288 ak:nual report Smithsonian iNsTiTUTioN, 1940 



HIBERNATION, ESTIVATION, AND OTHER PROVISIONS AGAINST 

 PERIODS OF FOOD SCARCITY 



Hibernation, the winter sleep of some animals, and estivation, 

 the summer sleep of a few others, are remarkable provisions of 

 nature to permit certain creatures to survive periods of cold, heat, 

 or food scarcity, with the minimum of hazard. These two peculiar 

 types of sleep appear to be essentially the same as ordinary sleep 

 so far as physiological phenomena are concerned; every gradation 

 seems to exist between them. But, in hibernation and estivation, 

 breathing is greatly reduced and in some instances of hibernation at 

 least, it may cease for hours at a time. The body temperature of 

 a hibernating animal may drop considerably, sometimes becoming 

 practically the same as the surrounding atmosphere. In other in- 

 stances, it remains at a fairly constant low level but above that of 

 the surrounding atmosphere. The heart beat is greatly reduced and 

 digestive processes are suspended. Some animals remain in this 

 condition for months at a time. Others for only a day or so at a 

 time. If disturbed while torpid, they generally soon regain activity. 

 Apparently animals must be in at least a fairly good state of flesh 

 before they can enter hibernation. If they were not, they might 

 not have sufficient vitality to survive a long sleep. 



Temperature is only one of the factors that determines when 

 animals should go into hibernation or estivation. In fact, although 

 extensive studies have been made of this phenomenon, much yet 

 remains to be learned. 



Some animals that live in the rigorous climates of the extreme 

 north do not hibernate, while others do. In the temperate regions 

 many hibernate. Some of these, notably the ground squirrels of 

 the western United States and probably those of Siberia as well, 

 continue their summer sleep into their winter sleep. In the arid 

 portions of both the Tropics and the temperate regions estivation 

 enables the creatures to avoid the intense heat and dryness and in 

 some instances, the food shortages or other adverse conditions such 

 as the drying up of water. Lung fish, northern turtles, and some 

 other animals undergo the same periods of torpor. Practically all 

 reptiles and amphibians in temperate regions hibernate, and some of 

 those in the arid Tropics estivate. 



Both hibernation and estivation are practiced by many forms of 

 invertebrate life, cliiefly insects, mollusks, and arachnids. No birds 

 are known to hibernate. 



The hibernation of bears wherein the animals become exceedingly 

 fat in the fall and select a sheltered den in which to "sleep" through- 

 out the winter is too well known to require comment other than to 

 point out that it is a remarkably good provision of nature to permit 



