HUDSON 



order for this species to dissipate all of its metabolic heat and 

 heat gained from the environment when the T exceeds the T 

 (Fig. 3). Since at high ambient temperatures, other species oi 

 birds comparable in size with the Poor-will become hyperthermic 

 and elevate their metabolism when panting (Dawson, 19 54; Dawson 

 and Tordoff, 1959), it is difficult to use comparisons for evaluating 

 the reduction in evaporative water loss accruing from the Poor- 

 will's reduced metabolism. However, it may be noted that if there 

 is no radiation-convection-conduction of metabolic heat, as would 

 occur in the Poor-will when the body and ambient temperatures 

 are equal (40 C), a 40 gram bird with a metabolism one-third of 

 normal saves 12 cc of water /day (assuming that one cc of oxygen 

 releases 4.8 calories and 1 mgm of evaporated water dissipates 0.58 

 calories). 



Scholander (1955) has suggested that evolutionary adaptation for 

 temperature regulation in homeotherms has principally involved heat 

 dissipation and that heat production has not been modified since all 

 species, regardless of habitat, typically follow the mouse to ele- 

 phant curve. Thus, arctic mammals at low ambient temperatures 

 keep their heat dissipation minimal by virtue of good insulation and 

 possess special means for dissipating heat during activity or at 

 relatively high ambient temperatures. In contrast to arctic mammals 

 and birds, animals from desert areas more frequently encounter 

 problems of maximizing heat dissipation when there is a small diff- 

 erence between body and ambient temperatures. For this reason, 

 it might be expected that at least some species, particularly those 

 which are diurnal, would demonstrate a reduction in the level of 

 basal metabolic heat production. The Poor-will is an example of 

 such a species, and thus it is an exception to Scholander's general- 

 ization that metabolism is not adapted to climate. Although Scho- 

 lander et al.(1950) relate the low metabolism of tropical caprimul- 

 gids to their capacity to hibernate, such a correlation does not 

 differentiate between cause and effect. Thus, hibernation may either 

 allow or follow a lowbasal metabolism. Also, considerable evidence 

 has accumulated to suggest that the low metabolic rate of many 

 hibernators may be attributable to the fat deposits which in them- 

 selves probably exert little effecton the overall metabolism (Barth- 

 olomew and Hudson, 1960; unpublished observations on Citellus 

 tereticaudus and Cercaertus nana) . Therefore, it is probably neces- 



426 



