106 



ANALYSIS OF THE ENVIRONMENT 



Among the characteristics of torpor in 

 mammals are the reduced rate of breathing, 

 with complete suspension of breathing for 

 several minutes at a time; lowered body 

 temperature; and persistence of the heart 

 beat, as in cold-blooded vertebrates, when 

 the animal is decapitated. 



Relations between hibernation and sleep 

 have been investigated somewhat. Sleep of 

 mammals in which heat regulation is imper- 

 fect, and in which hibernation can develop, 

 diflFers conspicuously from sleep of the nor- 

 mally winter-active mammal. Relation of 

 the period of hibernation to the sexual cycle 

 is far from clear, but there may be active 

 development of the gonads during hiber- 

 nation. This is especially conspicuous in 

 Columbian ground squirrels, in which the 

 long period of aestivation and hibernation 

 concentrates active life into about five 

 months. The remarkable interruption of 

 early stages of embryonic development in 

 various mammals exhibits no correlation 

 with hibernation, since it occurs in non- 

 hibernating mammals as well as in hibernat- 

 ing forms. 



The zoological dispersion of hibernation 

 among mammals is not especially illuminat- 

 ing, since closely allied forms (e.g., the true 

 squirrels and the ground squirrels) may 

 diflFer radically in this respect. Hibernation 

 is reported for the orders Monotremata, 

 Marsupialia, Insectivora, Chiroptera, Ro- 

 dentia, and Camivora. Hibernation of carni- 

 vores appears to diflFer in important respects 

 from that of other orders and might be re- 

 ferred to as pseudohibernation. The tenrec 

 of Madagascar, a remote relative of the 

 hibernating hedgehog, is often cited as an 

 aestivating mammal. Various ground squir- 

 rels of western North America have a well- 

 defined period of aestivation combined with 

 hibernation, since they disappear into their 

 burrows early in August and do not appear 

 until the following March. Ground squirrels 

 in Turkestan have the same habit. 



Among gradients connecting the hiberna- 

 tion habit with more normal life histories, 

 there may be mentioned storage of food by 

 nonhibemating rodents, the storage of con- 

 siderable supplies by some that hibernate, 

 and of small amounts, perhaps as bedding 

 rather than food, by those in which the 

 hibernating habit is fully developed. Aggre- 

 gation associated with hibernation is wide- 

 spread among tropical bats as well as those 

 of temperate regions. Aggregation of hiber- 



nating mammals is otherwise rare. It is 

 reported for family parties of skunks. 



Great diflFerences exist in the extent to 

 which the hibernating animal withdraws 

 from the effects of the environment. Some 

 bats merely enter crevices beneath loose 

 bark, while others congregate into vast win- 

 ter colonies in caves, where the temperature 

 is constant. Burrows of more familiar hiber- 

 nating rodents extend well below the frost 

 Une into relatively constant temperature, 

 while pseudohibernation of carnivores may 

 occur in hollow logs, in snow-covered de- 

 pressions, or even on level ground beneath 

 trees. 



We do not find an ecological study of 

 the north-south gradient in the hibernation 

 of mammals with a considerable latitudinal 

 range, but it is obvious from scattered in- 

 formation that such behavior clines must 

 exist, though less directly correlated with 

 environmental climatic factors than are 

 similar clinal gradients in overwintering be- 

 havior of poildlothermic animals. 



In general, it appears that several en- 

 vironmental factors combine with interna] 

 factors (which in turn have been modified 

 in evolutionary adaptation to hibernation) 

 to induce dormancy in mammals. Hiberna- 

 tion and aestivation are to be compared 

 with seasonal migration or with food storage 

 as evolutionary adjustments for the passage 

 of an unfavorable season. According to this 

 view, the partial poikilothermy of homoio- 

 therms with the hibernating habit is 

 secondary and, at least in higher orders, is 

 a degeneration. 



ENVIRONMENTAL TEMPERATURE AND THE 

 RATE OF BIOLOGICAL PROCESSES 



Metabolic rates increase almost to the 

 upper temperature limits at which the or- 

 ganism is normally active.* The possibility 

 that such ecological accelerations are funda- 

 mentally similar to the effect of heat upon 

 the speed of chemical and physical reac- 

 tions has attracted much attention. Chemi- 

 cal reactions in a laboratory test tube, and 

 rate of living of adult organisms, speed of 

 embryonic, larval, or pupal development, 

 rate of living of adult organisms, speed of 

 locomotion, and other behavioristic reac- 

 tions, are all accelerated with higher 

 temperatures. These results are more ob- 

 vious in poikilothermal organisms than in 



• For exceptions, see Barnes, (1937). 



