HIBERNATION IN MAMMALS— LYMAN and CHATFIELD 85 



fectly understood and, therefore, its putative functions have an aura of mystery. 

 Fawcett^^ has compared brown and white fat by a number of histochemical tech- 

 niques and concluded that the tissues are quahtatively similar but that brown fat 

 is a more active type of tissue, thus confirming the biochemical studies of Wer- 

 theimer,^^ Mirsky" and others. Other investigations have shown that brown fat 

 reacts to insulin and other substances that influence carbohydrate metabolism.^^- '"• 

 40, 41, 42 jj^yg evidence is accumulating that this tissue is indeed specialized for the 

 storage of fat, but that the stored fat may be derived principally from carbohydrate 

 rather than from ingested lipids. It has recently been shown that the storage and 

 utilization of this fat is profoundly influenced by its nerve supply.'*^ In conclusion, 

 brown fat appears to be a specialized adipose tissue, and as such may have some 

 significant function in animals such as hibernators which depend almost exclusively 

 on stored fat for energy; but we know of no evidence which justifies calling this 

 tissue the "hibernating gland." 



The endocrines. Along with other preparations for the hibernating state it is 

 generally accepted that the endocrine glands undergo an involution. This polyglandu- 

 lar involution has been observed by many investigators, and is the basis for one of 

 the theories of hibernation which will be discussed at the end of this paper. Kay- 

 sgj-is, 44 hag a^ijjy reviewed the literature on this subject. Briefly, it may be said that 

 at the time that mammals enter the hibernating state and during the hibernating 

 period, the thyroid and adrenal appear histologically to be in a quiescent state. The 

 ovaries and testes are atrophic before hibernation and remain so during the hiber- 

 nating state, although in the last few days of hibernation the gonads of some mam- 

 mals such as the woodchuck and ground squirrel show some signs of increased 

 activity. (See Growth, page 96. *^' ^'''^ ^') 



Involution of the thyroid and adrenal cortex upon exposure to cold is in direct 

 contrast to the result obtained in many non-hibernating mammals in which cold 

 causes a stimulation of the anterior pituitary, thyroid and adrenal cortex.'*^' "> ^° 

 Recently Deane and Lyman^^ have re-examined the question of polyglandular 

 involution during hibernation and have compared the reaction of the thyroid and 

 adrenal cortex of the golden hamster and the white rat on exposure to 5° C. Keep- 

 ing the animals at this temperature produced hyperplasia of the adrenal cortex and 

 thyroid gland in rats but caused no detectable increase of activity of these glands in 

 hamsters though the animals maintained a constant homeothermic l)ody temperature 

 (fig. 2). On the other hand, the pituitary-thyroid and pituitary-adrenal relation- 

 ship has been shown to be normal in hamsters.^-- ^^' ^'*' ^^' '"^ It was therefore con- 

 cluded that exposure to temperatures at which the animals normally hibernated was 

 not sufficiently stressful to produce an "alarm reaction." Deane and Lymau''^^ em- 

 phasized that the decline in thyroid and adrenal activity which was considered b}^ 

 some to be intimately associated with hibernation actually took place in the summer 

 time at the close of the breeding season*"- ^"' ^*' ^^' '''° which was often weeks or 

 months before the time of hibernation. It thus appears to be a peculiarity of hiber- 

 nators and some other wild species"^ that they do not respond to the stress of mod- 

 erate cold by enlargement and activity of thyroid and adrenal. This lack of response 

 of endocrines so intimately concerned with metabolism must be permissive to the 



