90 G. A. BARTHOLOMEW 



Wilke, 1956). The inability of these fur seals to prevent hyper- 

 thermia even at low air temperatures and low levels of solar radia- 

 tion may restrict the location of their breeding grounds to the Bering 

 Sea area. In contrast to such situations, wherein environmental 

 temperatures may be limiting, several species with very different 

 capacities for thermoregulation may successfully occupy the same 

 demanding environment. Brown Pelicans {Pelecanus occidentalis) , 

 Great Blue Herons (Ardea herodias), and Western Gulls {Lams oc- 

 cidentalis), although having comparable thermoregulatory abilities 

 as adults, differ markedly in this respect while young. The first 

 two species are altricial; the third is precocial. Nevertheless all 

 three species nest successfully in unsheltered rookeries on the 

 desert islands of the Gulf of California, where they are subjected to 

 high air temperature, intense solar radiation, and extreme aridity. 

 They are able to breed despite these unfavorable physical conditions 

 and despite the profound differences in the capacity for temperature 

 regulation of the young, because the parents in the two precocial 

 species are extremely attentive and shade the nestlings during the 

 hours of intense heat, thus behaviorally compensating for the phys- 

 iological limitations of the young (Bartholomew and Dawson, 1954). 

 There can be no doubt that in areas such as the deserts, polar 

 regions, and high mountains where the environment is so demanding 

 that life is extremely difficult or impossible, physiological capacities 

 and tolerances limit the distribution of all groups of vertebrates. 

 But if one considers the continental areas as a whole and amphibia 

 and amniotes only, it becomes surprisingly difficult to find distribu- 

 tional limits that are set by physiological tolerance to physical 

 factors in the environment, except for those species that occupy one 

 of the unusually demanding environments such as mentioned above. 

 Ordinarily one species replaces another geographically. Such replace- 

 ment may of course on occasion be caused by physiological differ- 

 ences between the forms in question. The problems of sympatry and 

 competition are so complex, however, that in the absence of detailed 

 ecological and physiological knowledge, it seems unwise to assume 

 that in an area of contact or overlap each of a pair of geographically 

 complementary species is distributionally limited by its physiological 

 capacities. (See Dumas, 1956, for a carefully analyzed study of the 

 ecological and physiological responses to temperature and humidity 

 in two sympatric salamanders.) Although the topic has been one of 



