb (300 kg) pre-hibernation.

When sockeye salmon migrate upstream in Alaska to spawn, they attract numerous hungry grizzly bears.

The sun bear, native to South East Asia, is the smallest species, and spends most of its time foraging in trees. It uses its long claws to tear into hollow trees, to reach bee nests and other foods. Like other Asian bear species, it has long been targeted by hunters as its body parts are used in traditional medicine—the

sun bear, the sloth bear, and the Asian black bear are all classed as Vulnerable in terms of extinction risk.

American black bears, like the other smaller bear species, are skilled tree-climbers.

The baffling bear The giant panda is a true oddity. Its unique black-and-white pattern and its diet of bamboo is just the start. This bear’s other peculiar traits include its “thumb,” an enlarged and elongated wrist bone that works as an opposable extra digit to grip bamboo, and the diminutive size of its newborn cubs —at just 4–8 oz (100–200 g) they weigh only 1/800th as much as their mother, making them proportionately smaller than any other newborn placental mammals. Wild giant pandas only live in a few remote forested regions of China, and illegal hunting as well as habitat loss had a devastating impact on their population through the 20th century. However, conservation and captive breeding has restored their numbers. The latter is no mean feat, given their incredibly low reproduction rate and their notorious reluctance to mate in captivity, but artificial insemination is now successfully used. Captive pandas were historically given by China to other nations as symbols of diplomacy.

Right In zoos, giant pandas always draw a crowd, with their endearing appearance and quirky habits.

// Big (and small) cats There are about 40 species of wild cats in the world. Of those, only five belong to the genus Panthera—the true “big cats.”. These five—the tiger, lion, leopard, jaguar and snow leopard—are famed and respected for their deadly power. But all cats are formidable predators in their own right, and in fact some of the smallest species are among the most effective hunters, in terms of the size of prey they will tackle, and their success rate. Most cats have tawny or golden coats—sometimes unpatterned, but more often marked with beautiful spotting, rosettes, or stripes. Camouflage is important as they are adapted for short, explosive attacks rather than endurance pursuit, so getting close to unwitting prey—either by hiding or stalking—is necessary. The fastest land mammal, the cheetah, takes just 3 seconds to accelerate to 60 mph (96.5 km/h) from a standing start, but it must bring down its target almost immediately—if the prey can evade capture for more than 20 seconds it will probably escape, and almost no chases last longer than a minute. When it catches prey, a cat usually makes a very quick kill with a powerful bite to the neck—its long canine teeth (fangs) and strong jaws are adapted to this task. Because most cats are liable to have their prey stolen by scavengers of various kinds, being able to make an efficient kill is a necessary skill. The prey can then quickly and easily be taken to a safer place—in cover, or up a tree.

In a straight line a cheetah’s speed is unmatched, but its quarry may escape if it can turn more sharply than the cheetah.

Once they reach adulthood, cats are solitary in the main, though lions live and hunt in social groups, and so sometimes do adult male cheetahs (female cheetahs go it alone). They live in a wide range of habitat types but most are found in forested areas. Their stealth and keen senses make them very challenging for wildlife watchers to see, but the larger species can occasionally be

dangerous to humans, especially in areas where human settlements have encroached into their habitats.

The panther paradox Melanism – a higher-than-normal amount of dark pigmentation – can affect any mammal but is unusually common in cats. It is so frequent in leopards and jaguars that their black variants have a name of their own – “panther”or “black panther” (used for black-furred individuals of both species). In deep forest, black fur can work just as well as a dappled coat for camouflage. However, the underlying spotted pattern is still visible on a panther’s coat in strong sunlight. Black forms are also documented in some of the small cats, including the serval, jaguarundi, Asian golden cat and Geoffroy’s cat – as well as the domestic cat, of course.

Many of the spotted cats, both large and small, sometimes occur in a melanistic form. Black jaguars like this are known as

panthers, as are black leopards.

Besides the big cats of Panthera, some other natural groupings of species include the lynxes and bobcat, which are long-legged with shortened tails and very long ear tufts; the Leopardus cats of Central and South America (eight species of agile, long-tailed tree climbers with boldly patterned coats); the puma and cheetah, largest of the “small cats” and each other’s closest relatives, despite the former living in America and the latter in Africa; and the small cats of the genus Felis. The domestic cat descends from the African wildcat, and feral, freeliving populations of domestic cats can be found in many parts of the world today

The margay, smaller cousin to the more familiar ocelot, spends almost all of its time climbing in trees.

The sand cat is a small desert species related to domestic cats, and is found in north Africa and southern Asia.

// Underwater hunters— otters, seals, and sea lions

Despite their name, some species of river otters are equally at home hunting in the sea.

Most of the mustelids, including weasels, stoats, martens, polecats, badgers, and the wolverine, are land hunters. However, the otters are adapted to hunt in water, with one species, the sea

otter, being effectively a marine mammal—the female even gives birth in the water and her single pup rides on her stomach. The seals and sea lions are also well adapted to live and hunt in water, though they do need to come to land to mate and give birth. Water bodies and oceans are rich feeding grounds for animals, and as we have seen earlier in the book, most animal groups are and always have been water dwelling. Given the ecological richness of water habitats, it is not surprising that the vertebrates that evolved on land—the reptiles, birds, and mammals—all have some lineages that have evolved back towards an aquatic life. There are about 11 species of otters around the world. Most of them live in and around rivers. They have very dense fur, webbed paws, and powerful, muscular tails. The sea otter, the largest species, is buoyant enough to float on its back without effort, so it rarely comes to land at all. It is noted for carrying a rock around, which it uses to break open the sea urchins and molluscs it brings to the surface on its foraging dives. It lives around northern and eastern Pacific coasts, and its control of sea urchin populations makes it a keystone species in the marine kelp forest ecosystems there (too many urchins can destroy the kelp forests).

Sea otter “rafts” typically comprise up to 100 resting animals, which tend to all be of the same sex.

Tusk master The walrus is the world’s largest seal, and is notable for its bare, wrinkled skin and the elongated and thickened upper incisors that protrude from the upper jaw. This Arctic animal can weigh more than 2.2 tons (2,000 kg). Both sexes have tusks and use them to make and maintain holes in the sea ice, through which the walrus can enter and leave the water. However, males’ tusks are larger and are used in fighting as well. The diet is varied but includes a lot of seabed molluscs, which the walrus detects by using the copious sensitive whiskers around its mouth.

Under a walrus’s almost hairless skin lies a layer of blubber which can be up to 6 in (15cm) thick.

Seals have their forelimbs and hindlimbs modified into swimming flippers, making them ungainly on land but fast and

agile in water. They can also seal their nostrils and ear openings to prevent water ingress, and they have generous subcutaneous fat to keep their bodies warm. The sea lions and fur seals can bring their hind flippers under their bodies and use them for locomotion, but the true or earless seals cannot, so effectively drag their rear bodies along when they move on land (however, their swimming is more efficient). Only one seal species, the Baikal seal of Russia’s Lake Baikal, is restricted to freshwater habitats. The others hunt mainly or exclusively at sea. They will “haul out” on to beaches, islands or ice floes to rest, and also to mate and to give birth. Typically, a female will give birth and then mate again quite soon afterwards, so each pregnancy lasts close to a full year. Many form very large breeding colonies, in which males (which are often much larger than females) compete with fierce fighting to win the largest harem of females. Seal milk is exceptionally rich and fatty, meaning the pups grow quickly. Some pups are born with a fluffy baby coat and avoid the water until they have molted into their adult-like coat, but others can swim soon after birth.

Seals close off their nostrils when swimming underwater, using sight, sound, and touch to find prey rather than smell.

Sealions, unlike true seals, have visible external ears, and are also more agile and acrobatic than seals.

// A farewell to land— cetaceans One of the biggest surprises that came out of our unraveling the evolutionary history of mammals is the origin of the cetaceans. These animals—the porpoises, dolphins and whales—are supremely adapted to a fully aquatic life. In their body shape and the way they live, they are much more akin to fishes than to any living mammals, though like seals and otters, they are primarily active, prey-chasing carnivores. Yet their lineage originally emerged from within Artiodactyla, the order comprising the even-toed, hoofed grazing mammals.

Dolphin intelligence The marine dolphins are noted for their playful behavior and co-operative hunting methods—both indicators of high intelligence. In particular, the common bottlenose dolphin has been much studied, both in captivity and in the wild. It has been found to be capable of tool use, to have a concept of self, to be able to innovate, and to show awareness of future events. In an impressive example of creative thinking, a captive dolphin that was rewarded with food when it removed any trash that fell into its tank learned to tear up the bits of litter, so that it could claim a reward for every individual small piece.

One of the most acrobatic species, spinner dolphins live in pods of up to a few thousand individuals, and range throughout tropical and subtropical seas.

Cetacean traits include an absence of hindlimbs and pelvic girdle, a dorsal fin, forelimbs modified into powerful swimming flippers, nostrils transmogrified into a blowhole on top of the head, and a horizontally flattened tail with two flukes. The taut, smooth, hairless skin covers a thick layer of insulating blubber, a flexible ribcage to withstand pressure in deep water, and blood that can carry far more oxygen than that of a land mammal. In

cetaceans, body structures such as ears, mammary glands, and reproductive organs have no permanently external parts, so as not to break the streamlined continuity of their body shape. Their sensory systems are adapted for underwater life and the different ways that light, sound, and chemical particles move through water as opposed to air. Most cetaceans are highly social, and they are also highly intelligent, with sophisticated communication systems.

A group of narwhals congregating at a gap in the sheet ice. The single spiraling tusk has earned the narwhal the nickname “unicorn of the sea.”

There are nearly 90 species of cetaceans in the world, most of them belonging to a group known as “toothed whales” (which

also includes all 40 or so species of dolphins, and seven species of porpoises). Between them, they can be found in all of the world’s oceans, and five species of river dolphin inhabit mainly estuarine rivers in warm tropical and subtropical parts of the world. River dolphins’ adaptations to their rather different environment of warm, shallow but turbulent and murky water include reduced blubber, poor eyesight, but well-developed hearing, and a very long rostrum or “beak” with touch-sensitive hairs.

The bulging forehead or “melon” of dolphins and toothed whales acts as a lens to focus the sounds that the animal makes. It is particularly pronounced in the beluga.

The largest of the toothed whales is the sperm whale, which can weigh more than 55 tons (50,000 kg). It dives more than 6,560 ft (2,000 m) underwater to hunt prey such as giant squids. The smallest species is the 100 lb (45 kg) vaquita, an extremely rare porpoise found only in the northern Gulf of California. The

toothed whales include the remarkable narwhal, in which males develop a single tusk from an overgrown left upper canine (the tusk’s purpose or function is still unclear), and the beluga or “white whale.” Both of these distinctive species are found in Arctic seas.

Living up to their name, killer whales, or orcas, regularly team up to hunt and kill much larger whales, as well as preying on seals and fish.

// The great whales The largest animals on Earth—not just now but throughout the whole of the planet’s evolutionary history—are the great whales. These animals are more properly known as the “baleen whales,” after the baleen plates in their mouths, which resemble combs or bristles and are used to trap the small sea creatures on which they feed, as they take in and expel mouthfuls of sea water. There are 13–15 species of baleen whales on Earth. They all have a distinctive facial appearance compared to toothed whales, with a proportionately very large lower jaw. They also have smaller dorsal fins than most toothed whales. The smallest species, the pygmy right whale, is about 20 ft (6 m) long, while the largest, the blue whale, can reach almost 100 ft (30 m) in length. Some species have a fairly restricted distribution, such as the grey whale, which occurs only in the north Pacific, but others, such as the fin whale, range through all of the world’s oceans.

The bowhead whale uses its broad head to break through sea ice. It is thought to be the longest-lived mammal, with some individuals living 200 years.

Baleen whales feed on all manner of sea animals—freeswimming crustaceans such as krill are very important in their diets but they also eat fish, squid, and occasionally even seabirds.

A feeding fin whale swims fast (about 11 mph/17.7 km/h) towards a concentration of prey, such as a shoal of fish, with its mouth open. In this feeding “lunge” it takes in a huge mouthful of as much as 2,472 cu ft (70 cu m) of water. It then forces the water out of its almost-closed mouth and any solid objects are trapped by the baleen plates and then swallowed.

Whale fall We often read in the news about whale strandings, usually involving unwell or emaciated animals. Many whales die far from land, though, and sink to the deep ocean floor. Here, their immense bodies may become entire miniature ecosystems in their own right. The soft parts of their bodies are eaten by scavengers relatively quickly, but their bones, which are rich in fat, become colonized by specialized bacteria that themselves provide food for a community of marine molluscs. A whale carcass can support other living things for 100 years or more before all of the organic material is consumed, and the remaining mineralized structure of the bones can provide habitats for other animals for even longer.

Whales that strand and die on shore provide food for all kinds of scavengers, great and small.

Older right whales bear prominent callosities (patches of roughened, calcified skin) which are home to colonies of whale lice and whale barnacles.

Whale hunting has long been practiced by some coastal communities. As the process became more industrialized, so its impact on whale populations became ever more severe. The two species of northern right whales (so named because they were considered the “right” species to hunt) were particularly affected,

and both are still endangered—there are fewer than 250 north Pacific right whales alive today and only about 400 north Atlantic right whales. This is despite all right whale species being granted global protection from all forms of hunting in 1937. Some illegal hunting has continued to occur, but these very long-lived animals are slow to reproduce, each female producing one calf every three years at most. They are also threatened by oceanic pollution, and military use of sonar equipment is likely to affect their natural behavior and breeding success.

With its very long flippers and habit of frequent energetic breaching, the humpback is one of the most distinctive great whales.

ECOLOGY AND CONSERVATION Ecology is the study of living things in their environment. Our planet is home to dazzling and dynamic biodiversity, from mountain summit to ocean trench and everywhere in between. However, with the dramatic growth in human population over the last few centuries, and humanity’s need to repurpose the land to meet its own needs, pressure on the world’s remaining wild places is intense.

Mule deer (Odocoileus hemionus), on the edge of Denver city. Balancing the needs of humans and wildlife has never been more challenging than it is today.

// What is ecology? We see the animal world as a collection of individual species, each with their own unique traits and qualities. Yet we can’t fully understand any species in isolation, because each one is a functioning part of a wider web of living things—not just plants, but animals, fungi, and microorganisms as well. This interconnected assemblage of living things, together with the latitude, topography, and seasonal patterns of the place where they live, is an ecosystem. The study of ecosystems and how they work is known as ecology. Animals, because of the way their bodies work, need to consume organic material in order for their cells to build the proteins and fats that make up their tissues, and to provide energy for their activities. This means that they are “consumers,” while green plants and other photosynthesizing organisms, which only need sunlight plus inorganic material (carbon dioxide, water, and nitrogen), are “producers.” Consumers may feed on the plants themselves, or on other consumers, or they may consume dead organic material of any origin. However, they cannot exist without the primary producers. This is why the richest communities of animal species are found in places where plant diversity is higher. Some areas of land with very little in the way of plant life still support large animal populations, but species diversity tends to be low. Every type of natural

environment, or biome, on Earth has its own particular assemblage of producer and consumer species.

This map shows the natural environment types or biomes of the planet Earth.

The soil that supports plant growth is a vital component of land ecosystems. The organisms that live in it break down organic material into simple compounds that the

photosynthesizing plants can absorb through their roots. In marine ecosystems, the primary producers are mainly freefloating, photosynthesizing microorganisms, but organic material is still recycled through the system thanks to other microbes that consume the remains of all organisms that die in the sea. Nutrients are cycled between land and sea constantly, through animals such as seabirds that inhabit both.

The gyrfalcon (above) and barred forest-falcon (left) share a fairly recent common ancestor. However, the gyrfalcon is adapted to an open, snowy, tundra biome—its dense white plumage provides

warmth and camouflage, while its stocky, powerful build is ideal for fast, sustained flight in open skies. The barred forest-falcon evolved in tropical forests—it has dark plumage for camouflage, and short, broad wings for agile flight in its “cluttered” environment.

Studying any animal species in the wild involves understanding its ecology. This is of paramount importance when working out a plan for conservation, as sometimes the causes of a species’ decline are not direct, but the knock-on effects of problems affecting other species in its ecosystem. For example, Iberian lynxes have declined through over-hunting, but also because of the disease myxomatosis affecting their rabbit prey. Even if the causes of the decline are simple and easily eliminated (for example, by removing a harmful invasive species from an island ecosystem), it is still important to pay attention to the state of the ecosystem, as the period of imbalance may have caused many changes. It often pays to take a broad view when planning conservation work. Effective protection and support for the ecosystem as a whole will almost inevitably deliver lasting benefits for the target species, too.

The gyrfalcon (above) and barred forest-falcon (left) share a fairly recent common ancestor. However, the gyrfalcon is adapted to an open, snowy, tundra biome—its dense white plumage provides warmth and camouflage, while its stocky, powerful build is ideal for fast, sustained flight in open skies. The barred forest-falcon evolved in tropical forests—it has dark plumage for camouflage, and short, broad wings for agile flight in its “cluttered” environment.

// How ecosystems work The concept of a food chain is often used to explain the interrelationships within an ecosystem. For example, a bush produces berries, a mouse eats the berries and a fox eats the mouse, forming a simple three-step food chain. We label the bush a “producer,” the mouse a “primary consumer,” and the fox a “secondary consumer.” If we add a bigger predator capable of hunting the fox, such as a puma, that becomes a “tertiary consumer”. In practice, though, the fox will happily eat the berries, too, and things only get more complex the closer you look. Just as there are omnivores, there are also extreme specialists, such as caterpillars that will only eat the foliage of one plant species, and parasitoid wasps that will only lay their eggs on one caterpillar species. And predator-prey relationships are not always one-way. An adult goshawk will kill and eat a squirrel, but a squirrel that happens upon an unattended goshawk’s nest might eat the eggs it finds. Omnivores like bears are primary, secondary, and tertiary consumers all at the same time, and they are also scavengers of dead animals. Then there are the scavengers and the detritivores or decomposers that break down nonliving organic matter of all kinds, and the parasites that complete their life cycles on or in the bodies of one or more living hosts. A linear chain cannot capture all of this complexity, so a food web is often used instead.

Curlews and dunlins are both shorebirds with curved bills for probing mud, but the much larger curlew’s “probe zone” is deeper than the dunlin’s, so they do not compete directly.

Removing any of the species in a food web has consequences for all of the other species, and the same goes for adding in a new species. If a predator disappears, for example, this may work to the advantage of prey species, but not necessarily, because competition is another element of the food web. If predator A and

predator B both hunt prey C, but predator B is more effective at hunting that particular prey, the disappearance of predator A could allow predator B to become more numerous, and prey C could end up under increased predation pressure as a result. Competition is kept in check to some extent by niche separation. This means that two different species will generally occupy slightly different ecological niches. For example, two grazing mammals living in the same area of grassland are likely to prefer slightly different lengths of grass and/or slightly older or younger leaves. Some species are highly specialized, using a narrow niche but doing so very effectively, while others are generalists, able to use a wider range of resources but less effectively. Specialists are less adaptable and therefore more likely to suffer when there are environmental changes, so are more vulnerable than generalists. And in general, predators are often more vulnerable than prey species, because they cannot survive unless there is already a sufficiently large population of prey.

A simplified food web, comprising producers (plants that harness the Sun's energy through photosynthesis and absorb nutrients from the soil), primary consumers (animals that eat the plants), secondary consumers (animals that eat other animals), and decomposers (animals and other organisms that break down dead living things and their waste, and return nutrients to the soil). In reality, some animals have much broader roles than others—for example, foxes eat almost anything, living or dead.

// Relationships between different species As we have seen, all the species in an ecosystem are connected, whether directly or indirectly, to one another. The nature of these interrelationships is often rather more complex than it may appear at first glance. Even a single encounter between two individual animals may not always have the expected simple and predictable outcome. The most obvious relationship between two animal species is that of predator and prey. Many animals will end up being eaten by another animal—even the “top-tier” predators of land and sea are likely to fall victim to other hunters if they become injured, or too old and weak to defend themselves, and this is arguably a “better” death than to slowly fade away from starvation or an infected wound. Competition for the same resources is another important type of interspecies relationship. These struggles are not necessarily over food—for example, many woodland bird species nest in holes in tree trunks, and these are not always easy to find so may be hotly contested.

Cleaner wrasses are small, slim fish that remove parasites and dead scales from the bodies of other larger fish.

Other types of relationships between species may be mutually beneficial, or may benefit one without significantly affecting the other. Many parasites do little or no harm to their host species, and virtually every larger animal species is host to a small community of parasites living on the outside and inside of their bodies, consuming everything from blood to dead skin to a share of the food that the animal itself is eating. A parasite “wants” its host to survive and stay in good health—there is, in fact, some evidence that a certain level of parasite load could actually be beneficial to health.

A dead caterpillar covered in the pupae of parasitoid wasps that developed as larvae inside its body.

That said, genuinely co-operative relationships between different animals are not very common in nature. The relationship between clownfish and anemones is one such association (see page 39). Another often-cited example is the small animals that eat the parasites and dead skin they find on the bodies of larger animals. However, in some cases the “cleaner” animal may take more than it should, opening wounds and drinking blood.

Stealing someone else’s fish supper is easier than catching your own. Kleptoparasitism is widespread among seabirds.

Then there is the parasitoid, which kills its host. Many solitary wasps and some kinds of flies are parasitoids. The female lays her eggs in or on a living host (often a caterpillar) and the larvae eat the host alive. They may also control its behaviors, for example causing it to stop feeding and travel to a suitable place for the larvae to emerge from its body and pupate.

Social parasites If food is difficult to find, why not just steal it from another? If you spend time watching animals feeding in a group, you will notice that most have no qualms about grabbing food right out of another’s mouth, but for some species, food thievery is a way of life. Scorpion flies regularly take the wrapped-up prey of spiders out of their webs—and some spider species do this to their fellow spiders as well. Skuas and frigatebirds chase and harass other seabirds into dropping their prey. This piracy is known as kleptoparasitism. Another form of exploitation is brood parasitism—animals sneakily using other animals to parent their young. Many species of cuckoos and several other birds only ever lay their eggs in other birds’ nests, and employ a range of tactics to ensure the subterfuge is not detected and that their own young will be prioritized. Cuckoo bees lay their eggs in brood cells in the nests of social bumblebees, and in some cases even kill the bumblebee queen and replace her.

When an old male lion is no longer able to defend his position in a pride, he must fend for himself, and is likely to die of starvation.

// Animal behavior Animals of all kinds have two simple goals in life: to survive in a healthy condition and to have healthy offspring. It is often said that they are acting under the unconscious directive of their “selfish genes,” which “desire” only to be propagated down the generational line for as long as possible. Their behaviors, by and large, further these goals in fairly obvious ways. When we observe animals going about their day, we’ll see them finding food, resting, avoiding predators, taking care of their bodies, and (in the case of sexually reproducing species) interacting with the opposite sex. We may see them parenting their young, which can be a very protracted activity, or we may see them producing young in such overwhelming numbers that it is a near certainty that at least a few of those young will survive. Animal behavior, of course, becomes more and more elaborate with the increasing complexity of the animal itself. We start to observe examples of innovative, thoughtful behavior, extremely complicated courtship rituals and sometimes some apparently perplexing behavior that appears on the face of it to go against the genetic directive, such as self-sacrifice or infanticide.

Male bowerbirds build and decorate bowers as part of courtship behavior. The bower has no purpose beyond this, yet female bowerbirds “require” this elaborate behavior from their prosective mates is not clear.

The male resplendent quetzal’s over-long tail may hamper his flight, but his ability to cope with this “handicap” proves to females how strong he is.

Tool use is not common in the animal world. This woodpecker finch is using a cactus spine to probe crevices for insect larvae.

When a new male lion takes over a pride, he will kill the young cubs. They are not his offspring, and killing them means the females quickly become ready to mate again. This is disadvantageous for the females, but they need the strongest possible male or males to associate with the pride in order to provide them with cubs and keep other males away, so they tolerate it. But what about when an animal kills its own offspring? The phenomenon of “brood reduction,” when a parent bird kills one or more of its weaker chicks, looks brutal but can help improve the survival chances of the strongest youngsters when resources are scarce. Conversely, a parent animal may fight

to the death to defend its offspring, but only if the youngsters are going to be able to survive on their own—if not, the sacrifice would be pointless, and the parent would be better off allowing its young to be killed and saving its energy for a new breeding attempt.

Male damselfish mate with several females and guard the resultant eggs, but also eat some of the eggs, a practice known as “filial cannibalism.”

Sexual selection The best attributes for survival are not necessarily the best attributes for appealing to a potential mate. A dazzlingly obvious example of this can be seen in the case of the Indian peafowl. Male peacocks erect a vast, shimmering fan of elongated tail coverts when they display to peahens, and a peahen chooses to mate with the male with the best plumage and most energetic display. That male, therefore, may father the most offspring. However, when it comes to taking flight quickly to avoid a leaping tiger, that great train of feathers will not expedite his escape. Growing the feathers in the first place is a considerable drain on bodily resources, which is also a challenge to survival. This paradox is explained by the “handicap principle”. The male’s plumage is an “honest signal” of his health and vigor, because he survives and performs his display well despite these encumbrances. He is therefore a good choice of mate—his offspring will inherit his strong genes, and his sons will inherit his attractiveness, too.

Long-term parental care is rare among invertebrates, but in some insect groups, including earwigs and cockroaches, the mothers do defend and nurture their young.

// Threats facing animals today Natural events are still a cause of animal declines and extinctions, and this will always be the case. For example, volcanic activity on the island of Montserrat in the 1990s almost wiped out the Montserrat oriole, a colorful songbird that lives only on this island. However, most of the animal species threatened with imminent extinction today are in this perilous position because of human activity rather than natural events. The biggest problem is destruction or degradation of habitats by human activity. Sometimes, wild habitats are directly destroyed—forests are clear-felled or wetlands are drained, for example, and replaced with cultivation or concrete. In other situations, the damage is incidental (though no less devastating), such as when seabed coral reefs (along with other sessile invertebrate life) is ripped up by bottom-trawling fishing fleets.

Plastics in the marine environment are a big problem for animals. For example, sea turtles mistake drifting plastic bags for edible jellyfish.

Intentional killing of animals is a major cause of many highprofile recent declines and extinctions. The extinct passenger pigeon, once the world’s most abundant bird, was shot in such huge numbers that its population fell from billions to just a couple of caged survivors in Cincinnati Zoo over just 50 years. The pigeon was adapted to nest in very large colonies, so its fate was sealed well before its population was reduced to this level.

Tigers and rhinos have declined catastrophically through poaching—despite being protected—because there is still a great demand for their body parts in traditional medicine. The great whales have historically been overhunted to an enormous extent, as have certain marine fishes. In some areas, wild animals of many different kinds are hunted for food (bushmeat), while others are killed simply because parts of their bodies are used as ornaments, such as the beautiful whorled shells of nautiluses.

The European turtle dove is rapidly declining, but is still legally hunted in some Mediterranean countries.

Pollution is a serious problem on land and even more so in the seas. In particular, the vast quantities of plastics in our oceans are killing sharks and sea turtles, albatrosses, and whales. Their

feeding habits are such that it is hard for them to avoid ingesting plastics that are suspended in the water column, and these can lodge in the digestive tract and eventually cause death.

Global average temperature 1850–2019

Climate change The Earth’s climate has changed continuously over its history. Geology and palaeontology reveal that at least five major ice ages have occurred over the last 2.5 billion years, with warm interglacial periods in between. The fact of this is sometimes used as an argument that the very rapid humancaused climate change affecting the world today is not a cause for concern (or even that it is not human-caused at all). However, scientists are in agreement that our industrialized ways are responsible. According to the National Oceanic and Atmospheric Administration’s 2019

Global Climate Summary, the combined land and ocean temperature has increased at an average rate of 0.13ºF (0.07ºC) each decade between 1880 and 1980. Since 1981, the average rate of increase has been more than twice as much. Consequences of climate change include rising sea levels, increased desertification, a reduction in the extent of polar sea ice, and changes to weather patterns. All of these affect animals, and also humans, and if the change continues as it is, the consequences for all life on Earth will be catastrophic.

Deforestation removes not only trees but also a huge number of other living things. Larger and more mobile animals may escape

temporarily, but less forest cover inevitably means fewer animals all round.

// Extinction An extinction occurs when the very last living individual of a species dies. When dealing with a wild animal, it is nearly impossible to say with 100 percent certainty that any species is definitely extinct, as there may be an undiscovered population somewhere—the discovery of a hidden dinosaur population, for example, is the theme of many a science fiction tale and the dream of many a palaeontologist. However, we can often be sure beyond all reasonable doubt that a species is gone for good. A species may also be declared “functionally extinct” if there is no possibility of the surviving individuals ever building a viable population again—for example, if they are all of the same sex.

“Qi-Qi,” a captive baiji or Yangtze river dolphin, died in 2002. He was the last known living baiji.

Extinction is a part of life and a part of evolution. The vast majority of species that have ever evolved are now extinct, and it is unusual, for example, for any mammal species to exist for much longer than a million years. Knowing the average lifespan of species from different taxonomic groups allows us to work out

a “background extinction rate” for them, and comparing this to actual extinction rates reveals whether we are losing species at the expected rate or not. For mammals, the background extinction rate is one species disappearing every 200 years. However, since 1600 we have lost nearly 90 mammal species rather than the expected two. We see similar discrepancies in other animal groups. Under background extinction rates, it should have taken 10,000 years for us to lose the number of amphibian species that have gone extinct since the year 1900. Before a species becomes completely extinct, it may become “extinct in the wild”—a population may still exist in captivity after all wild individuals have gone. This is the case with Spix’s macaw, a blue parrot that is prized as a pet and aviary bird. This led to its extinction, as the last wild birds were trapped for the pet trade, and it was declared extinct in the wild in 2000, though a few dozen still lived in captivity. This number had increased to more than 150 by the year 2020, and a release of captive-bred birds into the wild may be carried out in the near future.

Gilbert’s potoroo, a small kangaroo-like Australian marsupial, was rediscovered 1994—it had been thought extinct for at least 20 years prior to that.

Regional extinctions also occur, where an animal disappears from a country or region—for example, the wolf, lynx, and brown bear were all wiped out in the British Isles over the last few hundred years, and will never be able to return of their own accord. However, because those species survive elsewhere in the world, the term “extirpation” is often used instead of extinction.

Re-evolution? We consider extinction to be permanent, because the random elements of evolution mean that a species is unlikely to ever “re-evolve” in an identical form, even if its ancestral species is still living. However, scientists have recently uncovered a case of apparent re-evolution. The Aldabra atoll, in the Indian Ocean, has been completely submerged several times over the millennia, most recently some 136,000 years ago. Fossil evidence shows that a flightless rail lived on the atoll then—it would not have survived the disappearance of the atoll. Yet there is a virtually identical flightless rail there today. Both incarnations of the Aldabra rail are descended from a flying species, the white-throated rail, which colonized the atoll from other nearby islands.

The Aldabra rail has the unique (as far as we know) claim of having evolved twice.

// Conservation—principles Animal extinctions are not inevitable. Even in the direst situation there is often some action that can be taken to save a species. Conservation practices are aimed at saving species from the brink, but also, less dramatically but arguably more importantly, at halting declines in all species, including those that are still common as well as those that are already rare, and at saving, managing, and restoring habitats for the benefit of all the living things they support. If it becomes evident that an animal is in danger of extinction, the first stages of planning conservation action involve carrying out surveys to accurately assess its population and distribution, performing studies on its ecology, and working out the nature of the current threat or threats to its survival. This is sometimes a simple task. For example, with a seabird nesting on an isolated island, it is usually easy to find and count the breeding pairs, and monitor whether numbers are rising or falling, and how well the young birds are surviving. If it is clear that this bird is threatened by a population of introduced rats, for example, which destroy its eggs and chicks, the solution is to remove the rats. The population should then recover, if there are no other threats at work. However, many rare species are incredibly difficult to find and count, and the threats that they face are multiple and complex.

Found and lost It may seem today that every square inch of the planet is mapped and documented, but in truth we are still finding species new to science every year, including birds and mammals as well as tiny invertebrates and marine creatures. One of the most striking recent newcomers to the global species list is the blue-eyed spotted cuscus, a tree-dwelling marsupial discovered by researchers surveying two tiny islands off West Papua, Indonesia, in the early 2000s. It, and most other newly discovered species, was classified as Critically Endangered soon after its discovery, as it was considered highly likely to have a tiny population. Its home islands are threatened by deforestation, and it is quite possible that it will become extinct before scientists can build up any meaningful understanding of its population, ecology, and conservation needs.

Areas of the world where habitat is now legally protected. The pink areas are islands or undersea; green areas are national parks, wildlife sanctuaries and nature reserves.

If one prominent species is clearly struggling, it’s likely that many of the other species that are part of the same ecosystem are also in trouble. Habitat protection is therefore a high priority in any conservation program. This often means establishing legally protected areas (known variously as nature reserves, national parks or wildlife sanctuaries) where development, hunting and

other potentially damaging activities are forbidden. Depending on local conditions, the park may need to be patrolled by rangers to enforce this legal protection.

The Cape Melville leaf-tailed gecko, endemic to the Melville Range on Cape Melville in northern Australia, was only described to science in 2013.

Other general conservation measures include campaigning against damaging developments and practices, educating communities and encouraging eco-tourism, and raising awareness—and funds—in the wider world. Sometimes, captive

breeding is used to save a particular species, with the long-term goal of reintroducing it to its natural habitat, or establishing a new population in another, safer place. These endeavors require careful planning and a lot of time, and are often successful. However, some animals do not fare well in captivity and other options need to be considered.

Lundy island, off England’s west coast, was recolonized by Manx shearwaters after a successful program to eradicate rats on the island.

// Conservation—success stories In 1976, the entire world population of the Chatham Island black robin was caught by conservationists, and moved from Little Mangere island to Mangere Island, into an area of newly established habitat suitable for the birds. They were closely monitored, and the eggs laid by the only successful breeding pair were taken from their nest and cross-fostered by a related species, to encourage the robins to produce another clutch of eggs more quickly. Today, there are about 250 living black robins, all descended from this one pair, and a subpopulation has been introduced to a second island, to improve the species’ chances of long-term survival. This celebrated story shows what can be achieved by conservationists, given the will, resources, and imagination (and a good dose of luck as well). There are many other stories like this of species brought back from the brink, often involving some degree of intervention in the breeding process. They include the Rodrigues fruit bat, the California condor, the Panamanian golden frog, and the golden lion tamarin (a small South American monkey).

Captive breeding and reintroduction have saved the Hawaiian goose from extinction.

Other successes have been achieved through strict legal protection. The south Atlantic population of humpback whales had plummeted to below 500 by the 1950s due to overhunting, but since a complete ban on commercial whaling in 1986, numbers have recovered to some 25,000. In southern South America, nearly 10 percent of marine and coastal areas under the jurisdiction of Chile, Argentina, and Uruguay are now safeguarded as Marine Protected Areas, with mining, dredging, and fishing strictly regulated or entirely prohibited.

Population bottleneck

This diagram uses colored balls to represent genetic variants, and shows how passing through a population bottleneck will permanently remove some genetic variation from an animal population, even if that population returns to its former size.

Bottlenecks When a species is reduced to a tiny remnant population, it may still be saved from extinction but it will carry the legacy of the crisis into the future, in the form of drastically reduced genetic variety. This places the whole population at higher risk, as its combination of genes can determine how well an animal copes with disease. Also, harmful genetic mutations are more likely to become concentrated in a highly inbred population. By looking at the genes of present-day individual animals, we can work out whether a genetic bottleneck ever

affected their species. For example, cheetahs have extremely low genetic diversity due to two bottleneck events, the first 100,000 years ago and the second 12,000 years ago. When captive-breeding endangered animals, it is now possible (and highly recommended) to compare genomes and choose pairings between the most distantly related individuals.

Cheetahs have very low genetic variability because of a historic population bottleneck. One variant that persists is the “tabbypatterned” king cheetah.

Reintroductions don’t necessarily involve globally endangered species. Returning any species to an area where it used to occur is often successful, as long as the factors that caused its disappearance in the first place have been addressed. There can also be a range of unexpected additional benefits. In North America, for instance, the reintroduction of grey wolves to Yellowstone National Park in 1995, and their predation of the park’s very high elk population, has resulted in a cascade of ecological benefits, with increased species diversity across the board and recoveries in numbers of many rare plants and wetland bird species. Through this kind of work we see more and more evidence of the value of saving and boosting biodiversity in general, as well as helping individual threatened species.

The balance of large grazing mammals such as bison and elk in Yellowstone National Park has improved dramatically since wolves were reintroduced.

// The future for animals This planet, the birthplace of all living things that we know of, is increasingly hostile to life. Humanity’s amazing success as a species is down to our ability to gather resources for ourselves in increasingly sophisticated ways. We used to hunt and gather, but now we can also cultivate, with ever increasing efficiency (which means excluding all other living things from the land where we grow our crops or tend our livestock). We once made simple shelters or used natural ones, but now we can quarry stone and mix cement, and build whatever structures we want. Thanks to machinery of various kinds, almost no part of the planet is out of reach—we can mine the tundra, dredge the seabed, and hunt down the most elusive animals to get what we want. As a result, our population has increased enormously and (as explained on page 151) we and our livestock account for 96 percent of all animal biomass on Earth.

As our Sun ages, Earth will become too hot to support life. The gas giant planets are inhospitable to life as we know it regardless of temperature, though some of their rocky moons may not be.

The human animal evolved here, just as all animals did, and is motivated by the same driving impulses. But we can also look at what has changed, and to predict what will change in the future. We are in the throes of a mass extinction event caused by our own actions, and only concerted action by all nations will reverse it. Whether we will find the collective will to achieve this, though, remains to be seen. Our natural impulses condition us to prioritize our short-term needs over long-term efforts. Ironically, we have to rise above our own animal nature, and invest long term in rebuilding biodiversity.

Green rooftops planted with native species or useful crops offer a way for the built environment to reclaim a bit of nature.

Whatever humans can and cannot achieve over the next few decades, it seems a near certainty that many more animal species will disappear, but some others will thrive. The most resilient species will not only endure, but will also probably outlive us. Meanwhile, the natural process of evolution will carry on—new species will evolve even as others die out. Whether our descendants live to see it or not, this planet is likely to be home to animal life for millions of years to come. In 4 billion years from now, our expanding Sun will have destroyed the last of Earth’s living things. However, with an estimated 6 billion more Earthlike, potentially life-supporting planets in our galaxy alone, it is extremely likely that animals, or something very like them, will inhabit the universe for as long as there is a universe to inhabit.

Farmland can be a haven for wildlife or little better than a desert, depending on how it is structured and managed.

Towns and cities do offer homes for a handful of enterprising animals. In England, the red fox thrives in suburbia.

Save wildlife, save ourselves? Humans often speculate about what event or events might occur to finish off their own species. One strong contender is a pandemic—a deadly disease that spreads globally and resists treatment. In 2019, a pandemic did strike, and at the time of writing we are still battling it. The coronavirus that causes COVID-19 is believed to have come to us from wild mammals, trapped for food. Governments around the world introduced restrictions on human movement to try to halt its spread, which had the additional consequence of some environmental benefits, such as greatly reduced air pollution. In the wake of COVID-19, perhaps humanity as a whole will find a new respect for animals, nature, and the environment we all share.

Humans may leave their mark on the environment, but left to its own devices, nature eventually reclaims anything that we can build.

Further reading The Marine World: A Natural History of Ocean Life by Frances Dipper, Wild Nature Press, Plymouth, 2016. Life on Earth by David Attenborough, William Collins, London, 2018. Last Chance to See by Douglas Adams and Mark Carwardine, Arrow, London, 2009. The Origin of Species by Charles Darwin, Arcturus, London, 2012. The Natural History of Selborne by Gilbert White, Penguin, London, 1977. Other Minds: The Octopus and the Evolution of Intelligent Life by Peter Godfrey-Smith, William Collins, London, 2018. The Genius of Birds by Jennifer Ackerman, Corsair, London, 2017. A Buzz in the Meadow by Dave Goulson, Vintage, London, 2015. The Diversity of Life by Edward O. Wilson, Penguin, London, 2001. Animal: Exploring the Zoological World by James Hanken et al, Phaidon, London, 2018. Endless Forms Most Beautiful: The New Science of Evo Devo and the Making of the Animal Kingdom by Sean B Carroll, Quercus, London, 2011.

How to Build a Habitable Planet: The Story of Earth from the Big Bang to Humankind by Charles Langmuir and Wally Broecker, Princeton, 2012. A Perfect Planet by Huw Cordey, BBC Books, London, 2020. Handbook of the Mammals of the World (multiple volumes), ed Don E. Wilson, Russel A. Mittermeier, et al., Lynx edicions, Barcelona, 2009–2019.

Picture credits Axiom Maps 40, 103, 172, 180, 184 Nature Picture Library 8 Tui De Roy, 11 left Charlie Summers, 16 Thomas Rabeil, 20 Sinclair Stammers, 22 top David Shale, below Visuals Unlimited, 37 top Magnus Lundgren, 41 below right Brandon Cole, 43 below Constantinos Petrinos, 57 below Ingo Arndt, 61 below Paul Bertner, 63 top Melvin Grey, 72 Paul Hobson, 76 Wild Wonders of Europe/Lundgren, 83 top right Visuals Unlimited, below Solvin Zankl, 87 centre Daniel Heuclin, 94 below John Cancalosi, 102 left John Cancalosi, 103 top Julian Hume, 105 below Tim Laman, 107 below right Konrad Wothe, 111 below Klein & Hubert, 113 below left Andrew Parkinson, 114 top Baerbel Franzke/BIA, below Ron Bielefeld, 116 left David Tipling, below right Marie Read, 122 top Dave Watts, below Doug Gimesy, 123 below right D. Parer & E. Parer-Cook, 127 top Stephen Downer/John Downer P, 130 centre right Roland Seitre, 131 Michael Pitts, 143 Nick Garbutt, 147 Anup Shah, 149 Richard Du Toit, 155 Michael Durham, 157 Nick Garbutt, 163 Klein & Hubert, 167 centre Doc White, 167 top Flip Nicklin, 168 Tony Wu, 169 top Steven Kazlowski, below right Bertie Gregory, 170 Shattil & Rozinski, 173 below Melvin Grey, 174 Loic Poidevin, 177 below Sergey Gorshkov, 179 top D. Parer & E. Parer-Cook, 179 centre

Alex Mustard, below KimTaylor, 182 Roland Seitre, 183 below right Pete Oxford, 185 top Tim Laman. Shutterstock 11, 12, 13, 14, 15, 17, 18, 19, 21, 24, 25, 26, 27, 29, 30, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 58, 59, 60, 61, 62, 62, 63, 64, 65, 65, 66, 66, 67, 68, 69, 70, 71, 74, 75, 78, 79, 80, 81, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 107, 108, 109, 110, 111, 112, 113, 115, 117, 118, 120, 121, 123, 124, 125, 126, 128, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 173, 176, 177, 178, 180, 181, 183, 185, 186, 187, 187, 188, 189. Victor McLindon 17 below, 34 top, 34 top, 36 top, 39 top, 42 top, 44 below, 46, 48 top, 50 top, 52 top, 74 top, 80, 96 top, 104 top, 106, 116 top, 120 below.

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