HART 



metabolism to compensate adequately for the observed cooling. It 

 was suggested that mild hypothermia might be tolerated by musk- 

 rats in nature. 



In swine and in the true aquatic mammals with an insulating 

 layer of blubber, living tissues replace the fur as the effective insu- 

 lators, and there is pronounced cooling of peripheral tissues. In har- 

 bor seals exposed to ice water, there is a reduction of heat produc- 

 tion during the winter, signifying a seasonal change in insulation of 

 the living tissues. Arctic harp seals are superior to harbor seals 

 during the winter since they can tolerate ice water without lowering 

 body temperature or elevating heat production. 



When the body skin temperature of various species are com- 

 pared, very large differences are found in the temperatures corres- 

 ponding to elevation of heat production. In the fur-bearing land mam- 

 mals studied, heat production increased with only slight skin cooling 

 whereas in aquatic mammals a pronounced skin cooling was necess- 

 ary. The swine and muskrat were intermediate. The distinction in 

 the metabolic response to cold between land mammals and aquatic 

 mammals is much more apparent with respect to skin temperature 

 than to air temperature. 



The metabolic response to cold in different non- aquatic species 

 is related both to insulation of the fur and to body size. However, 

 while small mammals with thin fur are metabolically sensitive to 

 cold, large mammals may be sensitive or insensitive, depending on 

 the fur insulation. In aquatic mammals so far studied the fur insula- 

 tion was negligible and hence did not affect the metabolic response 

 to cold. Temperature regulation in aquatic mammals is effected 

 through physical regulation of heat loss in the general body surface 

 and particularly in the appendages. 



226 



