ARCTIC METABOLIC ECONOMY 343 



Irving, 1950). A red fox and porcupines captive at Anchorage had 

 critical temperatures closely conforming with the mean environ- 

 mental temperature in winter and in summer, for by changing the 

 length of their fur they suited their metabolism to the temperature 

 of the season in the locality where they lived. Their adaptive varia- 

 tion in insulation shows that seasonal and geographical variation in 

 critical temperature in individuals of some species of mammals 

 correlate reasonably well with the climate to which they are exposed 

 (L. Irving, H. Krog, and M. Monson, 1955) . 



We were surprised, however, to find that the critical temperature 

 for a brant {Branta nigricans) and a fish crow {Corvus caurinus) 

 did not change seasonally. The crow's critical temperature in winter 

 and summer at Anchorage corresponded to the January mean in 

 the warmer climate of Seward, where it had been captured from 

 among birds resident in the coldest part of their usual range. When 

 forced to remain in the colder winter of Anchorage, the critical tem- 

 perature of the crow and migratory brant remained the same as the 

 mean temperature of the warmer winter on the natural wintering 

 ground of the species. Neither bird showed any modification of its 

 critical temperature with season or with its exposure to a winter in 

 Anchorage much colder than it would naturally encounter. 



There are other indications that under domestication birds do not 

 adjust their critical temperature to the climate in which they are 

 placed by man. Domestic geese at Strasbourg and Belgrade had 

 critical temperature near 0° C, according to the figures of Terroine 

 and Trautmann (1927) and Giaja (1931). The critical temperature 

 of domestic geese was above the midwinter temperatures of central 

 Europe, but birds of the genus Anser would seldom encounter below 

 freezing temperatures in their natural range, which usually extends 

 only as far north as water remains unfrozen. 



Domestic fowl {Gallus) (Terroine and Trautmann, 1927; Mitchell 

 and Haines, 1927) and pigeons {Columha) (Kayser, 1930; Terroine 

 and Trautmann, 1927; Falloise, 1900) have high critical temperatures 

 in the temperate climate of central Europe and North America. 

 Domesticated fowl are considered to be derived from wild ancestors 

 living in warm climates and they appear to have retained the critical 

 temperatures appropriate to tropical birds. Nevertheless, fowl live 

 with some shelter in Alaska and we have kept pigeons outdoors dur- 

 ing winter in Barrow. In captivity the birds have an unlimited 

 supply of food available without effort on their part and they are 

 protected by cages from being disturbed. While relieving an animal 

 of the need for exertion to obtain food does not mitigate the cold, 

 it may reduce the effort necessary for existence so that the full natural 

 influence toward evoking climatic adaptation is lacking in captivity. 



469496—60 23 



