EVOLUTIONARY ASPECTS IN MAMMALS 



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great as 0.2 G per cm at ambient temperatures of 25 C to 



27 C. Such gradients are 10 times steeper than those in the 

 human arm under similar conditions. The same vascular struc- 

 tures are present iji the limbs of other living Xenarthra, the 

 anteaters and the armadillos. The present study, showing very 

 large temperature gradients at low air temperatures, was not 

 designed to verify the counter- current hypothesis and can give 

 no conclusive evidence in this respect. 



We note from the important works of Irving and Scholander 

 and associates that the insulative value of animals generally 

 increases as we proceed north and south to the arctic and ant- 

 arctic regions. The almost unbelievable insulation attained by 

 some of the larger arctic mammals, like the husky, the wolf, 



and the fox, makes possible a maintained resting metabolism down 



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to 60 G to 70 G below zero. This fact, I think, poses an inter- 

 esting question: How do these animals get rid of the excess heat 

 produced during the extensive exercise they necessarily have to 

 practice? You may think that panting is enough to keep the body 

 temperature at so-called normal levels of 37 G to 38 G. I have 

 done this winter some measurements on the exercising husky 

 sled dogs. The results show that shortly after the start of exer- 

 cise, both intramuscular and deep rectal temperatures reach 



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levels of about 41 G to 42 G in an air temperature down to 



-50 G. These surprisingly high temperatures did not in any way 

 impair the performance of the dogs, which could keep on working 

 for 6 to 8 hours at the same speed, allowing only brief periods 

 of rest. Studies done on dogs from warmer temperate regions 

 indicate an impairment of function and heat collapse at lower body 

 temperatures, down around 41 G. Although few data are availa- 

 ble so far, it seems probable that the unbelievably great insula- 

 tion of some of the arctic mammals has resulted in an adaptive 

 tolerance to an elevated body temperature. At least the husky 

 provides information in this direction. My studies on the heavily 

 insulated muskrat indicate an entirely different solution to the 

 problem. Time seems, however, to prevent us from going into 

 this. 



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