GENERAL ZOOLOGY 



can usually be maintained only for short periods of time. The capacity 

 to regulate body temperature by the use of heat derived from metabolic 

 activities, more or less independently of temperature in the external 

 environment, is developed to its highest point in the vertebrate classes 

 Aves and Mammalia, which are said to be endothermous or ''warm- 

 blooded." In these forms the body temperature is maintained at a gen- 

 erally favorable level for the species, despite changes in the environmental 

 temperature. The original development of cndothermy bv the ancestors of 

 modern birds and mammals was undoubtedlv correlated with the perfection 

 of hair and feathers as insulating materials, but more deep-seated phvsiologi- 

 cal mechanisms are also involved. Mammals, for example, rapidly adapt to 

 changing temperatures by complex compensatory reactions involving the 

 breathing mechanism, heart and circulation, skin, and endocrine control of 

 metabolism, integrated by the central nervous system (see p. 125). Long- 

 term anatomical and physiological adaptations are common in mammals 

 habitually exposed to low or to high temperatures. Mammals of the arctic 

 regions have thick fur and heavy, continuous blankets of subcutaneous adipose 

 tissue, and heat loss through radiation is often minimized by reduction of 

 such appendages as external ears. In contrast, mammals of the warmer 

 temperate zones, and particularly of the tropics, generally have less dense fur, 

 lack continuous subcutaneous fat deposits, and may have large, fan-like ears 

 which function effectively to enhance cooling through radiation (Fig. 19.2). 

 In any environment marked by unfavorable extremes of temperature, endo- 

 thermous animals frequently display adaptive behavior patterns, involving 

 hibernation, estivation, migration, retreat to shelter, and so on, which enable 

 them to avoid excessive exposure. 



The capacity for endothermous temperature regulation makes possible, for 

 birds and mammals, existence under a wider range of external temperature 

 conditions than is true of any other vertebrate group. Although mammalian 

 and avian species are structurally and functionally adapted to life between 

 customary maxima and minima of temperature, thev are not so seriously or so 

 rapidly affected as most amphibians or reptiles would be by external temper- 

 ature changes. The compensatory reactions of endothermous forms are not 

 without limits and are not perfect; therefore, even mammals and birds are 

 to a considerable extent dependent on favorable environmental temperatures. 

 In man, for example, the temperature of the extremities is often several 

 degrees below the deep body temperature; it is well known that humans may 

 freeze to death, or under other circumstances may suffer heat exhaustion or 

 sunstroke. 



The indirect effects of temperature acting through other physical factors 

 may be as significant to animals as the more direct effects on metabolism and 

 other vital functions. Notable in this connection is the fact that the solu- 

 bility of oxygen in water decreases markedlv with increasing temperature. 

 The exclusion of an aquatic species from waters above a certain temper- 

 ature might actually be an effect of oxygen deficiency, rather than a direct 



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