82 PHYSIOLOGY OF INDUCED HYPOTHERMIA 



its nietabulic rate. If the cold is too intense or of too long duration, exhanstiou 

 eventually takes place and the animal begins to cool. When a critical level of body 

 temperature is reached, death occurs. This sequence of events can occur in both 

 hibernators and mammals that do not hibernate, but death takes place at a much 

 lower body temperature in the former. ^'^ Similar conditions can be realized by anaes- 

 thetizing an animal and exposing it to cold. 



One important factor which characterizes the deeply hibernating state is that the 

 normal animal is capable of arousing from this state without the aid of heat from 

 external sources. Any hibernating mammal, having received a stimulus strong 

 enough to start the process of arousal, will begin a chain of physiological reactions 

 which result in a rapid production of heat and culminate with the animal fully awake 

 at the end of two or more hours. Failure to realize that this is typical of true deep 

 hibernation has led to some confusion in the past. For example, hibernators treated 

 with drugs such as pentobarbital sodium and then exposed to cold cannot be con- 

 sidered to be in the deeply hil)ernating state when their body temperature drops to 

 that of the environment, for they are incapable of arousing themselves from this 

 state no matter what the external stimulus. 



Hibernation as it is observed in the natural state takes place in the fall of the year 

 and, except in the case of bats, is associated with the autumnal drop in environ- 

 mental temperatures. However, the laboratory animals which normally hibernate in 

 the fall may be induced to hibernate at any season of the year by maintaining them 

 for a sufficient length of time under suitable conditions at an environmental tem- 

 perature a few degrees above zero centigrade.^*'- ^'' ^* It has been reported that cer- 

 tain animals "aestivate" during extremely dry or extremely wet seasons of the year. 

 Thus some of the American ground squirrels^^ and the Madagascan tenrec^° aesti- 

 vate during the summer months. We know of no body temperatures having been 

 taken on these aestivating animals, but it seems reasonable to conclude that this is a 

 modified form of hibernation in which the animal enters into some sort of torpid 

 state with body temperature reduced to the cool temperature of its nest. 



PREPARATION FOR THE HIBERNATING STATE 



Fat and food storage. It has been known for many years that most animals 

 which hibernate become enormously fat during the late summer months and enter 

 hibernation in an obese condition. The golden hamster^" and the European hamster^® 

 appear to be exceptions to this rule. W^ade"^ indicated that fat thirteen-lined ground 

 squirrels tend to hibernate before thin ones when exposed to cold simultaneously. 

 Johnson"- agreed and showed that fat animals hibernated longer. Animals such as 

 the woodchuck depend completely on their stored fat for nourishment during the 

 hibernating period, while ground squirrels store small amounts of food in spite of 

 their obese condition.^" Hamsters, on the other hand, store enormous amounts of 

 food in their burrows prior to hibernation.-^ Lyman-* showed that there was a long 

 delay in the onset of hibernation in hamsters that were exposed to cold but pre- 

 vented from storing food. Thus, depending on the s])ecies, the storage of energy 

 either as fat or fodder appears to influence the onset of hibernation. The si)ecies 

 difference also applies to the denial of food, for Johnson-- showed that stai*vation 



