PHYSIOLOGY OF HIBERNATION IN MAMMALS 



CHARLES P. LYMAN * and PAUL O. CHATFIELD 



In reviewing the various physiological problems of hibernation in mammals, one 

 is hampered by the fact that the word "hibernation" is not sufficiently precise in 

 meaning. Webster defines "to hibernate" as "to pass the winter in close quarters in 

 a torpid or lethargic state." Such a definition must include all groups of animals 

 which are somewhat less active in the winter than they are in the summer. The 

 present article concerns itself with a small number of mammalian species which at 

 some time of year undergo a profound drop in body temperature with a concurrent 

 decrease in metabolism and heart rate. Because there is no single word which de- 

 scribes this condition, it is perhaps best to refer to it as "deep hibernation," but 

 for the sake of brevity the word "hibernation" will be used in this restricted sense. 

 Good comparative physiological data on mammals that hibernate are scanty, and 

 it appears likely that many animals which are commonly included in this group can- 

 not be considered as "deep hibernators." For example, the brown bear, Euaixtos 

 americanus, spends part of the winter in a dormant state but shows no striking drop 

 in body temperature or metabolic rate.^- - ' Clapp's^' observation of vapor rising from 

 the nostrils of a dormant bear during the winter also indicates that the body tempera- 

 ture is much higher than that of the environment. It appears probable that there are 

 gradations of dormancy from the heavy sleep of the bear to the deep hibernation 

 typical of some smaller mammals, but only the latter category will be considered 

 here. Table I gives a list of the more available animals which are known to be deep 

 hibernators. Throughout the remainder of this article animals will be referred to by 

 their "common" English names. 



It will be noted that several species of bats have been included in this table. As 

 Hock^ has pointed out, this order of mammals stands alone among the hibernators, 

 for their body temperature and metabolic rate drop precipitously whenever they 

 become inactive. Apparently the hummingbird" and the poor-will^^ belong in the 

 same category as bats. On the other hand, the rodents and insectivores which hiber- 

 nate can be exposed for long periods to a cold environment without changing their 

 body temperature. After hours, or sometimes months, the individual animal aban- 

 dons its homeothermic state, the metabolic rate decreases and body temperature 

 drops, and hibernation begins. Because the factors involved in these two types of 

 hibernation may not be the same, this review will be limited for the most part to 

 hibernation in rodents and insectivores. 



It should be emphasized here that there has always been some confusion between 

 hibernation and hvpothermia. This confusion has grown with the advent of experi- 

 mental hypothermia for surgical purposes and the development of pharmacological 

 products which will iicrmit hypothermia.^- The differences between the two states 

 have been clarified at length by Popovic^'' and Giaja.^^ Any mammal, if exposed to 

 a sufficiently low temperature, will increase its muscular activity, shiver and increase 



* Associate Curator of Mammals, Museum of Comparative Zoology, Harvard University, 

 t See also Svihla, A. and Bowman, H. C. : Am. Mid. Nat. 52: 248, 1954. 



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