EVOLUTION OF AVIAN TEMPERATURE REGULATION 



other hand, it was sufficiently far below the lethal temperature level 

 (about 47 C for contemporary birds) that moderate elevations of 

 body temperature resultingfrom activity, for example, could be sus- 

 tained without injury. 



The fact that the central body temperatures of birds do fall in 

 a fairly narrow band indicates that relatively little diversification 

 of this physiological character has occurred in the evolution of birds 

 subsequent to the establishment of homeothermy. One consequence 

 of this conservatism has been to render the fundamental level of 

 body temperature non- adaptive to climate (Scholander etal., 1950a; 

 Irving and Krog, 1954; Scholander, 1955; and Irving, 1960), although 

 temporary hjqjothermia and hyperthermia appear to have roles in 

 short-term adjustments to cold and heat, respectively, in some spe- 

 cies. Steen (1958) found that freshly captured small birds adjusted 

 to winter conditions in Oslo, Norway. These included Titmice ( Parus 

 major). Green Finches ( Chloris chloris ). Bramblings ( Fringilla 

 montifringilla ) , House Sparrows ( Passer domesticus ) , Tree Spar- 

 rows ( P. montanus) . and Redpolls ( Acanthis flammea ) . They became 

 hypothermic by as much as 9 to lo C when exposed to cold at night. 

 However, hypothermia did not develop in these birds when they were 

 experimentally acclimated to -10 C. Bartholomew and Dawson 

 (1958) regard hyperthermia as a regular feature of the response of 

 birds to heat. The tolerance by these animals of as much as 4 C in 

 excess of normal levels allows establishment of a favorable condi- 

 tion for heat transfer from body to environment when environmental 

 temperatures rise to near the level of body temperatures maintained 

 in cool environments. This response is of great significance in arid 

 regions because it reduces the demands for evaporative cooling from 

 what they would be in hot weather if body temperatures were main- 

 tained constant. The statement concerning the non-adaptiveness of 

 body temperature of course pertains to central body temperatures 

 and not to the temperatures of the peripheral tissues, particularly 

 in the legs, of birds. Variation in the temperatures of these tissues 

 comprises an important component of physical thermoregulation in 

 these animals (Irving and Krog, 1955). 



The fact that the general level of body temperature adopted by 

 birds exceeds that of mammals may confer a slight advantage in 

 warm environments, but its effect on heat exchange is probably in- 

 consequential in cold ones. The difference in thermal levels for the 



47 



