HUDSON 



mental extremes of high temperature and limited availability of 

 moisture characteristic of the desert. Among these are nocturnality, 

 fossorial habits, aestivation and hibernation, and dependence on 

 succulent foods. The many "niches" available are correlated with a 

 variety of successful adaptive patterns found in desert birds and 

 mammals. It is not surprising then, that abroad spectrum of phys- 

 iological abilities for coping with high temperatures and limited 

 availability of water is found among desert inhabitants. A species 

 possessing physiological mechanisms meriting a subjective judg- 

 ment of "well adapted to the desert environment" is one which 

 occupies a "niche" where high temperatures and a limited avail- 

 ability of moisture must be contended with. The converse would be 

 true of "poorly adapted" species. 



The difficulty of precisely describing the niche of a small bird 

 or mammal has been the subject of much discussion among biol- 

 ogists. However, some insight into the delineation of the "niche" may 

 be acquired by examining the physiological performance of a species 

 in the laboratory as an index of the environmental parameters to 

 which it is adapted. From observation of the variety of adaptive 

 mechanisms so far found in mammals of the deserts, it is becoming 

 apparent that no two species which have overlapping distributions 

 have the same physiological responses and therefore probably do 

 not occupy the same "niche." Thus competition between these desert 

 species is minimized, a distinct advantage in an area where re- 

 sources of food and water may fluctuate either seasonally or yearly. 



The role of natural selection in fitting a particular species for 

 the desert environment is difficult to assess because of the complex 

 relationship between the phylogenetic background of the species, the 

 "niche" occupied by a species, and the rate at which evolution can 

 occur in response to a changing environment. However, it can be 

 pointed out that natural selections need only act in the direction of 

 effectiveness of solution for a particular "niche" and need not be 

 concerned with elegance of mechanism. 



While diverse behavioral and physiological adaptations for 

 coping with the desert environment have already been found among 

 birds and mammals living and reproducing in this region, many 



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