MAMMALIAN COLD ACCLIMATION 



nerve of the herring gull that these responses may be altered by ac- 

 climatization in the same species. 



The comparisons in Figure 12 also illustrate the fact that the 

 metabolic sensitivity to cold may differ, depending on whether the 

 oxygen consumption is correlated with skin or with air temperature, 

 e. g., swine and muskrat. This is because the fur provides the insu- 

 lation for the muskratbut not for the swine. Similarly, the thick fur- 

 red arctic mammals with warm skin may be as sensitive to lowered 

 skin temperatures as the rat, caribou, and man, even though the most' 

 severe arctic conditions canbe withstood without elevation of resting 

 heat production (Scholanderetal., 1950a). Therefore, arctic and tro- 

 pical mammals may be quite similar with regard to the skin cooling 

 required for a given metabolic response. 



Metabolic response to cold in relation to body size and insulation 

 of the fur. A summary of data published in the fourth Temperature 



Symposium of the Americanlnstituteof Physics (Hart 196 2b)is illus- 

 trated in graphic form in Figure 13. The slope of the temperature- 

 metabolism curve below the critical level is given as the ratio of the 

 temperature drop to the increase in metabolism, which is dimension- 

 ally comparable to an insulation rather than a conductivity function. 

 This is done to facilitate comparison of slopes with fur insulation 

 (open circles) for the few species for which data are available. 



It may be seen that the slope tends to increase with increase in 

 body weight, but that there is an enormous increase in species vari- 

 ability with increase in weight. This is because the small mammals 

 are all metabolically sensitivetocold while the large mammals may 

 be sensitive or insensitive. The least sensitive are the arctic mam- 

 mals with thick fur which give the three highest values for slope, e. 

 g., snow-shoe hare in winter, red fox, and white fox. The larger 

 mammals may also have little fur and lower values for slope, e. g., 

 dog, harbor seal in summer, and Brahaman bull. 



When slope is related to insulation of the fur (broken line), there 

 appears to be a much closer correlation. However, it is obvious that 

 the same correlation would notbe applicable for either bare-skinned 

 or aquatic mammals with subcutaneous fat and a thin fur cover. The 



223 



