^72 Comparative Animal Physiology 



perature is the initiating factor, although in some mammals hibernation can 

 occur without a change in temperature and a hibernating animal can some- 

 times be awakened by further extreme cold. Hamsters in the cold hibernate 

 in the presence of adequate food,i^^ and hamsters, ground squirrels, and other 

 animals occasionally wake for brief periods to eat and void.'^-^ 



The most striking characteristic of hibernation is "turning down the thermo- 

 stat." The rate of fall of body temperature may be very rapid. The tempera- 

 ture of a ground squirrel may drop 5° per hour."- The body temperature of a 

 hibernating mammal fluctuates very much and is usually one to a few degrees 

 above air temperature. Hedgehogs enter hibernal sleep at an air temperature 

 of about W, and thereafter the body temperature rises and falls with the air 

 temperature. Below an air temperature of 5.5° the body temperature some- 

 times remains at 6° C.,^^ or it may go even as low as 2° for a brief period, 

 although this low temperature is not usually survived.^*^ Bats in a cave at 

 11.8° averaged 11.9° body temperature. ^^^ Ground squirrels become lethargic 

 at body temperature below 30° C. and show great individual variabihty in 

 response to environmental conditions which influence hibernation.^'' A wood- 

 chuck (Marmota) in hibernation has a rectal temperature several degrees 

 lower than its heart temperature. •^- 



The rate of oxygen consumption declines with body temperature. In the 

 hamster metabolism in 4-5 hours reached a minimum of 3 per cent of the 

 non -hibernating level.' ^^ Data for the woodchuck^^ show the profound 

 changes during hibernation: 



Non-hihernating Hibernating 



Heart rate 80/min. 4-5/min. 



Rectal temperature 34-39° C. (air 10-35°) 12-13° C. (air 10°) 

 Respiratory rate 25-30/min. 0.2-1 /min. 



R.Q. (after 24-hour fast) 0.71 0.7 



Basal metabolism 410 17-27 



cal./lO w'^V24hr. 



I leart rate is slowed and irregular, breathing very slow. Blood sugar in 

 hibernating mammals is little below waking values.'^- A waking dormouse 

 produces 4 cal./kg./hr. at 29°, and 15.5 cal./kg./hr. at 10 to 15° C.,-'*' whereas 

 a hibernating dormouse produces only 0.07 cal./kg./hr. at 8.5°. The respira- 

 tory quotient in hibernation is low, indicating fat oxidation (Ch. 8). 



Recovery from hibernation may be initiated by moving or otherwise stimu- 

 lating the animal, as well as by increasing the external temperature. Body 

 temperature and oxygen consumption then increase, the latter very rapidly, 

 temporarily to exceed the normal metabolic rate (Fig. 99)."^ The respiratory 

 excess may be correlated with unusual activity caused by the "rage" of the 

 hamster during the first hours after awakening rather than with any appreci- 

 able oxygen debt accumulated during the long hibernation. The temperature 

 of a hedgehog rose 19° in 42 minutes on awaking.^'" 



I lihernation is a method of self-preservation or conservation of body reserves 

 when external conditions of temperature and food supply are unfavorable. 

 In birds and most mammals the heat-regulating center appears to lack the 

 lability to permit adjustment required for hibernation. 



