HETEROTHEBMY IN HOMEOTHERMS 



It is an interesting indication of the general pattern of climatic 

 adaptability of animals that while land animals shed fur in summer, 

 northern harbor seals ( Phoca vitulina) lose part of their physiologi- 

 cal insulation in warm summer water at Woods Hole (Hart and Irv- 

 ing, 1959). As furcovered mammals vary the thickness of their coats 

 to suit the season, seals reduce the effectiveness of their physiologi- 

 cal insulation inwarm weather. A number of examples illustrate that 

 thermal adaptations of individuals are reduced as well as enhanced 

 to suit seasonal climates. 



Varying temperature of superficial tissues can thus efficiently 

 adapt warm-blooded heat producing machines to operate economic- 

 ally in a variety of environments. In fact heat producing machines 

 cannot work without thermal gradients. Until we examined arctic ani- 

 mals, however, I did not appreciate the extent of the swift changes in 

 the thermal gradients of the tissues of warm-blooded animals. Now 

 I find this variability in tissue temperature to be one of the most in- 

 teresting characteristics of warm-blooded life, and Isuspectthat it 

 is the primary insulator of the warm-blooded mechanism and that 

 fur and feathers are secondary developments. 



At the start of intense activity in cold weather bare skin may 

 suddenly warm to nearly 40 C and then cool when rest is resumed. 

 From measurements of thermal gradients extending for 6 to 8 cm 

 beneath the skin surface in swine and seals, we have found that large 

 masses of tissues are frequently involved in extensive thermal 

 changes. In the temporal and topographic variations of superficial 

 temperature, the warm-blooded animals differ fundamentally from 

 the cold-blooded kinds. In warm-blooded forms only the center is re- 

 latively homeothermous, while the organism is heterothermous . 



Variations With Temperature in Activities of Cold- and Warm - 

 Blooded Animals 



In summer on the tundra near the arctic coast of Alaska at Bar- 

 row, I noticed that when the sun shone intermittently through the 

 clouds, the flies (Grensia ) which I was pursuing escaped by flying. 

 When the sun was covered by a cloud the flies became grounded, 

 slow, crawling insects that I could easily catch. I inserted fine ther- 

 mocouples in several of the flies and found that in shade they were 



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