lEVING 



o o 



about 8 C and in sunshine they warmed to 12 C. The change of a 



few degrees converted the flies from slow, crawling to alert, flying 



organisms. 



Not only do habits of living change critically in cold-blooded ani- 

 mals at certain temperatures, but many measurable physiological 

 frequencies and velocities of their activities commonly double in 

 warming 10 C. Aquatic cold-blooded animals do not usually survive 

 the quick changes through which heterothermous tissues of northern 

 warm-blooded animals rapidly and frequently pass, but some north- 

 ern terrestrial insects can survive large and swift changes in tem- 

 perature. If their activity changed continuously from C to 40 C 

 with the common Q of two, it would increase 16-fold, but the dis- 

 continuity in activity seen in the tundra flies shows that critical tran- 

 sitions in their cold-blooded activity occur at certain temperatures 

 and drastically alter their manner of living. 



The physiological systems of cold-blooded animals do not oper- 

 ate consistentlyoverwide ranges of temperature, but the heterother- 

 mous superficial tissues of birds and mammals act in continuous 

 coordination with the homeothermous centers so that each animal 

 steadfastly remains one individual operating in its characteristic 

 manner. This integrated action of heterothermous tissues maybe the 

 most informative distinction between warm- and cold-blooded life. 



CXir knowledge about processes in the heterothermous tissues of 

 warm-blooded animals is too sparse to provide profitable speculation 

 on how they are integrated in the continuous life of individual organ- 

 isms, but I can add some examples of heterothermous operation in 

 the adaptative reactions to cold of people living in northern climates. 



Cooling of Hands and Feet of People Adapted to Cold 



A few years ago I was fortunate in making the acquaintance of 

 members of a sect accustomed to going with light clothing and bare 

 feet in Alaska. Two of their members who were university students 

 have helped us to understand some thermal reactions by their ability 

 to manifest and describe their adaptation to cold (Irving, 1959). While 



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