DAWSON, W. R. 



The evolutionary history of those components of physical regulation 

 affecting the extent of insulation inbirds is obscure, but it is possi- 

 ble to delineate certain of the factors which must have influenced 

 their development. Whether the evolution of feathers was originally 

 related to heat conservation as suggested by Young (19 50) or to the 

 establishment of capacities for gliding and ultimately, flight, it is 

 obvious that both thermal considerations and aerodynamic require- 

 ments have influenced their characteristics. The dual role of 

 feathers appears to have imposed restrictions on the amount of vari- 

 ation permissible in the thickness of plumage. Irving detected no 

 major differences between arctic and tropical birds of comparable 

 size in the thickness of plumage (Irving et al., 19 55). Subsequently 

 he did note some minor structural differences between feathers of 

 migratory and resident small birds in Alaska, which seemed to in- 

 dicate that the latter had more effective insulation (Irving, 1960). 

 In contrast to birds, large differences in pelage thickness between 

 many arctic and tropical mammals are apparent (Scholander et al., 

 19 50c). 



The aspect of physical regulation dependent on vasomotor ac- 

 tivity could have been established in the avian line prior to the 

 development of feathers as an outgrowth of the improvement of 

 circulatory capacity necessitated by the intensification of metabol- 

 ism. However, the character of the insulation afforded by the 

 plumage probably provided a stimulus for the development of vaso- 

 motor mechanisms to their present high level of performance in 

 birds. Even with the inevitable wear and loss of feathers between 

 molts, the minimum insulation afforded by the plumage is consider- 

 able, and this must make heat loss from the naked or thinly feathered 

 portions of the body of great importance during vigorous activity or 

 hot weather. The thinly feathered sides are exposed by holding the 

 wings away from the body in warm environments (Bartholomew and 

 Dawson, 19 54b; Hutchinson, 1954). These areas may also serve as 

 important sites of heat dissipation during flight. The unfeathered 

 portions of the legs of various species appear to be important sites 

 of heat dissipation under appropriate conditions (Bartholomew and 

 Cade, 1957; Bartholomew and Howell, 1961). Combs and wattles of 

 gallinaceous birds are apparently important in this respect also 

 (see Yeates et al., 1941), although Hutchinson (19 54) does not agree 

 that this has been convincingly demonstrated thus far. 



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