EVOLUTION OF AVIAN TEMPERATURE REGULATION 



have appeared in this line as a legacy from its reptilian antecedents. 

 The early development of panting in young birds, e. g., albatrosses 

 (D iomedea immutabilis and D. nigripes ) , studied by Bartholomew 

 and Howell (1961); herons, studied by Bartholomew and Dawson, 

 (1954a); and House Wrens, studied by Kendeigh (1939), likewise sug- 

 gests that this mechanism is of considerable antiquity in birds. If 

 panting was inherited from the reptilian antecedents of birds, its 

 function was apparently bolstered by subsequentchangesinthe car- 

 diovascular and respiratory systems, associated with the intensifi- 

 cation of metabolism. Rates of evaporative water loss by panting 

 lizards are a fifth or less those of panting birds of comparable size 

 at the same body temperature (compare data on the birds Pipilo 

 fuse us , P . aberti , and Richmondena cardinalis (Dawson, 19 54, 1958) 

 and on the lizard Dipsosaurus dorsalis (Templeton, i960)). It would 

 be of considerable value to an understanding of the origin of panting 

 to determine whether or not the midbrain center controlling panting 

 in the Rock Dove (von Saalfeld, 193 6), and presumably in other birds 

 as well, is homologous to the neural apparatus governing this ac- 

 tivity in reptiles. 



The apparently universal distribution of panting among birds 

 contrasts to the situation in mammals, which may bring about eva- 

 porative cooling at high temperatures by panting, sweating, or be- 

 havioral means. It is probably significant that panting, though it is 

 not a highly effective means of heat dissipation, is the only one of 

 these mechanisms which would not interfere with flight by marring 

 the integrity of the plumage of birds. Flight has undoubtedly created 

 special problems for these animals because of the high level of heat 

 production which it involves. Dissipation of this heat must require 

 extensive evaporative cooling, owing to the effectiveness of the in- 

 sulation provided by feathers. The development of the avian res- 

 piratory system was probably influenced by this need for evapora- 

 tive cooling as well as by the requirements for gas exchange. Zeu- 

 then (1942) and more recently Salt and Zeuthen( 1960) have suggested 

 that the air sacs are important in producing the necessary evapora- 

 tive cooling during flight. This suggestion appears plausible, but 

 awaits experimental evaluation. 



55 



