HIBERNATION IN MAMMALS— LYMAN and CHATFIELD 113 



cortex nor the medulla is essential for the process of arousal. Popovic and V^idovic^^- 

 have shown that the hibernating- adrenalectomized luiropean ground scjuirrel will 

 waken normally when an adrenal graft in the anterior chamber of one eye has been 

 (juickly removed by enucleating the eye, and Lyman (unpublished) has observed 

 normal arousal in hamsters that were adrenalectomized in the first five minutes of 

 the waking period. Thus we cannt)t agree with Suomalainen and Ilerlevi'"''' that the 

 process of arousal necessarily induces the "alarm reaction" which is a chronic syn- 

 drome brought about by stress in which the adrenal cortex is of paramount 

 importance. 



PHYSIOLOGICAL SPECIALIZATIONS OF HIBERNATORS 



It is to be expected that animals that hibernate should possess certain physiological 

 modifications which are related directly or indirectly to hibernation. The ability of 

 hibernators to desaturate their fat has already been described, but more data are 

 necessary before this trait can be limited to hibernators alone. 



As early as 1881, Horvath^^ showed that hibernators could be chilled to much 

 lower temperatures than animals that do not hibernate, and this has been confirmed 

 many times. For example, woodchucks^^* and hamsters^ '"'^ can tolerate chilling to a 

 colonic temperature of about 3° C. Animals that do not hibernate, on the other 

 hand, rarely live when the rectal temperature drops below 15° C.^^'^ The study of the 

 toleration of mammals to hypothermia has recently received great impetus because 

 of the use of chilling in surgery, as well as the importance of exposure during war, 

 but this problem is a subject in itself. It is of interest, however, that young mammals 

 tolerate chilling to much lower temperatures than can adults of the same 

 species.'^^' ^^^ As the young develop better temperature control, they are less able to 

 tolerate hypothermia.^-''*' The relationship between these facts and the ability of hiber- 

 nators to tolerate hypothermia has not been clarified. 



The nerves of animals that do not hibernate cease to function at about 8° C, as 

 Forbes and Ray^^^ showed for the cat. On the other hand, the nerves must respond 

 at lower temperatures in the hibernator, for there is a considerable degree of homeo- 

 stasis at 5° C, and the hibernating animal responds to tactile stimuli, pain and tem- 

 perature (see In Hibernation, page 88ff.) . Greater resistance to cold in the peripheral 

 nerves of hibernators was first pointed out by Tait,^^® who showed that a phrenic 

 nerve-diaphragm preparation, as well as the excised heart from hibernating wood- 

 chucks and hedgehogs, exhibited activity at much lower temperatures than would be 

 expected were the preparations from non-hibernating animals. 



This problem was reinvestigated by Chatfield, et al.,^^^ who compared the effects 

 of cooling in vitro on the excised tibial nerves of adult golden hamsters and albino 

 rats. They found that nerves from hamsters functioned down to an average tem- 

 perature of 3.4° C, while nerves from rats ceased functioning at an average tem- 

 perature of 9° C. (fig. 24). When nerves from rats were cooled, the action potential, 

 conduction velocity and excitability decreased linearly with temperature. These A^aria- 

 bles decreased at a slower rate in hamster nerves and the action potential actually 

 increased in amplitude in the early stages of cooling and then declined. Although 

 some nerves of birds will adapt to the cold,^°° there was no indication of adaptation 



