Temperature: Metabolic Aspects and Perception 371 



MIGRATION AND HIBERNATION 



Some birds migrate with the seasons and some mammals hibernate in winter. 

 Migration is a complex response, initiated by changes in energy balance 

 coordinated with seasonal variations in air temperature and photoperiod. With 

 their high body temperature and high metabolic rate, small birds lose weight 

 rapidly during a few hours without food at low air temperature. The length 

 of the daylight period for feeding must be sufficiently long to permit the bird 

 to accumulate enough reserves to last overnight, or the birds are compelled 

 to migrate. During the winter, survival time of English sparrows without 

 food at —14° C. is 19 hours; at +34° C. it is 61 hours.'"' During the warmer 

 summer months the resistance of sparrows to hunger is decreased; at —14" C. 

 survival time is only 11 hours. For the house wren at —14° C. during the 

 summer survival time is less than 5 hours, and this is a migratory species. 

 Differences between species in survival time without food depends on the 

 relative amount of utilizable fat stored in the body and on the rate at which 

 it must be metabolized to maintain body temperature. Undoubtedly the factors 

 that control migration mav also establish climatic barriers limiting distribu- 

 tion.»7- 101 



When the temperature goes down slowly, some lower invertebrates encyst 

 and their metabolism is reduced essentially to zero; in some insects water 

 content is reduced as they are "hardened" at low temperatures; larger poikilo- 

 therms become dormant, surviving as long as their metabolism is not reduced 

 below some critical level. Dehydration is important in initiating estivation, 

 particularly in reptiles. 



Certain mammals can reduce the effectiveness of their temperature regu- 

 lation and enter hibernation. Hibernation has been well reviewed in the 

 literature. 1^' *''• "•'•^' '■"'• ^^^ Hibernation is best known in monotremes, bats, 

 rodents, and shrews. It is doubtful whether carnivores truly hibernate in the 

 sense of becoming virtually poikilothermic, although bears and skunks sleep 

 for long periods in the winter. Numerous heterotherms enter cold narcosis 

 which may be a form of hibernation. The body temperature of bats approaches 

 air temperature and metabolism is reduced by about one-half in their diurnal 

 sleep; in caves in the winter hibernating bats are easily disturbed and fre- 

 quently active. Sleep can be induced in bats at 10° C. and they can be kept in 

 a state of cold narcosis in a refrigerator for weeks. The differences between 

 their usual sleep, when they are poikilothermic, and true hibernation are not 

 clear. Tropical bats do not hibernate but become increasingly active in cold 

 air."^' '•'•'^ If a three-toed sloth is placed in cold air (10°-15° C.) its body tem- 

 perature falls rapidly to about 20° and the animal enters cold narcosis;-^ this 

 may not be true hibernation. 1 he responses of marsupials and edentates to 

 cold have not been adequately examined. There are probably several kinds 

 of sleep, and several ways of inducing the various kinds of sleep. 



There is no simple cause of hibernation. The primary stimulus for the 

 woodchuck is depletion of food supply.'' Food reser\'es, true fat and brown 

 fat, j;,-e usually essential and are used during hibernation. Lack of food is also 

 a stimulus for estivation by some desert mammals. The magnesium concentra- 

 tion in blood serum is high, for example, in a hibernating hedgehog, and 

 injection of magnesium chloride may induce hibernal sleep."''' Blood mag- 

 nesium is also elevated in hibernating snails." ^ In many mammals low tem- 



