HABT 



by Irving etal. (1955), in which metabolism did not increase until the 

 temperature fell below -10 G. 



The regulation of body temperature by arctic mammals and 

 birds has been thoroughly investigated by Irving and Krog(19 55), 

 who showed that it depended both on thick fur or feathers over the 

 body and on peripheral cooling of the thinly covered legs and other 

 exposed parts. The temperature distribution on the body skin and ex- 

 tremities of various arctic mammals at different temperatures is 

 shown in Figure 4. Warm skin is dependent on a temperature drop 

 through the fur; this phenomena has been described for certain arctic 

 mammals by Griffin et al. (1953). An example of such a temperature 

 gradient measured by thermocouples placed in parallel at various 

 depths is shown in Figure 5. 



In thinly fur-covered legs, tissues replace fur as insulators. 

 Heat exchangers are possibly located in the area of the base of the 

 limbs, which show a sharp temperature drop. Such heat exchangers 

 have been demonstrated in tropical sloths ( Gholoepus hoffmanni and 

 Bradypus griseus) byScholander (19 57) and may occur widely in fur- 

 red mammals (Scholander, 1955). However, the presenceof a marked 

 temperature drop in a limb or appendage does not necessarily signi- 

 fy the presence of a heat exchanger. 



The cooling of peripheral tissues, which suggests tolerance to 

 cold not shared by warm tissues, is one of the remarkable proper- 

 ties of homeotherms. The demonstration of functional differences 

 between cool and warm tissues is difficult, although suggestive evi- 

 dence has been found in the distribution of fats of lower melting 

 point associated with low temperature function. Irving, Schmidt- 

 Nielsen, and Abramsen (1957) have shown that the distribution of 

 low melting point fats in various animals is not related to the cli- 

 mate in which the animals live. Other adaptations to cooling in peri- 

 pheral tissues have been demonstrated by Chatfield et al. (1953) in 

 the ability of the leg nerve of the herring gull ( Larus argentatus) to 

 conduct at lower temperatures in distal than in proximal parts of 

 the nerve, and by Heroux (1959) in a capability of the ears of rats to 

 recover from non-freezing cold injury ( Rattus norwegicus ) during 

 prolonged cold exposure. Nevertheless, the pronounced retardation 



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