EVOLUTION OF AVIAN TEMPERATURE REGULATION 



antecedents. It appears reasonable to postulate that the mechanisms 

 controlling the extent of insulation were developed after consider- 

 able quantities of heat became available to the nascent homeotherms 

 with the intensification of metabolism. However, the bolstering of 

 circulatory function required in this intensification must have pro- 

 vided a preadaptation for physical regulation through establishment 

 of an efficient heat transport system. The requirements of gliding 

 and ultimately flight undoubtedly intervened in the development of 

 that component of physical regulation involving the plumage. Conse- 

 quently the role of the plumage as insulation is at least in part a 

 compromise between thermal considerations and aerodynamic re- 

 quirements. The nature of this insulation has undoubtedly lent great 

 importance to the perfection of circulatory mechanisms having to do 

 with control of heat loss from the thinly feathered and naked portions 

 of the avian body. 



If the order of events in the establishment of birds was as sug- 

 gested here, physiological temperature regulation must initially 

 have been a costly process energetically. However, this would have 

 been outweighed by the advantages which it conferred to the early 

 homeotherm over its poikilothermic prey and competitors. Perhaps 

 the diversification of the early homeotherms within the avian line 

 contributed to a selection for the perfection of mechanisms of physi- 

 cal thermoregulation; with this diversification competition among 

 homeotherms would have been intensified, with the most efficient 

 types having the advantage. 



Associated with the evolution of homeothermy in the avian line 

 was the restriction of the thermal requirements for development. 

 Satisfaction of these requirements was keyed to the development of 

 patterns of parental behavior. It appears that theprecocial mode of 

 development is the primitive condition inbirds. The evolution of the 

 altricial mode of development has apparently been of great signifi- 

 cance from the standpoint of bioenergetics because it requires a 

 smaller egg of lower energy content than inprecocial development, 

 because it restricts the utilizationof energy by the developing young 

 to a minimum consistent with those maintenance processes exclusive 



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