DAWSON, W. R. 



of avian size is reached onlyinaltricial species. The energetic con- 

 siderations relating to smaller egg size and to the fact that the young 

 are not required to assume the energetic obligauons of homeothermy 

 during a major portion of their development support this suggestion. 



DISCUSSION AND SUMMARY 



Any accountof the evolution of homeothermy in birds will neces- 

 sarily be highly speculative on the basis of the information now at 

 hand, but it can be useful in emphasizing what appear to be the prin- 

 cipal determinants of this evolution and in suggesting pertinent lines 

 of future research. The transition from poikilothermy to homeo- 

 thermy in the avian evolutionary line must have involved may steps, 

 some concurrent and some sequential. With the capacities for be- 

 havioral control of body temperature which were probably present in 

 the poikilothermic forms, this transition was probably more signifi- 

 cant with respect to extension of the range of conditions over which 

 body temperature could be held in the range suitable for activity than 

 to any primary emancipation from the thermal environment. The 

 initial steps in the establishment of physiological temperature regu- 

 lation were probably metabolic, involving an overall intensification 

 of metabolism with its far reaching demands on the structure and 

 function of the various organ systems and then the development of 

 chemical regulation with its underlying control mechanisms. Per- 

 haps the rudiments of all the control mechanisms governing tem- 

 perature regulation of birds were present in their poikilothermic 

 antecedents , serving to control the behavioral and physiological com- 

 ponents of the temperature regulation which these animals probably 

 possessed. Extensive investigation of the neural mechanisms con- 

 trolling thermoregulatory activity in birds and reptiles is needed be- 

 fore an evaluation of this suggestion can be undertaken. 



The various components of the physical regulation in birds ap- 

 parently became functional at different times. The presence of pant- 

 ing in various contemporary thermophilic reptiles indicates that this 

 process is not the sole property of homeo therms and raises the pos- 

 sibility that it is present in birds as a legacy from their reptilian 



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