CHAPTER V 



EESPONSES OF HOMOIOTHERMS TO VARIATIONS IN 

 TEMPERATURE 



For every animal there is apparently an optimum 

 range of body temperature within which life-processes 

 are adjusted to proceed smoothly. This range is com- 

 paratively wide in the poikilotherms, but rather narrow 

 in the homoiotherms. Since homoiotherms maintain an 

 almost constant and optimum internal temperature, they 

 are little affected by external temperature changes un- 

 less the alteration is great enough to overtax or break 

 down their heat-regulating mechanisms. Thus it appears 

 that all animals have an optimum range of external as 

 well as of internal temperature. Poikilotherms have a 

 relatively wider range of internal optimum temperature 

 compared with the homoiotherm, but a relatively nar- 

 rower range of external optimum temperature (Fig. 3). 



The Body Temperature of Homoiotherms.— The aver- 

 age temperature of mammals is about 39 deg. C. That 

 of man is about 37 deg. C. That of the duckbill (Orni- 

 thorhynchus) and spiny ant-eater {Echidna) is about 

 25 deg. C. and is much more variable than in other mam- 

 mals. Birds have an average temperature about 5 deg. C. 

 higher than the mammals. 



The variation in the body temperature of the more 

 primitive mammals is more approximately and imper- 

 fectly regulated in comparison with the higher mammals. 

 There are few references to transitional stages between 



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