ẆVhen the bats were asleep their rectal and skin temperatures were. changes of temperature which are seen in these animals. We bave investigated

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1 459 6I2.55:612*58 TEMPERATURE CHANGES AND WINTER SLEEP OF BATS. BY R. C. BURBANK AND J. Z. YOUNG. (Department of Zoology and Comparative Anatomy, Oxford.) (Received July 18, 1934.) THERE are numerous references in the literature to the remarkable changes of temperature shown by bats when they awake from hibernation, but very little attention has been given to the equally striking daily changes of temperature which are seen in these animals. We bave investigated the matter on horseshoe, noctule and fruit bats. EXPERIMENTS ON HORSESHOE BATS. Greater and lesser borseshoe bats (Rhinolophus ferrum equinum Schreber and R. hipposideros Becbstein) were obtained from the Cheddar Caves, and were kept hanging from the wire lids of large boxes in a dark cellar, whose temperature ( C.) was about that of the caves in whicb they live. In this way the bats could be kept in the laboratory for several weeks, remaining motionless during the day, but usually waking during the evening. They could also be wakened at any time by means of such stimuli as shaking, bright illumination, or high-pitched sounds, to wbich latter they are peculiarly sensitive. The temperatures of the bats were taken by means of thermocouples, used either in the form of converted syringe needles, with which rectal or under-wing temperatures could be read, or as small copper-constantan junctions attached, by means of sticking plaster, to the shaved skin of the back. This latter method enabled continuous readings to be taken, its only disadvantage being that, on account of the shaving, the temperature was probably somewhat lower under the junction than elsewhere on the skin ẆVhen the bats were asleep their rectal and skin temperatures were found to be very close to that of the surrounding air, and warming or 30-2

2 460 R. C. BURBANK AND J. Z. YOUNG. cooling of the room was accompanied by corresponding changes in the temperature of the animals (Table I). Pembrey [1895] also gives figures showing that the temperature of sleeping bats is close to that of their surroundings. TABLE I. Temperatures of sleeping bats. Animal Date Method of recording Air 0 C. Bat 0 C. B. ferrurm equinum, E 29. i. 33 Needle couple under wing i. 33,,, *8 11*3 31. i. 33,,, *2 B. hippo8idero8, H 12. iv. 33 Needle couple under wing (room slowly warmed) *5 19*4 R. hipposidero8, 1 6. xi. 33 Wire couple on skin of back The first response to any stimulus was a rapid flexion of the legs of the hanging bat, the legs remaining bent for some time before gradually relaxing. At the same time the breathing movements, which are usually imperceptible in the sleeping bat, became apparent. Prodding of a bat in this state was followed by turning of the head, bending of the body, chirping and biting, all of wbich movements can be performed while the temperature is hardly above that of the surroundings. The bat in this state may be said to be awake, although it is not warm. Even very slight movements, however, were soon followed by a rise of temperature. When the stimulus given was slight the rise might be only temporary, the temperature returning gradually to that of the surroundings, but if the stimulation was prolonged, so that the bat continued to move, then a very rapid and continuous rise of temperature began. The strength of the stimulus necessary to produce this full warming varied very much with the external temperature, and with the health of the bat, it being sometimes impossible to arouse the animal in this way when the temperature was below 15 C. It was also found to be easier to wake bats in the evening than at any other time of day. The course of the rise of temperature varied somewhat in the different experiments, but usually, as in the case known in Fig. 1, the warming was slow at first and then later became much more rapid. This steeper rise was usually accompanied by obvious shivering, and often also by a curious rhythmical flexing of the legs. It is therefore clear that mueb of the heat

3 TEMPERATURE OF BATS. is produced as a result of muscular contraction, but we have not been able to obtain evidence by which it could be decided whether special mechanisms, such as the release of hormones, are brought into play to sustain or augment the muscular contractions, or to supplement them by the initiation of combustion in other parts of the body ; d r D A R K LIGHT DARK LIGHT 15 Time, 10 minute intervals Fig. 1. Temperature chart of greater horseshoe bat. Manipulation during attachment of thermocouple to back has caused warming at beginning of experiment. Bat stimulated at points marked 8. The black rectangles mark periods during which the animal was shivering or moving. Room temperature C. The rate of warming of bats during their waking has already been studied by Pembrey [1895], who found rises of more than 1 C. per minute. In our experiments the rate of warming was often equally remarkable; in one case a greater horseshoe (15 g.) warmed itself from 20 to 390 C. in 30 min., that is to say at the rate of 0.63 C. per minute over this

4 462 R. C. BURBANK AND J. Z. YOUNG. whole period. In smaller bats the temperature rose somewhat less rapidly, as would be expected, since the rate of heat loss per unit of body weight must be relatively greater in the smaller animal. The rise of temperature was accompanied by more rapid and regular breathing, and by a gradual change in behaviour. At about 25 C. the movements of the head became almost continuous, the characteristic ear twitchings began, the eyes opened, and finally, when the skin temperature reached about , the wings were stretched and the animal was able to fly. The maximum temperature attained varied in an interesting manner with that of the external air (Table II). Thus with an air temperature of TABLE II. Maximum temperatures reached by wakened bats. Animal Date Method of recording Air C. Bat C. R. hippmoideroe, AB 27. i. 33 Needle, rectum B. ferrum equinum, E 28. i. 33 Needle, rectum Needle, wing i. 33 Needle, rectum Needle, wing i. 33 Needle, rectum Needle, wing 20*0 39*I Needle, wing 20*0 41'0 9. ii. 33 Wire on back ii. 33 Wire on back R. hipoeiderom, H 12. iv. 33 Needle, wing R. hippo8idero8, I 4. xi. 33 Wire on back xi. 33 Wire on back C. the rectal temperature of a fully active bat was C., whereas with an air temperature of 200 C. it was C. The highest temperature ever observed, 41 C., was recorded immediately after a bat had been allowed to fly freely about the room. We have not been able to discover accurately what form of heat loss prevents the attainment of even higher temperatures. There are no sweat glands in bats, and there can be no rapid loss of heat from the body on account of the thick fur. On the other hand, the wing surface must constitute a very effective cooling agent, and it was often observed that when a bat stretched its wings the temperature fell distinctly, rising when the wings were again folded. A further downward temperature regulation is presumably effected by the loss of water from the lungs, and the rate of breathing increases very rapidly with rise of temperature [see Pembrey, 1895]. It is clear from our observations that these bats do not maintain any constant maximum temperature. As soon as the movements of a bat

5 TEMPERATURE OF BATS. 463 decrease its temperature begins to fall, and every change in activity is accompanied by corresponding fluctuations of temperature. Fig. 1 shows a typical record of such changes. It will be seen that when the bat ceases to move its temperature falls rapidly and steadily to that of the surrounding air, any slight change in the position of the bat being followed by a temporary rise of temperature. This whole process of warming and cooling can be repeated several times during the day, and it occurs normally when the bat wakes up in the evenings. OBSERVATIONS ON NOCTULE BATS. Similar observations were made on the temperature of a female noctule bat (Nyetalus noctula Schreber) caught in Oxford on 9. v. 34. During the daytime this bat hung motionless in its cage, with the wings folded, the breathing being imperceptible. Its skin temperature, C., proved to be the same as that of the air, the fur feeling cold like that of a dead mouse. When removed from its cage and placed on the table the animal showed a " reptilian " type of behaviour, crawling slowly along on all fours but making no attempts to fly. Like the horseshoe bats in a similar state it would turn its head towards a sound, and could squeak and bite. This bat awoke during the evenings, its skin temperature rising to C., with an air temperature of 160 C. If released at this time it would fly about the room, and could be fed and watered like the horseshoes. OBSERVATIONS ON FRUIT BATS. In addition to the Microchiroptera, of which the horseshoe and noctule bats are representatives, there is a group of tropical bats, the Megachiroptera or fruit bats, which are much larger animals, and should therefore require a relatively smaller expenditure of energy in order to maintain a high temperature. We were fortunately able to study the temperatures of these animals, thanks to the assistance of the authorities at the Zoological Gardens in Regent's Park and at the Oxford Zoological Gardens. We were immediately struck by the fact that even when the air temperature was 200 C. the animals were continually shivering. Moreover, whenever we visited them they were always awake, and usually moving about: in fact the attendants both in London and Oxford reported that they had never seen them fully asleep. As will be seen from Table III, the temperatures of all the animals examined were fairly high, but with interesting differences. Thus the animal A, which was a large

6 464 R. C. BURBANK AND J. Z. YOUNG. TABLE HII. Temperatures of fiuit bats. Species Date Time Air 0 C. Bat0 C. Pterope geddei, A 21. v vi O Pteropu geddei, B 21. v vi Pterqpus geddei, C 13. vi Pteropua geddei, D 13. vi Pteropu8 giganteus 13. vi Pteropuw cotinus 13. vi O male, hung rather quietly in its cage, and was found to have a distinctly lower temperature than B, which, though smaller, was moving about very actively at the time. It is evident that these fruit bats in captivity maintain a moderately high but not very constant temperature, and that they succeed in doing this only by continuous muscular movement and shivering. CONCLUSIONS AS TO TEMPERATURE REGULATION OF BATS. We thus reach the surprising conclusion that the Microchiroptera have no special mechanism for the regulation of heat production, but that their temperature varies with their activity, so that they become cold whenever they go to sleep. On waking one of these bats is able immediately to move its head and legs, and to bite and chirp, but at first it is in a cold-blooded condition, and cannot fly until it has gone through a process of warming by means of rapid breathing, shivering, jerking of the legs and other muscular activity, perhaps assisted by the discharge of appropriate hormones. Such shivering and hormonic discharge would then represent an elementary upward temperature regulating mechanism, temporarily brought into play by the stimuli which are waking the animal. The larger Megachiroptera, living in tropical climates, are able by means of shivering and continuous movements to maintain a fairly high temperature, and they seem to have the rudiments of a true upward regulating mechanism, in that shivering is brought into play as the animal cools, whereas in the Microchiroptera it is used only to assist in raising the temperature when the animal awakes. Presumably the first step towards a warm-blooded condition was the development of fur to allow of the conservation of the heat produced during muscular contraction. Then nervous and endocrine mechanisms were added by which extra heat was produced by shivering or chemical means. At first this extra heat may have been produced in response to

7 TEMPERATURE OF BATS. 465 exteroceptive stimuli, as it is in the horseshoe bat, and only later as a reflex response to cooling of the blood and skin, as in mammals with a full temperature regulating mechanism. The bats, in order to fly, have mostly remained rather small animals, in which, on account of the high surface-volume ratio, the maintenance of a high temperature would in any case require a relatively large food consumption, the difficulty being still further increased by the presence of extensive wing surfaces. Since also the Chiroptera were a very early offshoot from that central insectivoran stock from which all other placental mammals have sprung, it seems possible that their mechanism for temperature regulation has remained at the simple stage which it had reached when they diverged from the main stem. GASEOUS EXCHANGES DURING WINTER SLEEP OF BATS. These facts have an interesting bearing on the question of the hibernation of bats. It is well known that they may emerge from their retreat on a warm day at any time during the winter, and there are a number of scattered observations which agree with our own in showing that the bats become cold not only during their so-called hibernation but also in their diurnal summer sleep. Thus Coward [1907] records that "....a sleeping bat in summer is almost as cold and lifeless as a bat which is hibernating and is frequently as difficult to rouse." Putting all these facts together it seems that we cannot speak of a special state of hibernation in bats, but that when they become inactive their temperature always falls to that of the surroundings, and if the latter is low then they may remain inactive for considerable periods. We have studied, therefore, the gaseous exchanges of "hibernating" bats, with the object of comparing them with those of other mammals. It was not found possible to transfer horseshoe bats from the caves to the laboratory without waking them, but they were moved with as little disturbance as possible, and then allowed to go to sleep again without being fed. The respiratory exchanges were measured by means of the Haldane-Pembrey method, the sleeping bat being hung carefully from the stopper of a large beaker, through which dry C02-free air was then passed. The carbon dioxide and water given off were collected and weighed, and the oxygen intake estimated by difference. The method is liable to several errors, particularly as the amounts of C02 and water given off were very small when the bats were asleep. However, the results, given in Table IV, were reasonably consistent.

8 466 R. C. BURBANK AND J. Z. YOUNG. TABLE IV. Gaseous exchanges of horseshoe bats. All weights in grams. Temp. after Hours Loss Wt. Condi- exp. of by Animal Date Food, etc. g. tion C. exp. bat CO2 02 R.Q. R. hippo- 28. i mealworms 4 Asleep idero8, on 24. i. 33 AB 9,, 30. i. 33,, Asleep R. hippo- 6. ii. 33 Collected 5 Awake idero8, 5. ii. 33, AC not fed,,9 7. ii. 33,, - Awake R. ferrum 9. ii mealworm 14 Awake equinum on 8. ii. 33 E oil 17. ii. 33,, Asleep R. ferrum 9. ii. 33 Collected 16 Asleep equinum, 5. ii. 33, G not fed,, 10. ii. 33,, Asleep 13 12j The interpretation of the results given for waking bats is complicated by the fact that it is not possible to say how much of the explosive discharge of CO2 recorded by Pembrey [1895] during waking actually falls during the period of the experiment. Though the respiratory quotients observed are low, yet they are not so low as some of those observed in the case of hibernating marmots and hedgehogs. Pembrey, who first observed these low values, interpreted them as indicating that stored fat was being converted into carbohydrate, but it has also been held that they are due to the retention of C02, consequent on the low temperature. The whole question has been discussed by Gorer [1930]. The present results show that a very low quotient does not necessarily accompany a low temperature, and may thus be held to confirm Pembrey's interpretation, and at the same time to show that, as we had expected on other grounds, these bats do not show any special hibernating metabolism. It follows that the bats must take food at intervals throughout the winter, and Mr Painter, who has been a guide at Gough's Cavern for many years, states that some are to be seen on the wing inside the caves almost every evening of the year, though they only fly outside during mild weather. On the other hand, bats are sometimes seen hanging in the same place for several weeks, so that, unless they return every day to the same spot, it is evident that they may remain inactive for considerable periods. There is plenty of potential food inside the caves, principally in the form of moths and spiders. Further, the bats which were kept in

9 TEMPERATURE OF BATS. 467 Oxford, though living at a temperature similar to that of the caves, rapidly became weak if not fed, but could be kept alive for weeks on a diet of meal worms. This all agrees with the conclusions arrived at from our study of the temperature, namely that horseshoe bats become cold whenever they cease to be active, and that their so-called hibernation does not differ essentially from their ordinary sleep. SUMMARY. 1. Horseshoe and noctule bats have no mechanism for the maintenance of a constant temperature. Whenever they go to sleep they assume the temperature of their surroundings, and when first waked they show a slow, reptilian type of behaviour and cannot fly. However, they rapidly warm up to a temperature of 3040 C., the maximum varying with the activity of the animal and with the air temperature. Every variation in activity is accompanied by a change of temperature, and when movement stops they cool off to air temperature. 2. The larger fruit bats maintain a fairly high but variable temperature ( C.). They do this, however, only by continual activity and shivering, never becoming fully asleep in captivity. 3. The respiratory quotient of horseshoe bats taken during the winter was found to be usually between 07 and 0-8, the lowest quotient observed being It is concluded that the so-called hibernation of these animals does not differ essentially from their daily summer sleep, and this is confirmed by the fact that they may wake and take food at any time during the winter. We wish to thank the authorities at Gough's Cavern, Cheddar, Dr J. R. Baker and Messrs C. Richardson and B. T. Parsons for assistance in obtaining bats, and Prof. E. S. Goodrich and Dr C. G. Douglas for reading the manuscript of this paper. REFERENCES. Coward, T. A. (1907). Proc. Zool. Soc. Lond. p Gorer, P. A. (1930). Biol. Rev. 5, 213. Pembrey, M. S. and White, W. H. (1896). J. Physiol. 19, 477.