HUDSON 



sary to use fat-free weights for comparing the basal metabolic 

 rates of hibernators and non-hibernators, in order to be certain 

 that a low metabolic rate is a phenomenon typical of hibernators. 



o 

 The high lower critical temperature (35 C) in the Poor -will 



means that much of the time this species lives outside its thermal 

 neutral zone. It is interesting to note that the Poor-will undergoes 

 seasonal torpidity when food is less available and when the main- 

 tenance of a normal body temperature would be metabolically ex- 

 pensive (Bartholomew, Howell, and Cade, 19 57). While there are 

 other species of birds which spend much of their time outside the 

 thermal neutral zone (Dawson and Tordoff, 1959; Scholander et al., 

 1950), torpor is particularly advantageous in the Poor-will, because 

 this species represents an unusual combination of specialized mor- 

 phological and behavioral adaptations for foodgetting, with its food 

 sources subject to marked fluctuation in availability. 



The low basal metabolic rate of the Poor-will is reflected in 

 a low heart rate (Fig. 4) at thermal neutrality. Birds which are com- 

 parable in size to the Poor-will but which possess a normal meta- 

 bolism (Odum, 1945) have heart rates about twice that of the Poor- 

 will. While both heart rate and metabolism increase when the 

 ambient temperature decreases below the lower critical tempera- 

 ture, the heart rate reaches its maximum level at a T . of about 



o A 



15 C, whereas the metabolism continues to increase as the T , 

 o A 



decreases below 15 C. 



Seasonal torpidity as a thermoregulatory adaptation for low 

 temperature is a well documented phenomenon among mammals. 

 Although numerous natural history accounts have suggested that 

 seasonal torpidity may also be a response to conditions of high 

 temperatures and limited availability of food and moisture, there 

 are only a few studies of the physiological performance of animals 

 which utilize summer torpor or aestivation (Bartholomew and 

 Cade, 1957; Bartholomew and Hudson, 1960; Bartholomew and Mac - 

 Millen, 1961). 



The ecological stimulus for aestivation is difficult to identify 

 precisely in all of the species known to aestivate because of the 



428 



