ADAPTATIONS TO DESEPTS 



The Poor-will has a basal metabolism which is one-third the 

 predicted value and is thus an exception to Scholander's general- 

 ization that metabolism is not adapted to climate. The combination 

 of a low basal metabolism and a gular flutter which does not sig- 

 nificantly increase metabolic heat production enables the Poor- will 

 to dissipate all its metabolic heat at high ambient temperatures, 

 with a minimum expenditure of water. At thermal neutrality, the low 

 basal metabolism of the Poor-will is accompanied by a heart rate 

 which is one-half the v.alue found in birds of comparable size. 

 Because of a low basal metabolism, the Poor-will also has a high 

 lower critical temperature and may therefore spend much of its 

 time outside the thermal neutral zone. It is significant that this spe- 

 cies hibernates during the winter when it would require a great deal 

 of food for maintenance of a normal body temperature. 



While the stimulus for the onset of torpidity in those species 

 of desert mammals known to aestivate is not clearly defined, lim- 

 itation of food in at least two species, Perognathus longimembris and 

 Perognathus californicus . causes periodic torpor. 



Hibernation and aestivation in the mohave ground squirrel illus- 

 trate the same physiological characteristics and are differentiated 

 only by the level of body temperature during torpor and the season 

 in which torpidity occurs. 



Under laboratory conditions, the round-tailed ground squirrel 

 ( C. tereticaudus) is intermittently torpid during the summer and 

 fall, but does not become torpid during the winter or spring. There- 

 fore, in terms of natural history, this species could be considered 

 to be an aestivator and not a hibernator. 



It is postulated that competition, in the sense of utilization of 

 a common resource which is in short supply, between the sym- 

 patric desert ground squirrels is minimal because of differences in 

 their patterns of metabolism. 



The antelope ground squirrel, which is not capable of torpidity 

 has a broad array of thermoregulatory mechanisms adaptive for its 



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