■in A Comparative Animal Physiology 



are largely dependent on the environment for their temperature. The net 

 mechanical efficiency of muscle is about 20 per cent, yet the heat which is 

 lost is not wasted; insofar as it raises body temperature it permits faster 

 enzymatic reactions. The efficiency of a heterothermic animal is good in that 

 elevated temperatures are present when needed and enzymes need not work 

 overtime when the animal is at rest or in hibernal sleep. This efficiency is 

 apparently more than compensated for by slowness of getting started after a 

 period of low body temperature. 



Temperature limits the distribution of animals, and success at the limits of 

 environmental temperature is characterized by a variety of physiological 

 adaptations. Among poikilotherms the most important of these are: selection 

 of a favorable temperature, regulation of water loss, variations in water dis- 

 tribution, and variety of metabolic reactions, the extreme being suspension of 

 active life processes at unfavorable temperatures. Among homoiotherms there 

 are differences in degree of insulation (vasomotor responses and thickness of 

 body coat), and in the magnitude of the metabolic response to changes in 

 environmental temperature. Elucidation of the role of temperature in control 

 of the distribution of animals and in the evolution of new species would be 

 promoted by examination of the responses to temperature stress of individuals 

 of a species living at the limits of the temperature range, and ascertainment 

 of whether any observed individual differences are physiological or genetic 

 adaptations. 



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