98 PHYSIOLOGY OF INDUCED HYPOTHERMIA 



total hibernating period. To our knowledge, the longest continuous period of hiber- 

 nation was reported by Kayser^^ for a common dormouse which hibernated for 

 114 days. Although there is no way of knowing how long continuous hibernation 

 may take place under completely natural conditions, it seems reasonable to assume 

 that the usual period of continuous hibernation is much shorter than this record 

 case. Observations in the laboratory indicate further that the lengths of continuous 

 hibernation vary from species to species. Thus personal observation as well as the 

 notes from other investigators^^ indicate that animals such as ground squirrels and 

 woodchucks probably hibernate continuously for several weeks at a time, while the 

 golden hamster wakens as often as once a week and has been recorded as hibernat- 

 ing continuously for 21 days at the maximum.-^ The hamster is known to eat during 

 the short periods when it is awake,-* and it might be noted that gastric secretion 

 continues in the woodchuck during hibernation.^^' 



It is also true that species of hibernators vary in their sensitivity to external 

 stimuli during hibernation, so that a stimulus which would start the waking process 

 in one species will have no observable effects in another. For example, the hibernat- 

 ing marmot^" will respond with uncoordinated muscular movements when poked 

 with a sharp object but will not start the waking process, and Chao and Yeh^^^ re- 

 ported that moving the quills of the hibernating hedgehog will cause the animal to 

 first curl into a tighter ball and later relax after several deep respirations. On the 

 other hand, such a stimulus in the golden hamster would immediately start the 

 waking process. Our own experience with the hamster seems to indicate that the 

 animals may be less sensitive to stimuli on one day than on the next, but the reason 

 for this difference remains obscure. 



The cause of the natural periodic awakening in the hibernating animal has yet to 

 be conclusively explained, for any experimental modifications of the physiology of 

 the hibernating animal may arouse it by the stimulation of pain alone. Thus Adler^^^ 

 injected material which he believed contained thyroxin into hibernating hedgehogs 

 and concluded that he had involved the thyroid glands in hibernation when the ani- 

 mals aroused from the hibernating state. Later Zondek^-" showed that warm physio- 

 logical salt solution would produce the same result and Trendelenburg and Krayer^-^ 

 pointed out that Adler's extract contained little thyroxin and other extracts con- 

 taining little or no thyroxin had the same effect. Bruman^-- was able to awaken 

 hibernating common dormice by injecting atropine and thus considered that he had 

 implicated the parasympathetic system in hibernation. Recently Svihla, cf (//.,^"'' have 

 aroused ground squirrels by injecting distilled water, using an intraperitoneal injec- 

 tion of mineral oil or air as a control and thus attribute hibernation to a lack of body 

 water. Since even isotonic saline is painful when injected intramuscularly, it appears 

 very unlikely that the animals did not experience pain which then started the process 

 of arousal. More delicate and better controlled methods nuist be developed liefore 

 the cause of spontaneous arousal is clarified. The reduction of some critical nutrient 

 as a cause of waking has not been explored. 



AROUSAL FROM HIBERNATION 



General. The process of waking from hibernation is the most dramatic ])hase of 

 the hibernating cycle, for it starts with an animal tliat is chilled and almost motion- 



