HUDSON 



niche. Among its adaptive patterns are:(l) tolerance of ambient tem- 

 peratures up to 42.6 C for periods of 2 hours, (2) a thermal neutral 

 zone extending from 30 C to 42.6 C without a marked upper criti- 

 cal temperature, (3) supplementary evaporative cooling by active 

 spreading of a copious secretion of saliva over the body when the 

 ambient temperature exceeds 39 C , (4j dependence on hyperthermia 

 even at low ambient temperatures (30 C) for radiative-convective- 

 conductive dissipation of heat, and (5) effective capacity for unload- 

 ing accumulated body heat, by periodically returning to the cooler 

 subterranean environment. 



In an ecological context, problems of thermoregulation for 

 desert birds and mammals become intimately linked to the complex 

 interrelationship between availability of moisture, level of pulmo- 

 cutaneous water loss, and capacity for water conservation. A com- 

 parison of renal concentrating capacity among several desert rodents 

 offers some insight into the extent of adaptation for water con- 

 servation. Ranking those species for which data are available in 

 order of ability to concentrate urine one obtains the list: D. 

 merriami > Citellus leucurus > C. tereticaudus >Neotomaalbigula. 



Only D. merriami , whicn is primarily gramnivorous, is able to 

 maintain body weight on a dry diet while the ground squirrels and 

 wood rats depend on availability of succulent foods to satisfy their 

 water requirements. 



444 



