HIBERNATION IN MAMMALS— LYMAN and CHATFIELD 91 



while (luring the 330 hours that it was awake it used 579 calories.* Rasniussen and 

 Rasmussen"*' reported a much greater proportional weight loss in hibernating wood- 

 cliucks, as did Johnson'"'' in ground squirrels, but Kayser's contention that this was 

 due to periodic arousals appears reasonable. 



In spite of the depressed metabolic rate and extremely low body temperature, the 

 hibernating mammal maintains a cnnsiderable degree of homeostasis. I'"arlv experi- 

 ments indicated that exposure to sub-zero temperatures caused the animal to wake 

 from the hibernating state, but Wyss^' was the first to show that there was an in- 

 crease in heat production in hibernating dormice when the environmental tempera- 

 ture was lowered to zero centigrade and that this higher heat production could con- 

 tinue for days without the animal waking from hil)ernation. Similar results have 

 been reported for European ground squirrels by Kayser.^* Lyman^' confirmed this 

 by taking simultaneous body temperatures and metabolic rates on hibernating 

 golden hamsters. When the environmental temperature was lowered from 5° C. to 

 0° C. the animals maintained their body temperature at approximately 3° C. and 

 tripled or quadrupled their metabolic rate. In some cases the animal remained in 

 the hibernating state in spite of this increase in metabolic rate; in others, the animal 

 awoke from hibernation and in a few cases, the animal was apparently unable to 

 respond and died at the lower environmental temperature. 



The nervous system. In deep hibernation, the cerebral cortex of hibernating 

 animals shows no spontaneous electrical activity, but the species vary in the tempera- 

 ture at which electrical activity ceases. Thus the European ground squirrel'^^ and 

 the woodchuck-'' show spontaneous activity at a much lower temperature than the 

 golden hamster-" (figs. 7 and 8). Rohmer, ct aJ.,'*^ using the European ground 

 squirrel, actually reported an "accident complexe," which we suspect is some sort 

 of evoked potential, at central temperatures as low as 5° C. Brain temperatures were 

 not given. 



Other species difi^erences exist in the sensitivity of central nervous structures to 

 cold. Kahana, et al.,^'^ who studied the round window response in the golden hamster 



A i8*c lOOjjV T I sec. , 

 B I5'C 



C I3C 



D ll'C 

 E 7*C 



"■j-^-^-^ y--y 



t 



Fig. 7. — Electrocorticograms of a hibernating woodchuck at various cortical temperatures as 

 indicated. The arrow in record E indicates a cortical response evoked by noise. Other deflec- 

 tions in record E are artefacts from EKG. Calibration in record A applies to all. 



#E. F. Adolph and J. Richmond (J. Appl. Physiol. 8: 48, 1955) have calculated that the act 

 of rewarming once every 11 days uses as much energy as the previous 10 days of hibernation in 

 the thirteen-lined ground squirrel. 



