332 U. S. NATIONAL MUSEUM BULLETIN 217 



The availability of food, the time applicable to obtaining food, and 

 the capacity of the metabolic processes set limits upon the extent to 

 which warm-blooded animals can, by increasing their metabolic pro- 

 duction of heat, adjust their lives to the cold arctic climate. These 

 prior considerations lead us to look particularly for the methods 

 by which metabolic heat may be conserved, as an essential to the 

 adaptation of warm-blooded creatures to arctic life, and to analyze 

 homoiothermism among arctic animals to see how the operation of 

 its component factors is suited to the arctic environment. 



Body Temperature of Arctic Animals 



The regular warmth of birds and mammals appears in sharp con- 

 trast with the temperature of arctic winter, which has been reported 

 as cold as —68° C. in eastern Siberia (Bartholomew and Herbertson, 

 1899; Court, Sissenwine, and Mitchell, 1949), and —65° C. in north- 

 western Canada (fig. 16). In experimentation at Barrow we found 

 that at temperatures frequently occurring in winter, arctic warm- 

 blooded animals could maintain their bodies some 80° warmer than 

 the air by the production of metabolic heat at their basal rate and by 

 its conservation with their natural insulation (Scholander, Walters, 

 Hock, and Irving, 1950) . 



We now have temperature measurements for many resident species 

 of Alaskan birds and most mammals except those of the smallest size 

 (L. Irving and J. Krog, 1954). In winter at —45° C. and in warm 

 summer the mean body temperature of 15 arctic and subarctic resident 

 species of birds averaged 41.3° C. The mean body temperatures 

 differed among the arctic species of birds by about as much as was 

 found by Wetmore (1921) among species from temperate regions, 

 and the means (fig. 17) for the arctic and temperate species exhibited 

 about the same differences. 



Body temperatures of homoiothermic species are phylogenetically 

 differentiated, but the differentiation shows no common regard for 

 geographical range, and their temperatures are more stable than the 

 climates of the earth, which have changed appreciably, while the tem- 

 peratures of the birds that inhabit them apparently have remained 

 fixed. Body temperatures of arctic species show no indication of 

 modifiability by climate and the small differences in temperature 

 among the species show no adaptive qualifications for arctic life. 



During our experimental study a number of arctic mammals were 

 observed to hold their body temperature with remarkable constancy 

 for several hours in air as warm as +30° C. or as cold as —50° C. 

 We have not as extensive a view of birds, but it appears that their 

 regulation of body temperature is about equally stable. Mammals or 

 birds which had been active showed temperatures elevated 2° or even 



