IRVING 



species of mammals that regulate their temperature well when at 

 rest, there is a differenceof only afew degrees. Body temperatures 

 do not now differ geographically, and the comparative view indicates 

 that little scope for variation in warm body temperature has oc- 

 curred in the courseof evolution. There maybe evidence for ancient 

 separate development of warm body temper atiore in birds and mam- 

 mals, but the heat producing machines of the two warm-blooded 

 classes operate at nearly the same temperature. The reptilian meta- 

 bolic system was already so elaborately developed that its evolution- 

 ary modification for warmer operation was limited within a narrow 

 range of temperature (Fig. 2). 



Eco nomy of Heat Among Arctic Animals 



Watching the caribou inwinter in Alaska, I have been impressed 

 by the large amount of time that they expend resting and carrying on 

 individual and social activities that bring them no food, for while liv- 

 ing in warmer climates I had thought that arctic mammals must feed 

 diligently in order to combat the arctic cold with metabolic heat. But 

 Scholander and I (1950a) found at the Arctic Research Laboratory 

 that even in the coldest temperatures the warmth of well- insulated 

 arctic birds and mammals could be sustained with metabolism at the 

 resting rate. Because of this economy the cost of maintaining bodily 

 heat for existence in arctic cold does not exceed the metabolic cost 

 of living in warm climates. Insulation adaptive to the vicissitudes of 

 the arctic climate opens the north for occupation by warm-blooded 

 animals without economic handicap. 



Natural populations of birds and mammals including man engage 

 in a great variety of time-consuming individual and social activities 

 which organize their societies in order to pursue their annual pro- 

 grams. Although these programs differ to suit seasons in various 

 environments, the organization of avian and mammalian populations 

 demands so large a share of each individual's time that only limited 

 periods can be utilized for feeding without endangering the structure 

 of the population, which is as complex in the arctic as in milder cli- 

 mates. 



136 



