ADAPTATIONS TO DESEBTS 



species occupying special niches remain to be studied. Recently 

 we have examined the physiological performances of the Poor-will 

 ( Phalaenoptilus nuttallii) and three species of ground squirrels 

 ( Citellus mohavensis. Citellus tereticaudus, and Citellus leucurus) . 

 Our results illustrate some additional types of adaptations to the 

 desert environment. The ground squirrels are fossorial and diurnal; 

 hence, they occupy an ecologically intermediate position between 

 the small nocturnal and large diurnal mammals. The Poor-will is 

 crepuscular, although it may nest and roost in areas of extremely 

 high temperatures. 



The Poor-will has a basal metabolic rate or^e-third of that 

 predicted from the equation: cc02/gm/hr =9.3 W ' . Because of 

 its low standard metabolic rate, the lower critical temperature of 

 the Poor- will is also very high (Fig. 1). This low metabolism 

 minimizes the amount of metabolic heat to be dissipated, a useful 

 adaptation when a small difference between body and ambient tem- 

 peratures precludes much radiation, convection, and conduction of 

 metabolic heat. 



In order to prevent the elevation of body temperatures to 

 lethal levels when high ambient temperatures are encountered, many 

 birds and some mammals pant. The metabolic heat production 

 associated with panting increases the evaporative water loss and is 

 expensive to the water economy, a major consideration for animals 

 of arid regions. However, unlike many birds, the Poor-will does 

 not pant and therefore has no marked upper critical temperature. 

 Although the thermal neutral zone begins at a rather high lower 

 critical temperature, it is also very broad (Fig. 1) and extends at 

 least to 44 C. At ambient temperatures above 40 C, the extensive- 

 ly vascularized gular area is fluttered rapidly with the mouth held 

 open. In this way, the bird is able to dissipate its metabolic heat 

 (in addition to dissipating heat gained from the environment when 

 the ambient temperature exceeds the body temperature) while sim- 

 ultaneously keeping its level of heat production virtually unchanged. 

 At high' ambient temperatures, the amount of water expended by the 

 Poor- will for evaporative coolingisless than that expended by other 

 birds of comparable size (Fig. 2). The combination of a low basal 

 metabolism and a gular flutter which does not increase the metabolic 

 rate necessitates only a modest level of evaporative water loss in 



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