PERIODIC FLUCTUATIONS IN BODY TEMPERATURE 81 



1928) but gradually become homoiotbermic as they grow 

 older. 



Hibernation. — Hibernation is a resting state during 

 which animals exist in a more or less torpid condition. 

 It is a response usually associated with cold, in contrast 

 to other states of inactivity such as aestivation, which 

 is a response to warm, dry periods, and to inactivity 

 which occurs at certain periods in the life cycle inde- 

 pendent of temperature variations, such as the pupal 

 stage of many insects. 



Hibernation is a term generally applied to a torpid 

 state which occurs concomitantly with low temperatures 

 and is widely distributed through various groups of the 

 animal kingdom. Protozoans, rotifers, annelids, and 

 copepods commonly rest within cysts; snails close the 

 mouths of their shells and remain dormant ; certain fishes 

 inclose themselves in cocoons of mud and slime; many 

 poikilothermal vertebrates burrow in order to remain 

 torpid through cold seasons. Among the homoiothermal 

 animals only a few representatives of the mammals hi- 

 bernate ; namely, certain insectivores, bats, rodents, and 

 carnivores. There are no hibernating birds, but many 

 of them evade unfavorable temperature conditions by 

 migrating. 



Whether hibernation is directly induced by low tem- 

 peratures or not is still a controversial question. Many 

 investigators believe that it is more likely a response to 

 lack of food and that temperature is only a coincident 

 factor. Animals like squirrels prepare for hibernation 

 either by storing up food, which they consume at inter- 

 vals when they awake, or by fattening before they become 

 dormant. Pembrey (1895) has shoA\Ti that there is an 

 extraordinary low respiratory quotient in hibernating 

 mammals. This means that there is a conversion of fat 



