The metabolism of bats with special reference to hibernation in Tadarida mexicana

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THE METABOLISM OF BATS WITH SPECIAL REFERENCE TO HIBERNATION IN TADARIDA MEXICANA

A Thesis Presented to the Faculty of the Department of Zoology The University of Southern California

In Partial Fulfillment of the Requirements for the Degree Master of Science

by Takayoshi Kawahara June 1942

UMI Number: EP67148

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T h i s thesis, w r i t t e n by

.TAKAXQ5HI..KAWAHAHA. u n d e r the d i r e c t i o n o f h . X s F a c u l t y C o m m i t t e e , a n d a p p r o v e d b y a l l i t s m e m b e r s , has b ee n p r e s e n t e d to a n d a c c e p t e d by the C o u n c i l on G ra du a te S tu d y and Research in p a r t i a l f u l f i l l ­ m e n t o f the r e q u ir e m e n ts f o r the degree o f

MASTER OF SCIENCE

Dean

S e c re ta ry

D a t e ..

y C o m m itte e t

ACKNOWLEDGMENTS The author is greatly indebted to Dr* Francis M. Baldwin and Mr* Ozro B* Wisell of the Department of Zoology for their sincere desire to aid the author whenever problem­ atic difficulties are confronted, to Mr* Kenneth Stager, Mammalogist, Los Angeles County Museum, and Mrs* Kenneth Stager, for their earnest effort in obtaining the bats from San Gabriel Canyon and Pasadena, for without their aid, this experiment cannot be accomplished, to Mr* George Willett, Mammalogist, Los Angeles County Museum, for the use of his library; and to Mrs. Tema S. Clare of the Department of Botany and Dr. Catherine V. Beers for their willing co­ operation to criticize and correct the contents of this thesis.

TABUS OF CONTENTS CHAPTER I.

pAGE INTRODUCTION ......................................

1

Classification of the American bats of the genus T a d a r i d a ..................................... Notes on the American bats of genus Tadarida

1

«.

1



3

Key to the American bats of the genus Tadarida Taxonomic position of Tadarida mexicana (Saussure)

.................................

5

...................................

5

Type l o c a l i t y ...............................

5

Geographic distribution

6

Diagnosis

II,

.....................

R e m a r k s .....................................

6

HIBERNATION OF B A T S .............................

7

Review of literature with some observations on hibernation by others



Original observations on Tadarida mexicana . III.

THE METABOLISM OF HIBERNATING BAT:

.♦

7 24

TADARIDA

M E X I C A N A .......................................

26

The technique of s p i r o m e t r y ...................

26

P r o c e d u r e ...................................

26

The compilation of experimental d a t a ..........

28

The interpretation of d a t a .....................

45

IV.

S U M M A R Y ..........................................

46

V.

C ONCLUSION ........................................

51

BIBLIOGRAPHY...........................................

52

LIST OF ILLUSTRATIONS FIGURE

1*

PAGE S p i r o m e t e r .........................................

27

CHAPTER I INTRODUCTION I.

CLASSIFICATION OF THE AMERICAN BATS OF THE GENUS TADARIDA

The experimental study of hats for comparative purposes has aroused the interest of a considerable number of mammalogists.

However, owing to the recency of this interest,

references concerning the physiology of bats are few, indeed. Bats belong to the class Mammalia, order Chiroptera. The bats on which I have done specific studies on hibernation and particularly metabolism belong to the genus Tadarida and species mexicana. However, to obtain a comprehensive knowledge of Tadarida mexicana, it is best to review notes on the American bats of th8 genus Tadarida by Shamel (1931). II.

NOTES ON THE AMERICAN BATS OF GENUS TADARIDA The bats of the genus Tadarida^ were formerly known

as Nyctinomus.

The American forms fell into two groups, one

^ Shamel, Notes on the American Bats of the Genus Tadarida, Proc. U. S. Nat. Mus., Vol. 78, Art. 19, pp. 1-2, 1931. 2

See Lyon, Proc. Biol. Soo. Washington, Vol. 27, pp. 217-218, Oct. 31, 1914.

of which was named Nyctinomops in 1902 by Miller,3 who later found that the characters which divide the American species into two groups do not so divide the genus as a whole, when its Old World members are taken into account*

The name

Nyctinomops has therefore been placed in synonymy.^ The genus Tadarida belongs to the group of bats known as free tail bats, because the tail extends for some distance beyond the edge of the interfemoral membrane*

Members of

this genus are readily distinguished, externally, from all other American members of the family Molossidae, to which it belongs, by the deep vertical grooves or wrinkles on the upper lip; all the other American genera have smooth upper lips*

According to Oldfield Thomas,5 the primary features

to be considered in the separation of the skulls of this genus from the skulls of nearly related genera are as follows first, the definite Z-shaped form of the last upper molar (m3); second, the separation of the premaxillae between the upper incisors* The material examined gives a very satisfactory survey 3

Miller, Twenty New American Bats, Proc* Acad. Nat. Sci., Philadelphia, pp. 393, Sept. 12, 1902. 4 Miller, Families and Genera of Bats, Bull. U. S. Nat Hus . , 57, pp. 251-263, June 29, 1907. 5

Thomas, Journ. Bombay Nat. Hist. Soc., Vol. aa, No. 1, pp. 90-91, Apr. 21, 1913.

3 of the genus, so far as its American forms are concerned, ex­ cept that sufficient specimens of T. macrotis, T. femorosacca, and T* aurispinosa have not been at hand to definitely settle their status#

Many more specimens of T. macrotis are needed

both from North America and South America, and good skulls from the West Indies, before one can decide how many of the names applied to this large Tadarida designate valid local forms.

Adult skins of T. femorosacca are needed to determine

whether the apparent difference in color between this form and its South American ally, T. laticaudata, is as great as it appears.

Specimens are needed from eastern Brazil to

decide the real status of T# aurispinosa, the type and only known specimen of which has no skill.

The differences in

this skin from skins of T. laticaudata are very slight. It might be well to observe here that the type of Nyctinomus orthotis proves to be a Eumops® and that Nyctinomus kalinowskii is now placed in the genus Mormopterus.7 III.

KEY TO THE AMERICAN BATS OF THE GENUS TADARIDA8

First or brasiliensis group.--Second phalanx of fourth finger long, 6.6-9.2 mm.; ears extending to or falling short of

6 Miller, Proc. Biol. Soc. Washington, Vol. 19, p. 85, May 8, 1906. 7 Miller, Bull. U. S. Nat. Mus., 57, pp. 254, 1907. ft

° Shamel, Notes on the American Bats of the Genus Tadarida, Proc. U. S. Nat* Mus., Vol. 78, Art. 19, pp. 3, 1931.

4 end of rostrum when laid forward, inner edges not united at base; slight indication of pocket in membrane at the angle of femur and tibia; skull with broad flat rostrum, considerably wider anteriorly than at point of least interorbital breadth; weak sagittal crest; upper incisors strongly converging at tips* Ear not reaching end of rostrum when laid forward* Total length of skull 17.0 mm. or over • • cynocephala Total length of skull always under 17.0 mm. Forearm 40.0-42.5 m m .................. murina Forearm 36.6-41.4 m m .................. antillularum Ear extending to end of rostrum when laid forward. Total length of skull never over 16.2 mm.; transverse area of maxillary teeth smaller . . • intermedia Total length of skull always 16.2 mm. or over; transverse area of maxillary teeth larger. Forearm frequently under 40.0 mm. . . . muscula Forearm always 40.0 mm. or over. Breadth at m2 frequently reaching 7.0 mm. Forearm 42.2-46.4 m m .................. brasiliensis Forearm 41.6-43.8 m m .................. constanzae Breadth at m2 always less than 7.0 mm. Color lighter, verona brown ........ . bahamensis Color darker, mummy brown ............ mexicana Second or macrotis group.--Second phalanx or fourth finger short, 2.0-4.4 mm.; ears extending well beyond end of ros­ trum when laid forward, inner edges united at base for about 2 m m . ; well-developed pocket in membrane at angle of femur and tibia; skill1 with slender rostrum, scarcely wider anteriorly than at point at least interorbital breadth; prominent sagittal crest; upper incisors parallel. Forearm always 58.0 mm* or over Total length of skull 22.2-24.0 mm. . • . macrotis Forearm 50.0 mm. or less Tibia over 15.0 mm. ...................aurispinosa Total length of skull 18.0 mm. or over. Occipital depth of skull 6.4-7.0 mm. laticaudata Occipital depth of skull 6.0-6.4 mm. femorosacca Total length of skull usually 18.0 mm. or less Basal length 13.5-14.2 m m ........... europs Basal length 14.6-15.2 m m ........... yuctaniea

5 IV.

TAXONOMIC POSITION OF TADARIDA MEXICANA (SAUSSUR2)9 Diagnosis*

The forearm is 40.4-46*6 mm. in length,

usually ranging somewhere between 42 and 44 mm. seldom reaches 45 and rarely goes below 42. from 11*4-13.0 mm., average about 12. 17.8 mm., usually 16.2-17.4 mm. when laid forward.

It only

Length of tibia

Length of skull 16.2-

Ear extends to end of nostril

When the skulls are compared with those

of T. brasiliensis the greatest difference is seen to be the narrower rostrum in the Mexican form.

In 18 skulls of T.

brasiliensis the width at m 2 ranges from 6.2 to 6.8 mm.

In

only one of the South American skulls measured is the total length under 17.0 mm., while in T. mexicana it is frequently less than 17.0 mm.

The tibia averages somewhat shorter than

that of T. brasiliensis. Type locality.

Ameca, Jalisco, Mexico.

The describers

select no specimen as type, but give as the habitat the plateau of Mexico.

Specimens are mentioned from Ameca, Jalisco, and

from Gofre de Perote, Vera Cruz.

In the United States National

Museum collection are three specimens from San Pedro, Jalisco, which is in the immediate vicinity of Ameca, and twenty-three others from various places in Jalisco; therefore Ameca, Jalisco were chosen as the type locality. 9

Shamel, Notes on the American Bats of the Genus Tadarida, Proc. U. S. Nat. Mus., Vol. 78, Art. 19, pp. 5, 1931.

6 Geographic distribution*

Mexico, Texas, New Mexico,

Arizona, California, Lower California, Utah, and Oregon. Remarks.

This species is plentifully represented in

the United States National Museum collection.

Its range

covers a territory in Mexico on the south from the state of Pueblo north to central Texas (Austin on the east, and as far north as Brazos, Palo Pinto County), thence west through New Mexico, Arizona, Utah (north-eastern), and from Southern California north to Chico in Butte County and Fort Dallas, Oregon.

This bat has a remarkably extensive range for a

single species and varies only slightly in intensity of color, mummy brown to Proutfs brown (Ridgway 1912).

These varia­

tions are never localized, and dark and light examples can be matched from almost any two localities.

CHAPTER II HIBERNATION OF BATS I.

REVIEW OF LITERATURE WITH SOME OBSERVATIONS ON HIBERNATION BY OTHERS

Bats have a specific characteristic of undergoing hi­ bernation for an indefinite period with the approach of cold weather. They usually find caves, old abandoned buildings, and undersides of bridges as the most suitable places to hi­ bernate. Allen (1939) describes very fully from his investiga­ tions on the hibernation of the various types of bats found in the United States in the following paragraphs. Allen and his associates investigated a cave in Chittenden County, Vermont, during the month of January. temperature of the cave registered 42 degrees F.

The

He describes

his investigations in the following quotation. "We made out a few bats hanging singly from the roof, their long fur beaded with moisture, giving them a silvery appearance, while beyond in an angle of the sloping roof, some four or five feet from the floor, were two solid masses of bats, hanging in the silence of a deep sleep. these carefully.

We examined

The smaller mass consisted of about three

8 hundred and the larger we carefully estimated to contain nearly a thousand individuals, of which the greater part latex* proved to he the cluster bat (Myotis sodalis), a small chestnut-brown species.

They felt cool to the touch, but

one’s fingers thrust into the mass disclosed a warmth from their bodies, Ey this time our lights and the disturbance incident to handling them, even though slightly, started waking them up.

Soon many were flying about the cavern, or settling in

new places on the ceiling, while others disappeared through a low passage to another part of the cave.

Eere was a smaller

and cooler chamber with a cool air current from it to the first chamber.

One little cluster of bats consisted of six

males; a neighboring group of fifteen was made up of six males and nine females, so that there was no definite segre­ gation of the sexes, while a series of twenty-four picked at random was almost equally divided between the two.

Further

search later showed no less than six species of bats winter­ ing in this cave.

Ey far the greater number of them were

the cluster bats, with scattering individuals of two related and closely similar species, the little brown bat (Myotis lucifugus), with yellowish belly and browner back, and the longer-eared M. keenii septentrional!s.

These may have been

associated with cluster bats more closely than we could determine in the dim light.

With these were a few of the

9 big brown bat (Eptesieus fuscus), a relative of the European Serotine bat. We next examined more carefully the smaller pockets and crevices of the rocky walls and presently discovered a fourth and much rarer species of Myotis, showing the yellow­ ish fur and deep black face and ears of the least brown bat (M, subulatus leibii).

These last were evidently of

solitary habits, tucked away one or rarely two together, apart from the main groups, lying belly down in a narrow chink or crevice instead of being suspended by the hind feet. Perhaps their very much smaller and more delicate feet may be correlated with this habit,

A few bats hung here and

there separately from the walls, their fur glistening in the light of the lamps, with little beads of condensed moisture* Some of these proved to be the eastern pipistrelle (Pipistrellus subflavus), again a rather unsocial species.

Here,

then, were no less than six species of three genera gathered in from the neighboring region for the winter.

The differ­

ence in their habits and liking for company was a noticeable characteristic,”10 "Although it was not possible to keep the wintering bats under observation during their dormant period, this has been

G-. M. Allen, Bats (Boston: Harvard University Press, 1939), pp. 266-268.

10 done by Hahn (1908) in Indiana.

Some of his notes made in

caves at Mitchell, Indiana, bring out several interesting points.

The place was an underground tunnel in limestone,

with a stream flowing in it, whereby the caverns had been dissolved out.

The roof had broken through in two places,

making large openings.

Temperature records were kept for two

years in the main chamber and showed a remarkable uniformity, varying from about 51 degrees F. in January to 57 degrees in September.

The air was always near the saturation point here.

During the summer the bats are absent but begin to come in by early November.

In the winter of 1906-190? at least five

hundred bats were hibernating here, representing five species, of which the most abundant was probably the cluster bat (Myotis sodalis).

Two other species of Myotis (M. keenii

septentrionalis, a long-eared species, end M. velifer, a larger one), as well as Pipistrellus subflavus and the longeared bat (Corynorhinus), were among the species wintering here.

This cave at a more southern latitude (about 37 degrees

north), was much warmer than the one visited in central Yermont, v/hich may account for the apparently greater activ­ ity of its inmates.

By carefully marking the spots where

individual bats hung dormant, Hahn found that they frequently awake during the winter months and changed about.

Thus, of

eighteen bats kept under observation from November 19 to December 3, fourteen had moved within a week, and not one

11 remained in the same spot for two weeks.

Later in the winter,

however, an exceptional bat stayed in one spot from February 4 to 27.

The pipistrelles seemed less active and averaged

about two weeks in a given spot, while one remained stationary for forty-four days.

In general those that come in during

the autumn are fat and well nourished and soon settle down for a prolonged period of lethargy.

As the winter advances,

the stored fat is drawn upon, and as the bats become thinner their active periods become more frequent.

Those seen in

clusters now are usually awake and chattering.

In late winter

and early spring they often fly to the mouth of the cave and may even hang up for further sleep near the entrance.

There

is thus a constant testing of the conditions at the entrance, until in April or May the outer temperature permits them to leave their retreat for the summerfs activity.

Since these

species pass the summer daylight hours in sleep and hibernate for the greater part of the cold season, Hahn concludes that they must spend nearly five-sixths of their life hanging head downward in the dark. Further studies of cave bats in southern Minnesota by Swanson and Evans (1936) confirm Hahn’s observations.

Here

the prevailing species was the big brown bat (Eptesicus fuscus), with smaller numbers of Pipistrellus and two species of Myotis (M. lucifugus and the longer-eared M. keenii septentrionalis).

Visits to the cave in January, February,

12 and March showed fluctuations in the numbers of bats to be counted*

They were readily disturbed and evidently changed

about from time to time*

Individuals varied greatly in the

degree of torpidity, some awakening much more quickly than others when examined and flying with little provocation, while others required as much as three quarters of an hour to com© back to consciousness, even when taken out into daylight* The cave temperature in this case varied during the winter from 42 degrees to 45 degrees F. An interesting experiment with a big brown bat is re­ lated by Dr* Alexander Wetmore (1936).

In early December, at

Washington, D. C . , as he let down an awning over his office window, one of these bats was dislodged from the folds and lay sprawling on the window sill, partly numb with the cold* Constructing a small wooden box, with a double layer of woollen blanket tacked on the inside, he hung the bat in this, upside down, within the folds of blanket.

The box was then

fastened to the sill and a thermometer placed in it, in such a way that the bulb rested beside the section of blanket con­ taining the bat, while the graduated stem projected from the box where it could be read.

A second thermometer fastened

close by gave the external air temperature.

While the box

was not shaded from the afternoon sun, it was exposed to the sweep of west and northwest winds. until March 16.

Here it remained unopened

By this time these bats have already begun

13 to fly in the latitude of Washington,

The box was therefore

taken inside and the lid carefully lifted.

The bat was found

alive and awake, although it had crawled outside the blanket fold, where it had been placed the previous December,

Care­

fully replacing the lid, Dr, Wetmore returned the box to the window sill for four more days, when, on opening it once more, the bat was found resting in a semitorpid state beneath a bit of the blanket on the floor of its prison.

The box

was left open, and at about half-past five that afternnon the bat was found to have flown.

The range of outdoor temperature

during the winter of its confinement was much the same as that within the box, varying from 9.6 degrees C. in December to as low as -14 degrees in January (that is, from about 49 degrees F, to about 7 degrees above zero).

Of course, being

enwrapped in the woolen blanket and sheltered from wind, the bat was able to conserve heat, which allowed it to withstand these lower temperatures, yet the experiment shows a great resistance to cold. This interesting habit of hibernation or practical sus­ pension of bodily activity for the cold season is not common to all bats, but occurs chiefly in those members of the family Vespertilionidae that range into the north-temperate zone. These include, therefore, species in the northern hemisphere belonging to the genera Myotis, Pipistrellus, Nyctalus, Eptesicus, Barbastella, Miniopterus, Plecotus, Corynorhinus,

14 and perhaps some others*

Probably some of the bats in the

more southern parts of the southern hemisphere hibernate, but no evidence bearing on this point is known to me*

The horse­

shoe bats of the Old World family Rhinolophidae, though mostly confined to the tropics, are represented in central Europe and as far north as the British Isles by a larger and a smaller species*

Of these at least the latter is known to

hibernate regularly, while of the former, the observations of Coward in England seem to indicate that although they spend the winter in caves, their sleep is not very deep. have been

This may

correlated with the comparatively warm temperature

of the caves, 50-52 degrees F . , for when visited they were quick to awaken and fly, while frequent observations showed that they moved about at intervals or even went outside in warm spells to feed*

A third family, the Molossidae, or free­

tailed bats, may probably be included among the hibernators* This group is represented in the tropics of both the Old and the New World, and a few of the species extend to the southern boundaries of the temperate zone*

It seems likely that these

last undergo hibernation for a shorter or longer time at least. Thus, Vernon Bailey, writing of the "guano bat" of the south­ western United States, says that great numbers resort to the Carlsbad Cavern to winter.

He did not arrive, however, until

March 11, when a few of the bats were already active, leaving their cavern on warm evenings; but a series of cold nights

15 following kept them in for a week or more, although a few would be found flying about inside the extensive underground chambers, from which they refused to go into the colder air outside•

He found the temperature near the floor of the cave

to be usually 55 degrees F . , and it was said to vary but little throughout the year*

A number of the bats were cap­

tured for further obsdrvation, and he found that when the temperature fell to 50 degrees, they became torpid at night, stiff and unconscious, with a body temperature the same as that of the air*

If gradually warmed to 60 degrees, they be­

came active, awakening slowly with slight movements of the stiff wings and legs* Another larger species of this same family, Eumops californicus, the California mastiff bat, is a northern member of the genus, the other members of which are confined to tropical America, including some of the larger West Indian islands*

Its habits have been studied by A* B. Howell (1920)

in San Bernardino County and a few other localities in Southern California.

He finds that they become torpid for

short periods in winter, though even in late November a colony under observation was still active*

One he kept in captivity

remained dormant from November 5 to December 15, appearing so inert that it showed no sign of life until he moved its wings* Yet in mid-February, when there was a skim of ice every night, one roosted in an improvised shelter for only three nights*

16 He speaks of often seeing them vibrate their wings while hanging up, doubtless a reaction for increasing the body temperature.

There may be short periods of cold days during

which the bats remain fasting in their retreats, even though they do not seem to undergo the deep lethargy of hibernating species. It will be interesting to review briefly the investi­ gations of various writers regarding the condition of the bats themselves when in this deep lethargy of hibernation. It comes on gradually with the cooling of the air in late autumn, and in general does not take place until the tempera­ ture outdoors fails to rise above 50 to 54 degrees F.

The

caves in which bats hibernate may, as we have seen, in mid­ winter show a more or less constant temperature that seldom falls below 42 degrees.

The hibernating bats themselves cool

so that the temperature of their bodies falls from as high as about 104 degrees when they are active (Barkow, 1846), to the temperature of the caves in which they rest.

Swanson and

Evans made many careful tests, using an accurate thermocouple, which makes it possible to test quickly a very small point of the surface without disturbing the torpid bat.

Thus, on

February 12, when the outside temperature fell to 35 degrees, one cave containing pipistrelles showed 45 degrees, while another, in which the big brown bats were hibernating, stood at 42 degrees.

A thermometer thrust among a group of the

17 latter registered 98 degrees F. after the bats had been slightly disturbed, indicating that a certain amount of heat was developed in the group*

When, however, they were thor­

oughly dormant with the cave temperature at 44 degrees, the thermocouple applied to both the surface of the back and just inside the mouth of a bat showed exactly the same degree of heat, 44 degrees.

Others were placed in a cabinet that main­

tained a constant temperature of 45 degrees, and by means of an ingenious device, their temperatures were taken at the same points without having to open the cabinet, with the result that both on the surface of the body and within the mouth the bats showed exactly 45 degrees.

The temperature

of this chamber was now gradually lowered until it stood at 27.6 degrees F. or about 4 degrees below freezing point, but now it was found that the temperature of the bats did not go so low, remaining from 1.5 to 4 degrees F. above that of the surrounding air in the chamber, but even then, it was only a fraction of a degree above 29 degrees.

Below this the ap­

paratus did not permit of cooling, but the authors believe that the bats would have died if further cooled.

In this

they were doubtless correct, for the same experiments have been performed in Europe. A Russian physiologist, using the noctule bat (Nyctalus noctula), the long-eared bat (Plecotus), and the little Daubenton’s bat (Myotis daubentonii), procured near Moscow in

18 June, subjected them to lowering of temperature and found that lethargy was induced as usual.

Ey further cooling, he even­

tually lowered the body temperatures to within 7 degrees F. of the freezing point (39 degrees F . ) and found that they would revive.

But if their bodily heat went as low as about

30 degrees F. (-1.2 degrees C.) ice crystals formed in the thin membranes of the wings.

Even then the bats revived, but

if ice crystals began to form, with further cooling, in the fluids of the lungs and heart, the tissue of these organs was destroyed and the bats died within about twenty minutes.

The

English physiologists, Burbank and Young (1934), and the German naturalist, Eisentraut (1934), almost simultaneously published the results of investigations independently carried on along these same lines.

They are remarkably in accord,

although the work of the former pair was carried on chiefly in the laboratory, while Eisentraut*s was mainly on cave bats near Berlin in their natural places of hibernation. All three agree that the hibernation of bats is merely a deeper sleep induced by lowered temperature and that bats have little power of controlling the heat of the body, but, like cold-blood vertebrates, take their temperature, when at rest, from that of the surrounding medium.

Even in summer

this holds true, for in their regular daily sleep, the body quickly cools to nearly or quite the room temperature.

This

is aided by the great expanse of naked membrane of the wings

19 and elsewhere, in which the blood comes close to the surface. Normally, in the latitude of Berlin, Eisentraut found that bats of several species begin to enter the caves in the latter part of November, the little pipistrelles first, with some variation according to the season, whether colder or warmer than average.

Of ten species he found wintering, only the

horseshoe bat (Rhinolophus ferrum-equinum) lives in the caves the year around, but in winter it seeks deeper levels below the frost line.

When normally active in summer bats may show

a body temperature as high as 104 degrees F. (40 degrees C.) or even a degree or two higher (to 105*7 degrees F . , according to Burbank and Young)*

When they commence hibernation, as in

ordinary sleep, this soon falls as the bat becomes inactive, and soon the temperature is that of the surrounding air*

The

critical temperature at which the usual nightly activity is suspended and deeper lethargy commences is from about 46 degrees to 50 degrees F* (8 degrees to 10 degrees C.), accord­ ing to Eisentraut.

Below this critical threshold ordinary

sleep passes uninterrupted into deeper lethargy and finally into complete torpidity, with the rate of respiration so low that it is hardly perceptible, and the respiratory movements so reduced that one may wait sometimes several minutes without seeing a sign of life.

Swanson and Evans found that in a

hibernating big brown bat breathing was irregular and there were long periods when motion completely stopped, lasting

20 from three to eight minutes, with an average of only 4*6 respirations in half an hour.

These resting periods would be

followed by intervals when for about three minutes or less there might be from 23 to 48 breathings a minute.

When grad­

ually awakened, thi3 rate rose to 200 a minute before the bat was able to fly.

As to the duration of this lethargy

Eisentraut found, as did the American observers quoted, that a short waking period may not be infrequent in captive indi­ viduals kept in cool chambers, though often it may be only a few hours before the bat relapses again into its hibernating state.

During this period it may fly about and feed a little

if cave insects or spiders are available.

Especially, will

it drink, since the thinness of the membranes allows consid­ erable evaporation of moisture and the bats wake up thirsty* For this reason, too, they prefer a cave with a moist atmos­ phere, though different species may vary somewhat in prefer­ ring very moist or drier situations.

Thus, Barbastella seems

to like the drier parts of caves, while the large Myotis myotis and its smaller relative M. daubentonii hibernate in such damp places that their fur may be beaded with drops of dew. Hibernation in bats has attracted the notice of European naturalists for more than a century, and many careful studies have been made of its causes and the changes it induces in the body.

Thus, Winiwarter (1926) found in hibernating bats in

Europe that the mucous lining of the larynx and windpipe

21 becomes thickened, so that less air is admitted in breathing, and the supply of oxygen is thereby reduced automatically* Delsaux (1887) has reviewed at length the observations of older writers on the subject and recounts his own experiments undertaken to determine the conditions under which the re­ duced activities of the body continue.

As early as 1808

Saissy noted the reduced breathing and stated that a hibernat­ ing bat could live for more than an hour in an atmosphere devoid of oxygen.

Delsaux worked with the long-eared plecotus

and the larger Myotis myotis from the grottoes of Maestricht and found them very sensitive to mechanical stimulation, quickly awakening when touched.

He observed the lowered rate

of respiration and the rapid rise of temperature with awaken­ ing.

He found that if a hibernating bat was deprived of

oxygen, it soon fell to the ground.

Later, Rulot (1901) made

some interesting studies of bats in hibernation from the same caves.

He found that at the end of the hibernating period

bats had lost about a third of their original weight, while the water content increased in proportion from 1.499 per cent of the weight in early February to 3.145 per cent by April 23. The proportion of fat is much greater in November at the be­ ginning of hibernation, when he found it was 19.4 per cent of the total weight. cent of total weight.

It gradually decreased to about 15 per The amount of glycogen contained in

the body he tested, but found only a very small amount, which

22 diminished slowly from November to March.

He concludes that

the amount of this substance is too little to be of much im­ portance as reserve food, but the consumption of stored fat and albumen is greater and is much more in late winter or early autumn than it is when the bat first enters upon hiber­ nation.

Further studies on the gases given out or taken in

during hibernation have been made by several authors.

After

careful experiments, Hari (1909) concludes that the exhalation of carbon-dioxide gas and the intake of oxygen are a little over 1 per cent of the amounts shown when the bat is in normal activity. Merzbacher (1903) has distinguished four stages in the normal awakening from deep hibernation.

The first is the

condition of rigidity shown in deep lethargy, with almost no response to handling.

The second stage is marked by the be­

ginnings of slight motion, especially observable if the bat is lying on its back, when it will slowly extend first one leg and then the other, as if endeavoring to find a foothold from which it may hang in the normal resting position.

This

is the so-called ,fhanging-reflex,” which will take place even though the fore part of the brain has been removed, and hence is owing to entrance into activity of the medulla oblongata. As the bat passes into the third stage, the forebrain begins to function, there is a more active response and the uttering of slight characteristic sounds, until finally, with fourth

23 stage, the bat is fully awake and flies away.

These stages,

however, are not very sharply marked, and the transition from one to the other is uninterrupted.

Every variation in in­

creasing activity is accompanied by a gradual rise of tempera­ ture of the body to the final active state. While carrying on their study of hibernating bats in England, Burbank and Young (1934) also made interesting notes on some large fruit bats, Pteropus, in captivity.

This

tropical species does not, of course, go into a hibernating state, but they found that it maintains a nearly constant body temperature of 33 degrees to 37 degrees C. (91 degrees to 98 degrees P.).

If the air temperature becomes cool, these

big bats, instead of becoming dormant, maintain their bodily warmth by continual activity, crawling about and shivering, and so by muscular movements producing heat through the chem­ ical processes involved. Thus, while the smaller bats of northern regions retain a very primitive, almost reptile-like, condition in their lack of any mechanism whereby the nervous system reacts to maintain a nearly even temperature, they nevertheless can survive cold climates by withdrawing locally into sheltered places for the winter and sleeping away the frigid season in a state so inactive that very little energy or food is re­ quired.

Eisentraut also points out that this ready response

to cool conditions may serve them even in summer, when a

24 series of chilly, rainy days reduces the activity of insects on which they feed.

For at such times they may hang up for

several days on end and, with lowered temperature and reduced bodily processes, undergo a long fast with little discomfort."^ II.

ORIGINAL OBSERVATIONS ON TADAH IDA MEXICANA

Observations on Tadarida mexicanawere made during their captivity.

I placed a group of sixteen bats in a screen cage

and left them in an ice refrigerator; the ice chamber was filled to the capacity at all times, thus maintaining a con­ stant temperature of 45 degrees F. in the compartment.

The

drainage was used for the admittance of air for purposes of ventilation. Immediately upon confronting this low temperature, they gradually became inactive.

They grouped themselves to­

gether and after two days, they had undergone lethargy.

How­

ever, upon opening the compartment door for four or five minutes, and admitting enough air (room temperature), and thus changing the temperature of the compartment, they grad­ ually became active and began walking from one end of the cage to the other. Refusal to feed during captivity is a very peculiar characteristic of Tadarida mexicana.

I have tried forceful

^ G. M. Allen, Bats (Boston: Harvard University Press, 1939)* pp. 268-279.

25 feeding by using medicine-dropper for liquid foods and forceps for solid food (insect) but obtained no success. Despite the fact that they refused to feed, they sur­ vived in that hibernating state at an average of seventeen days in the compartment at a temperature of 45 degrees F. The longest length of time of survival of a single specimen that I have observed was thirty-four days. They always hibernate in a downright position with the head hanging downward. The rate of breathing was hardly noticeable during lethargy; however, as they gradually became active, owing to the change in temperature, the rate of breathing became very noticeably rapid and, thus, very difficult to measure its frequency. They prefer a chamber with moisture rather than a dry one.

Their rate of activity in a dry chamber is more fre­

quent than in a moist chamber. In general, they hibernate best in temperature at 45 degrees F. or lower.

CHAPTER III THE METABOLISM OF HIBERNATING BAT: I.

TADARIDA MEXICANA

THE TECHNIQUE OF SPIROMETRY

The volume of air respired varies with the extent of the movements and the size of the animal#

This volume may be

determined readily in any given case by means of a spirometer# The construction of this apparatus is represented in page 27, Figure 1.

It consists of a cylinder (A) and a re­

ceiver (B) filled with water#

The cylinder A is counter­

balanced by a weight (D) so as to move up and down in the water of B with the least possible resistance#

The tube (G)

connected to the oxygen tank converges with tube (H) connect to the desiccator at point (I) and emerges to tube (C) which passes through the wall of B and ends in the interior of A above the level of the water# Procedure#

The animal is placed in the desiccator (for

active test— place desiccator within room temperature; for hibernation test— place desiccator in compartment of ice refrigerator); line the adjoining parts of the desiccator with vaseline to prevent leakage of oxygen.

The valve of the oxygen

tank is released very gradually; as oxygen is released it fills

12 W# H. Howell, A Textbook of Physiology (Philadelphia: ?/. B. Saunders Company, T940T, p T 657.

I

M

I

i I

both the desiccator and cylinder A.

The measuring lever (E)

lowers itself to 0*0 as cylinder A is being filled with oxygen at this point the valve is turned off. at point Z to prevent leakage of oxygen.

Use another stopper After five to eight

minutes have elapsed immediately after the oxygen tank valve has been turned off, check for leakage and if conditions prove favorable proceed with the test.

As the animal consumes

the oxygen, lever E will register the amount of oxygen (cc.) being consumed.

The carbondioxide expired by the animal will

be absorbed by the soda-lime placed on the bottom surface of the desiccator. Care must be taken to avoid any contact between the animal and the soda-lime, since the latter causes irritation to the tissues of the animal, thus resulting in an unsuccess­ ful test. II.

THE COMPILATION OF EXPERIMENTAL DATA

The following experimental data comprise a series of sixteen individual bats tested.

Since the life-span of this

species is very short-lived, a series of three separate tests have been given to a single bat both during their active and hibernating state.

The results obtained were very satisfac­

tory indeed. Data for bats numbers 10, 11, 13, 14, 15, and 16 in their active state were not taken, for these bats were already in a state of hibernation.

29 BAT NO. 1

(SEX* MALE)

NT. (GM. )

TIME (MIN. )

ACTIVE8, TOTAL 02 (CC.) CONSUMPTION

Feb. 3 3 3

9.3 9.3 9.3

60 60 60

100 101 100

10.75 10.86 10.75

4 4

9.2 9.2

60 60

102 104

11.09 11.31

Average

101.4

10.95

DATE OF TEST

CC» O2 PER GRAM PER HOUR

HIBERNATION6 CC. O2 TOTAL Os (CC.) PER GRAM CONSUMPTION PER HOUR

Feb. 7 7

9.0 9.0

60 60

10.0 11.4

1.11 1.26

9 9

8.8 8.8

60 60

11.0 12.0

1.25 1.36

10 10

8.8 8.8

60 60

11.0 10.0

1.25 1.13

12 12

8.7 8.7

60 60

10.0 11.0

1.15 1.26

10.8

1.22

Average

a Active*

at room temperature.

6 Hibernation*

at 45 degrees F.

30

BAT NO* 2

- •- Ti l l ..

(SEX* MALE)

1

DATE OP TEST

ACTIVE* TOTAL 02 (CC.) CONSUMPTION

CC. O2 PER GRAM PER HOUR

TNT. (GM.)

TIME (MIN.)

Feb* 4 4

10*0 10*0

60 60

102 104

10*20 10.40

5 5

10*0 10*0

60 60

100 103

10.00 10.30

6 6

9*9 9*9

60 60

104 102

10*51 10.30

Average

102.5

10.29

HIBERNATION5 TOTAL O2 (CC.) CONSUMPTION

CC*• ^2 PER GRAM PER HOUR

Feb. 7 7 7

9.9 9*9 9.9

60 60 60

12.00 10.00 12.00

1.21 1.01 1.21

8 8

9.8 9.8

60 60

12.00 13.20

1.22 1.35

11*84

1.20

Average

a Active:

at room temperature*

5 Hibernation:

at 45 degrees P.

31

BAT NO* 3

DATE OF TEST

ACTIVE8, TOTAL 02 (CC*) CONSUMPTION

(SEX: MALE)

CC. 02 PER GRAM PER HOUR

WT. (GM.)

TIME (MIN. )

Feb* 2 2 2

9.2 9*2 9.2

60 60 60

96 100 98

10*43 10.87 10.65

3

9.1

60

100

10.99

Average

98*5

HIBERNATION5 TOTAL 02 (CC.) CONSUMPTION

CC. 02 PER GRAM PER HOUR

10.74

Feb* 7 8

9.0 8.9

60 60

12 10

1.33 1.12

9

8.9

60

12

1.35

10 10

8.9 8.9

60 60

11 10

1.24 1.12

11

1.23

Average

a Active:

at room temperature.

^ Hibeniation:

at 45 degrees P*

32 BAT NO. 4

DATE OF TEST

ACTIVE* TOTAL 02 (CC.) CONSUMPTION

(SEXs MALE)

CC. 02 PER GRAM PER HOUR

TUT. (GM.)

TIME (MIN.)

Feb. 2 2

8.2 8.2

60 60

96 98

11.71 11.95

3

8.2

60

96

11.71

4

8.1

60

97

11.98

5

8.0

60

96

12.00

Average

96.6

11.87

HIBERNATION6 TOTAL 02 (CC.) CONSUMPTION

CC. 02 PER GRAM PER HOUR

Feb. 16 16 16

7.8 7.8 7.8

60 60 60

12 10 12

1.54 1.28 1.54

17 17

7.8 7.8

60 60

12 10

1.54 1.28

11.2

1.44

Average

a Actives

at room temperature.

6 Hibernations

at 45 degrees F.

33

BAT NO. 5 (SEX* MALE)

TUT. (OM.)

TIME (MIN. )

ACTIVE* TOTAL Og (CC.) CONSUMPTION

Feb. 4 4 4

9.1 9.1 9.1

60 60 60

120 118 119

15.19 12.97 13.08

5

9.0

60

118

13.33

Average

118.75

13.14

DATE OF TEST

CC. 02 PER GRAM PER HOUR

HIBERNATION6 TOTAL 02 (CC.) CONSUMPTION

CC. 02 PER GRAM PER HOUR

Feb. 13 13 13

8.9 8.9 8.9

60 60 60

8 10 12

•90 1.12 1.35

14 14

8.9 8.9

60 60

10 10

1.12 1.12

10

1.12

Average

a Active:

at room temperature

6 Hibernation:

at 45 degrees F.

34

BAT NO. 6

DATE OF TEST

ACTIVE* TOTAL 02 (CC.) CONSUMPTION

(SEXs MATE)

cc. o2 PER GRAM PER HOUR

WT. (GM.)

TIME (MIN. )

Feb* 2 2

10.4 10*4

60 60

90 92

8.65 8.85

3 3

10*3 10.3

60 60

92 92

8.93 8.93

Average

91.5

8.84

HIBERNATION^ TOTAL 02 (CC.) CONSUMPTION

CC. 02 PER GRAM PER HOUR

Feb. 12 12

10.0 10.0

60 60

12 12

1.20 1.20

13 13

9.8 9.8

60 60

10 10

1.02 1.02

11

1*11

Average

a Activei

at room temperature,

k Hibernation!

at 45 degrees F*

35

BAT NO. 7

DATE OP TEST

tut.

(GM.)

ACTIVE* TIME TOTAL 02 (CC.) CONSUMPTION (MIN. )

(SEXi MALE)

CC. 02 PER GRAM PER HOUR

Feb# 2

8.1

60

114

14.07

3

8.1

60

112

13.83

4

8.0

60

114

14.25

Average

113.33

14.05

HIBERNATION^ TOTAL 02 (CC.) CONSUMPTION

CC. 02 PER GRAM PER HOUR

Feb. 8 8

7.8 7.8

60 60

12 12

1.54 1.54

15 15

7.6 7.6

60 60

10 10

1.32 1.32

11

1.43

Average

&

Active*

at room temperature,

k Hibernation*

at 45 degrees P.

36

BAT NO. 8

(SEX* MALE)

WT. (GM*)

TIME (MIN* )

ACTIVE6 TOTAL 02 (CC.) CONSUMPTION

CC* Oo PER GRAM PER HOUR

Feb* 4 4

8.1 8.1

60 60

100 104

12.35 12*84

5 5

8.0 8.0

60 60

98 102

12.25 12.75

Average

101

12*55

DATE OF TEST

Feb. 17 17 17

7.8 7.8 7.8

HIBERNATION*5 TOTAL Og (CC*) CONSUMPTION

CC. 02 PER GRAM PER HOUR

12 12 10

1.54 1.54 1.28

11.33

1.45

60 60 60

Average

a Active*

at room temperature.

^ Hibernation*

at 45 degrees F,

37

BAT NO* 9

DATE OF TEST

(SEXt MALE)

ACTIVE® TOTAL 02 (CC*) CONSUMPTION

CC. 02 PER GRAM PER HOUR

TflT. (GM.)

TIME (MIN. )

Feb. 2

8*4

60

108

12.86

3

8.4

60

102

12.14

4

8.3

60

106

12,77

5

8.3

60

104

12.53

Average

105

12.58

HIBERNATION15 TOTAL 02 (CC.) CONSUMPTION

CC. 02 PER GRAM PER HOUR

Feb. 7 7 7

8.1 8.1 8.1

60 60 60

10.00 8.00 7.00

1.23 .99 .86

8 8

8.0 8.0

60 60

7.00 8.00

.88 1.00

8.00

.99

Average

g

Activet

at room temperature*

^ Hibernationj

at 45 degrees F.

38

BAT NO. 10

DATE OF TEST

wr. (GM.)

ACTIVE®’ TIME TOTAL 02 (CC.) (MIN. ) CONSUMPTION

(SEX: MALE)

CC. 02 PER GRAM PER HOUR

HIBERNATION*5 TOTAL 02 (CC.) CONSUMPTION

CC. 02 PER GRAM PER HOUR

Feb. 12 12 12

8.2 8.2 8.2

60 60 60

10 8 8

1.22 .98 .98

15 15 15

8.0 8.0 8.0

60 60 60

8 9 9

1.00 1.15 1.15

8.67

1.07

Average

A

Active* at room temperature. k Hibernation*

at 45 degrees F.

39

BAT NO. 11 (SEXi MALE)

DATE OF TEST

ACTIVE®TOTAL 02 (CC.) CONSUMPTION

HIBERNATION5 TOTAL 02 (CC.) CONSUMPTION

CC. ©2 PER GRAM PEP HOUR

60 60 60 60

6 8 8 7

.65 .87 .87 .76

60 60

10 10

1.11 1.11

WT. (GM.)

TIME (MIN. )

Feb* 19 19 19 19

9*2 9.2 9.2 9.2

21 21

9.0 9.0

CC. 02 PER GRAM PER HOUR

Average

Active*

at room temperature.

5 Hibernation*

at 45 degrees F.

8.17

.90

40

BAT BO. 12

DATE OF TEST

Feb. 4 4

WT. (GM.)

TIME (MIN. )

9.0 9.0

60 60

Average

Feb. 7 7 7

8.9 8.9 8.9

ACTIVE®TOTAL 02 (CC.) CONSUMPTION

(SEXi MALE)

CC. O 2 PER GRAM PER HOUR

98 98

10.89 10.89

98

10.89

60 60 60

Average

£

Activet

at room temperature.

^ Hibernation:

at 45 degrees P.

HIBERNATION1* TOTAL Oo (CC.) CONSUMPTION

CC. 02 PER GRAM PER HOUR

9.00 8.00 9.00

1.01 .90 1.01

8.67

.97

41

BAT NO* 13

DATE OF TEST

(SEX* MALE)

ACTIVE* TOTAL 02 (CC.) CONSUMPTION

CC,• 02 PER GRAM PER HOUR

HIBERNATION*5 TOTAL 02 (CC.) CONSUMPTION

CC. ©2 PER GRAM PER HOUR

ITT. (GM.)

TIME (MIN. )

Feb. 4 4 4

8.5 8.5 8.5

60 60 60

14.00 12.00 12.00

1.65 1.41 1.41

5 5

8.5 8.5

60 60

11.00 12.00

1.29 1.41

12.20

1.44

Average

a Active*

at room temperature,

k Hibernation*

at 45 degrees F.

42

BAT NO. 14

DATE OF TEST

ACTIVEX TOTAL O2 (CC.) CONSUMPTION

(SEXt MALE)

CC .Og PER GRAM PER HOUR

HIBERNATION6 TOTAL 02 (CC.) CONSUMPTION

CC. Ojj PER GRAM PER HOUR

(GM. )

TIME (MIN.)

Feb. 7 7 7

9.0 9.0 9.0

60 60 60

8.00 6.00 8.00

•89 .67 .89

8 8

8.9 8.9

60 60

6.00 7.00

.67 .79

12

8.7

60

6.00

.69

6.85

.77

ht.

Average

A

Active*

at room temperature

^ Hibernation*

at 45 degrees F.

43

BAT NO. 15

DATE OF TEST

WT* (GM.)

ACTIVE0TIME TOTAL Oz (CC.) CONSUMPTION (MIN. )

(SEXt MALE)

CC. 02 PER GRAM PER HOUR

HIBERNATION^ TOTAL 02 (CC.) CONSUMPTION

CC. Oj> PER GRAM PER HOUR

Feb. 8 8

9.1 9.1

60 60

8.00 8.00

.88 •88

9

9.1

60

9.00

.99

10

9.0

60

8.00

.89

8.25

.91

Average

a Activei

at room temperature,

^ Hibernationt at 45 degrees F.

44

BAT NO. 16 (SEXt MALE)

DATE OF TEST

ACTIVE11 TOTAL 02 (CC.) CONSUMPTION

CC. Og PER GRAM PER HOUR

HIBERNATION^ TOTAL 02 (CC.) CONSUMPTION

CC. Og PER GRAM PER HOUR

WT. (GM. )

TIME (MIN.)

Feb. 4 4 4

8.1 8.1 8.1

60 60 60

6.00 6.00 7.00

.74 .74 .86

5

8.1

60

6.00

.74

6.25

.77

Average

ft Active*

at room temperature*

k Hibernation*

at 45 degrees F*

45 III.

THE INTERPR2TATION OF DATA

Oxygen consumption of bats in the active state averaged 102.658 cc. per hour at room temperature, which when figured on the co., gram hour basis averaged 11.59 cc. in the active state. While hibernating at a temperature of 45 degrees F . , the average oxygen consumption per hour is 9.69 cc. of oxygen, or an average total oxygen consumption per gram (body mass) per hour of 1.126 cc. of oxygen. The difference in the average total oxygen consumption per hour between bats in their active state at room tempera­ ture and in their hibernating state at 45 degrees F. is 92.968 cc. of oxygen more per hour than those in hibernation. The average difference of cc. of oxygen per gram (body mass) per hour between the bat

in its active state at room

temperature and in its hibernating state at 45 degrees F. is 10.464 cc. of oxygen per gram (body mass) per hour.

This

accounts for the fact that an active bat at room temperature utilizes 10.464 cc. of oxygen per gram (body mass) per hour. Thus, it is definitely certain that the longevity of the hibernating bat is prolonged indefinitely, if they remain in this inactive condition during hibernation in captivity with the assumption that they do not feed, for in a state of activ­ ity, their bodies must utilize a greater amount of stored energy to maintain this activity.

CHAPTER IV

SUMMARY 1. A brief review of the classification of the American bats of the genus Tadarida with their key and the taxonomic position of Tadarida mexicana, a review of literature with some observations on hibernation by others, end original ob­ servations on Tadarida mexieana, the technique of spirometry, the compilation of experimental data and the interpretation of data are presented. 2. The bats experimented on belong to the genus Tadarida, species mexlcana. Diagnosis.

The forearm is 40.4-46.66 mm. in length;

length of tibia from 11.4-13.0 mm.; length of skull 16.2-17.8 mm.

Ear extends to end of nostril when laid forward. Type locality.

Ameca, lalisco, Mexico.

Geographic distribution.

Mexico, Texas, New Mexico,

Arizona, California, lower California, Utah, and Oregon* Remarks.

Its range covers a territory in Mexico on the

south from the State of Pueblo north to central Texas (Austin on the east, and as far north as Brazos, Palo Pinto County), thence west through New Mexico, Arizona, Utah (northeastern), and from Southern California north to Chico in Butte County

47 and Fort Dallas, Oregon.

This bat has a remarkably extensive

range for a single speeies and varies only slightly in inten­ sity of color, mummy brown to Prout’s brown (Ridgway 1912). These variations are never localized, and dark and light examples can be matched from almost any two localities. 3. Hibernation comes on gradually with the cooling of the air in late autumn and in general does not take place until the temperature outdoors fails to rise above 50 to 54 degrees F.

The habitat in which bats hibernate in midwinter

show a more or less constant temperature that seldom falls below 42 degrees.

The hibernating bats themselves cool so

that the temperature of their bodies falls from as high as about 104 degrees when they are active to the temperature of the habitat in which they rest. 4. Hibernation of bats is merely a deeper sleep induced by lowered temperature and that bats have

little power of

controlling the heat of the body, but, like cold-blooded vertebrates, take their temperature when at rest from that of the surrounding medium.

This is aided by the great expanse

of naked membrane of the wings and elsewhere, in which the blood comes close to the

surface.

5. When normally active in summer bats may show a body temperature as high as 104 degrees F. (40 degrees C.) or even a degree or two higher (to 105.7 degrees F . )•

When they com­

mence hibernation, as in ordinary sleep, this soon falls as

48 the bat becomes inactive, and soon the temperature is that of the surrounding air.

The critical temperature at which the

usual nightly activity is suspended and deeper lethargy com­ mences is from about 46 degrees to 50 degrees F. (8 degrees to 10 degrees C.). 6.

In hibernating big-brown bat, breathing is irregular

and there are long periods when motion completely stops, last­ ing from three to eight minutes, with an average of only 4.6 respirations in half an hour.

These resting periods are fol­

lowed by intervals when for about three minutes or less there may be from £3 to 48 breathings a minute.

When they gradually

awake this rate rises to £00 a minute before the bat is able to fly. ?. They prefer a habitat with moist atmosphere for their hibernation, owing to the thinness of the membranes which allows considerable evaporation of moisture. 8. In hibernating bats of Europe, the mucous lining of the larynx and windpipe becomes thickened, so that less air is admitted in breathing, and the supply of oxygen is thereby reduced automatically (Winiwarter, 1926)# 9. At the end of the hibernating period bats lose about a third of their original weight, while the water increases in proportion from 1.499 per cent of the weight in early February to 3.145 per cent by April 23.

The proportion of

fat is much greater in November at the beginning of hibernation,

49 for it comprises 19.4 per cent of the total weight.

It

gradually decreases to about 15 per cent of the total weight. The amount of glycogen contained in the body is too little to be of much importance as reserve food, but the consumption of stored fat and albumen is greater and is much more in late winter or early autumn than it is when the bat first enters upon hibernation. 10. The exhalation of carbon dioxide gas and the intake of oxygen are a little over 1 per cent of the amounts shown when the bat is in normal activity. 11. The four stages in the normal awakening from deep hibernation are as follows: A. First stage— is the condition of ridigity shown in deep lethargy. E. Second stage— marked by the beginnings of slight motion. C. Third stage— there is more active response and the uttering of slight characteristic sounds. B. Fourth stage— bat is fully awake and flies. These stages are not very sharply marked, and the transition is accompanied by a gradual rise of temperature of the body to the final active state. IE. The average total consumption of oxygen in the active state at room temperature per hour is 102.658 cc. of oxygen.

The average total cc. of oxygen per gram (body mass)

50 per hour is 11.59 cc. of oxygen. In the hibernating state at temperature 45 degrees the total average oxygen consumption per hour is 9.69 cc. of oxygen.

The average total oxygen consumption per gram (body

mass) per hour is 1.126 cc. of oxygen.

CHAPTER V CONCLUSION It is definitely certain that the longevity of the hibernating bat is prolonged indefinitely, if it remains in this inactive condition.

On the basis of these experiments

the bats in their active state consume 92.938 cc. more oxygen per hour than the bats in their hibernating state or the bats in their active state utilize 10.464 cc. more oxygen per gram per hour than the bats in their hibernating state.

This

proves the fact that hibernating bats utilize less oxygen to burn their stored energy owing to their reduced state of activity.

BIBLIOGRAPHY

BIBLIOGRAPHY Allen. G. M . , Bats. 1939.

pp. £66-279.

Harvard University Press,

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