EVOLUTION OF AVIAN TEMPERATURE REGULATION 



The beneficial role in the avian heat economy of vasomotor ad- 

 justments favoring extensive blood flow through thinly feathered or 

 naked regions of the body in warm environments is reversed in the 

 cold and in most aquatic situations. This difficulty has been met by 

 the development of means for restricting heat loss from them. Heat 

 loss from the lower portions of the legs is apparently minimized in 

 many species by curtailment of the blood supply (during inactivity 

 they can also be protected by the body feathers when the bird "sits" 

 on them). However, counter-current arrangements for heat exchange 

 are evident in some species, for example, wading birds such as 

 herons, cranes, and flamingoes (Hyrtl, 1863, 1864). In either case 

 pronounced longitudinal temperature gradients can be produced. 

 Irving and Krog (1955) found that leg temperatures in the Gull ( Larus 

 glaucescens) ranged from 37.8 C proximallyto C distally when 

 the animal was subjected to cold. Peripheral heterothermy, which is 

 so important to the maintenance of centralhomeothermy, has appar- 

 ently demanded thedevelopmentof mechanisms of temperature com- 

 pensation m the peripheral tissues which are reminiscent of those 

 occurring in poikilothermic animals (Bullock, 1955: and Fry, 1958). 

 The demonstration of acclimation of conduction of the metatarsal 

 portion of the peroneal nerve to cold in the Herring Gull ( Larus 

 a rgentatus) provides an excellent example of this temperature com- 

 pensation (Chatfield et al. , 19 53) . 



The temporal relation of the development of those components 

 of physical regulation affecting the extent of insulation of birds to 

 the actual appearance of homeothermy in the avian line is largely a 

 matter of deduction. These components may have been present in 

 incipient stages prior to the advent of this condition, but it seems 

 reasonable that their full development occurred afterwards and was 

 significant in conserving the increased amount of heat produced as 

 a result of the metabolic changes discussed previously. If the fact 

 that altricial birds (in which the events in the ontogeny of tempera- 

 ture regulation can readily be observed because they occur after 

 hatching) develop fairly effective control of body temperature through 

 chemical regulation while their insulation is still in a rudimentary 

 state (Pembrey, 1895; Ginglinger and Kayser, 1929; Baldwin and 

 Kendeigh, 1932; and Dawson and Evans, 19 57, 1960) has any phylo- 

 genetic significance, it would appear that the development of physical 



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