DISTRIBUTION OF TERRESTRIAL VERTEBRATES 93 



tion extremely difficult. In the light of present knowledge it appears 

 more reasonable to look for the determinants of distribution of the 

 higher vertebrates in behavioral and ecological factors rather than 

 in terms of physiological tolerances. Available knowledge of physi- 

 ology helps to explain how a vertebrate can live where it does, but 

 rarely reveals why it does not occur beyond the observed limits of 

 its distribution. Physiological tolerances are permissive in that they 

 set the environmental parameters within which a species can occur. 

 By habitat choice, seasonal and daily patterns of activity, selection 

 of appropriate microhabitats, and acclimation, however, a species 

 with sufficient ecological tolerance can assemble the environmental 

 conditions necessary for survival and reproduction out of remarkably 

 unlikely arrays of environmental factors. Consequently, assignments 

 of distributional limits on the basis of assumptions about the physi- 

 ology of an animal are unrealistic. An animal's distribution represents 

 an integration of all the factors — behavioral, ecological, competitive, 

 reproductive, or other — that limit its existence as a population. It 

 is, therefore, unrewarding to attempt to explain distribution solely 

 in terms of the data presently available from physiological studies, 

 which because of the orientation of most physiologists, have been 

 neither sufficiently varied taxonomically, sufficiently intensive from 

 the standpoint of species and populations, nor often enough oriented 

 toward ecology to yield data adequate for the analysis of problems 

 as subtle as those involved in distribution. 



REFERENCES 



Andrewartha, H B., and L. C. Birch. 1954. The Distribution and Abun- 

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Bartholomew, G. A., and T. J. Cade. 1956. Water consumption of house 

 finches. Condor, 58: 406-412. 



. 1957. Temperature regulation, hibernation, and aestivation in the 



little pocket mouse, Perognathiis longimembris. J. Mammal., 38: 60-72. 



Bartholomew, G. A., and W. R. Dawson. 1953. Respiratory water loss in 

 some birds of southwestern United States. Physiol. Zo'dl., 26: 162-166. 



. 1954. Temperature regulation in young pelicans, herons, and 



gulls. Ecology, 35: 466-472. 

 -. 1958. Body temperatures in California and Gambel's quail. 



Auk, 75: 150-156. 



Bartholomew, G. A., T. R. Howell, and T. J. Cade. 1957. Torpidity in 

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