344 U. S. NATIONAL MUSEUM BULLETIN 217 



Although we have not yet found any birds having the adaptability for 

 seasonal cold which shows so clearly in the seasonal changes in the 

 fur of mammals, it does appear that birds of some arctic species are 

 adapted to cold, as is shown by their low critical temperatures. 



Physiological Analysis of Insulation 



Arctic land mammals, if they are large enough, are well protected 

 from cold by the thick covering of fur characteristic of land mam- 

 mals adapted to arctic life, and in some cases the ability to produce 

 this thick fur is an inherited character not determined by the environ- 

 ment. Some breeds of dogs, like the German shepherd can thicken 

 their fur in northern cold weather. Others, like short-haired pointers, 

 do not noticeably thicken their coat in cold seasons or when trans- 

 ported to the Arctic. Collie dogs in warm climates still have thick 

 fur and arctic sled dogs retain thick coats when taken to warm 

 climates. So the climatic adaptability of fur length varies among 

 the breeds of dogs selected under domestication. 



We found seasonal adjustment of insulation to environmental tem- 

 perature in the arctic red fox and porcupine (L, Irving, H. Krog, 

 and M. Monson, 1955). In summer fur the red fox, with a critical 

 temperature of 8° C., was adapted to an environment only 30° colder 

 than its body. In winter at Anchorage its critical temperature was 

 — 15° C, or 53° colder than its body. Arctic white foxes in winter, 

 with a critical temperature of —40° C, have insulation adaptive to 

 a temperature about 80° colder than their bodies. The winter in- 

 sulation of the arctic white fox and the summer insulation of the 

 red fox at Anchorage compare as 80 to 30. Some good-sized tropical 

 mammals had critical temperatures only 10° colder than their bodies, 

 and the effectiveness of their insulation compared with the winter 

 arctic white fox was as 10 to 80. The difference in these examples 

 adapted by natural distribution can be related to the thickness of 

 fur, a condition which varies with the climates in which they occcur 

 (Scholander, Walters, Hock, and Irving, 1950). 



It is quite possible for large arctic mammals to have fur ten times 

 thicker than that of tropical animals without hindering their move- 

 ments. By measurements of heat conductance through samples of 

 fur as much as 10- fold differences in physical insulation distinguished 

 the fur of well adapted arctic mammals from those in tropical Panama 

 (Scholander, Walters, Hock, and Irving, 1950) . The critical tempera- 

 tures of the arctic glaucous gull and domestic fowl differ by nearly 

 70°. The combination of all the physiological processes which make 

 up animal insulation must differ some 8-fold in these two species. 

 But arctic gulls do not have a feather cover which is eight times 

 thicker than that of hens, as would be necessary if thickness of 



