Morphological and Physiological Adaptations 147 



species in different parts of the world may go into hibernation or 

 aestivation at quite different periods of the year (Fig. 5.7). 



Certain mammals, and at least one bird ( Jaeger, 1949 ) , also become 

 dormant with reduced metabolism under seasonal extremes of climate. 

 Since this type of dormancy involves a physiological change peculiar 

 to warm-blooded animals, a separate term should be employed to 

 describe it, but in the absence of a special word the terms hibernation 

 and aestivation are used for mammals (and birds) as well as for 

 lower animals (Lyman and Chatfield, 1950). In mammalian hiber- 

 nation the internal temperature drops to about 1 degree above the 

 temperature of the animal's surroundings, provided that the latter does 

 not fall lower than a few degrees above 0°C. This is not likely to 

 happen in the burrows or caves where the animals usually hibernate 

 (Fig. 5.8). In laboratory experiments a reduction of the external 



Photo by D. R. Griffin 



Fig. 5.8. Hibernating bats, Mijotis I. lucifugus, hanging in clusters head down- 

 ward by hind claws hooked to the rough Umestone in a Vermont cave. 



temperature below 0°C causes the hibernating mammal either to be 

 aroused from its dormant condition or to be killed by freezing. 

 Temperatures well below 0°C for long periods in the winter would 

 prevent any mammal from hibernating in the Arctic unless it could 

 find a sufficiently tempered retreat. Whether for this or other reasons 

 there are few hibernating mammals in the far north. The Arctic 



