HUDSON 



appears that the ground squirrels produce a urine less concentrated 

 than that of the kangaroo rat ( D. merriami ), but more concentrated 

 than the urine of the wood rat ( N. albigula ) . Also, the antelope ground 

 squirrel (C. leucurus) produces a urine more concentrated than that 

 of the round-tailed ground squirrel. 



Significantly, the kangaroo rat, which is the species producing 

 the most concentrated urine, is primarily a seed eater, and can 

 maintain body weight on a dry diet. It is suggested that under natural 

 conditions the daily water requirements of the diurnal ground squir- 

 rels are too large toallowdependenceonthe water content of a typ- 

 ical seed diet even if the kidney were better able to concentrate 

 urine. Thus, while ground squirrels cannot maintain themselves on 

 a dry diet, they have a renal concentrating capacity sufficient to 

 balance the routine water losses with the water available in their 

 diet of succulent foods. 



SUMMARY 



Birds and mammals living in the deserts utilize a variety of 

 physiological, morphological, and behavioral patterns which may be 

 subjectively judged as varying from "well adapted" to "poorly adap- 

 ted. " In all cases, the ability of a desert species to live and repro- 

 duce in its environment indicates adaptation regardless of the ele- 

 gance of the mechanisms utilized. The role of natural selection is 

 such that effectiveness of solution rather than any special mechanism 

 is the primary criterion. 



The multiplicity of adaptive mechanisms attests to the diversity 

 of niches available, and it may turn out that no two desert species 

 of similar distribution have identical morphological, physiological, 

 and behavioral adaptations. While there are many species which re- 

 main to be studied, data for the Poor-will ( Phalaenoptilus nuttallii ) 

 and three species of ground squirrels ( Citellus leucurus , Citellus 

 tereticaudus , and Citellus mohavensis) further demonstrate the 

 diversity of adaptive mechanisms. 



442 



