ARCTIC METABOLIC ECONOMY 347 



making garments that serve well both purposes. If clothing is warm 

 enough for rest it seriously hampers heat dissipation during activity. 

 Arctic animals have solved this problem, for while wearing feathers 

 or fur sufficient for rest in cold they can endure heat and violent pro- 

 longed activity without apparent overheating. Just after they have 

 come out in spring with thick fur arctic grizzly bears {Ursus arctos) 

 when alarmed can run far up the steep rough slope of a mountain 

 with undiminishing speed. Wolves and caribou run at high speed 

 for long distances. Late in winter the sun shining on the mountain 

 cliffs where mountain sheep {Ovis dalli) rest in their thick fur is un- 

 comfortably warm for a well clothed man. Arctic ptarmigan are the 

 swiftest flying grouse and make long flights during migration at 

 high speed. 



This flexibility of the arctic animal's insulation, designated one of 

 the important characteristics of physiological insulation (Scholander, 

 Hock, Walters, Johnson, and Irving, 1950), arouses the envy of arctic 

 man uncomfortably and even dangerously sweating from small 

 exertion in his clothing. How animals can in effect reduce their in- 

 sulation without removing their covering, and through what avenues 

 heat can escape in such variable and well regulated fashion, has been 

 one of the most interesting problems of physiology in the Arctic. 



Sweating effectively regulates the disposal of surplus heat for some 

 animals in warm climates. In the Arctic, man, physiologically a 

 tropical animal, sweats violently during exertion, but under heavy 

 clothing the process is inefficient and not very useful. The ability to 

 sweat varies among mammalian species and its use is difficult to meas- 

 ure, but my impression is that it is not much employed by arctic 

 animals in cold weather, although in warm weather they probably 

 make some use of it, because at no time have I found their fur or 

 feathers moist, nor does frost collect on them as it does with such 

 embarrassment on the clothing of sweating man. 



The Canidae utilize evaporative cooling by well regulated panting 

 in the Arctic, and in winter the cloud of moisture above a dog team, 

 like the "water smoke" over an open lead in the sea ice, is sometimes 

 visible at great distance. Caribou, reindeer, and men exhale moisture 

 which cools them in amounts varying with the amount of air breathed. 

 But without the well regulated cooling system involved in dogs' 

 panting, evaporation from the lungs does not serve as a flexible, well 

 regiilated means for disposing of excess heat. With their air sacs birds 

 have large surfaces, but it is not clear that these surfaces are employed 

 for regulated cooling. In fact, the whole important subject of regu- 

 lated cooling for arctic animals is obscure. However, one system for 

 varying the escape of heat has not until recently received much con- 

 sideration. In it I can see some interesting uses and important con- 

 sequences for arctic life. 



