EVOLUTION OF AVIAN TEMPERATURE REGULATION 



these steps in mammals bycomparisonof species representing pri- 

 mitive and more advanced levels (see Eisentraut, I960) in develop- 

 ment of homeothermy. This approach was employed in the classic 

 study by Martin (1903) and has most recently been utilized by Johan- 

 sen (1961). Such an approach is less useful in attempting to trace the 

 evolution of temperature regulation in birds because of the absence 

 in the contemporary avifauna of especially primitive forms with re- 

 spect to attainment of homeothermy (see ''The Historical Back- 

 ground"). In the subsequentdiscussion a good deal of dependence has 

 been placed on information concerning the ontogeny of temperature 

 regulation in birds and on data on the physiological responses of con- 

 temporary reptiles to temperature. Due regard has been given the 

 difficulties of deriving evolutionary interpretations from such in- 

 formation: Ontogeny may only recapitulate phylogeny when expedi- 

 ent, and contemporary reptiles are for the most part far removed 

 from any line of direct importance to the history of birds. 



Behavior of Significance in Temperature Regulation 



Significant behavioral patterns in management of temperature 

 relations are widespread and presumably of considerable antiquity 

 among vertebrates. The ability of fishes to select particular tem- 

 peratures in experimental gradients is well known, and it appears 

 that this type of behavior plays a role in the distribution of at least 

 some species in nature (Sullivan, 1954). More pertinent to a con- 

 sideration of the evolution of temperature regulation in birds is 

 the abilityof reptiles under favorable conditions to control their body 

 temperatures by behavioral means (Cowles and Bogert, 1944 ; Bogert, 

 1949a, 1949b; Norris, 1953; Fitch, 19 56; and Saint- Girons and Saint- 

 Girons, 1956). Selection of suitable microclimates and absorption of 

 solar radiation allow many species to establish characteristic and, 

 in some cases, very high levels of body temperature when they are 

 abroad and active. The extent of the control of body temperature 

 which can be achieved by behavioral means when sufficient solar 

 radiation is available is indicated by the Andean lizard ( Liolaemus 

 multiformis) , which Pearson (1954) found could maintain a tempera- 

 ture above 30 C by basking, even though nearby shade temperatures 

 were at or below freezing. 



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