Early Hominid Scavenging Opportunities: Implications of Carcass Availability in the Serengeti and Ngorongoro Ecosystems 9780860543657, 9781407344683

Table of contents :
Cover
Copyright
Dedication
TABLE OF CONTENTS
LIST OF TABLES
LIST OF FIGURES
PREFACE
CHAPTER 1 CHASED OR CHANCED-UPON?
CHAPTER 2 FIELD STUDIES: THE SETTINGS, METHODS AND SAMPLE
CHAPTER 3 PARAMETERS OF SCAVENGING OPPORTUNITY
CHAPTER 4 SCAVENGING OPPORTUNITIES IN THE SERENGETI AND NGORONGORO
CHAPTER 5 EARLY HOMINID SCAVENGING OPPORTUNITIES
BIBLIOGRAPHY

Citation preview

Early Hominid Scavenging Opportunities Implications of Carcass. Availability in the Serengeti and Ngorongoro Ecosystems

Robert

J.

Blumenschine

BAR International Series 283

1986

B.A.R.

5, Centremead, Osney Mead, Oxford OX2 ODQ, England.

GENERAL EDITORS A.R. Hands, B.Sc., M.A., D.Phil. D.R. Waiker, M.A.

B.A.R. -S28J, 1986: 'Early Hominid Scavenging Opportunities'

©

Robert J.Blumenschine, 1986

The author’s moral rights under the 1988 UK Copyright, Designs and Patents Act are hereby expressly asserted. All rights reserved. No part of this work may be copied, reproduced, stored, sold, distributed, scanned, saved in any form of digital format or transmitted in any form digitally, without the written permission of the Publisher. ISBN 9780860543657 paperback ISBN 9781407344683 e-book DOI https://doi.org/10.30861/9780860543657 A catalogue record for this book is available from the British Library This book is available at www.barpublishing.com

To

Mi di

and

Grier

TABLE OF CONTENTS

List of Tables List of Figures Preface

iii V

vi

CHAPTER 1:

CHASED OR CHANCED-UPON?

1

CHAPTER 2:

FIELD STUDIES: THE SETTINGS, METHODS AND SAMPLE

9

The Serengeti and Ngorongoro Ecosystems General Ecology and Settings Suitability for the Study The Field Study Conditions Study Areas Field Methods The Carcass Sample Consumers Observed CHAPTER 3: PARAMETERS OF SCAVENGING OPPORTUNITY Carcass Completeness Carcass Consumption Sequences Indexing Carcass Completeness Carcass Persistence CHAPTER 4: SCAVENGING OPPORTUNITIES IN THE SERENGETI AND NGORONGORO . Carcass Size, Consumer Type and Scavenging Opportunity Carcass Size and Persistence Carcass Size and Completeness Summary of Carcass Size and Scavenging Opportunity Competition and Scavenging Opportunities Competitors and Their Effect as Scavengers . Predator to Prey Ratios and Scavenging Opportunities Seasonality of Scavenging Opportunity in the Serengeti Habitat Type and Scavenging Opportunity Summary of ModPrn Scavenging Opportunity i

9 9 14 15 15 17 21 24 30

31 31 32 44 45 47 48 48 51

60 61 61 69 69 95 106

CHAPTER 5: EARLY HOMINID SCAVENGING OPPORTUNITIES Prehistoric Community Structure and Dynamics Applicability of Modern Observations Changes in East African Community Structure . Prehistoric Carcass Availability The Hominid Scavenging Niche Relevant Hominid Characteristics Potential Contexts of Hominid Scavenging . Foraging Tactics for a Hominid Scavenger The Archaeological Recognition of Scavenging Conclusions BIBLIOGRAPHY

• 109 • . . • • . • • • •

110 110 111 119 123 123 12 5 133 139 146

• 149

ii

LIST OF TABLES

T able 2 .1

Characteristics of s tudy areas in the S erengeti and Ngorongoro S pecies composition of the S erengeti carcass .

2 .2

.

.

.

.

.

.

2 .4

s ample and l ive herbivore community Species composition of the Ngorongoro carcass s ample and l ive herbivore community S ize and age composition of the carcass s ample.

2 .5

S ize

2 .3

s

3 .1 3 .2

a

m

p

and l

e

.

age .

s tratification .

.

A lternative means

.

.

the

.

4 .2

4 .4 4 .5 4 .6 4 .7

.

.

4 .9 4 .10

4 .12

2 9

.

.

s izes

.

.

3 6

.

.

.

.

3 9

.

.

of .

ages

of 4 0

.

4 1

g

.

4 9

r

o

u

p

s

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

S ize-specific proportion of carcasses observed to decompose S ize-specific proportion of carcasses s urviving multiple abandonments a s f ood s ources Consumer-specific completeness on abandonment of carcasses of different s ize and age Average f eeding group s izes of l ions and hyenas .

.

.

.

.

.

.

.

.

5 2

.

.

.

5 4

.

.

5 5

.

.

5 9

.

.

6 2

.

.

6 3

.

.

on carcasses of different s ize and age Completeness on abandonment of medium-sized j uvenile l ion kills s cavenged by hyenas Completeness on abandonment of medium-sized adult l ion kills s cavenged by hyenas Completeness on abandonment of medium-sized adult l ion k ills s cavenged by vultures

.

Completeness on abandonment of l arge adult l ion kills s cavenged by hyenas and/or vultures S ize-specific carcass persistence i n the S erengeti and Ngorongoro Total c arcass completeness on f inal inspection i n the S erengeti and Ngorongoro Average hyena f eeding group s izes in the S erengeti and Ngorongoro Completeness on abandonment of medium-sized adult c arcasses in the S erengeti and Ngorongoro .

4 .13

.

.

.

.

4 .11

2 6 2 8

.

.

.

4 .8

2 5

Consumption s equences f or medium-sized adults by individual c arnivore taxa P ersistence of carcasses of different s ize/age

.

4 .3

.

a general

.

4 .1

.

carcass

.

expressing

.

3 .4

of

carcass consumption s equence Consumption s equences f or three adult carcasses Consumption s equences f or three wildebeest and z ebra .

3 .3

of

.

1 8

.

i ii

.

.

.

.

.

.

.

.

.

.

6 4

.

6 5

.

7 1

.

.

7 3

.

.

7 5 7 6

Table 4 .14

Daily carcass yields in different s easonal contexts i n the S erengeti, and i n Ngorongoro Completeness of medium-sized adult l ion kills in different s easonal contexts i n the S erengeti Completeness of medium-sized adult l ion kills s cavenged by vultures in different s easonal contexts i n the S erengeti Completeness of medium-sized adult hyena k ills in different s easonal contexts i n the S erengeti, and in Ngorongoro Completeness of medium-sized adult l ion kills s cavenged by hyenas i n d ifferent s easonal contexts in the S erengeti, and i n Ngorongoro Completeness of medium-sized adult natural deaths s cavenged by hyenas and vultures in d ifferent s easonal contexts i n the S erengeti Habitat-specific f requency of carnivores a s initial consumers Habitat-specific f requency of carnivores a s initial or s ubsequent consumers .

4 .15 4 .16

.

4 .17

.

4 .18

4 .19

4 .20

.

4 .21 4 .22

.

.

4 .24 4 .25 4 .26

.

.

.

.

.

.

.

.

.

.

.

.

.

.

8 6

.

.

.

8 8

.

.

.

.

.

.

.

.

.

.

5 .2

5 .4

.

5 .5

.

.

.

.

.

.

5 .7

.

5 .9

.

.

.

.

.

.

.

.

i v

.

.

.

.

9 3 9 6 9 8 9 9 1 00 1 02

.

1 03

.

.

1 05 1 13 1 15

.

.

1 17

.

.

1 18

.

1 21

.

1 22

.

Characteristics and quality of early hominid s cavenging opportunities Characteristics of the two b est s cavenging opportunities potentially encountered by hominids S ize-specific f requency of s elected skeletal parts of FLK Z inj bovids .

5 .8

.

Relative taxonomic diversity of modern and f ossil herbivores and carnivores R elative t axonomic diversity of modern and f ossil l arge herbivores, and f lesh-eating a nd bone-cracking carnivores .

5 .6

.

9 1

.

S ize-specific taxonomic diversity of modern a nd f ossil East African herbivores Larger mammalian c arnivores of Plio-Pleistocene and modern East African s avanna-woodlands Taxonomic diversity of modern a nd f ossil l arger carnivores .

5 .3

.

adult l ion k ills i n the S eronera s tudy area Habitat-specific completeness of medium-sized adult l ion k ills s cavenged by hyenas Distribution of l ion k ills i n S eronera with respect to habitat type and s eason Larger mammalian herbivores of P lio-Pleistocene and modern East African s avanna-woodlands .

5 .1

8 4

.

S cavenging pressure by hyenas on l ion kills i n different habitats Habitat-specific persistence of medium-sized adult carcasses i n the S eronera s tudy area Habitat-specific persistence of medium-sized .

4 .23

.

8 1

.

.

.

1 26

.

.

.

1 31

.

.

1 45

LIST OF FIGURES

i qure 2 .1 2 .2

Northern T anzania, showing the l ocation of the S erengeti National Park and Ngorongoro Crater Main vegetation types and s tudy areas in the S erengeti Ngorongoro Crater . . Monthly r ainfall in S eronera during the s tudy period and preceding ten years .

2 .3 2 .4 2 .5 2 .6

.

.

4 .3

4 .5

.

.

5 .1

.

.

.

.

.

.

.

.

.

.

.

.

.

4 .6

.

.

.

.

.

.

.

.

.

V

.

.

.

.

.

.

.

1 0

.

1 1 1 3

.

1 6

.

.

.

4 .4

.

.

Distribution of habitat types in S eronera Distribution of habitat types i n the NaabiNdutu s tudy area Mean carcass s ize and persistence S ize-specific completeness of carcass consumption by d ifferent i nitial consumers Completeness of l ion kills s cavenged by h yenas and/or vultures Relationship between hyena f eeding group s ize and completeness on abandonment of medium-sized adult hyena kills i n d ifferent s easonal contexts Relationship between hyena s cavenging group s ize and the completeness of s cavenged l ion k ills of medium-sized adult carcasses i n different s easonal c ontexts Habitat-specific completeness of medium-sized adult l ion kills s cavenged by hyenas Cost-benefit model of s cavenging i n r iparian or open vegetation habitats .

4 .1 4 .2

.

.

.

1 9

.

.

.

.

.

.

2 0 5 0

.

.

.

5 6

.

.

.

6 6

.

8 9

.

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.

9 2

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.

.

1 04

.

.

.

1 36

PREFACE

This monograph r epresents the main r esults of a three year project that resulted in a Ph.D. dissertation a t the University of California, Berkeley, i n 1 985. Alterations to the original work are minor with the exception o f the addition of more recent c ensus data i n Chapter 2 a nd an expansion of discussion on the archaeological recognition of s cavenging i n Chapter 5 . L ike the original, this work emphasizes the ecology of s cavenging opportunities a s seen in the Serengeti and Ngorongoro. Numerous other d ata were collected during the eleven month f ield s tint which bear heavily on the taphonomy of s cavenging, and h ence the potential s ignificance of s cavenging to early hominids. A synthesis of ecological observations s uch a s that presented here i s s een as a necessary f irst s tep in any analysis of prehistoric subsistence, i ncluding potential modes o f meat and marrow acquisition by early hominids. I n general, r esearchers attempting to r econstruct prehistoric l ifestyles place too l ittle emphasis o n f irst gaining a detailed understanding of e cological process. I make this s tatement at the r isk of s lighting the efforts and achievements of the g rowing number of paleoanthropologists who are aware of this need. B ut too often s till, behavioral interpretations of prehistoric data proceed without due regard f or the e cological basis of the s ystem i n question, l eaving hominid behavior in a v acuum, devoid of inputs f rom the greater physical and biotic environment in which they f unctioned. As B ewes s tated over f ifty

years

ago:

The viewpoint of no s ingle o ne of the human s ciences i s broad enough to explain f ully why man i s as he i s and why he behaves a s he does. Ecology alone s tands apart. It i s not a branch of s cience i t i s only a viewpoint. The s tudent of man and h is -

works, whether he c alls himself archaeologist, e thnologist, anthropolog ist. or what you will, would do well t o become more of an ecologist and try t o adopt the more general viewpoint ( 1931:11). .

S ince 1 931, Bewes' plea has been heeded in many of t he s ub-disciplines of anthropology. But in paleoanthropology, where we now have the means to r econstruct paleoenvironments i n s ome detail, a nd where much work has been done to this end, the e ssential need t o link h ominids to the emerging p aleoecological picture has n ot been adequately s atisfied. No small portion of the d eficiency arises f rom a poor understanding of the ecology a s -

Vi

o bservable i n modern s ettings of the s ystems under i nvestigation. This i s why I devote the i nitial r ound of a nalysis of f ield data on s cavenging, much heavier overall o n t aphonomy a nd related bone s tudies than the current work -

r eflects, to an understanding f rom an archaeologist's point o f view of t he many f actors that affect s cavenging opportunities i n contemporary s ettings. A world of effort i n t aphonomy cannot hope to i lluminte accurately or f ully our understanding of early hominid l ifestyles unless the b asic ecology of the systems under s tudy i s f irst understood. This work makes a small contribution to this e nd. As essentially a Ph.D. dissertation, this work b ene f itted f rom the contributions of perhaps more i ndividuals than if written by one well along i n their a cademic career. The brevity of my acknowledgments of their contributions below i s not meant to underestimate their importance in the completion of the r esearch. Conversely, a ny inaccuracies and deficiencies that may remain are, of c ourse, my responsibility. Two people made immeasurably great contributions t o the design and completion of the project. G lynn I saac was i ntegrally involved i n all but the f inal write-up of the s tudy. Before h is tragic death, Glynn enthusiastically encouraged and promoted the development of the f ield s tudy, and discussed i ts r esults with an eagerness and i nsight which I and all of paleoanthropology s urely miss. J . Desmond Clark also made i nvaluable contributions to all s tages of the r esearch project. I am particularly grateful f or his many l etters to f unding agencies and to me i n the f ield, which are r esponsible in no small way f or the s uccess of the s tudy. The work i n Tanzania was conducted under the auspices of t he Tanzania Antiquities D ivision, to whom I am grateful f or providing the i nvaluable l ogistical and r esearch a ssistance of Prosper Ndessokia and Charles S aanane. The S erengeti Wildlife R esearch Centre provided housing and a base for f ield o perations, and I thank Coordinator K . N. H irji, Acting D irector M . Bruno, and the s taff at the Centre f or approving of, and in s o many important ways assisting in the f ield s tudy. I am grateful to the Tanzania National S cientific R esearch Council f or granting a permit to conduct the research, and the authorities of the Tanzania National Parks s ystem and the Nqorongoro Conservation Area Authority f or their cooperation. I n the Serenqeti, a number of other r esearchers t aught me s o many of the s ecrets of this f inest place on earth. I thank for their a dvice and a ssistance Alan and Joan Root, Tony Collins, Craig P acker, Anna Pussev, Chris Magin, Sue Canney, Feroz Khurji and e specially T im Caro. As well, my s tay in the S erengeti would not productive without my f riends V ii

have been s o enjoyable and at the S eronera Wildlife

Lodge. I n Nairobi, a number of i ndividuals provided a ssistance during breaks f rom the f ield. I thank R ichard L eakey, Harry Merrick and Kamoya Ki m neu f or their h elp with t he vehicle, and Fiona Marshall, Anne Vincent, Henry Bunn, E llen Kroll, Diane Gifford-Gonzales and Paul Whitehead f or l ogistical a ssistance and their interest i n the p roject's progress. The analysis and write-up of r esults commenced a t Berkeley. I thank F . Clark Howell f or his discussion on East African f ossil carnivores and Thelma R owell f or s haring her knowledge of the East African bush. Many hours of f ruitful discussion were s pent with Katie Milton, T im White, John Parkington, Jeanne S ept, P eter Glumac, Curtis Marean and especially Charlie McNutt, who r ehashed with m e r epeatedly appropriate s tategies and units of analysis. I am also thankful to the Quantitative Anthropology Laboratory f or providing computing f unds and f acilities, and the expertise of Gene Hammel, Madeline Anderson a nd Greer Murphy. Write-up was completed a t Rutgers University, where

the

processing

Department

of

Anthropology

provided

word

f acilities.

Insightful comments on drafts of this work were r eceived f rom J . D. Clark, F .C. Howell, T . Rowell and C . Marean. Leonard Blumenschine, j r. made s everal s eries o f exhaustive and expert editorial comments. Melinda Blumenschine assisted in the l abelling of s ome of t he f igures. I am also grateful to G eorge Frame f or sending census data of S erengeti and NgoronqorO herbivores a nd carnivores. Major f unding f or the f ield r esearch was provided by the National S cience Foundation ( BNS 8 3-20026) to J .D. Clark a s Principle I nvestigator. I am a lso t hankful t o N .S.F. f or the use of the Land R over they f unded through a grant to Glynn I saac. The L .S.B. L eakey Foubdation provided a l arge i nitial grant and l ater g ave most generously a n additional emergency grant to enable the c ompletion of t he f ield s tudy. The R egents of the University of C alifornia provided

a Travelling

Fellowship during

the

f ield period.

CHAPTER 1 CHASED OR CHANCED-UPON?

A major behavioral distinction of modern hunter-gatherers among all primates i s the important d ietary contribution made by t issues f rom l arger mammals. The transition f rom a mostly vegetarian d iet to one marked by r egular and n utritionally s ubstantial intakes of meat i s commonly viewed as a dietary i nnovation integral to the evolution of many unique f eatures of human t echnological abilities and s ocial s ystems ( e. g. Campbell 1 979, K . H ill 1 982, I saac 1 971, I saac and Crader 1 981, Thompson 1 975, 1 976, Washburn and Lancaster 1 968) Ample evidence f or meat-eating i s f ound throughout the whole s pan of the archaeological r ecord. Fossil bones bearing butchery marks i nflicted by the earliest s tone-tool-using hominids i ndicate the d ietary transition to have been made, at l east i n k ind, by two million years ago. .

Aside f rom the evidence f or meat-eating, the details of the dietary transition are l argely unknown. For instance, the proportional contribution of t issues f rom l arger mammals t o the whole diet of prehistoric hominids i s s till a matter of untestable conjecture. Whether meat-eating among pre-sapient hominids was a ssociated with the complex of social adaptations s een among modern hunter-gatherers is also open to question. The prevailing f ood-sharing model of Early P leistocene hominid adaptation, involving a distinct s exual d ivision of s ubsistence l abor and the s haring and delayed consumption of gathered f oods at a central place or home base ( Isaac 1 976, 1 978a, 1 978b), has been challenged r ecently by a number of r esearchers ( Binford 1 981, P otts 1 984, Shipman 1 983) , i ncluding I saac ( 1983) On a more f undamental l evel of i nquiry, the method(s) used by hominids to acquire meat i s s till unknown. .

Different modes of meat acquisition by omnivorous hominids imply d ifferent f requencies of meat-eating and distinct f oraging s trategies. Two general modes of meat acquisition, hunting and s cavenging, are available to a meat-eater. S cavenging, or f oraging f or and consuming animals f ound dead, implies meat-eating to be an opportunistic behavior conducted i rregularly and with a minimum of social cooperation. Hunting l ive quarry, on the other hand, denotes meat to be r egularly s ought and an integral component of hominid d ietary s trategies; hunting success would be greatly enhanced by s ocial adjustments including a high l evel of group cooperation during pursuit and c apture, if n ot also during consumption. I f the primary 1

mode of meat acquisition by prehistoric hominids can be determined, paleoanthropologists would have a f irmer basis f or evaluating the nutritional, r anging and s ocial aspects of hominid dietary s trategies. The r elevance of methods of meat a cquisition t o broader questions in prehistory has l ong been r ecognized. The earliest i nterpretations of a nimal bones f ound i n a ssociation with Plio-Pleistocene h ominid f ossils i n the S outh African c ave deposits a ssumed a primary r ole of hunting ( Dart 1 953) The assumption enhanced the h umanness of the " ape-men," an essential a spect of Dart' s a ssertion to an initially unconvinced anthropological community of the hominid s tatus of the australopithecines ( cf. Binford 1 981). Dart's proposal prompted i nterpretations of e arly hominid s ocial l ife a s one centered on hunting ( Washburn and Lancaster 1 968) ‚ and characterized by the modern human traits i ncorporated i n the f ood sharing model ( Isaac 1 978a, 1 978b, Lancaster 1 975) and the l ess honorable killer instinct-based hunting hypothesis ( Ardrey 1 976) The primacy of hunting gained broad acceptance in t he 1 960's and early 1 970's as f ield s tudies of non-human primates documented the small animal meat-eating of chimpanzees and baboons to be achieved almost exclusively by hunting ( Harding 1 973, Harding and S trum 1 976, Hausfater 1 976, S trum 1 976, Teleki 1 973, 1 975, 1 981, van Lawick-Goodall 1 968; s ee Butinsky 1 982 f or a r eview ) Hence, a side f rom pirating baboon kills ( Morris and Goodall 1 977) ‚ o nly r are s cavenging by chimpanzees been r eported recently f rom Tanzania's Mahale Mountains ( Hasegawa et al 1 983). Baboons also usually avoid s cavengeable f ood ( personal observation, S . Washburn pers. comm., but s ee H ausfater 1 976) and have been shown experimentally to avoid a ny provisioned carcass with which they had no previous experience ( Strum 1 983). The virtual absence of s cavenging among non-human primates has been interpreted to preclude the n ecessity of a transitional s cavenging phase i n human evolution ( Goodall .

.

.

and Hamburg 1 975, T eleki 1 975:179). R ather, the h unting of small or incapacitated prey has been s een a s an a ncient and s ufficient preadaptation to hominid predation of l arger animals ( DeVore and Washburn 1 963, Teleki 1 981) T he essential humanness of two million y ear old hominids s eemed i ndisputable well i nto the 1 970's. .

S cavenging by early hominids, too, has h ad i ts l ong-time proponents. Bartholomew a nd R irdsell ( 1953) and J .D. C lark ( 1959, 1 960) f irst proposed a r ole f or s cavenging among early hominids. Their s uggestions were based on the apparent l ack of anatomical and technological s pecializations in hominids f or hunting e specially l arge animals and the propensity of modern predators t o s cavenge: " Since f ew meat-eaters a re l oathe t o s cavenge, and the implementation which would allow the australopithecines to k ill s uch l arge animals i s not apparent, we

2

s uggest that s cavenging f rom kills of l arger carnivores may have been s ystematically carried out ( Bartholomew and B irdsell 1 953:490) .

L ikewise, n otes:

speaking

of

"

the

makers

of

the

Oldowan,

Clark

" A good d eal [ of meat] no doubt, e specially [ from] l arge animals, could have been obtained by s cavenging, and this i s s till a well-known practice among s ome primitive human s ocieties t oday ( 1960:311-312)." C lark continues by c iting Bantu-speaking p eoples of n orthern Z imbabwe:

r eports of the Mashi

s cavenging among R iver r egion of

" Juveniles are s till, or were until r ecently, s ent out in the early mornings.. . to l ocate where vultures were f lying, t hus pinpointing the place where a carnivore had made a would then r ecover the

k ill. The villagers meat ( 1960:312)."

These early s uggestions d id not deny a r ole f or, or even the primacy of, hunting. As s uch, s cavenging was readily incorporated into s ome general exposes of human evolution ( e.g. H owell 1 968, Pfeiffer 1 969). The s eemingly s ound r easoning behind Clark's and Bartholomew and B irdsell's s uggestion s till demands a r ole f or s cavenging ( e.g. Bunn 1 982, I saac and Crader 1 981) , but one possibly only supplemental to preying upon the moderately-sized animals that dominate archaeological bone a ssemblages. Very f ew r esearchers have assigned a primary or exclusive role t o s cavenging i n the evolution of hominid meat-eating. Louis Leakey ( 1967, in Teleki 1 981) proposed the existence of a s cavenging phase preliminary t o and bereft of hunting. S zalay ( 1975) s uggests t he dentition of the australopithecines t o he s pecifically adapted to s cavenging. Here, the thickly enameled, l arge cheek t eeth of Pliocene hominids i s s een as an adaptation t o bone c rushing, and their i ncisiform canines an a daptation f or detaching s craps of meat f rom bones f ound mostly defleshed. Jonas ( 1976, contra Wind 1 976) c ites the ability of humans to become tonically immobilized to d iscount a predatory a ncestry and s upport an exclusive s cavenging position. L ittle support h as been f ound f or any of t hese s ug gestions of exclusive s cavenging by early hominids. They ha ve, however, s erved to emphasize a phenomenon known f rom s tudies of most modern carnivores ( e.g. S chau er and Lowther 1 969) and human hunter-gatherers ( e. g. Woodburn 1 968 for the Hadza and A . Brooks pers. comm. f or the S an) namely, that hunting and s cavenging are not mutually exclusive behaviors.

3

With s cavenging being elevated to at l east a s r ealistic a behavior f or early hominids as hunting, a number of researchers have r ecently proposed criteria f or d istinguishing the two behaviors archaeologically. T he proposals r epresent the f irst a ttempts to go beyond s peculation and gross analogy to modern meat-eaters, and t o test d irectly the existence of hunting and s cavenging o n archaeological data. Most criteria h ave been based o n s ize, age or body part profiles expected t o typify f ossil bone a ssemblages accumulated through hunting or s cavenqing. T he criteria have been applied to a number of f ossil-bearing s ites i n eastern and s outhern Africa s panning much o f Pleistocene prehistorv.

and

Most tests of hunting and s cavenging yield equivocable often disparate r esults, even when applied to t he s ame

s et of assemblages. This i s particularly true of t he various South African cave s ites ( Vrba 1 975, 1 980, Read-Martin and Read 1 975, Shipman a nd Phillips 1 976) , f or which considerable doubt r emains over a more basic i ssue: the i dentity of the bone accumulators ( Binford and B ertram 1 977, Brain 1 981, Maguire et al 1 980, R ichardson 1 980) I t i s also true, however, of s ites certainly created a t l east i n part by hominids, i ncluding the e arly P leistocene s ites .

f rom Olduvai ( Binford 1 981, Bunn 1 982, Potts 1 984) , and even the Late P leistocene s ite of Mouth ( Klein 1 982, B inford 1 984)

1 983, Shipman Kiasies River

Two f actors, I f eel, are r esponsible f or t he inconclusiveness of the tests. One f actor i s philosophical, the other methodological. The philosophical f actor p ertains to the variable degrees of humanness implied by i ndications of the primacy of hunting versus s cavenging. T he traditional view of early Pleistocene hominids a s almost human i s being eroded by conclusions that s cavenging was a s l east as important a s hunting. The most provocative a nd consistent advocate of the " hardly human" s chool, Louis B inford, i s, i n these t erms not s urprisingly, the s trongest a dvocate of the s cavenging position. B inford has c hallenged I saac's f ood-sharing model of early hominid l ifestyles s ince the l ate 1 970's ( Binford 1 977), and s ince t he early 1 980's has been constructing the s oundest arguments a nd most emphatic conclusions i n f avor of t he primacy o f s cavenging: " The s igns s eem clear. Earliest man, f ar f rom appearing a s a mighty hunter o f beasts, s eems to have b een the most marginal of s cavengers ( 1983:59)." Binford's claims have r ecently culminated in a d etailed analysis of the Middle S tone Age f auna f rom the s ite of Klasies R iver Mouth on the S outh African Cape ( Binford 1 984) B inford concludes that even e arly H omo s apiens was to a l arge degree a s cavenger, hunting only small ungulates a nd s cavenging the medium and l arge s ize f orms a lso .

4

r epresented

at

the

s ite.

I am not implying that Binford's provocative c onclusion i s b iased by preconceptions of a very l ate appearance of the f ull s uite of human traits. Nor do I s uggest a bias t oward the traditional view by those f inding evidence f or hunting. I do contend, however, that part of the inconclusiveness of the tests s tems f rom an uncertainty r eaching to the c ore of paleoanthropological inquiry: When d id we become human? The methodological f actor i s, i n my view, more important. The c riteria f or d istinguishing hunting f rom s cavenging have an inadequately detailed, and in some c ases, inaccurate basis. The deficiency l ies primarily in a poor understanding of the ecology of s cavenging, rendering s tatements of i ts taphonomy and hence archaeological recognition insensitive to important variations in the nature of the s cavengeable f ood s upply. B inford ( 1984), f or example, has contributed the most to an understanding of the general taphonomy of s cavenging by i dentifying s ome of the carcass parts most usually available to a marginal s cavenger. H is resultant criteria, however, do not accomodate the f ull r ange of f eeding opportunities a hominid s cavenger might have encountered, including a s et of edible parts more typical of those available to a predator on a f resh kill. Many of the criteria proposed by other taphonomists, while being based on general principles of b ehavioral and population biology ( most notably Vrba's and Klein's), a re also insentitive to the range of meat-eating opportunities potentially encountered by early hominids. For s cavenging to be treated as a testable alternative to h unting, a detailed understanding must be acquired of the c haracteristics of s cavenging opportunity. The nature of f eeding opportunities f or a s cavenger will determine that s et of preservable traces indicative of s cavenging. There is a s econd benefit to be r ealized f rom a knowledge of the r ange . of f eeding opportunities available to a scavenger. I ts ultimate application i s to the behavioral and s ocial contexts of hominid meat acquisition. A s tep in this d irection can be provided by a k nowledge of the ecological c ontexts among which s cavenging s uccess varies. The f requency, yield and contexts of f eeding opportunities f or a hominid-like s cavenger are e ssential components of models of individual and group f oraging strategies ( cf. Kurland and Beckerman 1 985). One brief a nd preliminary s tudy has attempted t o assess the f eeding opportunities provided by s cavenging. Schau er and Lowther ( 1969) conducted two experiments t o illustrate the a vailability of s cavengeable and easily caught prey i n the S erengeti National Park. The one experiment on s cavenging was conducted over a period of seven days in a n area of prey abundance. S challer and

5

Lowther traversed the l andscape on f oot and i n a vehicle i n s earch of s cavengeable f ood to approximate the f ood y ields a prehistoric hominid s cavenger m ight encounter. They d iscovered the minimal r emains ( marrow bones and brain) of f our l ion k ills of wildebeest and z ebra, and substantial amounts of f lesh on a naturally dead buffalo. T hey also e ncountered two l ive but s everely i ncapacitated a nimals which could have been c aptured easily. S chau er and L owther c onclude that an exclusively meat-eating hominid group could s ubsist under s imilar conditions, but t his was enabled only by the d iscovery of the i ncapacitated prey. Otherwise: " The r esults s how that l ion k ills offer, a t best, an erratic s ource of f ood, with long l ean periods probably broken by occasional f inds providing a considerable amount of meat. Our d iscovery of the dead buffalo was a matter of l uck, s omething that would happen only i nfrequently ( Schau er and Lowther 1 969:328) . "

S cavenging s eems f rom these r esults to provide a very i rregular and l ow-yielding s ource of meat, especially i n comparison to hunting. Although f ew paleoanthropologists would contend that meat was a primary component o f early hominid diets, the r esults imply that only hunting could account f or the consistent a ssociation of l arge quantities of animal bone r efuse with s tone artifacts a t archaeological s ites. A f undamental question arising f rom S chau er and Lowther's experiment that d id not receive due emphasis i s: Are there s ufficiently varied f eeding opportunities f or a hominid-like s cavenger to account i n part or whole f or the compositions of archaeological bone a ssemblages, or does the t axonomic, age and b ody part d iversity of the a ssemblages demand the additional o r exclusive practice of hunting i n their a ccumulation? S challer and Lowther's s cavenging experiment, though brief and l argely anecdotal, i s informative with r egard t o this question. The experiment s hows that hominids could easily acquire s cavenqeable f ood, i t i dentified s ome of the ecological contexts within which the f ood c ould b e acquired, and i t identified s ome of the carcass t ypes a nd g eneral s eries of body parts available to a s cavenger. These r esults provided, in 1 969, the v ery types of c riteria useful f or d elimiting those components of archaeological bone a ssemblages a s cavenging mode of meat acquisition c ould a ccount f or. I t a lso provided u seful i nsight i nto a n ecological c ontext i n which s cavenging by hominids c ould b e profitable, i .e. during periods o f prey abundance ( see Chapter 4 ) . Unfortunately, these r esults were overshadowed by the apparent l ow y ield of the s cavenging experiment.

and

The n eed t o gain a detailed understanding of t he s cale characteristics of s cavenging opportunity was p erceived

6

by this writer in l ate 1 982 in consultation with Glynn I saac. A more s ystematic and comprehensive f ollow-up s tudy to S chau er and Lowther's experiment was designed t o identify, through observations of carcass use and availability i n contemporary, natural s ettings, the composition of the s cavengeable f ood s upply and the contexts among which the amount and kind of the s upply varies. I n s o doing, one would be i n a position to a ssess accurately the s cavenged or hunted origins of archaeological bone type(s) of f oraging practiced. This work questions: 1 ) 2 ) 3 )

4 )

i s

assemblages, and to strategies most l ikely

concerned

with

the

evaluate to have

f ollowing

the been

s et

of

How variable i s the availability of s cavengeable f ood? What are the s ources of the variation? What types of carcasses and carcass t issues are most r egularly available to a s cavenger i n different ecological contexts? What s et of s cavenging opportunities existing today might a prehistoric hominid s cavenger have encountered and profited f rom?

The two f ields of s tudy relevant to these questions, wildlife biology and taphonomy, alone or together do not currently provide s ufficiently detailed answers. S ome carnivore studies, f or example, document rates of whole carcass consumption and f requencies of carcass abandonment ( e.g. Eaton 1 974, Kruuk 1 972, Lamprecht 1 981, S challer 1 972). However, the s tudies do not ( and . d id not i ntend to) determine the quantity and anatomical l ocation of edible tissues abandoned by predators to s cavengers, two fundamental a spects of s cavenging opportunity. S imilar studies document the nature and i nfluence of competition for c arcasses on the f eeding s uccess of various c arnivore species ( e. g. Eaton 1 979, Houston 1 974, 1 979, K ing 1 975, 1 976, Kruuk 1 967, 1 972, Lamprecht 1 981, Schau er 1 972), but data on the effect of competition f rom the viewpoint of the carcass itself are wanting. For example, there exists very little information on the effects of competition in reducing edible yields f rom c arcasses of different s ize or age, and in different ecological contexts. King ( 1975), noting most predation and consequently s cavenging opportunities t o be nocturnal, concludes that d iurnally active hominids c ould not have profitted f rom s cavenging. The c onclusion, however, overlooks the possibility of the intensity of competition f or k ills being variable and a t times sufficiently l ow that nocturnal deaths could persist as s cavengeable f ood s ources through the next day. I ndeed, aspects of s easonality, habitat type and carcass s ize that affect carcass persistence are in n eed of clarification and documentation.

7

Taphonomists have addressed many of t he above anatomically-based deficiencies, but a lmost exclusively i n terms of the bone r emains of carcasses whose resource l ife has l ong s ince been exhausted. Most taphonomic r esearch i s i n these terms physically or mechanistically o riented; carcasses and their component bones a re treated a s physical objects differentially prone to mechanical f orces o f d ispersal ( e.g. Behrensmeyer 1 975, A . H ill 1 975, 1 979, H ill and Behrensmeyer 1 985, Voorhies 1 969) , destruction ( e.g. Brain 1 967, 1 969, 1 981, Binford and Bertram 1 977) , o r burial ( e.g. Behrensmeyer and Boaz 1 980). The mechanistic orientation has provided a good understanding o f many of the f actors affecting a bone' s s urvival over the largely physical

f ossilization process.

All bones, however, begin their taphonomic h istory a s s tructures that bear or contain nutrients actively sought by organisms in need of nourishment. This nutritional o r energetic orientation to taphonomy as yet has f ew s ubscribers ( Binford 1 978, Speth 1 983). I contend, however, that it i s the very process of carcass degradation during the resource l ife of the carcass that will be most i nformative about the diet and s ubsistence of ancient meat-eaters, including the modes by which they acquired meat. I n this view, the often dramatic a ffects of s ubsequent pre- and post-burial physical taphonomic agents are conditioned l argely by the brief and early energetic phases of the taphonomic process. The taphonomic component of a s tudy of s cavenging opportunities must therefore be based on observations of f resh carcasses, i .e. recently d ead animals, whether whole or partially consumed, t hat s till provide a f eeding opportunity to vertebrate carnivores. Such observations were made during an e leven-month f ield s tudy in Tanzania's S erengeti National Park a nd NgorongorO Crater. Of particular interest was the utilization of f resh carcasses by l arge carnivores and t he r esulting s cavenging opportunity the carcasses provided. After a description of the f ield s etting and methodology , and the carcass s ample observed ( Chapter 2 ), the d erivation of two parameters of s cavenging opportunities a re described ( Chapter 3 ) . Chapter 4 contains the main body of f ield r esults on the s cale and characteristics of modern s cavenging opportunity i n different ecological a nd physical c ontexts. Chapter 5 i s an application of these r esults to the s cavenging opportunities potentially encountered by E arly Pleistocene hominids of E ast Africa, a nd to the archaeological

recognition of

s cavenging.

8

CHAPTER 2 FIELD STUDIES:

THE

General

Ecology

THE SETTINGS,

SERENGETI

and

METHODS AND SAMPLE

AND NGORONGORO ECOSYSTEMS

S ettings

The S erengeti National Park and Ngorongoro Crater are tropical s avanna-woodland ecosystems l ocated in northern Tanzania ( Figure 2 .1). Both are protected wildlife r efuges administered r espectively by the Tanzania National Parks system and the Ngorongoro Conservation Area Authority. The two ecosystems experience a minimum of human a ctivity, restricted mostly to small numbers of tourists, and in the case of Ngorongoro Crater, occasional authorized i ncursions by Masai pastoralists with their l ivestock. Both ecosystems support naturally maintained populations of l arger mammalian herbivores and carnivores. The Serengeti: The wildlife and the ecology of th? Serengeti have been the s ubject of intensive s tudy s ince the l ate 1 950 1 s . Research conducted under the auspices of the S erengeti Wildlife R esearch Centre ( formerly the Serengeti Research I nstitute) r enders the S erengeti one of the world' s best known tropical ecosystems. Müch of the knowledge acquired i s summarized i n an excellent volume edited by S inclair and Norton-Griffiths ( 1979) ‚ to which the r eader i s r eferred f or a more detailed a ccount of Serengeti ecology than i s given below. The 2 S erengeti National Park has an approximate area of 2 5000 km I t i s best known f or i ts vast migratory herds of wildebeest, z ebra and Thompson's gazelle, numbering approximately two million individuals. Together with the adjoining Maasi Mara park in Kenya and game r eserves, controlled areas and conservation areas that adjoin i t i n Tanzania, the S erengeti i s a s elf-contained ecosystem with respect to i ts larger herbivore populations ( Sinclair 1 979a, Figure 2 .1). The migratory herds utilize the woodlands and plains of the S erengeti ( Figure 2 .2) according to a s easonal pattern of rainfall. As with most of E ast Africa, the S erengeti experiences a l ong dry s eason ( June October) , a l ong rainy s eason ( March May) , and an intermediate period ideally characterized by s hort rains ( November December) and a s hort dry period ( January February) In s ome years, the s hort rains persist through the s hort dry s eason and merge with and i ntensify i nto the long rainy s eason ( Sinclair 1 979a). The Ngorongoro .

-

-

-

-

.

9

F igure 2 .1:

Northern T anzania, a nd

s howing t he S erengeti N ational P ark

N gorongoro C rater

( bold l ines).

The dashed l ine

e ncompasses t he a nnual r ange o f t he S erengeti's m igrat ory h erds.

1 0

Figure

2 .2:

The S erengeti N ational Park, t ypes a nd

s howing main vegetation

t he l ocation of s tudy a reas.

1 1

H ighlands to the s outheast create a r ain s hadow over the s outhern plains of the S erengeti, which r eceives approximately 5 00mm of r ainfall annually. Greater amounts of r ain occur as one proceeds north toward Kenya and west toward Lake Victoria out of the rain s hadow, r eaching a maximum of roughly 1 000mm per year ( Sinclair 1 979a) The average annual r ainfall f or the S erengeti a s a whole i s about 8 00mm ( Coe et al 1 976). .

The migratory herds utilize the S erengeti in a predictable annual pattern ( e.g. Pennycuick 1 975, Maddock 1 979). During the l ong dry s eason the herds are d ispersed throughout the extensive western and northern woodlands, extending into s outhern Kenya. With the beginning of the s hort rains i n early November, the herds amass at the southern margin of the woodlands, and, if r ain has fallen on the plains, proceed to these prime grazing grounds. The herds r emain concentrated on the open plains throughout the s hort dry and l ong rainy s easons. With the end of the rains in l ate May, the herds proceed back to the r elatively well-watered woodlands. The s easonal movements of these three migratory ungulate species are a major component of S erengeti ecology ( e.g. Bell 1 971, McNaughton 1 976, 1 979, Jarman and S inclair 1 979) due to their great numbers ( over 9 0% of l arger herbivores) and biomass ( over 6 0% of larger herbivore biomass) ( e. g. Houston 1 979). Ngorongoro Crater: Ngorongoro Crater i s a caldera located roughly 4 0 km s outheast of the S erengeti ( Figu e 2 .1). The Crater covers an area of approximately 2 65 km I ts f loor l ies 1 700 meters above s ea l evel, f rom 4 00 to 7 00 meters l ower than i ts surrounding rim ( Hay 1 976). The Crater f loor s upports an extensive f lat plain s urrounding shallow s oda Lake Makat, two l arge swamps, and two small patches of .

f orest

( Figure

2 .3).

Approximately 9 00mm of rain f alls in the Crater each year ( Coe et al 1 976) in a s easonal pattern s imilar to t hat in the S erengeti. The western portion of the Crater i s noticeably drier than the middle and eastern parts, supporting mostly gazelle throughout the year. Dense concentrations of wildebeest and z ebra and smaller numbers of buffalo, eland, gazelle, warthog, hartebeest, waterbuck and elephant, to name the most c ommon herbivores, are f ound mainly i n the eastern three-quarters of the Crater. Because of the Crater's relatively h igh annual rainfall and the availability of permanent f resh water f rom s treams a nd s prings, herbivore numbers f luctuate only to a minor degree on a s easonal basis. During the wet s easons, a very small proportion of Ngorongoro' s ungulates ( estimated a t 9 % of all individuals by Frame, 1 986) temporarily moves a s hort d istance, l ess than 1 0 kilometers, out of the Crater ( Elliott and McTaggart-Cowan 1 978).

1 2

F igure

2 .3:

Ngorongoro Crater, Carcass

search

portion of west,

U

the

Mandusi

bounded by

Swamp

O ljoro Nyuki s outh.

I

3

f loor of

Crater,

and

I

the

concentrated

and

1 2

showing

was

and Munge

k

1 3

the

caldera.

the

c entral

L ake Makat

River

R iver and Gorigor

I

in

to the

Swamp to

to

the

north, the

east

S uitability

f or

the

F ield

S tudy

D ifferences in the s tructure a nd dynamics o f the mammalian herbivore and carnivore g uilds between the S erengeti and Ngorongoro and within t he S erengeti f orm the basis f or a wide range of s cavenging o pportunities. T his i s s een best by comparing mammalian predator to prey r atios, which correspond in a d irect manner t o the availability of carcasses to s cavengers. Ngorongoro Crater i s a r elatively s table ecosystem with r espect to l arger herbivore numbers. As a result, it s upports an extremely high density of l ions and s potted hyenas ( e.g. Elliott and McTaggart-Cowan 1 978, Kruuk 1 972) among the highest in Africa ( C. Packer pers. comm.). Estes and Estes ( 1979) estimate the ratio of l arger mammalian predators ( excluding j ackals) to ungulate prey to be approximately 1 :40. An independent a ssessment by Frame ( 1986) agrees well with this f igure ( 1:45 f or t he dry s eason, 1 :39 f or the wet s eason). A high proportion of herbivore mortality i n Ngorongoro r esults d irectly from predation, and carcasses can be expected to be subject to complete and rapid consumption by l ions and hyenas, both a s predators and s cavengers. The l ow availability of c arcasses to s cavengers in Ngorongoro i s attested to by the e xtremely small number of vultures i t s upports. The l arger mammalian predator t o prey ratio i n the S erengeti i s up to an order of magnitude l ower than t hat of Ngorongoro. I have c alculated the r atio f or the S erengeti a s a whole to be approximately 1 :340 f rom data presented by Houston ( 1979) based on c ensus data f rom the l ate 6 0's, and 1 :166 f rom data presented by Frame ( 1986) based on estimates f or 1 977. The disparity betw een these e stimates s eems to r esult primarily f rom a l arge increase in l ion and particularly hyena numbers s ince the l ate 1 960's, a s s upported by a May 1 977 census on the plains ( see H anby and Bygott 1 979 and S inclair 1 979b f or explanations of t his and o ther changes i n carnivore and herbivore numbers) The relatively small number of carnivores attained by either e stimate in comparison to Ngorongoro r esults f rom the s easonal migrations of the majority of S erengeti h erbivores, and the necessarily f ixed annual ranges of its breeding predators ( Schaller 1 972, Kruuk 1 972) P redator populations are therefore l argely l imited i n numbers to those sustainable by the s easonal availability o f only small numbers of r esidential prey ( e.g. S inclair 1 979b, Hanby and Byqott 1 979). Houston ( 1979) estimates o nly 3 0% .

.

of herbivore mortality in the Serengeti t o result from predation, ascribing the r emainder t o natural causes such a s d isease and s tarvation. This c reates a f undamentally h igher availability of carcasses r elative to t hat i n Ngorongoro, and corresponds to the l arge vulture populations existing i n the S erengeti ( Houston 1 972, 1 979).

1 4

Within the S erengeti i tself, the s cale of s cavenging o pportunities can be expected to vary on a s easonal basis d ependent on the l ocation of i ts migratory herds. I n areas s easonally occupied by mostly r esidential herbivores ( plains during t he l ong dry s eason, woodlands during the r ainy s eason) predator to prey r atios will be r elatively h igh and carcass availability to s cavengers l ow. The p resence of migratory herds, however, will expectedly i ncrease f eeding opportunities f or s cavengers owing to h igher rates of predation and especially more non-predator r elated mortality. The magnitude of d ifference can be a ppreciated through reference to Frame' s ( 1986) data on c arnivore and herbivore numbers on the plains during the d ry and r ainy s easons of 1 977. From his data, I calculate t he ratio of l arger mammalian carnivores ( excluding j ackals) to ungulate herbivores to be 1 :186 during the r ainy s eason, a nd 1 :67 during the dry s eason. Wet c onditions on the plains do r esult in an e stimated q uadrupling of l arger more dramatic i nflux

carnivore numbers, but, of the major migratory

due to the s pecies, a

t ripling of the predator to prey ratio. Such s easonal changes are not apparent in Ngorongoro, where Frame's ( 1986) 1 977 census data ( quoted earlier) indicate s easonal predator to prey ratios to remain s teady and perennially h igh. The s cale of f eeding opportunities f or s cavengers within the S erengeti and compared to Ngorongoro will be d iscussed f ully i n Chapter 4 . S uffice i t to s ay now that the two ecosystems chosen f or the s tudy are ideally s uited f or revealing a wide range of s cavenging opportunities and t he characteristics of e ach.

THE

FIELD

STUDY

Conditions The f ield s tudy was c onducted f rom August 1 983 through June 1 984. The eleven month period s pans the middle of the l ong dry s eason to the beginning of the next l ong dry s eason. 1 983/84 was the s econd of two r elatively dry years in E ast Africa. The monthly distribution of rainfall i n S eronera, on the woodlands-plains e cotone of the S erengeti, i s s hown f or the y ear encompassing the s tudy period, and i s c ompared i n Figure 2 .4 to average monthly f igures f or the preceding ten years. [ These data were provided by the S erengeti Wildlife R esearch Centre, which monitors rain gauges throughout the S erengeti on a monthly basis.] Rainfall amounts f or the l ong dry s eason ( June October) of 1 983 were f airly typical of the preceding t en years, totalling 2 06.5mm f or the f ive month period i n 1 983, a nd an average of 1 96.8mm during the t en year period. The f our month intermediate period between November and February was relatively dry i n 1 983/84 ( 246.0mm ) compared to the preceding ten years ( 303.1mm ), and the l ong r ains ( March -

-

1 5

I

N

C

( u Iu 1)

IT P3 UT

1 6

t e n y e a r s p r e c e d i n g 1 9 8 3 / 1 9 8 4

( S e r e n g e t i R e s e a r c h I n s t i t u t e g a u g e # 3 5 ) .

l o n g r a i n y s e a s o n

May) all but f ailed in 1 984 ( 97.6mm compared to the average 2 78.2mm f or the ten year period) Overall, the year encompassing my s tudy experienced over 2 00mm l ess rain i n S eronera than t he average of approximately 7 80mm f or the area. Except f or December, the month of my s tudy during which most rain f ell i n S eronera, rainfall data f or 1 983/84 show a l ack of the pronounced s easonality more typical of S eronera and E ast Africa i n general. The dryness of the s tudy period p erhaps r esulted in higher than usual herbivore mortality i n the S erengeti. This will effect the degree of s cavenging opportunity encountered but not the processes and a ssociated characteristics of the opportunity that are of interest. The l ack of pronounced s easonality i n rainfall, however, denotes that the extent of s easonally based differences i n carcass availability was l ess .

pronounced than that expected f or more typical years. The effect of this will be d iscussed more f ully i n Chapter 4 .

S tudy

Areas

Characteristics of the s tudy areas, including the intensity by which each was monitored, are g iven i n Table 2 .1. The main s tudy area was that around S eronera and my base at the S erengeti Wildlife R esearch Center. S eronera i s located on the woodlands-plains ecotone of the S erengeti ( Figure 2 .2). T he s tudy area includes a diversity of habitats in i ts roughly 3 00 km extent, including Acacia woodland, plains, and r iverine woodland ( Figure 2 .5) I n addition to s upporting a diversity of habitat types, Seronera i s ideally l ocated with r espect to the movements of t he migratory herds, a l arge portion of which pass through the s tudy area on their way to and f rom the plains. 1 03 d ays of carcass s earching s panning all eleven months of the s tudy period were s pent i n S eronera, y ielding 1 50 f resh carcasses, over h alf of the total carcass s ample. .

The other major s tudy area within the S erengeti was the Naabi-Ndutu p lains ( Figure 2 .2). Most of this area i s a vast, f lat open grass plain s urrounded by distant hills, but i t also i ncludes at i ts southern end the l acustrine woodlands surrounding perennial Lakes Ndutu and Masek within the western extremity of Olduvai Gorge ( Figure 2 .6). The N aabi-Ndutu a rea was s ampled s ystematically f or a total of twenty days during the months of January through May when the migratory herds were present in variable and usually very l arge numbers. Casual s earching f or c arcasses in t his area was conducted during periods when only very small numbers of the xerophyllic Grant' s and Thompson's gazelle were present. The s light prospects of f inding a carcass at these t imes deemed s ystematic monitoring unproductive and l ogistically impractical. 7 2 carcasses were d iscovered i n the Naabi-Ndutu s tudy area, on t he margins of the Ndutu woodlands and the the

p lains

proper.

1 7

3 2 within or r emainder on

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1

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0 b . Z

r d

z I

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18

r d. .Q O

a )

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F igure

2 .5:

T he S eronera s tudy a rea, o f

t he A cacia woodland,

l and h abitats.

1 9

s howing t he d istribution p lains a nd r iverine wood-

F igure

2 .6:

T he N aabi-Ndutu s tudy a rea,

i ncluding t he plains

t o t he s outh of Naabi H ill a nd t he s urrounding L ake Ndutu a nd L ake Masek.

2 0

woodlands

The Banagi and S imba Kopjes s tudy areas ( Figure 2 .2) of the S erengeti were monitored f or only s even days each, mostly during t imes of migratory prey abundance. Banagi i s within the woodlands proper of the S erengeti, characterized by open s tands of various Acacia, Co r nmiphora and other tree s pecies broken o ccasionally by small patches of l ightly treed grassland. Five f resh carcasses were discovered i n t he area of B anagi. S imba Kopjes i s on the S erengeti Plains, punctuated in this area by numerous and s ubstantial kopjes that s erve at l east s easonally as havens f or l ions. Fourteen carcasses were observed in this s tudy area. Five carcasses were discovered incidentally while travelling within the S erengeti. One was f ound at Bologonja i n the northern woodlands, three at Moru Kopjes on the plains, and one on the plains near Olduvai Gorge, on the s outheastern park boundary ( Figure 2 .2). The s tudy a reas in the S erengeti represent a f airly comprehensive s ample of S erengeti habitats. However, the western and northern woodlands of the S erengeti were, with the exception of Banagi, not monitored, owing to l imited availability of petrol. Therefore, I did not observe the expectedly highest annual mortality that occurs in the northern woodlands at the end of the dry s eason ( Houston 1 979) This deficiency, however, i s l argely compensated f or by observations in the Naabi-Ndutu area during l ate February and March, when unusually dry conditions l ed to very high l evels of natural mortality. .

The f inal s tudy area was i n Ngorongoro Crater. 2 5 days during December, January and June were spent in Ngor songoro, yielding observations on 2 8 f resh carcasses. S earch was concentrated i n the wetter and more heavily populated portion of the Crater to the east of Lake Makat, a s indicated i n F igure 2 .3. This i s where the majority of the Crater' s predation and s cavenging occurs.

Field

Methods

Carcass s earch was concentrated during early daylight hours. Search c ommenced a s c lose to f irst l ight as possible and continued throughout the morning or until an area had been satisfactorily covered. Occasional afternoon and evening s urveys were made, but with rare exceptions all predator and most other activity had been s upressed by heat, and the only carcasses to be f ound were those already discovered in t he morning. S earch f or and observations at carcasses were n ot carried out a t night, despite the nocturnal peak of predator and s cavenger activity. Aside f rom the l ogistical difficulties of nighttime observations, my exclusively daytime observations of s cavenging opportunity are consistent with those potentially encountered by d iurnal prehistoric hominids. 2 1

Search was continuous f rom a s low-moving ( 25 km/hr.) Land Rover, punctuated by careful, binocular-assisted rooftop s cans a s f requently as the d ensity of vegetation demanded. The ob - ects of s earch were concentrations of airborne or grounded vultures, the directed f light of i ndividual vultures, and aggregations of mammalian carnivores. The l atter clue to a carcass's presence was the only one available in Ngorongoro. The abundance of vultures i n the S erengeti r esulted i n their targeting of most carcasses f ound. By mid-morning ( 0830-0900) the a ir i s usually s ufficiently warm t o create thermals on which vultures r ise and congregate i n l arge numbers if over a carcass. A l arge c loud of vultures riding a thermal can be s een f rom over 1 0 km away, a nd was a certain clue to the presence of a c arcass below. Vultures provide other l ess conspicuous, through unambiguous cues t o the presence of a carcass. Vultures h ave difficultly f lying without the aid of updrafts. Early morning l abored f light of a s ingle vulture was an unmistakable s ign of a nearby carcass, and i ts marked directionality always provided a reliable and easily f ollowed quide to the carcass. An aggregation of as f ew as f ive vultures roosting in the lower or middle canopy of a tree was usually a c lear s ign of a l ion kill below. Vultures roosting in small numbers at only the crown of a tree, however, almost always were nesting or occupying a vantage p oint f rom which other vultures at or proceeding to a carcass could be l ocated. One quickly l earns to r ecognize these clues f or f inding carcasses. Rarely was a carcass f ound without the clue provided by s ome carnivore already in attendance. I n only a handful of cases was an unattended f resh carcass, whole or partially eaten, discovered. I n t he case of o nly two cheetah kills did I f ollow a predator waiting f or a kill t o be made. This method, though ensuring the observation of a whole consumption episode, was very i nefficient. I ndeed, of the over 2 00 k ills eventually d iscovered, I witnessed the kill on only f our occasions. By s earching f or kills already made, I maximized the coverage of an area f or all c arcasses present or potentially s o. Althouqh originally proposed, s earch was not c onducted a long s ystematic transects ( e.g. A nderson and P ospahala 1 970, Burnham and Anderson 1 976, Eberhardt 1 978, R obi nette et al 1 954, 1 974) S everal early attempts with this method proved to be unproductive and an extravagant use of petrol. Carcasses in tropical ecosystems have a very short persistence compared to those i n t emperate l atitudes, where, f or example, a winter's worth of deer mortality c an be estimated i n one s eries of springtime transects ( e.g. Robinette et a l 1 954) Rather, s earch was much more s elective, being concentrated when p ossible in areas where .

.

I knew predators particularly useful

to i n

be present. This method was l ocating l ion k ills i n dense 2 2

vegetation or c arcasses during very early daylight hours. Alternatively, a reas s elected f or s earch were those in the vicinity of the l arger herds of ungulates which were typically patchily distributed i n a s tudy area. The method adopted was appropriate, s ince I was more interested in the variable f eeding potential provided by individual carcasses than absolute, i nstantaneous carcass densities. Most carcasses being consumed could be observed within ten meters if approached s lowly and indirectly. This distance was adequate f or making detailed, binocular-assisted observations of the carcass parts being consumed. Carnivores were extremely tolerant of my presence and ginger maneuvering f or a better view, especially in the s tudy areas most intensively monitored. I n only a handful of c ases did my presence noticeably disturb a consumer, and i n only one case of a baboon f eeding on a Thompson's gazelle f awn d id I i nterfere with the normal pattern of carcass consumption. A number of r ecords were made consistently upon discovery of a carcass. These include the habitat, approximate l ocation, the types and estimated number of herbivores i n the area, the number and types of a ctual consumers and potential s cavengers present, and the distribution and amount of edible tissues on the carcass. All observations were made f rom within the vehicle if consumers were present, but if not, a much more detailed, out-of-vehicle i nspection of the carcass could be made. Whether conducted on discovery or after abandonment by consumers present on discovery, these inspections additionally i ncluded a determination of the s ex, dental age and, when possible, the quality , of the carcass' s marrow. Marrow quality was determined using visual c riteria ( e.g. Brooks et a l 1 977, Franzman and Arneson 1 976, Harris 1 945,

Ransom

1 965,

S inclair

and Duncan

1 972)

Observations on carcass consumption included minute-by-minute account of the parts being consumed removed f rom the carcass, and the identity and number consumers. Unless immediately s cavenged by hyenas,

a or of a

detailed i nspection of the c arcass was made upon abandonment by s uccessive consumers. I attempted whenever possible to make continuous f eeding observations until all edible tissues h ad been consumed. For l ogistical r easons s uch as extremely prolonged persistence of a carcass, this goal could not a lways be achieved. I n many cases, as well, s low and minimal f eeding and the presence of another carcass i n the area inhibited continuous observations. Efforts were a lways made t o r evisit carcasses and periodically monitor their r esource s tatus until they no l onger represented a s ource of s cavengeable f ood. I was much more successful at this i n the i ntensively monitored S eronera and Ngorongoro s tudy areas, than in other s tudy areas which were usually visited f or only one or s everal days

at

a t ime. 2 3

THE

CARCASS

S AMPLE

Observations were made on the consumption a nd s cavenging potential of 2 74 f resh carcasses. Again, a f resh c arcass i s defined as any recently d ead l arger mammal s till providing nourishment to vertebrate carnivores. Fresh c arcasses are in this way c learly distinguished f rom weathered bones of carcasses whose r esource l ife has been exhausted and which occur i n f ar g reater numbers on t he l andscape. From hereon, the term c arcass will be used t o mean exclusively a f resh carcass. S ix mammalian c arnivores are included i n the total carcass s ample, including one s potted hyena, two cheetah, two j ackals, and one bat-eared f ox. The carnivores are excluded f rom the f ollowing analyses, so that the total s ample of l arger mammalian herbivore carcasses numbers 2 68. This l arge s ample of f resh c arcasses permits f or the f irst t ime a detailed a ssessment of

the

many

f actors

affecting

s cavenging

opportunities.

The taxonomic composition of t he carcass s amples f rom the Serengeti and Ngorongoro are s hown i n Tables 2 .2 and 2 .3, respectively. Also presented i n each table are the l ate 1 960's and l ate 1 970's s ets of comprehensive population s ize estimates of the more common l arger mammalian herbivores provided by H ouston ( 1979), Elliott and McTaggart-Cowan ( 1978) , and F rame ( 1986). The data presented f or Ngorongoro i ndicate a marked d egree of s tability in herbivore numbers, a t l east d uring the penultimate decade. The l ive population s ize estimates f or the S erengeti, on the other hand, i ndicate s ome dramatic changes f or certain s pecies during the 7 0's. For example, there had been a doubling of the number of S erengeti wildebeest by the l ate 7 0' s ( see a lso S inclair 1 979b), and unpublished data collected by Marcus Borner indicate the Thompson' s gazelle population to have continued i ts decline of the 7 0's to a current estimated number of only 2 50,000. With these temporal differences in mind, the available l ive population s ize estimates are s till useful f or making broad species-specific comparisons with c arcass population s izes. Wildebeest are by f ar the d ominant species in the carcass sample. Wildebeest r epresent 6 0% and 8 2% of a ll carcasses f rom the S erengeti and Ngorongoro, r espectively. Their dominance r esults in part f rom their great l ive abundance in both ecosystems, and a dditionally f rom their s tatus as the preferred, easily c aught prey of l ions and hyenas ( Elliott et al 1 977, Kruuk 1 972, Schau er 1 972). Z ebra are the next best r epresented species in the S erengeti carcass s ample, occurring, l ike wildebeest, more f requently than expected f rom their relative l ive abundance. Thompson's gazelle, a s mall antelope, are, conversely, uncommon in the S erengeti carcass s ample relative to their great l ive abundance ( although here, a current l ive population s ize of 2 50,000 would l ower their r elative l ive abundance to a l evel approximating their 2 4

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-

r elative abundance as carcasses) Smaller taxa in general are underrepresented a s carcasses in both ecosystems, and e specially in Ngorongoro. This i s clearly not due to l ower mortality rates, but rather to the r apidity by which they are consumed r elative to l arger animals ( see Chapter 4 ). T he relative abundance of s pecies i n the carcass s ample i s therefore not r epresentative of total mortality, but i s r ather a s kewed profile corresponding more closely to the l ikelihood of a s cavenger encountering any one species as a carcass. This p oint will be r eturned to at the end of Chapter 4 . .

A number of taxa occurring in the two ecosystems are not represented i n the carcass s ample. These species in the S erengeti ( hartebeest, eland, hippopotamus) are relatively uncommon, though regularly s een in their preferred habitats. More common taxa poorly represented i n the c arcass s ample are all smaller s pecies under f ifty kilograms l ive adult weight. The s ize a nd age composition of the total l arger herbivore carcass s ample i s presented in Table 2 .4. The s ize class designation f or each taxon corresponds to that commonly used by archaeological f aunal analysts ( e.g. Brain 1 981, Bunn 1 982), with the important exception that young individuals of a particular taxon are placed in a smaller s ize category than adults. This i s necessary because l arge developmental s ize changes are expected to r esult in different treatment by consumers of young and adult carcasses of the same species. The age classes used correspond to the dental age a ssignments used by archaeological f aunal analysts ( e.g. Bunn 1 982) n eonates and all pre-juveniles ( Bunn' s Age 1 ) display at most s lightly worn deciduous premolars, with at most the f irst of the molars e rupting. Juveniles ( Bunn's Age 2 ) have permanent premolars, but only s lightly worn permanent molars and i ncompletely erupted third molars. Adults ( Bunn' s Age 3 a nd Age 4 ) d isplay a f ull complement of permanent dentition, all of which i s in wear. The s ample has been s tratified into s ix s ize/age categories. The carcasses comprising each category are shown in Table 2 .5. The number of categories created reflects an attempt to maximize the s ample s ize while minimizing the s ize and age range of each. The categories comprise the basic carcass analytical units to be used throughout the presentation of r esults i n the f ollowing chapters. Two s ubadult categories have been created. The " small s ubadult" category i s dominated by Thompson's gazelle f awns and juveniles a nd neonate wildebeest; i t contains 3 4 carcasses, or 1 2.7% of the total carcass s ample. The "medium-sized j uvenile" category i s comprised exclusively of j uvenile wildebeest and z ebra, with the exception of one buffalo calf. 2 8 c arcasses, or 1 0.5% of the total s ample, 2 7

Table 2 .4: Size and age composition of the total larger mammalian herbivore carcass s ample. F or the age column, N = n eonate or pre-juvenile, J = j uveniles, SA = all s ubadults, A = adults, T = total. Bunn's ( 1982) s ize groups a re those commonly used by archaeological f aunal analysts. The s ize d esignations used here take account of l arge developmental s ize changes in any one species.

Adult Weight ( kg)

Bunn' s S ize Group

My S ize Group

Carcass n

S ample

Species

Age

Thompson's gazelle

SA A T

1 5

1 1

1 1

1 1 1 3 2 4

4 .1 4 .9 9 .0

Grant' s gazelle

N A T

4 0

2 2

1 2

3 3 6

1 .1 1 .1 2 .2

Impala

J A T

4 0

2 2

1 2

1 2 3

0 .4 0 .7 1 .1

Warthog

A /T

4 5

2

2

1

0 .4

Top!

N A T

1 00

3 3

1 3

1 1 2

0 .4 0 .4 0 .7

A T

1 23

3 3 3

1 2 3

Waterbuck

J /T

1 60

3

2

1

0 .4

Z ebra

N J A T

2 00

3 3 3

1 2 3

4 1 1 3 6 5 1

1 ,5 4 .1 1 3.5 1 9.1

N A T

4 50

4 4

2 4

1 8 9

0 .4 3 .0 3 .4

Giraffe

A /T

7 50

5

5

2

0 .7

Rhinoceros

A/T

8 16

5

5

1

0 .4

Elephant

A /T

1 725

6

6

1

0 .4

TOTAL

SA A T

Wildebeest

N J

Buffalo

*

excludes

1 wildebeest

1 5 1 4 1 37 1 66*

6 2 2 05 2 67 carcass

determined.

2 8

f or which

age

5 .6 5 .2 5 1.3 6 2.2

2 3.2 7 6.8 1 00.0 c ould

not be

T able 2 .5: S ize a nd age s tratification of the total carcass s ample. The s ize/age categories, based on data in Table 2 .4, are the basic carcass analytical units to be used throughout.

C arcass S ize/Age Groups

S pecies

and Age

Composition

Small s ubadults

Thompson' s gazelle s ubadults Grant's gazelle neonate ( 3) T opi neonate ( 1) Wildebeest neonates ( 15) Z ebra neonates ( 4)

Small adults

Thompson's gazelle ( 13) Grant's gazelle ( 3) Impala ( 2*) Warthog

Medium-sized j uveniles

Impala ( 1*) Wildebeest ( 4) Waterbuck ( 1) Z ebra ( 11) B uffalo neonate T opi ( 1) Wildebeest Z ebra ( 36)

Large

Buffalo

Very large adults

Subtotal n /(%)

( 11) 3 4 ( 12.7)

1 9 ( 7.1)

( 1)

Medium-sized adults

adults

( n)

2 8 ( 10.5) ( 1) 1 74 ( 65.2)

( 137)

( 8)

8 ( 3.0)

G iraffe ( 2) Rhinoceros ( 1*) E lephant ( 1)

4 ( 1.5)

TOTAL

2 67

* i ndicates poached individuals. The two impala were s nared and d iscovered before poachers, while poachers had s everely i nterfered with t he rhino before my discovery of i t; the

rhino

is

excluded

f rom the

f ollowing

2 9

analyses.

are

classified

as

medium-sized

j uveniles.

Four adult s ize categories have been created. S ize 1 and S ize 2 adults are l umped i nto a " small adult" c ategory to create a sufficiently l arge s ample of 1 9 carcasses, or 7 .1% of the total. The small adult c ategory i s dominated by Thompson's gazelle, and also includes a dult Grant's gazelle and impala. "Medium-sized adults" a re by f ar the l argest c ategory, i ncluding 1 74 carcasses or 6 5% of the total s ample. Adult wildebeest dominate this category, while the r emainder of the carcasses are all z ebra adults with the exception of one adult topi. " Large a dults" are r epresented exclusively by e ight buffalo, comprising 3 % of t he total s ample. "Very l arge adults" include only f our c arcasses, one of which ( the rhinoceros) was poached and too s everely interfered with by poachers to contribute to the f ollowing analyses. The very l arge adult c ategory is t herefore represented by two giraffe and one e lephant, less than 2 % of the total s ample. Most of the s ix s ize/age categories are s ufficiently l arge f or evaluating general s izeand a ge-based differences in carcass utilization. Only the medium-sized adult category, however, contains enough individuals to permit many of the detailed, multiple variable analyses presented in Chapter 4 . CONSUMERS

OBSERVED

Lions, s potted hyenas and vultures were t he most f requently observed carcass consumers. Other c onsumers observed were cheetah, j ackals ( three s pecies) ‚ bat-eared f oxes, baboons, wild dogs, maribou storks, a nd tawny eagles. These other consumers were either i nfrequently observed at c arcasses, or, as with j ackals and maribou s torks, were commonly present but made immeasurable contributions to carcass consumption. Leopards and s triped hyenas occur in the S erenqeti in very small numbers. T hese s pecies were s een on only two occasions each and never at a carcass. The term hyena therefore will be used throughout t o refer exclusively to the s potted hyena. A diversity of smaller mammalian carnivores occurs i n the Serengeti and Ngorongoro, but these carnivores are mainly n octurnal s pecies never observed at carcasses. The term vulture i s used i n a generic s ense throughout. The vulture taxa most f requently present a t carcasses and in the l argest numbers are the griffon vultures, i ncluding the white-backed vulture and R uppell's qriffon vulture. Other vulture s pecies f requently present at carcasses but always in s mall numbers are t he l appet-faced vulture and the hooded vulture. The Egyptian vulture was rarely observed a t carcasses i n the S erengeti, but was present in Ngoron oro.

on

a number

of

3 0

occasions

s ingly

o r

i n

pairs

CHAPTER 3 PARAMETERS OF SCAVENGING OPPORTUNITY

I am concerned here with the development of two parameters of s cavenging opportunity. These are the completeness of carcasses available to s cavengers, and the duration over which carcasses persist as s cavengeable f ood s ources. The bulk of discussion concerns an analysis of the s equences by which carcass parts are consumed, the r esults of which are critical f or establishing a meaningful measure of c arcass completeness. The two parameters are used i n the f ollowing chapter in an analysis of variations i n the s cale of s cavenging opportunity among particular physical and ecological contexts.

CARCASS

COMPLETENESS

The completeness of a carcass determines the types and amount of edible t issues available to s cavengers. Scavengers typically encounter incomplete c arcasses; upon discovery, 2 58 of 2 68 carcasses in my s ample had been or were being consumed. The l ess complete this i nitial consumption, the greater the diversity of parts available to a s cavenger, the greater i ts total nutritional r eward, and, in general, the greater the s cavenging opportunity. For my purposes, a measure of carcass completeness must be informative on two f ronts. F irst, the measure must reflect the f raction of a carcass's r esource potential remaining f or s cavengers. S econd, the measure must be s ensitive to the s pecific carcass parts comprising the f eeding opportunity, and i n particular the s eries of skeletal elements bearing the r emaining edible t issues. The l atter requirement i s essential to the ultimate application of o bservations on modern carcass use and availability to those f eeding opportunities a hominid s cavenger might have encountered, and to the development of criteria f or the archaeological r ecognition of s cavenging. The ability of a measure of completeness characterize a ccurately both the amount and d istribution

to of

edible carcass parts depends on the degree of uniformity i n carcass consumption patterns. An invariant s equence of carcass part consumption by a ll carnivore taxa on all carcass types would r ender as highly specific the yield and s eries of carcass parts r epresented by a s ingle completeness value. A random or inconsistent consumption pattern, on the other hand, would r ender the value insensitive to the parts comprising i t, and c onsequently of 3 1

l ittle use f or archaeological predictions. To evaluate t he extent t o which one c an a ccurately s peak of a general c onsumption pattern, data are presented below on t he s equence by which various carnivores h ave been o bserved t o consume the parts of variously s ized a nd aged c arcasses. Although presented here f or methodological purposes, i t will be s een i n Chapter 5 that the c onsumption s equence i tself provides a basis f or c riteria f or distinguishing a rchaeological a ssemblages a ccumulated through s cavenging a s opposed to hunting.

C arcass

Consumption

S equences

Constructing A S equence: Carcass consumption s equences a re constructed through a cross-sectional analysis o f the p art by part completeness of all carcasses. Per-part completeness i s that encountered upon my discovery of a carcass. These data were collected whether o r not t he c arcass was being consumed a nd r egardless of t he s tage consumption had r eached i n a way described f ully i n Chapter 2 . On-discovery data are used i n o rder to ensure the b est possible discrimination among t hose parts typically consumed early i n the s equence, a nd f or which f eeding observations are the l east f requent. I n a number of cases I was unable to appraise accurately t he completeness of a ll parts on d iscovery, s o that d ata f rom the earliest s ystematic i nspection conducted between f eeding bouts of s uccessive c onsumers are used. For o nly 8 of 2 68 carcasses d iscovered d id c ircumstances prevent an a ccurate a nd s imultaneous a ssessment of completeness at s ome p oint before the carcass's r esource l ife h ad been exhausted. M ost uncertainties were r estricted to t he completeness of o nly one or two parts, a nd these were u sually unambiguously cleared up by consulting r ecords o n s ubsequent f eeding observations or s ystematic i nspections. F ield r ecords on c arcass completeness o n discovery were encoded f or c ompüter analysis by a ssigning a whole number c ompleteness value to e ach of a s eries o f 2 6 c arcass units. The 2 6 parts c onstitute the whole of a c arcass m inus the tail and body s kin ( see, f or e xample Table 3 .1). W ith the exception of the l ower v iscera ( digestive o rgans) a nd upper viscera ( heart, l ungs, d iaphram, esophagus) , e ach unit c orresponds to the bone or bone s et that bears or contains the edible t issue. The carcass parts used were usually e asily d istinguished during f eeding o bservations. This i s c ertainly t rue f or t issues c ontained by bone ( limb bone marrow, brain and head pulps) ‚ and f or most f lesh parts a s well. S light ambiguity arose only i n d istinguishing p elvic g irdle f lesh f rom both proximal f emur f lesh a nd posterior l umbar

f lesh.

3 2

The specificity of the parts used corresponds to the manner by which carcasses are consumed by carnivores. R ib f lesh and thoracic vertebrae f lesh are considered as one u nit, the r ibcage ( unlike B inford, e .g. 1 978) . This i s b ecause carnivores do not, and I was unable to, d istinguish b etween proximal r ibs and thoracic vertebrae during the c onsumption of dorsal r ibcage f lesh. For s imilar r easons ( and again unlike B inford) ‚ s acral f lesh i s not d istinguished f rom pelvic f lesh ( being l abeled pelvic f lesh) , nor are the tarsal and carpal bones distinguished f rom the metapodials ( the unit being r eferred to a s e ither metatarsals

or metacarpals)

The average rank in the consumption s equence of paired, appendicular parts i s used. Upper and middle l imb bones ( humerus, f emur, radius-ulna, tibia) ‚ however, are e ach considered twice to a ccount f or and distinguish between f lesh borne by the parts and the marrow contained by them. The f lesh of a given l imb bone i s typically eaten much earlier than its marrow. Foot bones ( metapodials and phalanges) ‚ however, bear negligible to no f lesh an are c onsidered only once, f or their marrow. The head, l ike upper l imb bones, bears f lesh and contains edible t issues, s o that a distinction i s made between head f lesh parts ( tongue, mandible, maxilla) and head contents ( brain, and nasal, mandibular and f rontal pulp) . Postcranial axial units, on the other hand, are considered only f or their f lesh, as the quantity of grease in these bones i s small a nd could not have been extracted by hominids before the l ate advent of f ire f or cooking and containers in which g rease

most a s

could be

r endered.

A whole number value of 0 to 4 , ranging f rom l east to complete, was assigned to each unit of all c arcasses,

f ollows:

o

=

1

=

2 3

= =

the edible tissue i s associated bony part i s

f inished, and either present

the or

absent. the edible t issue i s almost f inished, with l ess than approximately 2 5% remaining. the edible t issue i s approximately half eaten. the edible t issue has been s tarted, but i s greater than approximately 7 5% complete.

4 = the

edible

tissue

i s

complete.

Additionally, i f the completeness of a carcass part was n ever accurately assessed, the part was assigned a value of 9 and

excluded

f rom

subsequent

analysis.

Five grades of per-part completeness were used i n r ecognition of r epeated observations that many parts are, t o varying degrees, i ncompletely consumed before f eeding c ommences or even i s f inished on other parts. The use of a s impler presence/ absence or three tier coding s ystem would obscure

this

important

tendency 3 3

of

s cavengers

to

encounter

partially

consumed parts.

The completeness values f or l ike parts among a s ample of c arcasses were analyzed i n a cross-sectional manner. F or e ach part a mean completeness value a nd a f requency d istribution of values were obtained a s a lternative bases f or r anking parts i nto a consumption sequence. T he c ross-sectional method assumes the s tate on d iscovery o f e ach carcass to be r epresentative of a d iscreet s tage i n t he consumption s equence. The method has t he advantage over the use of l ongitudinal f eeding observations at a s eries o f i ndividual carcasses of enabling t he use of all c arcasses r ather than a much smaller s ample f or which the c arcasses' whole or nearly whole r esource l ife was observed. T he major d isadvantage of the c ross-sectional method i s i ts r equirement of r elatively l arge s ample s izes to o btain a n adequate l evel of d iscrimination among parts of s imilar r ank in the consumption s equence. Mean completeness values are u sed to r ank c arcass parts i nto a consumption s equence. A part with a mean completeness value of, f or example, 1 .6 i s eaten o n average earlier than a part with a mean completeness value of 1 .9. Parts with equal mean completeness values are e aten o n average s imultaneously and g iven equal r ank values. T he meaning of these values, however, i s ambiguous. T he ranking neither refers to the s equence by which parts a re s tarted ( as would be the c ase i f the r anking was based on t he f requency by which parts were encountered complete), nor t o the s equence by which parts are f inished ( as would r esult f rom the use of f requency values of parts e ncountered f inished) R ather, the r anking based on mean values i ndicates a s equence by which parts are partially eaten, which has the advantage over o ther methods of being s ensitive to the c ommon partial u se of parts t hat a re either abandoned a s s uch, or not c ompletely consumed until l ater i n the s equence. This i s particularly the c ase w ith nasal pulp, the anterior portion of which, owing to t he s oftness of the nose, i s t ypically s tarted very early i n the s equence, but not f inished until more a ttractive within-bone t issues a re . consumed ( Table 3 .1). I t i s a lso true f or the tongue, which i s t ypically s tarted after mandible f lesh, but r apidly f inished before r emaining h ead f lesh i s completely consumed ( Table 3 .1). Minor c ases o ccur with humerus f lesh, metatarsal marrow and the brain ( Table 3 .1). With these f ew and usually minor exceptions, t he s equences based on mean, f requency-complete a nd f requency-finished values are very s imilar, with the m ean value r anking being i ntermediate i n c ases where t hey d iffer .

( Table

3 .1).

I t s hould be n oted that the o rdering of c arcass parts i n Table 3 .1 and s imilar t ables i s based on t heir anatomical proximity to one another, r ather than s trictly by t heir a ctual r ank i n the c onsumption s equence. T his arrangement

was

deemed

easier 3 4

to

i nterpret

than

i f

parts

were ordered i rrespective of anatomy c orrelated r elationship with r ank.

to

obtain

a perfectly

T he General C arcass Consumption S equence: The consumption s equence f or a ll 2 60 herbivore carcasses f or which data were available i s s hown in Table 3 .1. The s equence i s a g eneral consumption s equence ( G. C.S.) because i t d isregards d ifferences i n c arcass and consumer type. Edible t issues o ccurring outside of bones f lesh) are consumed before t issues within bones h ead contents) Within these two t issue c arcasses are c onsumed f rom the back to the three distinct body regions. S ix s tages c onsumption, l isted f rom the earliest to l ast r ecognized: .

1 )

2 ) 3 ) 4 ) 5 ) 6 )

( viscera and ( marrow and categories, f ront among of carcass eaten, are

hindquarter f lesh, including viscera and f lesh posterior to the r ibcage, with the exception of tibia f lesh, which i s consumed in the next s tage. f orequarter f lesh, i .e. post-cranial f lesh anterior to and i ncluding ribcage f lesh. head f lesh, including the tongue. hindlimb marrow. f orelimb marrow. head c ontents.

With the exception of t ibia f lesh, all parts within any one of the s ix body r egions are completely consumed before parts of the r egion begun next i n the G .C.S. The exception provided by t ibia f lesh results f rom the l ate consumption of the distal half of the unit, which i s t ightly encased i n s kin and eaten only after more easily extracted f lesh parts. Expected Deviations f rom the G . C.S.: The possibility of consumer- and carcass-specific deviations f rom the G .C.S. has important a rchaeological consequences, a s mentioned earlier. Field o bservations and theoretical considerations i ndicate that consumption patterns should vary according to two major f actors: 1 ) different mechanical constraints imposed on the c onsumption of carcasses of different s ize, and the d ifferential ability of consumer taxa to overcome these constraints, and 2 ) variations i n the r elative nutritional attractiveness of parts related to carcass age differences. D ividing the t otal s ample i nto i ndividual c arcass and consumer categories r educes s ample s izes to the extent that i t i s not always possible to obtain s ufficient discrimination among parts i n all potentially r elevant consumer and c arcass groupings. Nonetheless, consumption s equences f or a number of s uch groupings could be established, as s hown i n Tables 3 .2 3 .4. -

Table 3 .2 s hows consumption s equence.

the effect of Carcass s ize i s 3 5

carcass s ize on an expected basis

the f or

Table 3 .1: The consumption s equence f or all c arcasses ( n=260) based on a ranking of 1 ) mean values of p er part completeness on discovery of the carcasses, 2 ) t he f requency by which parts were discovered complete, and 3 ) the f requency by which parts were d iscovered f inished. Note the general s imilarity among the three s equences, with major exceptions being restricted t o the rapidity by which the tongue i s f inished ( a) , and t he early initiation of f eeding on nasal pulp ( b) The rank based on mean v alues i s used f or the general consumption s equence ( G. C. S.). .

RANK

Carcass

Unit

l ower viscera upper viscera H INDQUARTER FLESH pelvis l umbar f emur t ibia FOREQUARTER FLESH r ibcage s capula humerus radius-ulna cervical HEAD FLESH tongue mandible maxilla H INDLIMB MARROW f emur t ibia metatarsal p11c t 1 d11 y

I N CONSUMPTION SEQUENCE f requency f requency mean values complete f inished 1 4

1 5

1 4

2 5 3 9

2 4 3 9

3 5 2 9

6 8 7 1 1 1 0

6 7 8 1 1 1 0

8 7 6 1 2 1 1

1 2 1 3 1 4

1 3 1 2 1 4

1 0a 1 3 1 4

1 5 1 6 1 8

1 6 1 7 2 0 1 9

1 5 1 6 1 8 1 7

1 9 2 1 2 2 2 0

1 8 2 1 2 3 2 2

1 9 2 1 2 2 2 0

2 3 2 5 2 4 2 6

1 5 2 4 2 5 2 6

2 4 2 5 2 3 2 6

b

FORELIMB MARROW humerus r adius metacarpal phalanges HEAD CONTENTS nasal pulp mandible pulp brain f rontal pulp

3 6

deviations f rom the G .C.S. because l arge carcasses potentially contain parts that pose physical constraints to consumption not encountered on smaller carcasses. Only adult carcasses a re considered in order to control possible age-related variations. Consumer type, however, could not be c ontrolled without reducing the s ample s izes of the small adult and l arge adult classes to i nadequately small numbers. Medium-sized adults ( wildebeest and zebra) are by f ar the l argest s ample ( n = 1 68 carcasses) and as a r esult show the greatest s imilarity to the G .C.S. Large adults ( buffalo) are r epresented by only e ight i ndividuals, resulting in poor r ank discrimination of parts eaten l ate in t he s equence ( marrow and head f lesh) This problem i s not s een with the small adult sample ( mostly Thompson's and Grant' s gazelle), which numbers 2 4 carcasses. Very l arge adult carcasses ( giraffe and elephant) are excluded f rom the t able due t o s mall s ample s ize ( n = 3 ). .

The effect of carcass age on the consumption s equence i s s hown i n Table 3 .3. Carcass age i s an expected basis f or deviations f rom the G .C.S. primarily because of age-related differences in t he nutritional content of marrow bones relative to other parts. Marrow cavities of neonate and young juvenile ungulates f unction exclusively a s s ites f or the production of r ed blood cells and therefore contain only hematopoietic f luid ( e. g. deCalesta et al. 1 977, Hanks et a l. 1 976, personal observations) Marrow cavities of adults, however, f unction primarily as f at reservoirs f or use by the animal during nutritionally s tressful periods ( Brooks 1 978, Brooks et al. 1 977, Dunham and Murray 1 982). The t ransition i n marrow cavity f unction occurs during the juvenile period ( personal observation) ‚ s uch that marrow bones of carcasses designated j uvenile contain variable proportions of h ematopoietic f luid and f at. Because f at has a much higher nutritional yield, one might expect marrow bones of young a nimals to be ignored until consumption i s completed on other parts r anked l ater i n the adult-dominated G .C.S. The consumption s equences f or carcasses r epresenting neonates, j uveniles and adults are shown in Table 3 .3. Carcass s ize within any one age class is c ontrolled by c onsiderinc i the three developmental s tages of wildebeest and z ebra only. Consumer type could not be held constant without s everely reducing s ample s izes. .

Consumer type i s the f inal major f actor expected to effect variations f rom the G .C.S. The greatest difference i s e xpected in t he differential ability of carnivores to gain a ccess to t issues within bones. Hyenas are noted f or their ability to e xpose nutrient-rich bone cavities i n even the l argest animals, and are the primary consumers of marrow and head c ontents i n Africa. The l ess powerful j aws of f eu ds permit o nly l imited access to these t issues, and all vulture species ( with the exception of the l ammergeier, a r are species which drops bones f rom heights onto rocks) are c ompletely unable to do s o. Carnivores able easily to extract within-bone nutrients might be expected to consume 3 7

these parts at an earlier G .C.S., which i s composed carnivore t axa observed.

s tage t han predicted of carcasses eaten

by by

t he a ll

The consumption s equences f or l ions, h yenas a nd vultures on medium-sized adult carcasses i s s hown i n Table 3 .4. Only those c arcasses are i ncluded whose c ontext a nd condition ( e.g. degree of bone modification a nd d isarticulation) permitted a high l evel of c ertainty that only one i dentifiable consumer taxon had f ed. The c onsumer s pecific analysis i s necessarily l imited to t he l arge s ample of medium-sized adult c arcasses. S ample s izes f or other age and s ize c lasses were t oo s mall to yield useful r esults. I ncidental comments, however, will be made o n these other carcass types, and on the cheetah, a n ot rarely observed consumer r estricted to small prey. Deviations f rom the G .C.S.: The s pecific c onsumption s equences s hown i n Tables 3 .2 3 .4 a re a ll broadly s imilar to the G .C.S. in Table 3 .1. All f lesh parts, with t he occasional exception of f lesh on t he anterior p ortion o f the body, are consumed before marrow and head c ontents. Numerous d eviations occur among i ndividual carcass parts within any of the s ix body r egions o utlined f or t he G .C.S. These are usually s light and are p robably due l argely t o s ampling and s tatistical l imitations. The notable deviations f rom the G .C.S. are those i nvolving a p artial o r whole s equential switch between adjacent members o f the s ix body regions, as d iscussed below. -

The three f lesh bearing r egions a re eaten among a ll consumer and c arcass c ategories i n a pattern v ery consistent with the G .C.S. The major d eviations are t he consumption of head f lesh before s ome f oreguarter f lesh parts. Vultures are l ate consumers of most f orequarter f lesh relative to h ead f lesh on medium-sized adults ( Table 3 .4), and, i t was observed, on a ll c arcasses i n g eneral. This r esults f rom the l imited ability of vultures to g ain access to f lesh beneath s kin except f rom n atural orifices, i ncluding the anus, genitals, ears, e yes, a nd mouth. Parts close to orifices ( head f lesh, pelvic, l umbar a nd f emur f lesh) are therefore consumed by v ultures before hidden t issues ( upper viscera, f orelimb, t ibia and c ervical f lesh) Head f lesh i s also consumed b efore cervical f lesh, and to a l esser extent r adius-ulna f lesh, in a n umber o f c ontexts o ther than vultures f eeding a lone. Cervical f lesh on l arge adult ( buffalo) c arcasses i s covered with particularly thick s kin which i nhibits access to t he anterior portion of the n eck r egion until head f lesh i s consumed ( Table 3 .2). A s imilar c ircumstance was observed on the two g iraffe and one e lephant c arcass discovered. T he consumption of c ervical f lesh after h ead f lesh on small adult carcasses ( Table 3 .2) i s caused by the t endency o f l ions, cheetah and vultures to peel a protective l ayer o f body s kin over the neck while f eeding f rom the r ibcage a nd s houlder. Hyenas on smaller carcasses, however, owing t o .

3 8

T able 3 .2: Consumption s equences f or three s izes of adult carcasses. The r ank of parts in the G . C.S. is also shown f or c omparison. Note the l ate consumption of cervical f lesh i n s mall adults a nd l arge adults ( a), the early consumption of phalanges f rom l arge adults ( b) , and the consumption of nasal pulp before all marrow in small adults. S ee text f or details.

RANK

Carcass

Unit

G .C.S.

I N

CONSUMPTION

small adults ( n=24)

med-sized adults ( n=168)

SEQUENCE l arge adults ( n=8) 1 3

l ower viscera upper viscera H INDQUARTER FLESH

1 4

3 6

1 4

pelvis l umbar f emur t ibia FOREQUARTER FLESH

2 5 3 9

1 4 2 5

2 5 3 1 0

2 5 4 6 .5

r ibcage s capula humerus r adius-ulna c ervical HEAD FLESH

6 8 7 1 1 1 0

8 7 9 1 0 a 1 4

6 8 7 1 1 9

8 9 6 .5 1 1 a 1 8.5 .

tongue mandible maxilla HINDLIMB MARROW f emur t ibia metatarsal phalange FORELIMB MARROW humerus r adius metacarpal phalanges HEAD CONTENTS nasal pulp mandible pulp brain f rontal pulp

1 2 1 3 1 4

1 1.5 1 1.5 1 3

1 2 1 3 1 4

1 1 1 1 1 8.5

1 5 1 6 1 8 1 7

1 6 1 7 2 3 2 4

1 5 1 6 1 8 1 9

1 8.5 1 8.5 1 8r D 5 1 3

1 9 2 1 2 2 2 0

1 8 2 0.5 2 0.5 2 0.5

1 7 2 0 2 1 2 2

1 5 1 8.5 1 81 5 5 1 4

2 3 2 5 2 4 2 6

1 5 2 0.5 2 5.5 2 5.5

2 3 2 4 2 5 2 6

2 3 2 4.5 2 4.5 2 4.5

3 9

Table 3 .3: Consumption sequences f or three ages of wildebeest and zebra. The rank of parts in the G . C. S. i s shown for comparison. Note the early consumption relative to

marrow

bones

of

the

brain

and

f rontal

pulp

( a) and to a lesser extent in juveniles ( b) details and other deviations from the G . C.S.

RANK Carcass

Unit

l ower viscera upper viscera HINDQUARTER FLESH

i n See

.

I N CONSUMPTION

G . C. S.

neonates ( n=20)

juveniles ( n=25)

1 4

3 5

1 5

neonates text

SEQUENCE adults ( n=168) 1 4

pelvis

2

3

3

2

l umbar f emur

5 3

3 1

5 2

5 3

9

6

6 8

7 8 .5

7

8 .5

tibia FOREQUARTER FLESH ribcage scapula humerus radius-ulna cervical HEAD FLESH

1 0

1 0

5 7 .5 7 .5

6 8 7

1 1 1 0

1 6 1 0

1 1 9

1 1 9

1 2 1 3

1 2 1 2

1 3 1 2

1 2 1 3

1 4

1 2

1 4

1 4

f emur

1 5

2 1

1 7

1 5

tibia metatarsal

1 6 1 8

1 9.5 1 9.5

1 6 1 9

1 6 1 8

1 7

1 7

20

1 9

1 9

2 3

1 8

1 7

2 1 2 2 2 0

2 4.5 2 4.5 2 2

22 23.5 2 3.5

2 0 2 1 2 2

tongue mandible maxilla HINDLIMB MARROW

phalanges FORELIMB MARROW humerus radius metacarpal phalanges HEAD CONTENTS nasal

pulp

mandible pulp brain f rontal pulp

2 3

1 8

2 5 2 4 2 6

2 6

a

26

1 4 5

a

21 25

1 4.5

4 0

1 5

2 3 b

f or

2 4 2 5 2 6

Table 3 .4: Consumption s equence f or medium-sized adults by i ndividual carnivore t axa. The rank of parts in the G . C.S. i s s hown f or c omparison. C onsumption by vultures s tops at the last f lesh unit ( radius-ulna), while the only within-bone t issues eaten by l ions on this s ize c lass are nasal pulp and occasionally one extremity of f emur marrow. Note the early c onsumption of hindlimb marrow before all head f lesh by hyenas ( a), and the early consumption of head f lesh relative t o most f orequarter f lesh by vultures ( b).

RANK I N hyenas only Carcass

Unit

l ower viscera upper viscera H INDQUARTER FLESH p elvis l umbar f emur t ibia FOREQUARTER FLESH r ibcage s capula humerus r adius-ulna c ervical HEAD FLESH tongue mandible maxilla HINDLIMB MARROW f emur t ibia m etatarsal phalanges FORELIMB MARROW humerus r adius metacarpal phalanges HEAD CONTENTS n asal pulp mandible pulp brain f rontal pulp

CONSUMPTION l ions only

S EQUENCE vultures only

G .C.S.

( n=43)

( n=38)

1 4

1 2

1 3

2 8

2 5 3 9

3 5 4 7

2 5 4 1 0

1 4 3 1 1

6 8 7 1 1 1 0

6 9 8 1 2 1 0

6 8 7 1 1 9

5 1 2.5 1 2.5 1 4 1 0

1 2 1 3 1 4

1 6 1 7 2 2

1 3 1 2 1 4

1 5 1 6 1 8 1 7

1 1a a 1 4 a 1 5 a

1 6

1 9 2 1 2 2 2 0

1 8 1 9 2 0 2 1

2 3 2 5 2 4 2 6

2 4 2 3 2 5 2 6

1 3

4 1

1 5

( n=10)

7b 9b

their powerful tearing j aws, were observed the back to f ront pattern s een i n the G .C.S.

to

c onform

t o

All specific s equences s ave one c onform to t he G .C.S. with r espect to the consumption of marrow after a ll f lesh. The exception i s the hyena, which i s shown t o consume h indlimb marrow before h ead f lesh i n medium-sized adults ( Table 3 .4). On smaller adult c arcasses, hyenas were observed to consume hindlimb marrow even earlier, before most f orequarter f lesh. The early c onsumption of hindlimb marrow by hyenas i s partly an artifact of the t endency of individual hyenas to r emove r ecently defleshed h indlimbs f rom the highly competitive f eeding that characterizes hyenas at a carcass ( see also Kruuk 1 972:125). Hence, although hyenas do not actually f inish consuming hindlimb marrow until l ater than indicated i n Table 3 .4, t he tissues are nonetheless r emoved f rom the pool of potentially s cavengeable parts at a s tage earlier than predicted by t he G .C.S. Hyenas consume f orelimb marrow at a stage more i n keeping with the G . C.S., l eaving until later among f lesh parts only maxillary f lesh. The conformity of the f orelimb marrow parts results f rom the n eed f or these to b e mostly defleshed before removal of the marrow bones, which typically occurs l ate in the consumption of a ll f lesh parts. L ions on small adults, small s ubadults, a nd occasionally medium-sized j uveniles are the only other measurable consumers of marrow bones in the S erengeti. [ Wild dogs are c ertainly c apable of exposing marrow cavities of animals at l east this l arge, but o n the o ne occasion they were observed to f eed, the c arcass was abandoned at an early s tage in f avor of joining another hunt.] L ions were never observed to gain access c ompletely to the marrow cavities of any bone of medium-sized adult o r l arger carcasses. Lions f requently do gnaw the e piphyses of especially the humerus and f emur of l arge carcasses, but can expose at most the very top or bottom of t he marrow cavity. The l ower r ank of the f emur compared to other marrow bones on medium-sized adult carcasses eaten by l ions ( Table 3 .4) results f rom s everal occasions where t he distal epiphysis of the f emur had been heavily gnawed, a nd a s mall portion of the marrow r emoved i n the process. C heetah a nd vultures were never observed to crack marrow bones of even the

smallest

carcass.

I n most s pecific cases, a ll hindlimb marrow i s consumed before any f orelimb marrow, a s i n the G .C.S. T his i s certainly true f or hyenas, the consumer responsible by f ar f or most marrow consumption, on medium-sized adults ( Table 3 .4) and other c arcasses. Exceptions, h owever, do occur. On medium-sized j uveniles ( Table 3 .3) a nd medium-sized adults ( Tables 3 .2 and 3 .3), humerus marrow i s c onsumed before metatarsal and phalange marrow of t he hindlimb. The r easons f or this are uncertain, but the pattern i s s een to a greater extent f or small adults ( Table

4 2

3 .2) ‚ where all f orelimb marrow i s consumed before the marrow of the metatarsus and phalanges of the hindlimb. This pattern was observed on s everal occasions of l ions consuming Thompsont s gazelle adults. The preferred t issue of l ions i s clearly f lesh, and consumption of marrow bones on small carcasses, or the gnawing of their epiphyses on larger carcasses, appears to be an activity carried out a s an afterthought to the meal j ust consumed. I n these terms, the s equence of marrow part consumption by l ions might be expected to f ollow a more random pattern than that practiced by the bone-crunching-specialist hyenas. The f inal exception i s s een on l arge adult carcasses ( Table 3 .2), where marrow f rom both the hindlimb and f orelimb phalanges is c onsumed before any other marrow. A s imilar pattern was observed on the two giraffe carcasses discovered, and r elates to the manageability of phalanges compared to the other, exceedingly l arge and difficult to crack marrow bones of l arge carcasses. Head contents are generally l ow yielding and difficult to extract carcass parts eaten l ast, as a group. This i s invariably the c ase f or mandibular pulp, and in all but one case f rontal s inus pulp. Nasal pulp i s the most r eadily exposed head c ontent, being protected only by s kin and l aminar bone. I t is consumed before all or most marrow in small adult ( Table 3 .2) and all s ubadult ( Table 3 .3) carcasses. This pattern results f rom the tendency of l ions to c onsume most nasal pulp early i n the consumption sequence on all k ills, as documented here f or medium-sized adults ( Table 3 .4). Aside f rom nasal pulp, l ions were never observed to consume head contents of medium-sized adult and larger carcasses. Vultures ( Table 3 .4) and cheetah were never observed to consume any head content on any s ize carcass. The brain i s the other head content to s how deviations f rom the G .C.S. In all adults ( Table 3 .2) and f or all consumers ( Table 3 .4), the brain i s eaten after marrow bones, as predicted by the G .C.S. Adult brains are s ecurely encased i n thick cranial bones, and i n many bovids protected additionally by s trong f rontal bosses. When eaten, the brain of adult carcasses i s typically exposed by gnawing a circular hole in the occiput. The cranium of subadults, and particularly neonates, i s thin and unprotected by bosses. The brain i s an easily exposed tissue, and relative to unfatty l imb bones of s ubadults, i s nutritionally more rewarding. Accordingly, the brain i s consumed before and f requently marrow bones in n eonates ( Table

to the 3 .3) by

exclusion l ions and

of all hyenas,

by crushing the superior cranium and i n the process, consuming the f rontal s inus r egion. The variable f at content of j uvenile marrow bones i s also apparently responsible f or the earlier consumption of the brain compared to l ower f orelimb marrow bones. Unlike neonates, however, the brain of medium-sized j uveniles i s exposed by l ions and more f requently hyenas in the f ashion typical f or 4 3

adults, a s reflected pulp ( Table 3 .3).

I ndexing

Carcass

by

the

l ater

c onsumption

of

f rontal

Completeness

The important deviations f rom the r egional carcass consumption s equence of the G .C.S. described above require carcass completeness to be expressed u sing s everal values. The consumption s equence of the three f lesh r egions was shown to be consistent f or most c arcass and consumer categories. Upper viscera, however, t end to be eaten l ater than most hindquarter f lesh. An i ndividual completeness value s hould therefore be used to d escribe the amount of viscera r emaining on a carcass, while a s eparate a nd s ingle value can accurately characterize the amount a nd d istribution of all r emaining f lesh. Marrow and anterior body f lesh were shown to be eaten i n differing orders depending on consumer type. L ikewise, head contents, particularly the brain and nasal pulp, were s hown to be consumed at various s tages r elative to marrow bones depending on carcass age. S eparate c ompleteness values a re therefore required f or marrow bones and head contents. Within any one of the f our t issue types ( viscera, f lesh, marrow, head contents) , a particular completeness value will correspond to a f airly predictable s eries of constituent parts, depending on the consumer and carcass type being considered, and according to the specific consumption s equences presented in tables 3 .2 through 3 .4. An index i s used to characterize the completeness of each of the f our carcass components. To s implify analysis, an index i s used rather than estimates of the absolute weights of r emaining t issue. The i ndex was constructed f or each f ollowing way: whole number values of 0 -

carcass in the 4 were assigned

to each part as in constructing consumption sequences. The s um of the values of all parts within each of the f our c arcass components was . obtained. This sum was then divided by the value of the component if complete. The quotient obtained i s the percentage i ndexed c ompleteness value f or the component. Using viscera a s a s imple example, a carcass whose l ower viscera have been completely consumed ( completeness value of 0 ), but which remains with approximately half of the upper viscera ( value of 2 ) h as a total completeness of 2 . Relative t o the whole visceral completeness of 8 , the percentage indexed c ompleteness value f or viscera of this carcass i s 2 5%. A s imilar procedure was f ollowed f or other t issue components of the carcass. Unlike the consumption s equence, however, both members of paired, appendicular parts were counted, y ielding 1 8 f lesh parts with a total possible c ompleteness of 7 2, 1 6 marrow parts with a total c ompleteness of 6 4, and 4 head content parts with a total c ompleteness of 1 6. By s umming the completeness values of all f our components 4 4

( total possible of 1 60 f or 4 0 c ompleteness value was obtained

carcass parts) ‚ an i ndexed f or the whole carcass.

The index a s constructed i gnores drastic differences i n the yield of individual carcass parts. Because the G .C.S. proceeds f rom generally high yielding to l ow y ielding parts, ( see Blumenschine and Caro, i n press) this m ethod overestimates the actual completeness of thoroughly e aten carcasses r elative to more complete ones. However, g iven the broad a pplicability of the G .C.S. and a knowledge o f the important deviations f rom i t, a particular indexed c ompleteness value will correspond to a f airly specific s et o f parts. For i nstance, an indexed completeness value f or f lesh of 4 5% denotes r emaining f lesh to be represented mostly by the e ight f lesh units eaten l ast. For virtually a ll consumer and carcass types, the eight units include the t hree head parts, c ervical f lesh, and f lesh f rom both t ibiae and radius-ulnae. S imilarly, a total carcass i ndexed c ompleteness value of 3 0% denotes remaining edible t issues t o be restricted mostly to the l ast twelve parts consumed. T hese include, through reference to the consumption s equences, head c ontents and marrow mostly of the f orelimb, w ith the brain being i ncluded if juveniles and adults, but n ot neonates, are being considered. The indexed c ompleteness values therefore provide a f airly accurate and s imply derived means of expressing the amount and d istribution of e dible t issues r emaining on a carcass. The completeness values refer to the s tate of a c arcass upon abandonment by consumers. Completeness on a bandonment i s u sed instead of completeness on d iscovery b ecause the f ormer event provides the only s tandard r eference point in the r esource l ife of a s eries of c arcasses. Having a s tandard r eference point i s essential f or comparing the s cavenging opportunities provided by c arcasses in different ecological and physical contexts. C arcass abandonment i s also the point at which any s cavenger, regardless of i ts rank on the competitive h ierarchy, at l east potentially has a ccess to the carcass. S ince carcasses a re often abandoned and taken over by other c onsumer taxa s everal times before i ts r esources are e xhausted, s everal useful reference points of s cavenging o pportunity

exist

f or most

carcasses.

CARCASS

PERSISTENCE

The duration over which a carcass persists as a s ource of s cavengeable f ood provides another measure of the s cale of s cavenging opportunity. I f continuously consumed f rom the time of death, the persistence of a carcass would be d irectly proportional to i ts completeness at any point in i ts resource l ife. Many carcasses, however, are abandoned b y i nitial and s ubsequent consumers i n a partially consumed 4 5

s tate, and can r emain unattended by s cavengers f or v arying amounts of time. The l onger this interval, the more l ikely a c arcass i s to provide a f eeding opportunity to a particular s cavenger. Carcass persistence i s measured i n hours f rom t he time of death until all edible parts have been consumed. The death of only 4 of 2 68 c arcasses was witnessed, r equiring e stimates to be made of the i nterval between d eath and d iscovery of the carcass. The d ay of death f or most c arcasses could be determined easily b ecause most p arts of a particular s tudy area were monitored on a daily b asis. A s well, i nformation on carcass s ize and completeness o n d iscovery, and the number and types of c onsumers p resent ensured that most carcasses were l ess than one d ay old. O ccasionally, a carcass was d iscovered two or more days a fter death, a s i ndicated by the dryness of r emaining t issues, the presence of maggots and the g eneral d egree o f t issue decomposition. D eaths of the previous n ight were a ssumed to have occurred at m idnight, corresponding a pproximately to the earlier of t wo peaks of n octurnal predator activity documented f or l ions and hyenas i n the S erengeti and Ngorongoro ( Kruuk 1 972, S challer 1 972). The r esource l ife of a c arcass was deemed e xhausted when a ll edible parts were observed t o have been c onsumed, or when the marginal r emains were observed to b e t ransported f rom the d eath s ite, usually by hyenas c arrying s craps to a den. A partially c onsumed c arcass eaten i n f ull or transported away f rom the death s ite at n ight was a ssumed to have persisted until midnight. I n many c ases, I was unable f or l ogistical r easons t o observe or e stimate a ccurately the end of a carcass's r esource l ife, a nd these a re necessarily excluded f rom analyses of c arcass persistence.

46

CHAPTER 4 SCAVENGING OPPORTUNITIES IN THE SE RENGETI AND NGORONGORO

A number of f actors have been f ound to effect the s cale of s cavenging opportunity i n the modern S erengeti and Ngorongoro ecosystems. These f actors can be grouped within c ertain physical and ecological contexts among which s cavenging s uccess varies. The contexts of s cavenging are i n this way viewed as independent variables that condition c arcass persistence and completeness on abandonment by i nitial and subsequent consumers. Two s ets of contexts have been f ound to have, i n their combined effect, a major influence on the s cale of s cavenging opportunities. These are carcass s ize i n r elation to the s ize and number of i nitial consumers, and the degree of c ompetition f or carcasses. Each s et of contexts i s based on s ound principles of uniformitarianism s uch that their i nfluence on modern s cavenging opportunity s hould be s imilar in kind to that i n the past. Carcass s ize in r elation to the number and s ize of i nitial consumers defines a f undamental physical context within which the resulting f eeding opportunity f or a s cavenger should vary in a predictable manner. A c arcass of a g iven s ize provides a certain quantity of f ood and imposes certain mechanical constraints upon consumption. The extent to which a carcass i s consumed i s directly dependent on individual and group s atiation l evels, and the ability of consumers to overcome mechanical constraints to the consumption of carcass parts. These constraints will be greatest f or edible t issues within bones, especially on l arger carcasses. Larger carcasses are also l ikely to be l ess completely e aten before consumers are s atiated and thereby to survive as f ood r esources through multiple f eeding episodes. Carcasses consumed by weak-jawed carnivores, too, are more l ikely to persist a s f ood r esources through one or s everal f eeding episodes, and hence provide a b etter f eeding opportunity f or s cavengers. Among the consumers regularly observed, hyenas are l east constrained by physical obstructions to consumption owing t o their powerful j aws. Weaker-jawed f elids have only l imited access to t issues within bones, and vultures have the most r estricted access to unexposed edible tissues. The l argely physical basis of these expectations, including j aw s trength relative to bone s trength, and carnivore gut s ize relative to the weight of edible carcass t issues, i ndicates 4 7

modern observations on s ize determinants of s cavenging opportunity to be an accurate informant of those carcass s izes most f requently available in t he past. The intensity of competition f or carcasses d efines a basic ecological context within which s cavenging s uccess s hould vary in a predictable manner. Greater competition f or carcasses should r esult in l ower average per carcass completeness when encountered by a s cavenger, and g enerally l ower encounter rates s temming f rom s horter average carcass persistence. The i ntensity of competition f or carcasses c an he approximated by l ocal consumer to carcass ratios. Although the relative numbers of consumers and carcasses i n an area proved impractical to determine, t hey are nonetheless expected to vary directly with ecosystem type, s eason, and habitat type. Within each of t he three contexts, the completeness by which carcasses are c onsumed and the duration carcasses persist a s f ood resources will be shown to vary in a predictable manner according to the i ntensity of competition. By using competition as the f undamental basis f or identifying s ome of the e cological contexts within which s cavenging s uccess varies, determinations of ecologically-based variations i n prehistoric s cavenging s uccess are s implified.

CARCASS Carcass

S IZE,

S ize

and

CONSUMER TYPE AND

SCAVENGING OPPORTUNITY

P ersistence

The average persistence of carcasses of different s ize i s shown in Table 4 .1, Persistence, again, i s measured f rom the estimated t ime of death until the carcass n o longer provides a f eeding opportunity. For all medium-sized adult and smaller carcasses, the l atter point corresponds exclusively to the complete consumption of a ll edible t issues. For adult buffalo and l arger carcasses, s cavenging opportunities ceased to exist when decomposition o f remaining t issues i nhibited vertebrate carnivores f rom deriving f urther nourishment f rom t he carcass. S ince I was not always able to observe the end of a carcass's r esource l ife, the data in Table 4 .1 are a s ubsample of t he total carcass s ample. Also contained in Table 4 .1 i s the weighted average of the estimated l ive weights of the c arcasses comprising each s ize/age c lass.

one and

The average persistence of carcasses increases f rom hour in small s ubadults to 2 72 hours in adult giraffe elephant, well after the onset of decomposition. Small

adult and medium-sized j uvenile carcasses persist f or l ess than s even hours. This i s at l east t hree t imes shorter than medium-sized adult persistence, which on average i s approximately one day. Adult buffalo persisted f or a n average of 1 00 hours, the approximate t ime d ecomposing microorganisms and maggots r equire to become well established i n a carcass. 48

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F igure

4 .1:

C arcass

s ize a nd p ersistence.

s ize g roup a re s hown. a

=

1.2,

o small

b

=

1 .1,

r

M ean v alues

f or e ach

For t he r egression l ine d rawn =

. 99.

s ubadults

•

s mall a dults

f l

medium-sized j uveniles

•

medium-sized adults

A

l arge a dults

l o g p e r s i s t e n c e ( h i s )

very l arge adults

1 0

1 000

1 00 l og l ive w eight

5 0

( kg)

1 0000

Figure 4 .1 s hows the r elationship between c arcass s ize and persistence t o be nearly l inear and highly c orrelated on a double l ogarithmic s cale. Logarithmic transformations of s ize and persistence are appropriate because r ates of consumption decrease exponentially a s progressively l ower yielding and harder to extract tissues are c onsumed, and because the allometry of weights of anatomical parts are c ommonly best d escribed by l ogarithmic f unctions. The s trong correlation between average carcass s ize and persistence obscures many important variations in persistence within any one s ize c lass ( see s tandard deviations i n T able 4 .1). For the moment I am particularly concerned with i llustrating those variations c aused by the type of consumer r esponsible f or the i nitial s tages of carcass consumption. For example, the average persistence of medium-sized adults f rom the S erengeti consumed i nitially by hyenas i s 8 .2 hours ( n = 1 8, SD = 6 .3). Occasionally I observed hyenas f inishing an adult wildebeest or z ebra i n l ess than an hour. On the other hand, medium-sized adult carcasses i nitially consumed by l ions and only s ubsequently f inished by hyenas persisted f or an average of 3 4.5 hours ( n = 3 0, SD = 3 3.2). The s ignificantly l onger persistence of l ion k ills ( Mann-Whitney U -test, p < . 00001) i s partly due to s lower rates of consumption and smaller f eeding group s izes than s een among hyenas, but primarily r esults f rom the i nability of l ions to extract marrow and most head contents. These parts persist on average f or more than a day until they a re l ocated and s cavenged by hyenas. The manner by which hyenas reduce carcass p ersistence, and s cavenging opportunities in general, will b e f ully d iscussed i n the s ection on competition. Other f actors effecting carcass persistence will be also be d iscussed. Suffice i t t o s ay f or now that carcass s ize i s a f undamental determinant of c arcass persistence, but one whose effect i s greatly modified by the ecological s etting of the carcass.

Carcass

S ize

and Completeness

Most carcasses in the S erengeti and Ngorongoro are eventually completely consumed by vertebrate carnivores. Of the 2 32 carcasses f or which s uch data are available, only 1 0 ( 4.3%) were n ot completely c onsumed, being allowed to decompose; most o f these were l arge c arcasses ( Table 4 .2). No small s ubadult or small adult carcasses were observed to decompose, being completely consumed well before maggots and microorganisms became e stablished. Only 4 % of medium-sized j uveniles and 2 % of medium-sized adults were not completely c onsumed by vertebrate carnivores. On the other hand, 4 3% of the adult buffalo and a ll of the very l arge adult carcasses f ound r emained with e dible t issues that

eventually d ecomposed.

5 1

Table 4 .2: Percentage of carcasses of each s ize class observed to decompose. Carcasses a re excluded f or which persistence was known to be unusually l ong, but which could not be confirmed to have decomposed or to h ave been completely consumed by vertebrate carnivores.

Carcass S ize Group

Total

n

C arcasses n

Decomposing %

Small

s ubadults

2 4

0

0 .0

Small

adults

1 6

0

0 .0

2 6

1

3 .8

1 56

' 3

1 .9

7

3

4 2.9

3

3

1 00.0

Medium-sized

j uveniles

Medium-sized

adults

Large Very

adults l arge

adults

5 2

The rarity of incomplete carcass consumption does not d enote minimal s cavenging opportunities or their r estriction to o nly the l argest carcasses. Many carcasses s urvive two or more abandonments by l ions and/or hyenas, the frequency of which i s partly s ize-dependent ( Table 4 .3). Less than 2 5% of medium-sized adult and smaller c arcasses survive two or more abandonments, while all l arge a dult and l arger carcasses do. These data i ndicate that a s maller proportion of parts of l arger carcasses will be c onsumed in any one f eeding bout. More importantly, they s how that some medium-sized adult and smaller c arcasses s urvive initial and s econdary consumption, and are therefore available to s cavengers. A more detailed understanding of the s ize-dependency of scavenging opportunity i s provided by data on carcass c ompleteness on abandonment by initial consumers. Comparisons are n ecessarily restricted to carcasses f ed on by the s ame initial consumer taxon. Table 4 .4a-d and F igure 4 .2a-d show the mean percentage indexed completeness of variously s ized carcasses f ed on initially and only by l ions, hyenas i n a s ingle f eeding bout, vultures, and cheetah.

r ange

Lions were observed of carcasses, and

to to

f eed f rom the l argest s ize do s o with a completeness

dependent on carcass s ize. The mean indexed completeness of the total carcass on abandonment by l ions i ncreases f rom approximately 1 2% f or small subadults to almost 7 0% f or l arge adults ( Table 4 .4a, Figure 4 .2a). Although l ions were known to have f ed on one of the very l arge carcasses in my s ample, their effect to the exclusion of other consumers was

not

observed.

Virtually a ll viscera are consumed by l ions r egardless of carcass s ize. Lower viscera, s urrounded by deposits of mesenteric f at, are almost always completely eaten f irst. Upper viscera a re consumed s omewhat l ater ( refer to Table 3 .4) ‚ and occasionally, a s r eflected by the small amount of visceral remains of medium-sized adults i n Table 4 .4a, a s ingle

l ung

i s

n ot

consumed.

The remaining carcass components ( flesh, marrow, head c ontents) show s imilar trends toward i ncreasing completeness on abandonment with increasing carcass s ize ( Table 4 .4a, F igure 4 .2a). Negligible amounts of f lesh are abandoned by l ions on the three smallest carcass s ize c lasses. Medium-sized adults are typically abandoned with f lesh remaining o n the head and the distal r adius-ulnae and d istal tibiae. B uffalo adult carcasses usually s urvive l ion f eeding bouts with f lesh on much of the anterior half of t he body. This i s the case even when l ions r emain with the c arcass f or s everal days to obtain s everal meals f rom i t.

( with

As s tated i n Chapter 3 , marrow bones and head c ontents the exception of nasal pulp) were n ever observed to 5 3

Table 4 .3: S ize-specific proportion of carcasses s urviving a s f ood resources a fter two or more abandonments by l ions and/or hyenas.

C arcass S ize Group

Total

n

C arcasses Surviving Multiple Abandonments n

Small

s ubadults

2 4

3

1 2.5

Small

adults

1 6

3

1 8.8

2 6

5

1 9.2

Medium-sized

j uveniles

Medium-sized

adults

L arge Very

adults l arge

adults

1 56

3 8

2 4.4

7

7

1 00.0

2

2

1 00.0

5 4

i

c-

r -1

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0

H r d

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0 0

O J 4 4

0 E i

H H

N

Lf l

N O H

c a r c a s e s

0 ) ( L ) U ) C

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r d U )

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L

H

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5 5

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C H E T A H O N L Y

W

Figure

4 .2:

different

Size-specific

i nitial

V = viscera, carcass.

4 -

F

=

completeness

consumers. f lesh,

small

subadults

f -

small

adults

k

medium-sized

M

of carcass

consumption

Accompanying data appear =

marrow,

=

Table

head Contents,

T

by

4 . 4a-d . =

total

HC

T

medi um - s ized adults _

juveniles

HC

in

_

_

0

_

_

0

l arge

Q

v ery

_

adults large

adults

a )

l ions

c )

hyenas

b )

vultures

d )

cheetah

1 00

F

5 6

M

b e consumed by l ions on medium-sized adult and l arger c arcasses ( Table 4 .4a, F igure 4 .2a). Marrow bones are only i nfrequently c onsumed f rom medium-sized j uveniles, being r estricted usually to upper l imb bones. L ions occasionally e xpose the brain of j uvenile wildebeest and z ebra, but a side f rom nasal pulp, do not c onsume other h ead c ontents o f this s ize c lass. Data i n Table 4 .4a s how l ions to c onsume marrow bones to a s imilar extent i n s mall s ubadult a nd small adult carcasses. This apparently violates the e xpectation of r elatively incomplete l imb bone c onsumption i n subadults owing to l ow f at c ontent ( see Chapter 3 ). C ompared to h ead contents, however, the c onsumption of m arrow bones i s much l ess complete i n s mall s ubadults than i n small adults. Head contents of medium-sized j uveniles a re also, on average, more completely c onsumed by l ions t han marrow bones. The r eversed s equence of marrow and head contents c onsumption i n a dult and s ubadult carcasses will be s een to a pply to hyena c onsumption patterns a s well. The pattern c an be a scribed to both nutritional and physical c auses: marrow bones of adults are preferred f or their h igh f at, h ence high energy content over l ower yielding a nd harder to e xtract h ead c ontents. The r anking of carcass components i s s witched i n s ubadults and particularly n eonates owing both t o the l ow to n il f at content of marrow bones and the r elative ease by which the brain i s exposed. Despite important d ifferences i n the c omponents c onsumed, there r emains a s trong s ize d ependency i n the c ompleteness of total carcass consumption by l ions. The amount of viscera and f lesh consumed by l ions appears to d epend on s atiation l evels, which, a ssuming constant f eeding group s ize, are r eached a t an earlier s tage of the consumption s equence i n l arger carcasses. The amount of marrow and head c ontents consumed by l ions i s a lso a f unction of s atiation, but additionally of their extremely l imited ability to expose these t issues. H ence, the expected preference f or marrow bones over h ead contents s een f or l ions on small adults i s not r eplicated f or the medium-sized adult c ategory s imply because of the l ion's i nability to c rack marrow bones of adult wildebeest and zebra. That l ions are necessarily l imited t o f lesh-eating on medium-sized adult and l arger c arcasses i s l argely responsible f or the h igh completeness by which within-bone t issues a re abandoned t o s cavengers. Vultures a lso s eem to be c onstrained by the mechanics of l arger carcasses and a s a r esult produce t he e xpected r elationship between carcass s ize a nd c ompleteness on a bandonment ( Table 4 .4b, F igure 4 .2b). Vultures c onsumed all viscera on the f ew naturally d ead medium-sized j uveniles and adults on which they were o bserved a s i nitial consumers, but were able t o gain a ccess to only approximately h alf of the visceral parts of the two very l arge adult c arcasses. The r emaining viscera were all

5 7

i naccessible upper viscera. A s imilar trend is s een with f lesh. Very l ittle f lesh, r estricted t o the l ower l imbs, i s a bandoned on medium-sized j uveniles a nd adults; almost three-quarters of the f lesh parts of very l arge c arcasses, however, c annot be exposed by vultures f eeding alone. R egardless of c arcass s ize, vultures c annot expose tissue within bones. Vultures are therefore effective f lesh consumers on all but the l argest c arcasses, but will always a bandon marrow and head contents to o ther s cavengers. The c ompleteness by which hyenas a s i nitial c onsumers a bandon carcasses of d ifferent s ize i s s hown in T able 4 .4c and F igure 4 .2c. Although hyenas were i nvolved w ith the c onsumption of c arcasses f rom a ll s ize c lasses, I only observed them a s i nitial consumers t o abandon c arcasses f rom the three s ize groups s hown. Unlike l ions and vultures, h yenas do not show a c onsistent s ize-dependent completeness of carcass c onsumption. The expected relationship holds to a slight d egree between medium-sized j uveniles and medium-sized adults. H ence, hyenas were always observed to consume a ll viscera and f lesh of j uvenile wildebeest and z ebra, and t o consume virtually a ll head contents a s well. Hyenas often l eave small portions of viscera and f lesh o n adult wildebeest and z ebra, and s ubstantial amounts of head c ontents, i ncluding the brain i n 9 of the 3 6 cases. Small s ubadults, however, are abandoned by hyenas with a higher average completeness than l arger c arcasses. The d eparture f rom expectation i s partly a r eflection of the higher c ompleteness by which nutrient-poor marrow bones of young animals are abandoned ( which i n both s ubadult categories i s h igher than head content completeness, a s expected ) Small s ubadults, however, are also abandoned with more f lesh than the two l arger carcasses c lasses. .

The l ack of a s ize-dependent c ompleteness o f c onsumption by hyenas i s a r eflection of their unconstrained c onsumption of virtually a ll c arcass parts of even the l argest carcasses ( Coe 1 980, personal observations) The extent t o which hyenas c onsume a c arcass, r ather, s hould depend exclusively on s atiation l evels and the number of hyenas f eeding. T able 4 .5 shows the average f eeding group s ize of h yenas on their k ills. A n average of 2 .2 hyenas f ed on e ach s mall s ubadult c arcass, while the values f or medium-sized j uveniles and a dults was 1 2.1 and 1 0.9, r espectively. The s izes of hyena f eeding groups a re r elated to the t ypes of prey available ( Kruuk 1 972). Juvenile and adult wildebeest a nd z ebra were usually observed t o be f ed on by a whole c lan, a s c apture of these prey usually r equires a cooperative group effort. Foraging f or easily captured very young prey, whether cached gazelle f awns or n eonate wildebeest, i s an i ndividual pursuit, a nd the c arcass i s quickly c onsumed by a s ingle individual or a pair of hyenas. .

5 8

Table 4 .5: Average number of hyenas or l ions f eeding a s initial consumers on carcasses of different s ize and age. Number of carcasses f or which f eeding group s izes could be determined are l ower than the actual number of carcasses known to have been consumed initially by each of the two consumers. The n umber of l ions f eeding excludes very young cubs; half-grown cubs each count as one half of an adult. All hyenas are c ounted equally, a s very young i ndividuals do not appear at carcasses. Compare average group s izes f or each carcass s ize c lass with the completeness of consumption shown in Table 4 .4a f or l ions, and Table 4 .4c f or hyenas.

Carcass S ize Group

HYENAS ONLY mean # 1 n carcass

LIONS ONLY mean # 1 n carcass

Small

subadults

5

2

4 .0

Small

adults

0

1

1 .0

5

6 .8

Medium-sized

juveniles

Medium-sized

adults

Large Very

adults l arge

adults

2 .2

7

1 2.1

3 3

1 0.9

1 8

0

1

0

0

5 9

6 .7 1 6.0

T able 4 .5 a lso s hows the s ize-specific average number of l ions f eeding per c arcass. L ion f eeding group s izes a re d irectly proportional to carcass s ize, but i nversely r elated to the completeness of c onsumption. These data s upport the s uggestion that mechanical constraints imposed by the carcass are a major d eterminant of the completeness by which l ions consume carcasses, with s atiation p laying a s maller r ole. The completeness by which cheetah consume carcasses i s s hown i n Table 4 .4d a nd F igure 4 .2d. Although I was able t o observe the effect of s olely cheetah consumption on only one s ize class, small adults, these data are presented t o i llustrate the importance of r elative consumer t o carcass s ize on the completeness of consumption. The r eader c an c ompare the completeness of cheetah consumption o f Thompson's gazelle adults ( Table 4 .4d) t o that by t he l arger l ion on comparably s caled-up medium-sized adults ( Table 4 .4a) A general s imilarity i n completeness of each c arcass component, and among the r atios of all c omponents f ed on by the two f eu ds, i s apparent. This i ndicates carcass s ize i n r elation t o consumer s ize to b e a good predictor of c arcass part completeness on abandonment b y c arnivores with s imilar r elative extractive potentials. I t a lso s upports the s uggestion that t he physical principles underlying the c ompleteness of carcasses encountered b y s cavengers in modern s ettings will a pply to the past. .

S ummary of Carcass

S ize

and

S cavenging Opportunity

I have d emonstrated c arcass s ize to be a s trong determinant of the completeness of c onsumption by initial c onsumers and hence of the s cale of f eeding opportunity encountered by s cavengers. Larger c arcasses persist longer and will more f requently s urvive multiple abandonments b y c onsumers. I f f ed on by c onsumers contrained b y obstructions to edible parts, a l arger carcass will also b e abandoned more complete. Hyenas are n ot s o c onstrained, b ut r ather consume at l east medium-sized adult and smaller c arcasses to an extent d ependent o n f eeding group s izes. Although data a re not available o n the completeness b y which hyenas alone consume adult buffalo a nd larger c arcasses, they were observed a s s econdary, s cavenging consumers of g iraffe and e lephant c arcasses to abandon most marrow bones a s well a s s ubstantial amounts of f lesh. This i s to be expected on the basis that hyena f eeding group s izes and appetites are f inite a nd l ower than t he huge quantities of f ood provided by very l arge c arcasses.

6 0

COMPETITION AND

C ompetitors

and Their

S CAVENGING OPPORTUNITIES

Effect As

S cavengers

The major s cavengers in the S erengeti and Ngorongoro, a nd in African s avanna woodlands in general, are hyenas and vultures. Jackals are present as s cavengers i n small n umbers ( usually l ess than f ive) at most carcasses, but t heir impact on the r eduction of carcass completeness was n ever observed t o be measurable. Lions are known to be r egular s cavengers especially of hyena k ills, usually u surping these s oon after the kill i s made ( Schaller 1 972, Kruuk 1 972) However, I observed l ions to s cavenge only .

three times, a ll f rom cheetah k ills of Thompson' s gazelle. I n l ight of the l ion's propensity to s cavenge, c ertainly s ome carcasses I discovered being f ed on by l ions with hyenas waiting t o s cavenge were actually hyena kills s tolen by l ions. If this were the case, the l ow degree of d isarticulartion and bone damage that clearly distinguishes l ion from hyena meals indicates l ions to have s cavenged the carcasses very e arly in the consumption s equence. Hence, a ll carcasses l abelled " lion kills" were, if not actually l ion victims, f ed on primarily by l ions and s how the consumption pattern typical of true l ion kills. The s cavenging abilities of hyenas and vultures can be evaluated by examining their effect on abandoned l ion kills. Lions were r esponsible f or the initial consumption of 4 4% of all c arcasses f or which I could identify the i nitial consumer. On all but the smallest carcasses, l ions always abandon s ubstantial r emains to s cavengers. Cheetah and vultures, t oo, abandon many edible parts, but their uncommonness a s initial consumers ( 10% and 5 % of the sample, r espectively) r enders them an i nfrequent contributor to s cavenging opportunities. Hyenas are f requent initial consumers ( 38% of the total s ample), but usually reduce c arcass completeness to very l ow l evels s ave in s ome c ircumstances to be d iscussed l ater. The total and component part completeness of abandoned l ion kills s cavenged by hyenas and/or vultures are s hown in Tables 4 .6 through 4 .9 and Figure 4 .3a-d f or three carcass s ize groups. T hese include medium-sized j uveniles, medium-sized adults and l arge adults. I nsufficient data are available to show the effect of s cavengers on other carcass categories. For c omparison, data presented previously are included on the completeness of i nitial consumption by l ions, and of c arcasses initially eaten by the s cavenger being examined. I t should be noted that the individual carcasses comprising the s cavenged s ample of each t able are all not necessarily a s ubset of those i nitially consumed by l ions. Some l ion kills were not discovered until they had been or were being s cavenged. These could nonetheless be unambiguously i dentified as l ion kills or carcasses usurped very early by l ions due to the presence of f ull-bellied 6 1

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l ions i n the i mmediate vicinity, or by the degree of bone modification and d isarticulation exhibited by the c arcass. The data i n Tables 4 .6 4 .9 therefore do n ot describe the exact r eduction i n carcass c ompleteness by s cavengers over t hat done by l ions. -

The extent to which hyenas s cavenge abandoned l ion k ills i s s hown i n Table 4 .6 a nd F igure 4 .3a f or medium-sized j uveniles, and Table 4 .7 and F igure 4 .3b f or medium-sized adults. Hyenas u sually c ompletely scavenge the s ubstantial remains of abandoned l ion k ills of both s izes. T otal c arcass c ompleteness i s r educed f rom approximately 4 0% and 5 5% a s abandoned by l ions, to 4 % after hyena s cavenging f or both medium-sized j uveniles a nd medium-sized adults, r espectively. Marginal-yield h ead content parts, i ncluding mandibular and f rontal pulp, dominate t he parts of medium-sized adults i gnored by s cavenging hyenas; v irtually all marrow and any v iscera a nd f lesh abandoned by l ions are s cavenged. Hyenas also completely s cavenge viscera and f lesh of medium-sized j uveniles, a nd, a s expected, r educe the completeness of head contents to a g reater extent than marrow parts. I t i s i nteresting to note that the absolute completeness values of component c arcass parts, and the r elative d ifferences among the c omponents are very s imilar between the hyena-scavenged s ample and t he hyena-hunted s ample f or medium-sized adults, and v irtually i dentical f or medium-sized j uveniles. Hyenas, as p redators o r s cavengers, c onsume carcasses i n a s imilar manner, a nd one that virtually obviates a f eeding opportunity f or other s cavengers. The r eduction i n y ield of m edium-sized a dult l ion k ills by s cavenging vultures i s much l ess s evere t han that by hyenas ( Table 4 .8, F igure 4 .3c). ' The small amounts s cavenged by vultures are l imited t o the already small quantities of viscera and f lesh abandoned b y l ions. Vultures consume about half of the f lesh parts abandoned b y l ions, l eaving f or other s cavengers f lesh s cattered i n s mall pockets beneath s kin of the f orequarter, u sually o n the d istal radius-ulnae. As with h yenas, the c onsumption patterns of vultures a s i nitial consumers a nd s cavengers o f l ion k ills are very s imilar. Unless confronted with a c omplete natural death the i mpact of vultures o n r educing s cavenging opportunities f rom medium-sized a dults i s u sually minimal. The combined effect of vultures and hyenas s cavenging l ion k ills of adult buffalo ( Table 4 .9, F igure 4 .3d) i s s imilar i n k ind t o those f or medium-sized adults. T he r eduction i n t otal carcass y ield by vultures only i s greater f or l arge adult c arcasses owing to t he larger amount of f lesh a bandoned by l ions on this s ize c lass. T he f lesh not s cavenged by vultures i s that protected by t he thick s kin of the buffalo, i ncluding, typically, c ervical f lesh a nd d istal l ower l imb f lesh. O n those l ion k ills o f a dult buffalo s cavenged by hyenas a nd vultures, total 6 8

c arcass c ompleteness i s reduced f rom almost 7 0% to nearly 2 5% due to hyenas s cavenging most marrow bones. [ The c ompleteness of f lesh is higher than that after s cavenging b y vultures alone because different carcasses are i ncluded i n the two s amples.] Note again how, as in all adults, h yenas concentrate on marrow to the virtual neglect of head c ontents. Hyenas s cavenging a s econd t ime ( usually the next d ay) f inish the r emaining marrow and r educe head content c ompleteness by almost a third, l eaving mandibular pulp i ntact i n all three cases, maxillary pulp b rain and f rontal pulp in only one case.

in

two,

and

the

In s ummary, hyenas are effective s cavengers on all c arcass s izes e xamined. As s cavengers t hey typically a bandon n egligible remains to other s cavengers, l imiting t hese usually t o head contents. Vultures, on the other h and, can s cavenge virtually all r emaining f lesh, but their i mpact on s cavenging opportunities i s l ower owing to the a lready l ow quantities of f lesh abandoned by initial c onsumers. Their greatest impact would be f elt on those n atural d eaths t o which vultures have early access, but h ere too, their l imited ability to expose tissue beneath s kin renders their s cavenging s low and, on l arge carcasses, i ncomplete. I n a nalyzing the effect of competition on s cavenging opportunities, i t i s therefore most r elevant to c oncentrate on c ompetition as applied by hyenas, and to i dentify those e cological contexts within which s cavenging p ressure by hyenas i s variable. The s eries of contexts to b e examined i nclude those based on 1 ) ecosystems d ifferences in predator to prey ratios, 2 ) s easonal d ifferences i n s cavenger to carcass ratios, and 3 ) micro-habitat d ifferences in the intensity of s cavenging p ressure applied by hyenas.

P redator

to

Prey

R atios

and

Scavenging

Opportunity

E xpectations and General Observations: A high ratio of predators to prey in a given area should correspond d irectly to h igh s cavenger to carcass ratios. This i s particularly true of Ngorongoro Crater, which s upports one of the highest b iomasses of mammalian predators in Africa, a nd a predator t o prey ratio at l east f our times higher t han that i n the more typical S erengeti ( see Chapter 2 ). H igh predator to prey ratios do r esult in high rates of predator-related mortality, but the t endency of most predators, particularly hyenas, to s cavenge results in a v ery high degree of competition f or k ills. Per-herbivore predation rates i n the S erengeti, on the other hand, are r elatively l ow, a nd much mortality ( estimated at 7 0% by H ouston, 1 979) i s related to s tarvation or disease. Given r elatively f ewer hyenas and more deaths, competition f or c arcasses i n the S erengeti should be much l ess i ntense than i n Ngorongoro.

6 9

This expectation i s s upported by the uncommonness o f vultures i n Ngorongoro i n general, a nd a t carcasses there in particular. Vultures were i nvolved i n the consumption o f only one of 2 1 medium-sized adult c arcasses observed i n Ngorongoro, but on 6 5% of those i n the S erengeti. V ultures are easily dominated and excluded b y two o r more h yenas a t a c arcass, s uch that the exclusive s cavenging adaptation o f vultures i s viable only i n contexts where c ompetition with hyenas i s s ufficiently l ow to p ermit their f requent exclusive access to carcasses during a t l east a p ortion o f the c arcasses' r esource l ife. The s mall number of v ultures i n Ngorongoro attests to the r arity of these c onditions there, while l ow hyena to carcass r atios i n the S erengeti permit vulture numbers to be extremely h igh ( see Houston 1 972, 1 979 f or excellent discussions on the e xclusive s cavenging adaptations of S erengeti vultures). Carcass Persistence i n the S erengeti and Ngorongoro: The d ifferent predator t o prey ratios i n Ngorongoro a nd t he S erengeti are a ssociated with d istinct d ifferences in the duration over which carcasses persist a s f ood r esources. Table 4 .10 s hows the mean persistence in hours of t he three s ize classes of carcasses common t o the Serengeti a nd Ngorongoro s amples. Among medium-sized adults, the carcass type observed most f requently in both ecosystems, c arcasses persisted almost f our t imes l onger i n the S erengeti than i n Ngorongoro. The one buffalo c arcass d iscovered in t he Crater persisted f or l ess than half the average t ime of t he l arger s ample f rom the S erengeti. Medium-sized j uveniles are s hown t o persist f or approximatley the s ame a mount o f t ime i n both ecosystems. The r easons f or this are u nclear, but could well be an artifact o f the method u sed t o estimate t ime of death, which i n both ecosystems was assumed to be midnight. G iven t he pattern s een among medium-sized adults, i t i s probable t hat most medium-sized j uvenile c arcasses d iscovered unfinished i n the C rater were k ills of the hour j ust before daybreak. S imilar b iases may be operating f or the l arge carcasses, but their overall l onger persistence than smaller c arcasses ( Table 4 .1) r educes the impact of this s ource of e rror. T he longer persistence of c arcasses i n the S erengeti c learly r esults f rom i ts l ower hyena to c arcass r atios, and the c onsequent r educed rate a t which i ncompletely c onsumed carcasses a re encountered by hyenas. I n Ngorongoro, on t he other hand, I never observed a c arcass to he a bandoned by n on-hyaenid i nitial consumers without hyenas b eing present t o s cavenge immediately most i f not a ll of the r emains. With f ew if a ny partially eaten c arcasses in Ngorongoro experiencing a n i nterval of unattendence by hyenas, carcass p ersistence will always be r elatively s hort. Carcass Completeness i n the S erengeti a nd Ngorongoro: A very s trong impression f rom observations i n Ngorongoro was the extreme r arity of hyenas f inally abandoning e ven t he most marginal head contents. I n the S erengeti, on t he other hand, these and o ther c arcass parts were c ommonly o bserved

7 0

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to s urvive one or two hyena f eeding bouts. The impression i s s upported by the high proportion of carcasses ( 65%, or 1 7 of 2 6) I observed to be c ompletely consumed i n Ngorongoro compared to the much l ower proportion ( 23%, or 6 2 of 2 06 carcasses) f or the S erengeti. I t i s a lso c onsistent with the drastically l ower completeness of carcasses on f inal inspection in Ngorongoro ( Table 4 .11). The mean total completeness values f or three carcass s ize classes on f inal inspection in Ngorongoro are a ll near or below 5 %, indicating the r emains to be r estricted usually to f rontal and mandibular pulp. C ompleteness values on f inal inspection in the S erengeti are comprised, on average, not only of all head contents, but s ubstantial amounts of marrow a s well. Both indications of relatively thorough carcass consumption in Ngorongoro, however, are b iased f or methodological reasons directly related to the a ctual much more rapid consumption of carcasses i n Ngorongoro. During any morning' s observations in the Crater, I was much more l ikely to observe complete carcass c omsumption. S ituations arose in the Serengeti, on the other hand, that prevented me f rom monitoring the actual f inal abandonment of carcasses s imply because of their l ong persistence and the a ssociated l ong intervals during which i ncompletely consumed carcasses were unattended by s cavengers. Hence, the differences in the overall c ompleteness of carcass consumption presented above are artificially divergent, a nd to an uncertain degree more s imilar. The data serve only to corroborate the evidence presented above s hort carcass persistence in Ngorongoro.

f or

extremely

There i s, however, a more accurate indication that carcasses are ultimately abandoned i n a l ess complete s tate in Ngorongoro than the S erengeti. I nventories of the body parts represented by weathered bones distributed on the l andscape were taken on s everal e xhaustive transects i n Ngorongoro and the S eronera s tudy area of the S erengeti. These data are s imilar in kind t o those collected by Behrensmeyer ( Behrensmeyer 1 983, B ehrensmeyer and Boaz 1 980) in Amboseli National Park. They s how the d iversity of s keletal elements s urviving hyena gnawing to be h igher i n the S erengeti than in Ngorongoro. For i nstance, c omplete or epiphyseal l imb elements account f or 2 5% ( 49 of 1 94) of a ll weathered elements encountered on the S eronera t ransects, but only 1 2% ( 22 of 1 86) of those i n Ngorongoro. At l east one epiphysis of a l imb bone i s destroyed by hyenas when extracting marrow. The l ow occurrence of l imb epiphyses i n Ngorongoro indicates that carcass c onsumption proceeded past the marrow bone s tage of the consumption s equence more f requently than i n the S erengeti. Among crania, typically the l ast parts destroyed by hyenas, only 2 4% ( 7 of 2 9) remained with the brain case i ntact i n Ngorongoro while 5 2% ( 12 of 2 3) d id in S eronera. These data confirm what my observations on f resh carcasses c ould not, namely, that carcasses are f inally abandoned by v ertebrate carnivores i n 7 2

T able 4 .11: Total carcass percentage i ndexed completeness on f inal inspection in the S erengeti and Ngorongoro. C ompleteness values do not necessarily correspond to the quantity of t issues f inally abandoned by vertebrate c arnivores. The values do show the greater l ikelihood of c arcasses of each s ize group being observed to be c ompletely consumed in Ngorongoro than i n the S erengeti.

C arcass S ize Group

TOTAL CARCASS SERENGETI n mean + S . D.

Medium-sized j uveniles

2 2

3 6.3

±

3 9.3

Medium-sized adults

1 35

2 9.6

±

3 7.2

1 8.6

±

6 .5

Large

adults

6

7 3

COMPLETENESS NGORONGORO n mean + S . D.

4

2 1

1

5 .5

+

5 .5

3 .5

±

8 .6

4 .0

a more complete Ngorongoro.

condition

i n

t he

S erengeti

t han

i n

Carcass completeness on abandonment by t he s ame c onsumer or l ike s eries of consumers s hould differ between the two ecosystems only if the number of consumers, hence group appetites, differ. Kruuk ( 1972) notes significantly l arger average hyena f eeding group s izes in Ngorongoro than i n the S erengeti. For wildebeest a nd z ebra carcasses, Kruuk f ound average f eeding group s izes to be less than 1 5 individuals in the Serengeti, a nd more than 2 0 i n Ngorongoro. On the carcasses I observed, however, t he range of hyena f eeding group s izes between the two ecosystems was s imilar, and, given my sample, their average s ize was s tatistically indiscernible ( Table 4 .12). The average f eeding group s ize of hyenas a s initial consumers o f medium-sized adults i s, in f act, l arger i n the S erengeti, while the average s ize of f eeding groups of h yenas a s s cavengers i s only s lightly l arger in Ngorongoro. The average hyena f eeding group s ize on a ll medium-sized adults f or which data are available i s j ust over ten i ndividuals in both the S erengeti and Nqorongoro. The s light differences in hyena f eeding group s izes observed correspond directly to t he completeness by which c arcasses were observed to be consumed by hyenas ( Table 4 .13). The completeness of medium-sized adult l ion kills s cavenged by hyenas i s s lightly h igher i n Ngorongoro than in the S erengeti ( Table 4 .13), but this corresponds to a l ower average number of hyenas per carcass in N gorongoro ( Table 4 .12). The completeness on abandonment o f medium-sized hyena kills in the C rater i s slightly lower than in the S erengeti, but this is a ssociated with s lightly h igher hyena f eeding group s izes. A f inal i llustration of the dependency of completeness of consumption o n f eeding group s izes i s provided by episodes of hyena f eeding that permitted a subsequent f eeding opportunity for v ultures. This c ircumstance occurred only once in Ngorongoro but 3 5 times in the S erengeti, and always r esulted i n carcass consumption atypically incomplete f or hyenas. T hese high values of completeness ( Table 4 .13) correspond t o s ignificantly l ower hyena f eeding group s izes ( 6.4 per carcass) than are typical f or the S erengeti ( over 1 0 p er carcass, Table 4 .12). The above carcass group very i ncompletely c onsumed b y hyenas and s cavenged by vultures r eveals a context in t he S erengeti in which competition by hyenas i s l ow and the r esulting f eeding opportunity f or other s cavengers high. The rarity of this context in Ngorongoro i s responsible f or the observation that only 2 4% ( 5 o f 2 1 f or which d ata a re available) of medium-sized adult c arcasses persisted a s f ood resources after hyena f eeding i n Ngorongoro, w hile 6 0% ( 72 of 1 21 carcasses) s urvived hyena f eeding in t he S erengeti. Contexts of l ow s cavenging pressure by h yenas i n the Serengeti has a basis in s easonal variations i n levels

7 4

T able 4 .12: Average f eeding group s izes of hyenas a s i nitial consumers or s cavengers on medium-sized adult c arcasses in the S erengeti and Ngorongoro. No consistent patterning between ecosystems i s s een. S ee text f or a d iscussion of the l ow average hyena f eeding group s ize in t he "hyenas and vultures" category f or the S erengeti. C ompare these d ata to the completeness of c onsumption by hyenas presented in Table 4 .13. No inter-ecosystem d ifferences i n f eeding group s izes within individual c onsumer categories are s ignificant to the . 05 l evel ( Student t-test, 2 -tailed probabilities).

Consumer ( s)

H yenas

only

Hyenas l ions

f ollowing

n

2 4 7

FEEDING GROUP SERENGETI mean + S . D.

1 0.7

±

7 .0

1 6.0

±

8 .6

Hyenas and vultures

1 1

6 .4

±

8 .6

Total

4 2

1 0.4

±

8 .3

7 5

S IZE NGORONGORO n mean + S . D.

9 1 0

1 1.9

±

8 .6

9 .5

±

1 0.1

0

1 9

1 0.6

+

9 .5

Table 4 .13: Completeness of medium-sized adult carcasses on abandonment by various consumers i n the Serengeti ( A) a nd Ngorongoro ( B) . Compare like consumer categories between the two ecosystems. No differences between ecosystems in total carcass completeness among l ike pairs of consumer types a re s ignificant to the . 05 l evel ( Mann-Whitney U-test). Differences in completeness between ecosystems correspond directly to average hyena f eeding group sizes ( Table 4 .12). The very high completeness of the "hyena and vulture" category in the Serengeti corresponds to low feeding group s izes and a special circumstance of carcass glut ( see text).

MEDIUM-SIZED ADULT PERCENTAGE INDEXED COMPLETENESS: MEAN! ( S.D.) Head Consumer ( s) A ) Hyenas

n

Viscera

Lions

only

f ollowed

hyenas

2 7

1 .9

Hyenas

Lions by

1 4

3 .9 ( 10.9)

0

0

once

Hyenas and vultures

B )

Marrow

Contents

Total

SERENGETI

( 7.4)

by

Flesh

3 5

5 .4 ( 19.0)

8 .7 ( 20.8)

2 6.4 ( 38.7)

8 .0 ( 16.4)

2 .5

1 4.7

2 .4

( 9.0)

( 30.4)

( 6.2)

1 9.1

6 1.4

8 7.9

4 2.3

( 26. 5)

( 37.7)

( 24.3)

( 25.8)

7 .1 ( 19.6)

1 9.4 ( 36.4)

4 .8 ( 10.6)

NGORONGORO

only

f ollowed

9

1 1

0

0

0

0

hyenas

Hyenas and vultures

1

0

0

7 6

1 1.7

2 1.6

( 24.9)

( 34.3)

0

8 7.5

6 .4 ( 12.5) 8 .8

o f natural b elow.

S easonality

mortality

of

and

S cavenging

l ive

prey

abundance,

Opportunity

in

the

as

described

S erengeti

E xpectations: S easonality of rainfall and the l arge-scale d istribution of migratory herds in the S erengeti s hould e ffect l ocal s cavenger to carcass ratios, hence s cavenging o pportunities, i n predictable ways. Levels of rainfall c orrespond on a s easonal basis to l evels of natural mortality among h erbivores, and, on a shorter t erm basis, t o the degree of herbivore concentration and dispersal in a n area. The l arge-scale, ecosystem-wide distribution of m igratory herds i s also s easonal, resting ultimately i n the p eriodicity of r ainfall and concomitant f luctuations i n f orage quality. Seasonal l evels of rainfall are inversely proportional t o the l evel of nutritional s tress experienced by h erbivores. High s tress l eads t o high l evels of natural mortality, such a s that which occurs at the end of the dry s eason. Houston ( 1979) estimates over half of annual h erbivore mortality to occur in the f inal three months of t he dry s eason ( August through October). With the addition of natural mortality to predator-related mortality, s cavenger to carcass ratios should reach their l owest point a t the end of d ry periods, r esulting in glut s ituations when scavengers experience a superabundance of f ood. I t i s a t these t imes when vultures, as exclusive s cavengers, are c onfronted with the highest annual f ood availability, and t he lowest l evels of competition with hyenas. Wet periods, h owever, i nduce the growth of good quality f orage, which, more than water availability, maintains herbivores i n good c ondition ( Sinclair 1 979a). The resulting l ow l evels of n atural mortality during wet periods l eads to a high degree of competition among s cavengers f or the t issues abandoned o n the relatively f ew predator kills that occur. I was unable to monitor the expectedly highest annual l evels of mortality that occur a t the end of the dry s eason i n the northern and western woodlands of the S erengeti e cosystem. S till, the dryness of the short dry s eason and the essential f ailure of the f ollowing l ong rainy s eason i n 1 983/84 enabled me to observe unusually h igh l evels of s tress-related natural mortality on the s outhern plains of t he Serengeti. The s easonal movements of migratory wildebeest, z ebra a nd Thompson' s gazelle produce dramatic s easonal changes i n the availability of carcasses i n any one area of the S erengeti. A g eneral observation in the f ield was the c onsistent association of high carcass densities with high d ensities of migratory herds, r egardless of the amount of r ecent r ainfall. Part of the increase i n carcass availability r esults f rom the influx of mobile,

7 7

non-breeding predators that f ollow h erds i nto an a rea, and the consequent addition of k ills to t hose m ade b y r esidential, breeding predators. S ince mobile l ions and hyenas usually occur i n much smaller groups than r esidential prides and clans ( Schau er 1 972, Kruuk 1 972), the additional kills s hould be l ess c ompletely consumed a nd offer a better opportunity f or s cavengers than k ills b y r esidential predators. As well, the mere presence o f large numbers of herbivores increases the chances of m ortality f rom old age, disease or accident occurring a nd adding f urther to the s cavengeable f ood s upply. I n areas s easonally devoid of m igratory animals, local herbivore numbers are very l ow, c omposed of r esidential h erbivores only. R esidential breeding predators e xperience their most s tressful s eason at t hese t imes ( Hanby a nd Bygott 1 979, Kruuk 1 972, S challer 1 972, S inclair 1 979b). Most deaths are l ikely to be predator k ills and t hese a re l ikely to be thoroughly consumed. Any r emains s hould b e s ubject to very r apid and thorough s cavenging by h yenas a t these t imes. Vultures are well adapted to c onditions of s easonal f luctuations i n carcass abundance. Their efficient f light a llows them to travel l ong distances daily to t rack the movement of migratory herds over t he whole annual round. Vulture to carcass r atios might therefore be expected to b e r elatively c onstant throughout the year c ompared to t he expected variability i n hyena to carcass r atios. T he effect of r elative herbivore abundance on s cavenging opportunities might therefore be expected to be f elt l ess on f lesh, t he f ood of vultures, than on marrow and h ead c ontents t hat a re c onsumed mostly by hyenas. Smaller-scale d ifferences i n the l ocal density d istribution of h erbivores r esulting f rom s hort term variations i n rainfall amounts s hould also effect s cavenging opportunities. R egular r ainfall r eplenishes small temporary pools of water that enable h erbivores to b e d ispersed. Dry conditions, however, quickly reduce water availability to f ew s ources around which herbivores a re f orced to c oncentrate. S uch short t erm a nd small s cale concentration and d ispersal was often observed in t he S eronera s tudy area during periods of i rregular r ainfall. S ince the r anges of r esidential p redators are f ixed b y t erritorial boundaries ( e.g. B ertram 1 979, H anby a nd Bygott 1 979, Kruuk 1 972, S challer 1 972), t he degree of h erbivore concentration s hould be i nversely proportional to t he number of consumers at any one c arcass. A higher l ocal concentration of c arcasses s hould t herefore r esult i n l ess complete c arcass c onsumption, a nd better s cavenging opportunities. The co-effects of r elative r ainfall and h erbivore a bundance i n the S erengeti s hould p roduce l arger v ariations i n s cavenging opportunity than that r esulting f rom any o ne

7 8

effect alone. S cavenger to c arcass r atios and c ompetition f or c arcasses s hould be l owest during dry periods of h igh natural mortality and herbivore c oncentration, and i n areas temporarily s upporting l arge numbers of migratory ungulates. This s ituation characterizes the northern a nd western woodlands during the l ong dry s eason, and the period which I o bserved i n February through April i n the southern plains. Conversely, the h ighest s cavenger t o carcass ratios a nd competition s hould occur during wet periods in areas s upporting only small numbers of residential herbivores, s uch a s during the r ainy s eason i n the woodlands. Houston ( 1979) , a ssuming 2 % annual mortality, e stimated the density of mortality f or the s easonal habi*ats of the S erengeti to be as f ollows: 1 ca cass/33 km o n the plains during the wet s eason, 1 '300 km on the plains during the dry s eason, and 1 /412 km i n the woodlands d uring the wet s eason. Houston does not provide an estimate of carcass densities f or the woodlands in t he dry s eason, but i t must approach i f not exceed that f or the wet s eason on the plains owing to high l evels of natural mortality. Stratifying the C arcass S ample S easonally: Daily r ecords were kept on the i ncidence and i ntensity of s howers i n e ach study area being monitored. The most complete r ecords a re those f rom the i ntensively monitored S eronera s tudy area. In s tudy areas visited i nfrequently and f or s hort durations, s hower data were collected f rom r eports of other researchers r ecently i n the area of interest, and by the conditions I encountered upon visiting the area, i ncluding the presence or a bsence of t emporary pools of water, the color of the grass, and the amount of dust. E ach day of carcass s earch could thereby be c lassified i nto r elatively wet or relatively dry days. Dry days are those f ollowing a week i n which a t most one or two l ight s howers f ell. This amount of r ainfall always proved i nsufficient to produce t emporary s urface accumulations of water. Most days c lassified a s dry were preceded by a t l east one week of no rain producing extremely dry, d usty conditions, which, i f persisting, turned the grass brown. Wet days are those on which a t l east one heavy s hower or s everal c ontinuous days of l ight showers f ell during the preceding week. This l evel o f rainfall usually produced many t emporary pools of water a nd a f lush of green on previously brown expanses of grass. The c lassification of wet and dry days does n ot necessarily correspond to the r ainy and dry s easons. This i s a necessary consequence of the l ack of pronounced s easonality i n r ainfall during the year encompassing the f ield s tudy. S ome dry days occurred within a period of relatively heavy r ainfall, and occasionally wet days occurred i n predominantly dry periods. As a r esult, t he rainfall c lassification used i s n ot h ighly correlated with expected l evels of natural mortality. Of the 2 7 c ertain

7 9

natural deaths encountered i n the S erengeti, 1 9 o ccurred on dry days, but e ight of these were drowned wildebeest. I encountered 8 natural deaths on wet days, which i s n ot s ubstantially l ower than the 1 1 unaccidental deaths observed on dry days. The r ainfall c lassification u sed, however, i s s ensitive to changes i n l ocal density d istributions of herbivores. Such changes are rapid r esponses to the availability of water s ources. Whether the mechanism i s the degree of l ocal herbivore dispersion or the l evel of natural mortality, r ainfall-based variations in s cavenging opportunity s hould have a s imilar effect f or both. The presence or absence of migratory herds i n a s tudy area was a lso monitored on a daily basis. I nfluxes of migratory herds to a s tudy area were extremely c onspicuous, r eaching abundances of tens of thousands of animals i n the r elatively small area. The departure of the migratory herds l eft an area devoid of all but small numbers of Grant's gazelle, topi, hartebeest, buffalo, g iraffe a nd warthog. Days of herbivore abundance were those when v ery l arge c oncentrations of wildebeest and z ebra, whose migratory movements are i n concert, were present. Thompson's gazelle are more dry-adapted than wildebeest and z ebra and h ave s omewhat d ifferent s easonal movements ( e.g. Maddock 1 979). An area s upporting many tommies but f ew or n o l arger migrants was considered to be r elatively devoid o f animals. Most days of herbivore s carcity, however, were those during which only small numbers of r esidential ungulates were present. S easonality of Carcass Availability:. The effect of s easonal variations on the availability of c arcasses i s i ndicated by data i n Table 4 .14. The data d escribe the average number of carcasses discovered per day of r elative r ainfall and/or herbivore abundance. The number of c arcasses d iscovered per day disregards differences i n daily s earching distance, which r anged f rom 3 0 t o 8 0 l inear k ilometers. The data also are not a measure of t he absolute number of carcasses present per day i n a g iven s tudy a rea, a s s urely s ome carcasses, particularly small ones, were never discovered. S till, daily c arcass yields a lmost a lways corresponded to my s ense of the potential availability of carcasses, and the data i n T able 4 .14 a re l argely consistent with the expectations described above. Most carcasses i n the S erengeti were d iscovered on days and i n a reas s upporting l arge c oncentrations of wildebeest and z ebra. An average of 2 .9 carcasses were d iscovered on s uch days of herbivore abundance c ompared to only 1 .1 per day on days of prey s carcity. The d ifference i s h ighly s ignificant. Among days o f h erbivore a bundance, a s ignificantly h igher number of c arcasses were d iscovered on dry days than on wet days. However, r elative amounts of r ainfall alone, according t o my c lassification, d o not s eem

8 0

Table 4 .14: Average number of carcasses d iscovered per day i n the Serengeti during periods of different r ainfall ( dry vs. wet) and/or l ive herbivore abundance ( many vs. f ew), and in Ngorongoro. Using S tudent's T-test, a ll paired d ifferences within the S erengeti are s ignificant to a t l east the . 05 l evel, except f or dry/few vs. wet/few, and a ll wet vs. all dry. Ngorongoro i s s ignificantly d ifferent f rom the whole of the Serengeti, and each of the combined rainfall/herbivore abundance categories except f or wet/few.

S easonal Context

Days of S earching

Carcasses D iscovered

Carcasses/Day mean + S .D.

Dry/Many Dry/Few Wet/Many Wet/Few

2 4 2 2 3 6 4 0

8 4 2 0 9 2 4 8

3 .5 0 .9 2 .6 1 .2

± ± ±

±

1 .9 1 .5 1 .9 1 .1

All Many All Few

6 0 6 2

1 76 6 8

2 .9 1 .1

± ±

2 .0 1 .3

All Dry All Wet

4 6 7 6

1 04 1 40

2 .3 1 .8

T

+

2 .1 1 .7

1 22

2 44

2 .0

+

1 .9

2 5

2 8

1 .1

+

1 .0

Total S erengeti Total Ngorongoro

81

t o effect c arcass availability measurably. The f undamental s easonal-based availability of carcasses i n the S erengeti s eems to be dependent mostly on the presence or a bsence o f migratory herds rather than on t he r elative d ryness o f p eriods preceding days of carcass s earch. Whether t his i ndication of the f undamental availability of c arcasses c orresponds to the a ctual, or r ealized, s cale of s cavenging opportunity, as measured by c arcass c ompleteness a nd persistence, will be evaluated l ater. Table 4 .14 a lso provides a comparison of the average d aily carcass yield in the S erengeti t o that in N gorongoro. Compared to the S erengeti a s a whole, s ignificantly f ewer c arcasses were discovered per day i n Ngorongoro ( 2.0/day v s 1 .1/day, r espectively). D espite the h igh predation r ates o f the Crater, the short persistence of carcasses there i s apparently r esponsible f or the l ow numbers remaining to b e d iscovered on my early morning carcass s earches. I ndeed, i t was a c ommon ( and f rustrating) observation to s ee f ull-bellied l ions and hyenas a t d awn i n the C rater, b ut a bsolutely no trace of the carcass. Note, however, t he s imilarity i n daily carcass yields f rom Ngorongoro and t he S erengeti i n contexts of herbivore s carcity ( both at 1 .1 c arcasses d iscovered per day). I t i s precisely a t t imes o f l ow prey availability i n the S erengeti that competition f or c arcasses i s expected to be the h ighest, and apparently t o a d egree approaching that s een i n N gorongoro. I n s ummary, data on the number of carcasses d iscovered per day s uggest that c arcass persistence and completeness on abandonment s hould be highest i n the S erengeti during periods of herbivore abundance a nd l ow r ainfall. S cavenging opportunities s hould be the poorest during periods of prey s carcity and s imilar to those i n the always highly c ompetitive Ngorongoro Crater. Methodological B iases i n S easonality Data: Carcass persistence data are s ubject to methodological b iases t hat produce r esults i nsensitive to s easonal variations. During t imes of c arcass abundance, only a small percentage o f carcasses could be monitored until consumption was c omplete. Among medium-sized adult carcasses, I observed only 2 3% ( 9 of 3 9) until complete consumption d uring d ry periods of herbivore a bundance, but 5 6% ( 9 of 1 6) during wet periods of herbivore s carcity. These data d o suggest that c arcasses will persist l onger during periods of l ow competition, but they are a t l east i n part an a rtifact o f my tendency to l eave the carcasses before f eeding was complete i n f avor of visiting another carcass i n t he area. S uch an opportunity was more l ikely to a rise, o f course, during periods of carcass abundance. Contributing f urther t o the bias i s the f act that carcass abundance was usually encountered in s tudy areas monitored f or only o ne or t wo days at a t ime ( Naabi-Ndutu and S imba Kopjes) T his reduced my chances of r evisiting a carcass b efore l eaving the s tudy area. Furthermore, the Naabi-Ndutu and S imba Kopjes s tudy .

8 2

areas are primarily open plains habitats devoid of f eatures on the l andscape permitting e asy r elocation of carcasses. Hence, of the 3 0 medium-sized adult carcasses d iscovered during periods of prey abundance a nd r emaining with edible parts on f inal i nspection, 6 3% occurred i n the two plains s tudy areas. At the other extreme, during periods of carcass s carcity ( wet, f ew l ive herbivores present) only one of the s even c arcasses i n this s ample i s f rom outside of i ntensively monitored S eronera. The effect of these biases i s to r educe artificially the observed persistence of the average c arcass during periods of c arcass abundance. The f ew c arcasses o bserved i n this context until the end of their resource l ife were, f or one r eason or another, very quickly c onsumed r elative to the majority of carcasses f or which persistence c ould not be calculated. The available, biased data s how carcass persistence during periods of carcass abundance t o be s lightly l ower than during periods of prey s carcity. I could conceive of no way to correct f or the c ontributing biases. Data on s easonality of c arcass completeness a re n ot s ubject to the a bove biases. C omparisons of carcass completeness are made within groups of l ike consumers that were observed t o a bandon c arcasses with edible t issues. Such abandonments o ccur over a much s horter t ime s cale than does complete c arcass consumption which i s the basis f or measures of p ersistence. I f, owing to l ogistical considerations, the i nitial or l ater a bandonment of a carcass was not observed, that c arcass i s necessarily excluded f rom analyses of c arcass completeness. S easonality of C arcass Completeness: The completeness by which medium-sized adult carcasses are abandoned by l ike consumers i n d ifferent contexts of r ainfall and herbivore abundance i s s hown i n Tables 4 .14 4 .19. The t ables include data on the average number of consumers primarily responsible f or carcass consumption if these c ould be accurately determined. Note again that s ample s izes f or which f eeding g roup s ize was determined a re s ometimes smaller than those f or the c orresponding completeness data because I was not a lways able t o observe c onsumption but could nonetheless unambiguously i dentify the c onsumer(s) responsible. The number of carcass c onsumers was s hown previously i n c omparisons between the S erengeti and Ngorongoro to be an important i ntervening variable conditioning c arcass completeness, a s i t i s s hown to be -

here

i n

d ifferent

s easonal

contexts.

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a bundance s eems t o have a s tronger i nfluence on the amount of f lesh a bandoned than do r elative amounts of r ainfall. Lions tend to abandon c arcasses with s ubstantial quantities of f lesh on the head, d istal l imbs a nd n eck during periods of prey abundance ( Table 4 .15). Prey s carcity, however, i s a ssociated with t he consumption by l ions of virtually a ll f lesh, with the o ccasional exception o f maxillary f lesh and more rarely f lesh on the extreme d istal portion of l imbs. Viscera are also always c ompletely c onsumed a t these t imes, while small amounts r emain during p eriods of prey abundance. Within c ontexts of prey a bundance, s lightly more viscera and f lesh are a bandoned during wet periods. The completeness of f lesh consumption f rom medium-sized adults by l ions s eems to be d ependent i n part on the s ize of t heir f eeding groups ( Table 4 .15). Feeding g roup s ize i s i nversely proportional to t he completeness of f lesh abandoned by l ions f or all f our s easonal c ategories s ave that of prey s carcity during wet periods. The l atter c ontext i s that d uring which f lesh consumption t ends to be most complete, b ut f or which the smallest average group s ize of l ions was observed. The r eason f or this unconformity i s n ot known, but i t i s r esponsible f or the l ack of c orrelation between average f eeding group s ize and f lesh completeness when r elative prey abundance or r elative r ainfall are c onsidered a lone ( Table 4 .15). The l ack of a c omplete correlation i s curious i n l ight of expectations that the presence of migratory h erds i s u sually a ssociated with an i ncreased l ikelihood of encountering l ion k ills made by small groups of mobile, non-breeding males that f ollow t he herds ( cf. S challer 1 972, H anby a nd Bygott 1 979) L ion kills of medium-sized adults during periods of migratory herd a bsence, on the other hand, are more l ikely to be made exclusively by l arge r esidential breeding prides which should more c ompletely consume carcasses r egardless of the amount o f r ainfall and consequent d ispersion of .

prey. Relative prey abundance r ather than r elative amounts of rainfall also s eems t o have a greater effect on the completeness by which vultures s cavenge l ion k ills ( Table 4 .16) The pattern, however, i s r eversed f rom that s een f or l ions alone. Flesh abandoned by l ions i s more completely s cavenged by vultures during periods of prey abundance than when f ew prey a re in the area. I n the l atter c ontext, s cavenged l ion k ills are a bandoned by vultures with f lesh r emaining partially eaten over most of the f orequarters. This pattern c an be explained by the s trong c orrelation between prey abundance and vulture abundance, a phenomenon caused by the a bility of most vultures, n esting or n ot, to f ollow migratory herds over l arge areas. Vultures a re extremely r are i n a reas of the S erengeti s upporting only residential ungulates, and appear a t c arcasses i n very s mall numbers. An i nflux to the area of migratory herds, .

8 5

Table 4 .16: Completeness of medium-sized adult l ion kills after s cavenging by vultures i n different s easonal c ontexts of relative rainfall ( wet vs. dry) and l ive h erbivore abundance ( many vs. f ew). Note the virtual c omplete s cavenging of viscera and f lesh by vultures in contexts of herbivore abundance f rom the substantial amounts l eft b y l ions ( Table 4 .15).

MEDIUM-SIZED ADULT PERCENTAGE I NDEXED C OMPLETENESS: MEAN! ( S.D.) Head Viscera Flesh Marrow Contents Total

S easonal Context

n

Dry/Many

5

0

2 .5

1 00.0

9 5.0

50.6

Dry/Few

2

0

( 3.3) 2 7.1

1 00.0

( 7.3) 9 6.9

( 2.0) 6 1.9

Wet/Many

9

0

Wet/Few

1

0

( 27.1) 5 .7 ( 5.7) 0

9 9.3 ( 1.9) 1 00.0

( 3.1) 9 4.5 ( 5.5) 8 1.3

( 12.5) 52.3 ( 3.3) 4 8.1

4 .5 ( 5.2) 1 8.1

9 9.6 ( 1.6) 1 00.0

9 4.7 ( 6.2) 9 1.7

5 1.7 ( 3.0) 5 7.3

( 7.8)

( 12.1)

9 5.6 ( 6.4) 9 3.1 ( 6.5)

5 3.9 ( 8.6) 5 1.9 ( 3.3)

All Many All

Few

1 4 3

0 0

( 25.6) All

Dry

All Wet

7 1 0

0 0

9 .5 ( 18.5) 5 .1 ( 5.7)

8 6

1 00.0 9 9.4 ( 1.9)

h owever, i s a ccompanied by an extremely conspicuous and d ramatic i ncrease i n vulture numbers. L ion k ills draw l arge g roups of vultures at these t imes, which s wamp and v oraciously c onsume most r emaining f lesh and viscera i mmediately upon abandonment by l ions. In s ummary, the r elatively l arge amounts of f lesh o bserved to be a bandoned by l ions during periods of prey a bundance will be quickly and thoroughly s cavenged by v ultures. Vultures have a l esser impact a s s cavengers i n a reas s upporting mostly r esidential herbivores, but i n this c ontext, r elatively l arge l ion f eeding group s izes r esult i n the i nfrequent abandonment of any f lesh. Unattended l ion k ills of medium-sized adults will therefore provide l ittle t o no f lesh at a ll t imes of the year i n the S erengeti, with f urther f eeding opportunities being r estricted to t issues w ithin bones. Hyena kills of medium-sized adults a re consumed i n a p attern s imilar to that observed f or l ion k ills ( Table 4 .17) Although h yenas always consume their k ills much more t horoughly than l ions, total and c omponent part c ompleteness s how the greatest d ifferences within the c ontext of r elative prey abundance. R elative r ainfall has a s maller but s till measureable effect. During periods of p rey scarcity, c arcasses are abandoned by hyenas with only marginal head contents; hyenas are more l ikely to l eave not o nly head contents and marrow, but s ome f lesh a nd viscera i n contexts of prey abundance. Wet periods, too, witness h yenas abandoning carcasses r elatively complete, while k ills made on dry days are abandoned with l ittle marrow and h ead contents, i n a pattern s imilar t o that s een i n Ngorongoro. This pattern i s unexpected i n l ight of the h igher expected l evels of natural mortality during dry periods. The carcasses comprising the whole s ample i n Table 4 .17, however, were a ll deemed t o be hyena k ills that o ccurred i n S eronera. Natural mortality was not observed i n S eronera to the extent that i t was on t he plains t o the s outh. Prey abundance i n S eronera was u sually a ssociated with relatively wet periods that drew l arge h erds of wildebeest and z ebra f rom the north to the margin of the plains. I n this c ontext, hyena f eeding group s izes were on average small, r esulting i n r elatively i ncomplete c onsumption. .

Figure 4 .4 s hows the r elationship between mean hyena f eeding group s izes and t otal carcass c ompleteness on abandonment. The two variables are i nversely proportional. A s imilar pattern i s s een f or head c ontent a nd marrow completeness a s well. The f igure c learly s eparates total carcass completeness a long l ines of r elative prey abundance, with thorough consumption o ccurring during periods of prey s carcity when average f eeding group s izes of 1 3 or more were observed. Large group s ize i n c ontexts of p rey s carcity i s consistent with the e xpectation that mostly l arge, r esidential c lans will o ccur i n areas

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88

Figure 4 .4:

R elationship b etween m ean hyena f eeding g roup s ize a nd

the c ompleteness abandoned

able d ata a re negative

o f medium-sized a dults a re

s hown f or N gorongoro,

i n t he S erengeti.

a nd more c omplete c arcass c onsumption

s carcity

C ompar-

which c onform well t o t he g ood

c orrelation s een f or t he S erengeti s ample.

group s izes with p rey

b y w hich their k ills

i n d ifferent s easonal c ontexts

i n t he S erengeti.

H igher f eeding a re a ssociated

Accompanying data a ppear i n

Table 4 .17.

2 0

0

dry, many a nimals

•

d ry,

f ew a nimals

f l

w et,

many a nimals

wet,

f ew a nimals

A

N gorongoro

-

n

C 5 6

I

8

I

1 0

A

I

I

1 2

1 4

r

1 6

1 8

mean n umber of h yenas per c arcass

8 9

2 0

2 2

occupied only by r esidential ungulates. With the migration of l arge herds of wildebeest and z ebra into Seronera c ome s olitary or paired mobile hyenas which are l ikely to f eed on carcasses in smaller groups. Average group s izes during periods of prey abundance are under t en, resulting in t he r elatively incomplete consumption of the average carcass. Hyena k ills in Ngorongoro are s hown to be c onsumed by groups of intermediate average s ize, and to be abandoned with a completeness that conforms well to the seasonal pattern described by the S erengeti s ample. The s eason-specific completeness of medium-sized adult l ion kills s cavenged by hyenas i s s hown in Table 4 .18. T he usually minimal s cavenging by vultures ( Table 4 .16) i s i ncluded to an unindicated degree. Greater d ifferences in the i ntensity of hyena s cavenging are a ssociated with relative amounts o f rainfall r ather than relative prey abundance. This pattern is t he converse of that s een f or l ions on their kills, vultures s cavenging l ion k ills, and hyenas on their kills. Hence, the thoroughness of hyena s cavenging i s greatest d uring wet periods, r egardless of the abundance of prey. The complete s eries of marrow bones and head contents abandoned by l ions are virtually a ll s cavenged by h yenas, while d uring d ry periods, hyena s cavenging i s l ess thorough. As with other consumer categories, the thoroughness of hyena s cavenging i s inversely proportional to the number of hyenas i n the s cavenging group. F igure 4 .5 s hows t he relationship f or total carcass completeness only, but i t a lso holds f or the completeness by which marrow and h ead contents are s cavenged. Ngorongoro clusters c losely with the wet s ample f rom the S erengeti, whete s cavenging group s izes are close to or exceed ten i ndividuals, and t otal carcass completeness i s r educed to roughly 6 %. T he pattern i n the S erengeti might be explained by the greater dispersion of prey during wet periods, which produces f ewer l ion k ills per hyena territory, and a l arger number of hyenas to s cavenge each abandoned c arcass. The p attern d oes not s eem to result f rom higher r ates of natural mortality and the consequent l ower competition f or, and c ompleteness on abandonment of, carcasses during dry periods. If t his were the case, one would expect the head c ontents of natural deaths i nitially consumed by l ions to b e preferred over marrow bones by s cavenging h yenas i n a p attern n ot unlike that s een in s ubadults at a ll t imes. T his results f rom the depletion of f at in marrow bones of nutritionally s tressed ungulates which, if s evere enough, l eads quickly t o death ( e. g. H arris 1 945). Hyenas and vultures comprise the f inal set of c onsumers to be considered i n t he analysis o f seasonal variations in c arcass completeness ( Table 4 .19). T his s ample i s drastically d ifferent f rom others involving h yena c onsumption f or a number of r easons. 3 1 of the 3 4 carcasses

90

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F igure 4 .5:

R elationship b etween

t he c ompleteness

b y which

hyena s cavenging g roup s ize and

t hey abandon

k ills

i n d ifferent s easonal c ontexts

d ata

f or Ngorongoro

a re

s hown

i n

amounts o f r ainfall,

adult l ion C omparable

to c onform

c orrelation a mong t he S erengeti s amples. a nd more t horough s cavenging

m edium-sized t he S erengeti. t o the g ood

n egative

L arger s cavenging g roups

a re a ssociated

w ith r elatively h igh

r ather t han prey s carcity a lone.

Accompanying

data a ppear i n Table 4 .18.

0

d ry,

many animals

•

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f ew animals

0

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many animals

w et,

f ew animals

A

N gorongoro

2 0-

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mean number of h yenas per c arcass

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i n this s ample occurred during periods of prey a bundance, and 2 5 were d iscovered on the S imba K opje and N aabi-Ndutu p lains during the very dry period o f J anuary through March when the migratory herds were extremely concentrated. O f t he 2 2 c arcasses f or which the c ause of death c ould b e c onfidently determined, 1 8 were n atural deaths and o nly f our were probable h yena k ills. Most carcasses were d iscovered f irst by vultures and only s ubsequently b y h yenas. These c ircumstances d escribe the t imes o f greatest c arcass abundance encountered during my s tudy p eriod, when a s many a s e ight very complete natural d eaths were d iscovered on any s ingle day of s earching. Carcass a bundance was s ufficiently h igh at t hese t imes t o produce a g lut f or s cavengers, possibly on the order of that expected f or the northern and western woodlands during t he end of the dry s eason. The main consequence of the c arcass g lut is a n a typically h igh completeness by which hyenas abandon c arcasses to other s cavengers. Consumption by h yenas often d id not proceed beyond viscera or s mall a mounts o f h indquarter f lesh. Most of the r emaining f lesh was consumed by vultures, ihose effect on r educing carcass c ompleteness t o the values s een i n Table 4 .19 i s probably e qual to o r greater than that of h yenas. The s mall amount c onsumed by hyenas i s d irectly r elated to a small average f eeding group s ize of approximately f our i ndividuals. I n this c ontext, I f requently observed hyenas f oraging and f eeding either s ingly or i n pairs. The small f eeding group s izes are n ot the r esult of l ow hyena densities, but rather of the l arge n umber Of available c arcasses. Many of the c arcasses occurred on the Naabi-Ndutu plains i n the vicinity of a t l east two a ctive hyena dens. I observed o n s everal occasions one or two hyenas f eeding f rom a recent natural d eath while the r est of the c lan l ay f ull-bellied a nd uninterested a s hort d istance away. These data i llustrate n icely how l ow h yena to carcass r atios c an l ead to g ood f eeding opportunities f or non-hyaenid s cavengers. Carcass glut, though o ccurring i n the s hort dry s eason d uring my s tudy, s hould i n years of more normal r ainfall b e r estricted to the very end of the l ong dry s eason. S ummary of S easonality: S easonality i n both r ainfall a nd h erbivore abundance has d istinct effects on t he completeness by which c arcasses are a bandoned to s cavengers i n the S erengeti. I nitial consumers, whether a s predators or s cavengers of n atural deaths, a bandon carcasses of a c ompleteness that s eems to d epend more on prey abundance than on r elative amounts of r ainfall. D espite t he addition of k ills made by mobile predators to those made by r esidential c lans or prides, the a ssociated i nflux o f migratory h erds t o a n area i s a pparently s ufficient t o d ecrease s cavenger t o carcass r atios. T he higher availability of c arcasses i s a ssociated with smaller average f eeding group s izes and t he i nitial abandonment of a greater amount of e dible t issues t o s cavengers. Adding

9 4

the effect of d ry periods s tressful to herbivores to contexts of prey abundance f urther reduces s cavenger to carcass ratios, creating the best s easonally-based s cavenging opportunities. I t i s in f act only at these times that scavengers are l ikely to encounter unattended carcasses remaining with l arge quantities of f lesh. Hyenas as s econdary consumers, s een here a s s cavengers of l ion kills, s eem instead to consume r em -.ins to a degree dependent on r elative amounts of r ainfall. S cavenging group s ize again was s een to be an intervening variable affecting the completeness of consumption. Lion kills s cavenged by hyenas almost a lways provide the most marginal opportunity f or other s cavengers, which i s only s lightly better during wet periods than dry ones. Wet periods in general, however, were seen to offer a poorer opportunity to s cavengers unless accompanied by the presence of l arge numbers of prey and potential c arcasses.

Habitat

Type

and

S cavenging Opportunity

A striking observation in the S erengeti was the r arity of s ightings of hyenas in r iparian woodland. R iparian woodlands in the S erengeti s tudy areas include both riverine and l acustrine woodlands. R iparian woodlands are in g eneral more densely wooded than the open Acacia woodland that dominates the northern and western portions of the Serengeti. R iverine woodlands f requently monitored f or carcasses occur along the S eronera and Wandamu r ivers of t he Seronera s tudy area ( Figure 2 .5) The two r ivers are perennial and are bordered by narrow s trips of dense to open woodland a s l ittle a s 1 00 meters wide, and no more than three quarters of a kilometer wide. Much denser and extensive r iverine woodland occurs along l arger r ivers i n .

the S erengeti t o the north, but these were not monitored. Denser and more extensive l acustrine woodland occurs along the f lanks of and within Olduvai Gorge around perennial lakes Ndutu and Masek within the Naabi-Ndutu s tudy area ( Figure 2 .6). On the handful of occasions I observed hyenas in r iparian woodland, they were present i n r elatively small numbers ( less than t en, and usually around f ive individuals) and conspicuously wary, apparently of encountering

l ions.

Hyenas, however, are very common i n open plains and open Acacia woodland throughout the S erengeti. Hyenas will also forage without hesitation up to the margins of riparian woodland. The d isinclination of hyenas to f requent denser riparian woodland i s consistent with Kruuk's ( 1972) observation of their preference f or open vegetation habitats. These

observations

are

s upported

by

data

on

the

habitat-specific f requency of hyenas and other carnivores as i nitial consumers ( Table 4 .20). Hyenas were observed or 9 5

C

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1 9 . 1 C D

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c o W o rd 0 ) H H I OrI C d r 1 c

I N I T I A L

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U ) C d c !P

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a ) N

H • '

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r -4

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n 4 H O

96

Ic

C d Z . •H s . 4

C d

r 4

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c

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q v

i mplicated a s i nitial consumers on approximately half of a ll carcasses f ound in open plains, open Acacia woodland, a nd in Ngorongoro, which i s mostly plains. On riparian woodland margins or within r iparian woodlands, however, h yenas were i nitial consumers on l ess than 1 0% of all c arcasses. L ions s how a r eversed pattern, a ccounting f or t he initial c onsumption of 8 8% of a ll carcasses f ound w ithin r iparian woodland, and more than half of those on i ts margins. Vultures are rare i nitial consumers i n all h abitats, and c heetah were i nitial consumers only 1 2 t imes, a ll on the plains. Other carnivores were r arely r esponsible f or initial consumption. L ions are therefore the principle c onditioner of s cavenging opportunities i n r iparian woodland, while h yenas play the r ole i n more open habitats. G iven the typical complete marrow and h ead c ontent r emains of l ion k ills on their preferred, medium-sized adult prey, a nd the r are c ircumstances i n which hyenas abandon more t han minimal r emains to s cavengers, the expectation arises t hat much better s cavenging opportunities will be e ncountered i n r iparian woodland than i n more open h abitats. As major s cavengers, however, hyenas have a l arge i mpact on r educing carcass yields i n all habitats. Table 4 .21 shows the h abitat-specific f requency by which various c arnivores were i nvolved, i nitially or a s s cavengers, i n c arcass consumption. Hyenas a te f rom over 7 5% of a ll c arcasses i n a ll habitats r egardless of vegetation c over, a nd to a degree a pproaching or s imilar t o that observed i n t he highly competitive Ngorongoro Crater. Vultures consumed f rom over 5 0% of all c arcasses i n a ll h abitats, while the f igures f or l ion and cheetah, which were r arely if ever observed to s cavenge, are s imilar to those f or i nitial c onsumption. The a pparent contradiction between the r arity of hyena s ightings and t heir s cavenging of most carcasses i n r iparian woodland i s r esolved by c onsidering the r apidity by which hyenas s cavenge i n d ifferent h abitats. Table 4 .22 s hows such data f or l ion k ills. The proportion of S erengeti l ion kills discovered by me that had yet t o be s cavenged by hyenas was highest within r iparian woodlands and l owest i n Acacia woodlands. C omparable d ata f or Ngorongoro are s imilar to those f or Acacia woodland. Moreover, of the l ion k ills s till being consumed by l ions, i n only one i nstance within r iparian woodland were hyenas i ntent on s cavenging p resent. O n the o ther hand, hyenas were present and waiting t o s cavenge l ion k ills immediately i n over half the cases o n the plains, a nd in all c ases i n Acacia woodland and Ngorongoro. These data s uggest there to be a l onger i nterval between a k ill by l ions and i ts d iscovery by h yenas in r iparian woodland than i n more open habitats. The s uggestion i s confirmed by data on carcass persistence presented i n Table 4 .23. Only medium-sized a dults within the S eronera s tudy area are considered. The

9 7

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a )

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co

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U ) U ) r d

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r d I A O U ) 4 J

G )I -

a ) I i $ - H

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a ) U ) H r d P 0 E -

0 1 0 b 0 I A 0 Z

T able 4 .22: S cavenging pressure by hyenas on l ion k ills in d ifferent habitats of the S erengeti, and in Ngorongoro. C olumn B r efers t o the proportion of all l ion k ills yet to be s cavenged by hyenas upon my discovery of the carcass. Column C i s a s ubset of column B , r eferring to the proportion of k ills s till beinç consumed by l ions at which hvenas were present and waiting to s cavenge on abandonment by l ions. Note both the high p:oportion of k ills yet to be s cavenged and the l ow proportion r eady to be s cavenged i n r iparian woodland proper compared to more open habitats.

Habitat Acacia woodland P lains R iparian woodland margin R iparian woodland proper

Total # of l ion k ills

9 4 4

9

B Kills unscavenged by hyenas n

1 1 7

4

C Kills r eady to be s cavenged n

1 1.1

1

1 00.0

3 8.6

9

5 2.9

4 4.4

U

2 2

1 3

5 9.1

7 .7

Total S erengeti

8 6

3 5

4 0.7

3 1.4

Ngorongoro

1 4

1 4.3

1 00.0

2

99

Table 4 .23: Average persistence of medium-sized a dult carcasses in different habitats of the S eronera s tudy a rea. Carcass s amples f rom which persistence was calculated are those f or which complete consumption was observed. The habitat-specific proportion of c arcasses observed until complete consumption i s not correlated to p ersistence, i ndicating a l ack o f bias in the extensiveness of carcass monitoring among habitats. Using the Mann-Whitney U-test, all paired differences i n persistence are significant to the . 05 l evel except f or r iparian woodland margin versus r iparian woodland proper.

Habitat

Total Carcass

Acacia woodland

1 0

P lains

5 4

R iparian woodland margin R iparian woodland

7

1 4

Carcasses observed until completely consumed n n

8

P ersistence ( hours) mean + S .D.

8 0.0

1 0.3

+

1 4.9

5 1.9

1 8.0

±

8 .4

5

7 1.4

3 3.4

±

1 5.2

7

5 0.0

7 0.7

±

4 8.8

2 8

proper

1 00

a nalysis i s r estricted t o S eronera due t o the i rregularity o f c arcass monitoring i n other s tudy areas. That data f rom S eronera are n ot s ubject to methodological biases i s i ndicated by the l ack of correlation between h abitat-specific persistence and the proportion of c arcasses observed until c onsumption was complete ( Table 4 .23) Medium-sized adult carcasses persist f or a lmost f our t imes l onger i n riparian woodland ( mean of 7 1 hours) t han i n A cacia woodland ( 10 hours) or grass plains ( 18 hours). C arcass persistence on the margins of r iparian woodland ( 33 h ours) i s s ignificantly l ower than in r iparian woodland p roper, but s till h igher than i n more open habitats. The l ong persistence of c arcasses i n r iparian woodland i s in part d ue to the dominance of l ions a s i nitial c onsumers in this habitat and their t endency t o " guard" c arcasses between meals until all f lesh has been consumed ( e.g. Schau er 1 972). To t est the impact of this possible b ias, persistence data f or only l ion k ills i n S eronera are presented in T able 4 .24. By r emoving those carcasses very q uickly eaten by hyenas a s i nitial consumers, persistence i ncreases s lightly f or the plains and Acacia woodland h abitats, but n ot to a d egree s ignificantly'affecting the d rastic d ifferences s een earlier. L ion k ills are therefore a bandoned and c ompletely s cavenged by hyenas within a day i n open vegetation habitats, but on average persist unattended f or a f ar l onger i nterval in r iparian woodland. When hyenas f inally d id s cavenge l ion k ills i n r iparian woodland, they l eft the c arcass more complete t han i n more open habitats ( Table 4 .25, Figure 4 .6). I n Acacia woodland, hyenas completely s cavenged the three l ion k ills observed. Only s ome head contents were occasionally i gnored by h yenas s cavenging l ion k ills on the margins of r iparian woodland. L ion k ills were s cavenged l ess thoroughly by h yenas on the plains, but s till to a s ignificantly greater e xtent than i n r iparian woodland proper. Small s cavenging group s izes of h yenas i n r iparian woodland are apparently r esponsible f or their abandonment of s ome of the f lesh a nd marrow, and most of the head c ontents l eft by l ions. C ombined with the much l onger persistence of carcasses i n r iparian woodland, this h abitat offers by f ar the best f eeding opportunity f or non-hyaenid s cavengers. There i s a n important s easonal c omponent to the s cavenging opportunity d escribed f or r iparian woodland. L ion kills, which a ccount f or most deaths i n r iparian woodland, are most l ikely t o be made in this habitat during the dry s eason ( Table 4 .26). Approximately 8 0% of l ion k ills within or on the margins of r iverine woodland i n S eronera o ccurred during dry days, while most l ion k ills ( 75%) in open h abitats away f rom water occurred on wet d ays. During dry periods, ungulates are f orced t o drink f rom perennial water s ources within r iparian woodland where 1 01

Table 4 .24: P ersistence of medium-sized adult l ion kills in different habitats of the S eronera s tudy area. By considering l ion kills only, habitat-based differences i n persistence are only s lightly l ess than those presented i n T able 4 .23. Average persistence in r iparian woodland proper i s s ignificantly greater ( .05 l evel) than that i n both Acacia woodland and plains ( Mann-Whitney U -test, direction predicted).

P ERSISTENCE ( hrs.) n mean + S .D.

Habitat Acacia woodland

3

Plains

1 4

1 8.0

±

2 1.9

+

1 2.1

1 5.2

9 .1

R iparian woodland margin

5

3 3. 4

±

R iparian woodland proper

7

7 0.7

± 48.8

1 02

r d 4 J

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0

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C dQ U )

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0

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o

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• Lf l

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•

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C )

M E D I U M S I Z E D A D U L T

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0-H 0 4 - ( 1 5 ( I ) C d 4 - C ) r d 0 ) • . r d E r A 4 (1 I ( N C d P HH • b U ) ( 1 ) C d P H 0 ) C d4 4 E E n a 0 ) 0 ( I ) C d > I C 4 P P E i - Q, . 4 J I J a C d

L

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P E R C E N T A G E

' I G ) b d

H

4 ' C d 4 ' H £ ) C d

r d C d H O r d 0

C d H r d O 0

U ) z H C d H 04

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103

r d C d H r d PrI H C d r d d 0 4 H O C d

C d H C d 4r 4 U J r d r d a a o o H O P 04

F igure 4 .6:

C ompleteness o f medium-sized a dult l ion k ills a fter

s cavenging

by h yenas

S ee Table

i n d ifferent habitats

o f t he S erengeti.

4 .25 f or a ccompanying d ata.

0

0

o

o plains

A

A

r iparian w oodland margin

E I

E I

r iparian w oodland proper

Acacia woodland

6 0

5 0

1 0

v iscera

f lesh

m arrow

h ead c ontents

1 04

t otal

T able 4 .26: D istribution of l ion kills in the Seronera s tudy area with r espect to habitat type and r elative amounts of rainfall. Note the greater l iklihood of kills to be made in riverine woodland 2 during dry periods, and on the plains during wet periods. X = 1 7.12, d . f. = 1 , p

i W

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T h e q u a l i t y o f e a r l y h o m i n d s c a v e n g i n g o p o r t u n i t y i n d i f e r e n t c o n t e x t s .

U E -9 . 1

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have afforded early hominids the opportunity to u tilize a ll carcass parts, i ncluding the within-bone t issues of medium-sized and l arger carcasses abandoned b y f eu ds. Stone tool use may therefore h ave given h ominids a competitive edge over vultures i n exposing f lesh well protected by skin in l arger carcasses, and over h yenas i n exposing tissues within bones of medium-sized a nd larger carcasses. The advent of s tone tool use i n carcass processing, i n this s ense, i s s een a s a critical f eature i n the origin of meat- and marrow-eating among hominids. That the t iming of the origin of tool use coincides broadly with the marked r eduction of hyaenid d iversity from the Late P liocene high to the current l ow s uggests it t o be t he novel adaptation enabling hominids to enter the a pparently r ecently underexploited s cavenging n iche. I t may e ven prove correct that s tone tool use enabled hominids t o exclude many hyaenids f rom a s cavenging n iche, and to b e a t l east partly responsible f or the extinction of many Pliocene hyaenids. Within the context of consumer type, c arcasses initially consumed by f eu ds are s hown to provide the best opportunity f or a hominid s cavenger ( Table 5 .7). Feu d kills are common today, and perhaps would have b een more common and of a l arger, more equable s ize distribution i n the past. This r esults f rom the greater diversity of Plio-Pleistocene f elids, and their suggested i ndividual prey type specializations within the whole range of larger herbivores. I t has been s uggested on the basis of j aw s tructure that all f eu ds of the Plio-Pleistocene would have been unable to consume marrow and head contents ( save nasal pulp) of carcasses at l east the s ame s ize as or l arger than medium-sized adults. Carcasses c onsumed by f elids would have a l ong average p ersistence, r esulting i n f eeding opportunities f or a hominid s cavenger of a ll marrow, most head contents, and, on very l arge c arcasses, s ubstantial amounts of f lesh. Vultures and hyenas would provide poorer opportunities to a hominid s cavenger. As documented e arlier, vultures cannot expose t issues within bones, but except on t he l argest carcasses are very effective f lesh eaters. Vultures are rarely initial consumers, and when they are, i t i s on natural deaths l ikely to have very poor quality marrow. T he tendency f or hyenas to f eed i n l arge groups a nd their ability to consume all parts of even the l argest c arcasses make them poor candidates f or providers of s cavengeable f ood f or hominids. The good quality opportunities f or a hominid s cavenger provided by l arger carcasses and by f eu d kills can be viewed within contexts of variably intense competition a s applied by hyenas. The s cale of hominid s cavenging opportunity would be poor in contexts of high hyena t o carcass ratios, as r epresented by Ngorongoro Crater at a ll times of the year, and within the S erengeti during times of

1 28

l ow migratory prey a bundance, h igh r ainfall, and u sually i n open vegetation habitats away f rom water s ources. I n d ispersal s ystems, l ow prey abundance c orresponds to the r ainy s eason. D ry periods, however, would witness the a ggregation of a t l east water-dependent s pecies ( including hominids) within the area of permanent water s ources where c arcass d ensities would be r elatively h igh, and c ompetition f or carcasses l ow. Very dry periods of prey adundance i n a ll ecosystems, a s indicated i n Table 5 .7, provide a good opportunity f or a hominid-like s cavenger. I n these c ircumstances, carcass abundance approaches a point where l ocal s cavenger populations are s aturated with f ood, and c arcasses persist f or r elatively l ong periods with l arge quantities of f lesh. I t i s only a t these t imes that open vegetation h abitats away f rom r ivers and l akes a re c onsidered to provide a good opportunity f or a hominid s cavenger. Wetter periods, with l ower c oncentrations of h erbivores and l ower rates of natural mortality, provide f ewer carcasses a nd r esult i n the more r apid a nd thorough c onsumption of c arcasses typical of hyenas i n open vegetation habitats. Aside f rom high dry s eason mortality, the other e cological context providing a good opportunity f or a h ominid-like s cavenger i s f rom f eu d k ills i n r iparian woodland. This s etting has been shown t o afford l ong c arcass persistence in the absence, or a t l east extreme s lowness, of hyena s cavenging. Most f elid k ills i n r iparian woodland occur during the dry s eason when h erbivores drink f rom well-vegetated perennial water s ources providing numerous ambush points f or f elids. L ike modern f elids, the s aber tooth a nd f alse s aber tooth cats of the P lio-Pleistocene probably r elied on s tealth and ambush t o t ake down prey a fter a r elatively s hort c hase. T he extent t o which extinct f elids were open plains f orms l ike the c heetah, or, whenever possible t ook advantage of d enser vegetation f or ambushing, i s not known. The weak-limbed Megantereon, however, probably t ook advantage of vegetation c over to s urprise prey. I t can be s uggested that r iparian woodlands were the • favored hunting grounds of most P lio-Pleistocene f elids during the dry s eason. Hyenas, a s t oday, might have ventured i nfrequently i nto these habitats t o s cavenge. At any r ate, the evidence f or hominids c entering a ctivities i n r iparian woodland s uggests they would have had quick access to any f elid k ills abandoned t here. In s ummary, two opportunities emerge a s the most l ikely to have been available to a hominid s cavenger. O ne i s f rom abandoned f elid k ills of medium-sized a nd l arger p rey in r iparian woodland during the dry s eason. The other i s encountered d uring periods of prey abundance when a dditionally natural mortality i s h igh. B oth opportunities a re primarily dry s eason phenomena, with comparable a nd o ther opportunities during t he wet s eason not b eing n early a s

r egular

or

profitable.

The

two

1 29

opportunities,

however,

d iffer i n a number of respects that have important implications f or the degree t o which any one was encountered and taken advantage of by hominids. These d ifferences implications

are outlined i n Table 5 .8 a nd their f or hominid s cavenging are d iscussed below.

A major difference between the two opportunities i s the habitat type in which the opportunity occurs. I n my experience in the S erengeti, virtually all stress-related natural mortality occurred outside of r iparian woodland i n open vegetation habitats. Dry conditions are s tressful t o herbivores not s o much because of thirst, but b ecause of the increasingly l ow quality f orage they consume during prolonged dry periods ( Sinclair 1 979a). Natural mortality among grazers such as the alcelaphines, antelopines a nd equids that dominate the l arger herbivore communities of s avanna-woodlands, i s most l ikely to occur i n their preferred, open vegetation habitats away f rom riparian habitats. This has also been observed in Nairobi National P ark during a s evere drought i n the early 1 970's b y Hillman and Hillman ( 1977). Natural deaths among browsers ( e.g. g iraffe) and e specially herbivores preferring riparian habitats ( e. g. elephant, hippopotamus, buffalo, waterbuck) might on the other hand be expected to be concentrated i n riparian habitats. This has been d emonstrated f or elephants during a prolonged drought in the T savo National Park by Corfield ( 1973) As well, the natural deaths of o ne giraffe and one elephant I observed during my s tudy both occurred i n the dense Acacia woodlands around L ake Ndutu. Mesophyllic species, however, occur i n f ar s maller l ive numbers than xerophyllic grazers in s avanna woodland ecosystems. As a result, s cavenging opportunities f rom s tress-related natural deaths will be most f requently r ealized in non-riparian, open vegetation h abitats. A hominid s cavenger that centers activities i n riparian habitats would therefore i ncur l ower travel a nd s earch costs when f oraging f or abandoned f elid kills i n riparian woodland than i t would by f oraging i nto open habitats away f rom water f or natural deaths ( Table 5 .8) A s well, the r isk of exposure to predators would be higher in o pen vegetation habitats than i n r iparian woodland which provides many trees f or easy escape. .

.

While s earch and r isk costs may be l ower i n riparian woodland, the benefit r ealized f rom s cavenging abandoned l ion kills i s also l ower than that expected f rom v ery complete natural deaths. Abandoned f eu d kills usually provide only l imited quantities of f lesh in a ddition t o marrow bones and head contents. Natural deaths, c onversely, usually provide a bonanza of f lesh, especially i f occurring i n l arge numbers. For example, on a very productive day of c arcass s earch in the S imba Kopjes s tudy a rea on the plains, I discovered s ix c arcas es of adult wildebeest a nd z ebra in an approximately 4 0 km ' a rea. All of the c arcasses r emained with at l east marrow bones and head contents, a nd three were either abandoned to vultures or d iscovered

1 30

Table 5.8: Characteristics of the two best scavenging opportunities potentially encountered by hominids.

Characteristic

Riparian Woodland Felid Kills

Natural Deaths In Open Vegetation Habitats

Length of foraging trip

short, within areas of sites

long, into habi­ tats away from sites

Risk of exposure to predation

low, many trees for escape

high, few opportunities for escape

Scavengeable tissue yield and type

low, mostly marrow and head contents

high, large amounts of flesh

Nutritional content

high fat, low protein

high protein, no fat

Duration of opportunity

long, throughout all dry periods

short, at end of prolonged dry periods

131

h aving been f ed on only by v ultures, r emaining with s ubstantial amounts of f lesh on a t l east the a nterior h alf of the body. A s imilar magnitude o f f eeding opportunity was observed on the Naabi-Ndutu plains during s everal weeks of unusually dry weather when z ebra e specially were dying i n l arge numbers. There are important nutritional d ifferences i n the parts c omprising each of the two s cavenging oportunities. Marrow bones of animals killed when otherwise h ealthy a re a g ood s ource of calorically r ich f at. On only 7 o f 2 7 medium-sized adult l ion kills i n r iparian woodland d uring t he dry s eason was marrow observed t o be l ow i n f at. O n the r emaining carcasses I f ound the marrow on visual i nspection ( Sinclair and Duncan 1 972) t o b e h igh in f at, or c ould s afely i nfer their f attiness because they were s cavenged by hyenas [ on many other occasions, h yenas were o bserved to i gnore marrow bones of s tress-related deaths, often a fter only l icking or puncturing the c ancellous epiphysis of the bone]. I have determined the twelve major l imb bones of a h ealthy adult male wildebeest, c ommon prey o f l ions, to provide approximately 3 50 grams of f at. This amount i s equivalent to over 3 000 calories ( 9 c al/gram of f at) ‚ which i s well over the daily caloric r equirements of a s mall adult modern human ( e.g. L ee 1 968). S tress-related n atural d eaths, on the other hand, provide much f lesh high i n protein but devoid of f at. Wild ungulates in g eneral h ave very l ow intramuscular f at l evels ( usually between 2 % and 5 % by weight, Ledger 1 968) , a nd the early use of intramuscular f at among all body f at deposits a s an e nergy r eserve by s tressed a nimals ( e.g. H arris 1 945, S inclair a nd Duncan 1 972, Smith 1 970) would c ompletely r emove a ll f at f rom f lesh in n atural deaths. When nutritionally s tressed, a s i s l ikely to occur during the dry s eason, f at contained i n marrow bones i s the l ast energy r eserve t o be u sed by the animal. I f s tress continues, a ll marrow f at i s depleted and the animal s oon d ies. S tress-related n atural d eaths would therefore provide f at n either f rom f lesh nor l imb bone marrow cavities. S peth and Speilman ( 1983) d escribe the n utritional complications, i ncluding s tarvation, that a rise f rom an exclusive protein d iet. Even a h igh protein d iet, t o be nutritionally s ound, must be c omplemented by l arge amounts of c arbohydrates or f ats to h elp metabolize t he proteins. The potential s everity of t he problem, i n f act, h as l ed S peth ( in prep.) to a rgue that f lesh-eating b y hominids during the dry s eason would be a n unviable nutritional s trategy. This would c ertainly be t rue i f hominids a ttempted to s ubsist exclusively on the f lesh of s tress-related natural deaths f or a prolonged p eriod. The extent to which hominids consumed c arbohydrates and p lant f ats, however, i s not known. Only r ecently have two s tudies been c onducted on the s easonal a nd h abitat-based availability of s uch plant f ood nutrients ( Sept 1 984, Vincent 1 985) . What i s certain, however, i s t hat f oraging 1 32

f or abandoned l ion kills of medium-sized adult and l arger carcasses i n r iparian woodland would provide a concentrated s ource of c alories, and one perhaps l arge enough to permit the consumption of l arge amounts of no-fat f lesh f rom natural d eaths. A f inal important difference between the dry s eason s cavenging opportunities encountered within a nd outside of r iparian woodlands i s the f requency a nd duration over which t he opportunities exist ( Table 5 .8). P eriods of high natural mortality are usually r estricted to the f inal two months of the d ry s eason ( Houston 1 979) As well, f or natural deaths to s aturate other s cavengers, l arge numbers of h erbivores must be i n the area i n which conditions are s tressful. Herbivores, however, a re f orced t o drink f rom permanent water s ources not only throughout the dry s eason, but a lso during predominantly wet periods that experience a week or more of n o r ain. Abandoned f elid k ills in r iparian woodland s hould b e available f or a t l east s ix months of every year in a r egion that r eceives s imilar amounts of r ainfall a s the S erengeti. The a ctual number of abandoned l ion kills available to a s cavenger i n r iparian woodland d epends on the number of l ions present. Along a 1 0 k ilometer s tretch of the S eronera R iver r egularly monitored during my s tudy, two l ion prides were present and hunting t hroughout dry p eriods. From each pride, I l earned to expect to f ind a t l east one k ill every two or three days. O ccasionally, multiple k ills would be made. One c ould therefore expect t o f ind at l east one abandoned l ion k ill p er d ay per ten k ilometer s tretch of the S eronera. As well, the k ills were very predictably l ocated. The movements of a pride are very c onspicuous, a s they would have been to an early hominid group, and k ills t end to be l ocalized f or a p eriod of t ime a long a short s tretch ( approximately one k ilometer) of r iparian woodland. The presence of c arcasses c ould be easily monitored by an early hominid group, and quickly s cavenged upon abandonment by l ions. That there i s n o s cavenger today s pecializing on the within-bone r emains of l ion k ills i n r iparian woodland s uggests the availability of a s cavenging n iche that might have been f illed i n the past by hominids s upplementing a mostly plant f ood diet with h igh energy f ats f rom marrow bones. .

F oraging

T actics

f or a Hominid

S cavenger

The characteristics of the two g ood s cavenging opportunities d escribed above can be v iewed a s c omponent c osts and benefits of a lternative f oraging s trategies used by hominid s cavengers. The purpose of viewing the two o pportunities i n t erms of costs and benefits i s to a ssess t heir relative v iability and to i dentify the f oraging p attern most l ikely a ssociated with e ach. With r espect to the f oraging pattern used, p articularly i nterested i n modeling t he c ircumstances 1 33

I am under

which a hominid s cavenger would have f oraged most profitably in a " feed-as-you-go" ( Isaac 1 980, 1 981) or " routed" ( Binford 1 984) mode typical of nonhuman primates, or a central-place f oraging mode unique among p rimates t o hunter-gatherers. The type of f oraging s trategy practiced by early hominids has polarized views among paleoanthropologists. The central place or home base f oraging s trategy i s r epresented most explicitly by I saac ( e. g. 1 978a, 1 978b). Here, early hominid groups are s een to f orage f rom a nd r eturn to on a daily basis a f ixed point on the l andscape where gathered f oods are distributed among group m embers i n a way s imilar to that s een i n many hunting and gathering groups today. The f ixed point, or central place, is s een archaeologically as the many dense c oncentrations of animal bone and stone tools referred t o as s ites ( or more appropriately " mega-sites", I saac et a l. 1 981). S uch concentrations clearly could not have been produced i n a day, but rather can and have been e xplained as r epresenting the debris l eft over repeated daily visits to t he locale, as might be expected f rom a central-place mode o f f oraging. This view has been supported by other researchers i ncluding Bunn ( 1982) I saac ( 1980, 1 981, 1 983) has more recently s uggested a number of other behavioral t actics t hat could result in s uch dense concentrations of artifacts and f ood refuse without i nvoking the classical home base mode of s ubsistence. .

The f eed-as-you-go f oraging mode f irst s uggested by I saac a s an alternative to central-place f oraging by early hominids, has r ecently been r elabeled as routed f oraging and championed by Binford ( 1984) R outed f oraging is l ess organized than central-place f oraging. I t involves traveling among a number of r esource points on t he l andscape each day, and remaining a t or near the l ast r esource point at the end of the day. No effort i s expended to return to the s tarting point, which might not b e revisited f or l ong periods. Routed f oraging is t ypical of all nonhuman primates not t ied down t o nesting s ites ( prosimians) or s leeping s ites ( hamadryas baboons) , a nd implies f or early hominids a s imilar l ack of cooperation i n f oraging and an absence of f ood s haring a mong group members. The routed f oraging position explains t he f ormation of archaeological mega-sites a s having occurred periodically over a r elatively l ong period of t ime, a nd r epresenting activities conducted s pecifically at t he r esource point being exploited. Aside f rom i ts proximity t o the resource point ( raw material s ource f or s tone t ool making, f ruiting trees, etc.), the s ites have n o special s ignificance a s s ocial f oci at which hominid group members assemble on a r egular and planned basis. .

The type of f oraging mode assigned t o early hominids clearly corresponds to the general degree of humanness granted to them. Central place versus routed f oraging i s 1 34

one more ingredient, similar to the hunting versus scavenging issue, of the differing views on the timing of the origins of unique human traits discussed in Chapter 1. It therefore becomes appropriate and important to evaluate the foraging mode most likely to be associated with a scavenging mode of animal food acquisition. This can be done through a cost-benefit analysis. The cost-benefit analysis necessarily views hominids as ex clusive scavengers. That is, they are purposefully foraging for and consuming only animals discovered dead in the two contexts of good scavenging opportunity. I will for the sake of simplicity and clarity make no attempt to integrate into the model the relative costs and benefits of foraging for plant foods. Hence, the model loses realism (in the sense of Maynard Smith 1974) - hominids certainly have always been omnivores deriving most nutrients from plant foods but gains specificity by being able to integrate into strategies many of the relevant characteristics of the two good scavenging opportunities potentially encountered by hominids. Figure 5.1 depicts a model of the costs and benefits of scavenging within and outside of riparian woodland as a function of foraging distance. The origin of the graph represents a point in riparian woodland (e.g. sleeping trees) from which a hypothetical early hominid group commences foraging for carcasses on a given day. Two straight line cost curves (C and Cb) are shown. Curve C represents e x clusively the Bost of travel incurred whil� searching for abandoned felid kills in a routed foraging mode within riparian woodland along a course more or less parallel to a river channel or lake_ shore. The curve attains its highest point at distance D , which represents the max imum possible foraging distan��x that can be covered in a day. The other cost curve, C , has a slope b twice that of C to account for the higher costs associated with several circumstances, including 1) routed foraging for natural deaths outside of riparian habitats where a risk cost is added to account for the scarcity of trees from which to escape predation, 2) central-place foraging in non-riparian, open vegetation habitats for natural deaths, where return trip travel to the central place essentially doubles travel costs (Orians and Pearson 1979, Hamilton and Watt 1970) but reduces risk of e x posure to predation by minimizing time spent in areas providing few means of escape from predators, and 3) central-place foraging for abandoned felid kills in riparian woodland, which again is twice as costly as routed feeding owing to return trip travel costs. The model does not incorporate costs associated with carcass processing as these are assumed to be similar for carcasses found in both habitats. The cost of transporting carcass parts to a central place are included in the additional cost associated with return trip travel. 135

F igure

5 .1:

C ost-benefit model o f

s cavenging i n r iparian o r open

v egetation habitats a s a f unction o f

B p lains max

f oraging d istance.

-

B rw max

C O S T O R B E N F I T

B p lains min

B r w m in

D max

D. m in

s tarting point

FORAGING D ISTANCE B

=

benefit a ccrued

f rom s cavenging a bandoned f elid k ills

Bb

=

w oodland. b enefit a ccrued f rom s cavenging natural deaths habitats away f rom water

i n r iparian

i n o pen vegetation

s ources.

C

=

c ost o f routed f oraging i n r iparian w oodland.

C

=

cost o f c entral p lace

f oraging,

o r r outed f oraging i n n on-riparian

h abitats w ith a dded r isk c osts. D. i s t he minimum f oraging d istance at which c osts a re balanced by min b enefits a ccrued i n r iparian woodland habitats ( Br w m in ) a nd o pen vegetation habitats d istance woodland

c orresponding ( B

rw max

p l a i n s m i n) .

( B

Dm ax i s

t o t he maximum

) and plains

( B

b enefit a ccruable

p lains max

1 36

the maximum d aily f oraging

) .

i n r iparian

Two benefit curves, B and �b' are shown. Their positive slope reflects the '.incre asing yield accrued with incre asing for aging dist ance. Their sigmoid shape reflects the low chance of finding a carcass at the beginning of a foraging trip, and the increa �ed ch ances of finding a particul arly high-yielding c arcass as foraging progresses from the st arting point. Benefit curve B refers solely to the benefits accrued by consu ming tissue/of ab ando ned lion kills discovered in riparian woodland. This curve attains its highest poi nt and levels out at a point on the y- ax is (B ) corresponding to the max i mu m benefit available ax wiHhJTl a d ay's max i mu m travel (D ) within ripari an woodl and. Benefit curve B represent��e benefit realized b by for aging for carcasses i n open vegetation habit ats during dry periods of prey abund ance and high natural mortality. Its slope is shown to be two ti mes that of B to reflect the minimu m extra yield provided by n atural de�ths bearing flesh. Curve B reaches its highest point at b on the y- ax is, corresponding to max imu m daily B . ax ance (D · ). B o 1 a i ns maist foraging is much closer to, or m 1 . exceeds, the group s��! atioH �6r5t �fi an is B rw max

The two benefit curves are dr awn so as to intersect the corresponding cost curves at the s ame for aging distance. This has been done to si mplify the predictions arising fro m the model. The points of intersection on the x - ax is. This correspond to foraging distance D m . point represents the distance at i1hch be nefits accrued b alance the costs of for aging, and hence represents the mini mum dist ance required to be covered by an e x clusive scavenger in e ach h abit at to make scave ngi ng profitable. At shorter for aging dist ances, scavenging i n e ach habitat is unprofitable (costs outweigh possible benefits) and should not be pr acticed. The max imu m profit, or net benefit, is realized in both h abitats at a for aging distance (not shown) betwee n D . and D where the correspondi ng benefit and cost c�rOes are m��� disparate. Two i mplicatio ns for ho minid for aging arise fro m the as constructed. First, for scavenging natur al de aths in open veget ation habit ats to be as profitable as scavenging ab andoned lion kills in ripari an woodl and, the mini mum benefit accrued must be twice as high as that accrued within ripari an woodl ands (B . . n versus B . o n the y- ax is). This is due to th�½f:t�e? icosts deriv�� f}Bm addition al risk or return trip travel not e ncountered with routed for aging in ripari an woodl and. If ho minids were tied to a centr al pl ace and if risk costs were gre ater than shown, costs of foraging in open veget ation h abitats away fro m w ater might be so high as to neg ate a profit and to prohibit scavenging outside of rip ari an woodl and. High costs and low benefits were, in f act, the b asis for suggesting that ho minid scave nginq in hye na-frequented habit ats during times of herbivore scarcity and low n atur a l would be u nprofit able. Here, however, it is mortality exclusively the high costs incurred that would render the model

137

opportunity provided by natural deaths poor a nd t he c entral-place f oraging s trategy u nprofitable. W hether o r not this i s the case might d epend t o a great extent on t he d egree of group cooperation i n anti-predator defense exhibited by early hominids. I f effective, s uch defense c ould minimize the r isk a ssociated with f oraging in o pen habitats, and make s cavenging f or natural deaths by a c entral-place f oraging mode profitable. The s econd implication arising f rom t he model i s t hat anything that i nceases f oraging c osts i n r iparian woodland will negate any benefits r ealized f rom the relatively l ow y ielding l ion k ills f ound there. F or i nstance, f oraging f rom a c entral place i n r iparian woodland would i ncur costs described by curve C , which at a ll f oraging distances i s h igher than the benefit curve f or r iparian habitats. I t m ay well be that f oraging f or abandonded l ion k ills i n r iparian habitats would be i nsufficiently beneficial to f inance a c entral-place f oraging mode there. Rather, the c arcasses c ould be most efficiently s cavenged by practicing a routed f eeding mode typical of nonhuman primates. I n more r ealistic terms, s cavenging abandoned l ion k ills i n r iparian habitats would best have been an opportunistic pursuit, with any c arcasses being d iscovered in t he course of f oraging f or plant f oods b eing t aken f or their c alorically r ich marrow bones and brain. I n s ummary, the cost-benefit model s uggests s cavenging i n r iparian woodland to be best c onducted through a routed f oraging mode. The r elatively l ow y ield of the r emains f rom a bandoned l ion k ills d iscovered i s s uggested to b e insufficient to compensate f or a dditional transport a nd r eturn trip travel costs t o a c entral place. Scavenging f or natural deaths outside of r iparian woodland, on t he other h and, was argued t o be profitable o nly if c onducted through c entral-place f oraging which additionally f eatures a h igh d egree of group cooperation i n a nti-predator d efense. Hence, despite the greater y ield of s cavengeable f ood available f rom natural deaths outside of r iparian woodland, acquiring these i s s uggested to b e a more costly activity i n t erms of r isk and energy expended than s cavenging i n r iparian woodland. I f early hominids d id s cavenge, t he most readily a ccessible s ource a cquired i n a manner departing t he l east f rom the f oraging patterns of nonhuman primates, a nd presumably that of pre-stone tool u sing h ominids, would b e that provided by abandoned f eu d k ills i n r iparian woodland. This s ource a s well would s ubject hominids to t he l owest l evels of competition with other scavengers. I t would provide mostly medium-sized a dult c arcasses, occasionally l arger ones a bandonded by f eu ds, a nd more r arely carcasses of l arge mesophyllic animals f ound d ead f rom natural c auses. The opportunity f or a s cavenger i n r iparian woodland would be encountered primarily d uring t he dry s eason a s a r esult of the h ighly s easonal p attern o f

1 38

f eu d predation and natural mortality within r iparian woodland. The opportunity would provide most r egular access to marrow bones and head contents, with f lesh being available i nfrequently and i n very small pockets except on the r are l arge c arcass f ound. F inally, i t i s precisely i n these riparian s ettings that the greatest d iversity and abundance of plant f oods potentially e aten by hominids occurs ( Sept 1 984)

The

Archaeological

R ecognition of

S cavenging

The characteristics of hominid s cavenging opportunity s uggested above have important implications f or the archaeological r ecognition of s cavenging. Archaeological f aunal a ssemblages a ccumulated through s cavenging s hould have the f ollowing characteristics: 1 )

2 ) 3 )

4 )

5 )

a predominance of adults over j uveniles or younger i ndividuals, with most young being those of l arge s pecies. a predominance of medium-sized adult and l arger carcasses over those of smaller s ize c lasses. a predominance of f ragmented l imb and cranial parts over elements f rom the axial r egion of t he s keleton. a high proportion of a ll defleshing cut marks occurring on head and l ower l imb bones, with i ncreasingly l ower f requencies being f ound on parts defleshed progressively earlier i n the c onsumption s equence. smaller t axa being r epresented by a greater predominance of l atterly c onsumed s keletal parts, and displaying d efleshing cut marks on a more l imited s eries of body parts than l arger taxa.

Characteristic # 1 above i s consistent with the s uggestions of Vrba ( 1975, 1 980) and Klein ( 1982), but f or r easons d ifferent f rom those presented h ere. Vrba c ites different f requencies of j uveniles over a dult a nimals i n the diet of hyenas when s cavenging versus when hunting t o s upport a contention that j uveniles s hould be underrepresented i n a ssemblages accumulated primarily through s cavenging. Klein, f ocussing on the a pparent a ssociation between hunting and c atastrophic mortality profiles ( as r esults i n game drives) on the one hand, a nd s cavenging and a ttritional mortality profiles on the other, a lso states that young i ndividuals s hould be poorly r epresented i n a ssemblages accumulated through s cavenging. The basis f or the d istinct age profile of s cavenged a ssemblages s uggested here, r ather, r ests on the much more r apid and thorough consumption by i nitial c onsumers of young versus a dult i ndividuals, and the r esulting predominant availability of adults t o s cavengers. Only young of particularly l arge s pecies are l ikely t o s urvive

1 39

initial consumption f requently a nd be a component of a s cavengers diet. Age data f rom Plio-Pleistocene archaeological s ites at Koobi F ora and Olduvai are now available ( Bunn 1 982, Potts 1 982). No summary data are provided, but most assemblages analyzed by Bunn do s eem to be dominated by adults. Whether the proportions of subadult to adult individuals i s s ufficiently d istinct f rom t hat expected f rom age-mortality profiles of l ive populations i s a question that r equires f urther r esearch ( cf. K lein 1 982). Characteristic # 2 above, that assemblages a ccumulated by s cavenging should be dominated by l arger s ize classes of herbivores, i s consistent with expectations of many r esearchers. I mentioned in Chapter 1 the reasoning t hat the mere s ize of many of the animals represented at archaeological s ites s eemingly prohibited their hunting, and hence must have been s cavenged. The data presented h ere on s ize s pecific carcass persistence and completeness of initial consumption provide the f irst positive s upport f or this l ong-held expectation. These data indicate that a s cavenger will rarely encounter edible t issues r emaining on carcasses of small taxa, and, owing to t he greater abundance of medium-sized adults over l arger taxa in modern and past ecosystems, will derive the most r egular s cavenging opportunity f rom medium-sized adult carcasses. Vrba ( 1975, 1 980) has also suggested a s ize-based criterion f or distinguishing assemblages a ccumulated by s cavenging, noting that, compared to hunted assemblages, scavenged a ssemblages s hould contain a l arger s ize range of animals. This s uggestion i s based on the narrow s ize r ange of prey preferred by individual predator taxa, and the expected l arger r ange of carcass s izes available to more eclectic s cavengers. Vrba's criterion must be qualified i n l ight of the f indings presented here, to account for t he typical complete consumption by predators of small t axa, and the availability to s cavengers of incompletely consumed carcasses of primarily l arger taxa. Bunn' s ( 1982) data f rom archaeological s ites at Koobi F ora and Olduvai show the bone assemblages to be dominated by medium-sized and l arger taxa. G iven the good preservation of r emains a t most s ites analyzed by Bunn, and the presence of s ome i ndividuals of smaller taxa, these data are consistent with a primarily s cavenged mode of accumulation. That archaeological f aunal assemblages a ccumulated through s cavenging should be dominated by l imb and h ead parts ( characteristic # 3 above) r equires some d iscussion. B inford s ets of

( 1984) and Potts ( 1983) have proposed different body parts to be characteristic of a ssemblages

accumulated

through

s cavenging.

Binford s uggests that cranial bones and f oot bones ( metapodials and phalanges) are the l ast edible c arcass parts to be utilized and s hould therefore dominate s cavenged assemblages. He uses this r easoning t o ascribe to s cavenging the f oot and head bone dominated l arger b ovid 1 40

component of Middle S tone Age a ssemblages f rom the s ite of Kiasies R iver Mouth. These observations are s imilar t o mine with one very i mportant exception: i n addition to f oot bones, upper l imb bones ( humeri, f emora, r adius-ulnae, t ibiae) s hould b e equally characteristic of s cavenged assemblages. Hyenas do usually consume l imb bone marrow f rom the proximal to the d istal ( foot) e nd ( Table 3 .4). Although s tatistically this might be a basis f or expecting a m ore f requent a vailability of f oot compared t o upper l imb marrow bones, i n a ctuality, hyenas r arely s tart chewing a complete l imb without consuming marrow f rom a ll parts, i ncluding f oot bones. R egardless, hyenas are very infrequent providers of marrow bones f or s cavengers. Most marrow bones available are those abandoned by l ions, which, on medium-sized and l arger c arcasses, abandon a ll l imb bones with marrow cavities i ntact. I n l ight of these observations, I s uggest that the f oot bone to upper l imb bone dominance among the l arger bovids a t Kiasies R iver Mouth cannot be e xplained s olely by hominid s cavenging. I f hominids d id s cavenge most l arger bovids a t the s ite, they almost certainly obtained a s many upper l imb bones a s f oot bones, such that s ome additional f actor operated t o r educe the number of upper l imb bones that were originally brought to o r preserved a t the s ite ( see Blumenschine 1 986) Potts ( 1983) uses H ill's ( 1975) c arcass disarticulation s equence a s a basis f or predicting the carcass parts most l ikely to be available to a s cavenger. Specifically, Potts s uggests that a ssemblages a ccumulated through s cavenging s hould be dominated by parts that disarticulate r elatively l ate i n the s equence. These parts include bones f rom the postcranial axial r egion of t he skeleton r elative t o l imb bones, and among l imb parts, hindlimbs r elative to f orelimbs. Assemblages a ccumulated through s cavenging, according to P otts, s hould h ave a high proportion of axial parts r elative to l imb parts, and a high proportion of h indlimbs r elative to f orelimbs. I n both cases, the l atter s et of parts are those elements that disarticulate f rom a carcass earlier i n the s equence. There i s, however, a f undamental problem with these i nferences. Much of the disarticulation s equence occurs well after the r esource l ife of a c arcass has been exhausted. After this point, the main d isarticulating agents are physical weathering, microorganisms and insects that d egrade l igaments that c onnect bone. These l ate s tages of the disarticulation s equence are i rrelevant t o what i s available to a s cavenger s eeking nutrients. What i s r elevant i s the s equence by which edible t issues a re consumed ( Chapter 1 , and B lumenschine 1 986). According t o the general consumption s equence ( Chapter 3 , T able 3 .1) most axial elements s ave the head are completely u tilized ( i.e. defleshed) before t issues within bones ( limbs and head) are consumed. A h igh r epresentation of postcranial axial parts i n an archaeological bone a ssemblage i s therefore indicative of access by the bone a ccumulator t o f lesh parts that are e aten early i n t he c onsumption 1 41

s equence. S ince these are u sually consumed b efore a bandonment by i nitial consumers on a ll but t he l argest carcasses, such an a ssemblage i s not l ikely t o have b een a ccumulated through s cavenging. L ikewise, hindlimbs tend to be defleshed and then destroyed before f orelimbs. A dominance of f orelimb over h indlimb parts i n an a rchaeological accumulation i s therefore more l ikely t o be i ndicative of r elatively l ate a ccess to carcasses ( i.e. s cavenging), where if any l imb parts r emain with or without f lesh, these are most l ikely t o be f orelimbs. The b ody parts expected to dominate s cavenged bone assemblages on the basis of the consumption s equence ( limbs, a nd e specially f orelimbs, and the h ead) are, i n f act, t he c onverse of those expected on the basis of t he d isarticulation s equence ( axial bones versus f ew l imbs, many hindlimbs r elative to f orelimbs). Bunn ( 1982) notes a very high r epresentation of h ead and l imb parts over postcranial axial parts in m ost analyzed a ssemblages f rom Koobi Fora a nd O lduvai. B unn a scribes this pattern to the r elative durability a nd i dentifiability of the parts. I t i s i nteresting to n ote that i t i s precisely the more durable carcass p arts, i .e. those encased in thick bone, which are most l ikely t o i nhibit consumption by many i nitial consumers a nd consequently be available to a hungry hominid scavenger wielding a hammerstone. [ This s imple c orrelation between d ifferential bone preservability and the accessibility of edible t issues borne by or contained in t he bone was r ecognized l ong ago by Washburn ( 1957).] G iven t his, i t c an b e s uggested that the l imb and h ead bone dominance at m any e arly archaeological s ites i s more consistent with a s cavenged than a hunted mode of a ccumulation. Data on f orelimb to h indlimb ratios for s ix Olduvai B ed I f aunal accumulations presented by Potts ( 1983) a re a lso more c onsistent with a s cavenged mode of a ccumulation. Among small and l arge bovids, f orelimb:hindlimb r atios v ary i n unexplained ways among the s ites. However, f or t he numerically dominant medium-sized taxa, all s ites s how a c lear dominance of f orelimbs over hindlimbs. C ontrary to Potts' i nterpretation based on t he carcass disarticulation s equence, this pattern i s s uggestive of s cavenging o n t he c arcass s ize c lass s uggested h ere to have been m ost r egularly available to hominids. While a s cavenged r ather than hunted mode of a ccumulation s eems to predominate at t hese s ites f rom the g eneral body part data c ited a bove, this conclusion i s r egarded a s t entative until the a ffect of other taphonomic f actors unrelated to hominid selection of parts are f ully considered ( Blumenschine 1 986). The f ourth s uggested c haracteristic of b one a ssemblages accumulated through s cavenging relates t o t he s keletal distribution of defleshing cut marks i nflicted by h ominids using s tone tools. G iven the i nfrequent availability to s cavengers of s ubstantial q uantities of 1 42

f lesh, one can e xpect s cavenged a ssemblages to d isplay most d efleshing cut marks on bones d efleshed l ast by i nitial c onsumers, i .e. l ower l imb, h ead and neck bones ( Tables 3 .1 3 .4). This i s especially the case if s cavenging i s c oncentrated on medium-sized adults. I n that l arger quantities of f lesh occasionally r emain on c arcasses, cut marks s hould be progressively l ess common on bones d efleshed i ncreasingly earlier i n the consumption s equence. -

Bunn ( 1982, 1 983) provides s ome data on the f requency d istribution of cut marks on different bones of small a nd l arge bovids f rom the P lio-Pleistocene s ite of FLK Z inj a t O lduvai. Head parts and d istal l imb bones are among the most c ommonly cut-marked bones, bearing much higher i ncidences of c ut marks than postcranial axial bones. This i s consistent with a s cavenged mode of a ccumulation. H owever, upper l imb bones, which are defleshed r elatively e arly i n the consumption s equence by modern carnivores, a re a lso f requently cut-marked at FLK Z inj. This might s uggest more common a ccess to f lesh than has been s uggested t o be available to a hominid s cavenger. I t might, on the other h and, be a r esult of the r emoval of small s craps of f lesh that are abandoned by most consumers commonly d istributed a ll over the body. Alternatively, the h igh f requency of c ut-marked upper l imb bones might not r eflect d efleshing, but rather d isarticulation of bones a cquired a lready d efleshed. Currently, these a lternatives cannot be t ested [ notwithstanding S hipman's ( 1983) oversimplified c orrelation of cut marks l ocated n ear j oints or on s hafts with d isarticulation and d efleshing, r espectively ; experiments are c learly n eeded to d istinguish cut marks i nflicted f or d efleshing versus d isarticulating, a nd f or r emoving whole muscle bundles versus small s craps of f lesh. I t i s i nteresting to note, however, that among upper l imb bones f rom FLK Z inj, humeri are much more f requently c ut-marked than f emora of both small and l arge bovids. This pattern might r esult f rom s cavenging by hominids, a s f emora a re commonly among the f irst parts to be d efleshed by modern c arnivores, with humeri p ersisting with a ll f lesh until the r ibcage and all parts posterior to i t ( save the t ibiae)

have

been

s tripped of their meat.

The f inal s uggested c haracteristic ( #5 above) of bone a ssemblages a ccumulated through s cavenging i s a qualification of numbers 3 a nd 4 above r elated t o s ize d ifferences among taxa. Here, smaller t axa a re expected t o b e represented more exclusively by those s keletal parts e aten f rom l ater i n the consumption s equence than l arger t axa. Additionally, smaller t axa s hould bear d efleshing c ut marks on a more l imited s eries of body parts ( i.e. those d efleshed l ast by modern c arnivores) than l arger taxa. I n both cases, the greater l ikelihood of a s cavenger e ncountering l arger c arcasses i n a more c omplete r esource s tate than s maller ones f orms the basis f or these p redictions. Data on the r elative f requency a nd placement of cut marks cannot be a pplied c urrently f or the r easons 1 43

c ited above. However, amenable to the above provided by Bunn ( 1982)

available s keletal part data a re test, as conducted below on d ata f or bovids at FLK Z inj.

Table 5 .9 s hows the number of s elected skeletal parts f or s ize 1 and 2 bovids, and s ize 3 and 4 bovids at FLK Z inj. The body parts used are grouped into two c ategories. The f irst category contains those parts typically available with edible t issue only early in the consumption sequence. These parts, i ncluding the pelvis, vertebrae, ribs, and s capula, provide only f lesh and no within-bone edible t issues, and are expected to h ave been more commonly available on l arge carcasses. The s econd category includes bones that are eaten f rom only l ate i n the c onsumption s equence, and, as such, are n ecessarily restricted to matapodials and phalanges. Upper l imb and head parts a re excluded f rom the analysis as these bear f lesh a nd contain edible tissues that are eaten at different s tages in the consumption sequence. For all parts used, the t otal values of l efts and r ights provided by Bunn are used. For vertebrae, ribs and metapodials, only the most common portion of these parts given by Bunn are t abulated to minimize artificial and possibly d isproportionate inflation of per-part numbers r esulting f rom f ragmentation. The data i n Table 5 .9 show a marked, s ize-based a symmetry in the f requency of the two body part classes. S ize 1 and 2 bovids are represented by nearly equal numbers of the two s keletal c ategories, while l arger bovids a re much more f requently r epresented by parts available as f ood s ources only relatively early in the consumption s equence. I n other words, r elative to l arger bovids, the s mall bovid s keletal profile shows a greater degree of skewing toward those parts most f requently available to a s cavenger on a ny c arcass s ize c lass. The h ighly s ignificant asymmetry of the data are consistent with s uggested characteristic # 5 of s cavenged bone assemblages, where s cavengers are more l ikely to encounter smaller carcasses with a very l imited s eries of bones r emaining with edible t issues. The application of the f ive s uggested characteristics of s cavenged a ssemblages to s ome Plio-Pleistocene archaeological bone a ccumulations are mainly consistent with a s cavenged mode of animal f ood acquisition. The r esults, however, must be viewed a s t entative f or two main r easons. F irst, the criteria were applied in only a general manner due to the current availability of data i mperfectly s uited to the t ests. For instance, the r esults o btained in Table 5 .9 may r esult not f rom s cavenging, but f rom s ize-dependent d ifferential f ragmentation; this a lternative ( deemed l ess l ikely than the s cavenging interpretation) c annot be tested with currently available data. S econd, the above discussion makes no explicit attempt t o evaluate other processes of accumulation ( e.g. h unting) or d isturbance that may have operated on the a ssemblages. B oth s ources of i nconclusiveness n eed to be more f ully 1 44

Table 5 .9: S ize-specific f requency at FLK Z inj. The s elected s eries available with edible t issues only sequence, and those eaten f rom consumption s equence. Data are f rom The d ifference i n t he highly significant ( X

of bovid s keletal parts of body parts are those early in the consumption only very l ate i n the Bunn ( 1982, Table 4 .28).

s ize distribution of grouped = 2 8.1, d . f. = 1 , p < . 001).

NUMBER OF S ize Parts

eaten

f rom o nly

l+ 2

1 0 1 5 1 7 6

Sub-Total

4 8 ( 48%)

eaten

f rom only

i s

ELEMENTS S ize

3+ 4

early

P elvis Vertebrae ( centra only) R ibs ( proximal only) s capula

Parts

parts

1 8 3 2 3 5 1 4 9 9 ( 77.3%)

l ate

Metapodials ( proximal only) Phalanges ( complete and e piphyseal r emains)

2 5 2 7

S ub-Total

TOTAL

1 45

1 7 1 2

5 2 ( 52%)

2 9 ( 72.7%)

1 00

1 28

considered ( e.g. B lumenschine 1 986). Still, the c onsistency of the r esults with predictions i s s uggestive of a possibly important role f or s cavnqing in early hominid s ubsistence.

Conclusions The nature of s cavenging opportunities suggested f or early hominids has s ome general implications for o ur understanding of early hominid adaptation, and t he meaning of the early archaeological r ecord. If i t can be conclusively demonstrated that s cavenging by early hominids generated most components of archaeological bone a ssemblages, s everal s uggestions would apply. First, it c an be suggested that marrow bones and the brain made a much more s ignificant dietary contribution r elative to f lesh than i s commonly assumed. Most paleoanthropologists assume f lesh to have been the important n ew source of nutrients available with the advent of animal f ood consumption by hominids. Marrow and the brain have generally been overlooked as an important and r egularly available source of nutrients. I ndeed, Shipman ( 1984) and P otts a nd Shipman ( 1981) , noting the high f requency of s tone tool cut marks on meatless f oot bones at early archaeological s ites, propose the exploitation of these carcass p arts f or non-dietary uses, such as the n eed f or tendon in t ool manufacture. While this might be t rue to an extent, i t i s clear f rom unpublished experimental s tudies I have carried out that to extract marrow within f oot bones, they must f irst be skinned, the tendon r emoved a nd u sually t he p eriosteum s craped away. All of these activities geared t o extracting marrow produce cut marks on the bone. O nly B inford ( 1981, 1 983) has previously recognized t he potentially l arge role of marrow a nd the brain i n hominid d ietary s trategies. I t i s the marrow and head c ontents, i n f act, f or which there appears to be an open s cavenging niche in certain ecological contexts today. I f hominids s cavenged exclusively f rom a bandoned f eu d k ills i n r iparian woodland, f lesh would i ndeed have been r elatively unimportant. If s cavenging was r egularly practiced in t he a ccumulation of archaeological bone a ssemblages, i t can b e s uggested that the novel s et of behaviors resulting i n d ense concentrations of s tone a rtifacts and bone were distinctly dry s eason activities. S peth and Davis ( 1976) have previously s uggested many of the early s ites a t Olduvai to be dry s eason s ites owing to the p resence of l arge numbers of carapaces of turtles available o nly during the dry s eason. The f indings on s cavenging s uggest that a ll good f eeding opportunities, whether f rom f eu d k ills o r natural deaths, consistently occur during the d ry season. I t therefore becomes possible that archaeologists s tudying the Plio-Pleistocene r ecord are being informed o nly of d ry s eason a ctivities, and have l ittle or no information o n activities conducted during r ainy p eriods. 1 46

Finally, i f Plio-Pleistocene hominids c an be demonstrated to h ave s cavenged, i t i s possible that central-place f oraging and the s uite of other h uman traits associated with the use of a home base had yet to evolve. This has been s uggested to be the c ase if hominids s cavenged f rom the most r eadily accessible s ource, i .e. abandoned f eu d k ills i n r iparian woodland. The r emains of f eu d kills of medium-sized adults, though providing substantial nourishment f or a s ingle i ndividual, i s suggested to come in i nsufficiently l arge packets both to f inance central-place f oraging a nd to afford the s urplus s o essential to r eciprocity s ystems of the f ood s haring/home base model of e arly hominid l ifestyles. Archaeological evidence that would be c onsistent with s cavenging exclusively within r iparian woodland i nclude the f our criteria presented earlier, with emphasis placed on a strong predominance of cut marks on bones d efleshed very late by modern f eu ds. Although the current evidence s hows cut marks to have a wider s keletal d istribution, i t i s uncertain which of these were i nflicted to disarticulate defleshed bone, t o r emove small s craps of f lesh, or to strip away whole muscle bundles. S cavenging, h owever, n eed not a lways imply routed f oraging. During dry periods of h igh natural mortality, f lesh can be s cavenged i n quantities many t imes l arger than individual daily a ppetites. The added energetic c osts of central-place f oraging could not only be f inanced by the bonanza provided by natural d eaths, but the carcasses would also provide a l arge s urplus of f ood that could be brought to t he home base and d istributed among group members. Archaeological evidence consistent with the r egular s cavenging of natural deaths would i nclude a l arger r ange of body parts than those a cquired by s cavenging abandoned f elid kills alone, particularly a h igh proportion of a xial parts in addition to l imb parts. T he greater d iversity of parts should also d isplay a more even s keletal d istribution of c ut marks i nflicted while r emoving muscle bundles. Finally, l imb bones s hould be r ecovered l argely complete, as marrow of natural d eaths i s of poor quality and an unattractive s ource of nutrients both t o hominids and s cavengers of abandoned hominid s ites. S cavenging n atural deaths through c entral-place f oraging does n ot r equire the existence of a s exual division of s ubsistence l abor a nd t he a ssociated d aily f issioning of a group i nto f oraging s ub-groups that reassemble to exchange f oods c ollected during the day. Scavenging natural deaths i n open vegetation habitats, i n f act, was s uggested to h ave r equired organized anti-predator tactics, the effectiveness of which would be enhanced by l arge group s izes. R isk of e xposure to predation would s till place a premium on r eturn t o a riparian r efuge, but here, the hominid group would move a s a whole unit throughout the daily round. I t may be that only the advent of r egular hunting, often r equiring

1 47

d iscreet s talking t actics, that s elected f or t he s uperimposition upon central-place f oraging of d aily f issioning of groups i nto s pecialized s ubgroups and o ther s ubsistence-related s ocial adjustments s een i n modern hunter-gatherers. C entral-place f oraging c an b e viewed i n this way a s a preadaptation origins i n s cavenging.

to

1 48

s uccessful

hunting,

w ith

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