HETEEOTHERMY IN HOMEOTHEBMS 

 The Variable Insulation of Arctic Animals 



The thick fur of arctic animals is obviously an insulator that 

 protects them from excessive loss of heat (Scholander et al., 1950b). 

 In fact fur is so effective an insulator that a man clothed in winter 

 caribou (Rangifer tarandus) fur becomes overheated when he walks , 

 and we do not yet know how animals with thick fur get rid of the heat 

 generated by their long, swift running. A portal for the exit of sur- 

 plus metabolic heat is provided by the thin covering of the limbs and 

 noses. When active, these extremities become nearly as warm as the 

 body, but at rest their skin cools. We found that the bare skin of the 

 toe pads of arctic sled dogs and the hooves of caribou at rest might 

 be near the freezing temperature. The large webbed feet of Alaskan 

 Glaucous- winged Gulls (Larus hyperboreus) arenearly ascold as the 

 icy water in which they swim (Irving and Krog, 1955). When we tried 

 to measure the heat emitteci to cold water from the extensive webbed 

 foot of an Arctic Gull, the amount was so small that at first we re- 

 garded the results withsuspicion(Scholanderetal., 1950b) (Fig. 3). 



Effective conservation of heatby cold skin is shown by the cold- 

 ness of the entire surfaces of swine (Sus scrofa) in Alaskan winter 

 weather (Irving, 1956). Thevalue of their cool skin as an insulator is 

 apparent in the practicability of raising hogs outdoors in Alaska, 

 where our estimate indicated that they consumed about the same a- 

 mount of food as in temperate climates (Irving, Peyton, and Monson, 

 1956). 



An even more impressive indication of the insulating effective- 

 ness of changing the temperature ofbare skin was shown by the hair 

 seals ( Phoca vitulina and P. groenlandica ) that J. S. Hart and I ex- 

 amined in winter atSt. Andrews, N.B. (Irving and Hart, 1957). In ice 

 water their skin was only a degree warmer than the surrounding 

 water, and their metabolic production of heat was little greater than 

 in warm air. Thus cooling of bare skin provides insulation against 

 excessive loss of heat in arctic waters , which have the greatest ca- 

 pacity of any inhabited environment for removing heat. This thermal 

 economy allows great numbers of seals, walrus, and narwhals to live 

 throughout the year in the icy arctic seas. 



137 



