DISTRIBUTION OF TERRESTRIAL VERTEBRATES 89 



these species, but the desert comprises only part of the perimeter of 

 their ranges. On other parts of the perimeters, different factors must 

 be Hmiting. 



Locally the distribution of many amphibians and reptiles is often 

 determined by aridity and temperature, but even these animals, 

 which are relatively dependent on climate, are able by their diurnal 

 and seasonal patterns of activity to select from apparently unfavor- 

 able physical circumstances the environmental conditions that do 

 not exceed their particular physiological tolerances. A result is that 

 while it is possible to reason from physiological data to the conditions 

 necessary for survival, it is not possible to reason from distributional 

 data to physiological capacities in the absence of detailed ecological 

 knowledge. Two examples may be cited. Thorson (1955) found that 

 the spade-footed toad, Scaphiopus hammondii, which occupies arid 

 regions, actually takes up water more slowly than do frogs from 

 more moist environments. In western Australia, frogs of the genus 

 Neohatrachiis frequent clay soils in which they cannot dig deep 

 burrows, whereas all species of the genus Heleioporus occupy friable 

 soil in which they can dig deep burrows. In the various species of 

 Neobatrachus, rate of water uptake increases with increasing aridity 

 of habitat, whereas in Heleioporus no such correlation can be demon- 

 strated. Presumably because of the microhabitat occupied during 

 estivation, rapidity of water uptake by Neobatrachus is of selective 

 importance, whereas for Heleioporus it is not selectively important 

 because the latter can dig deep enough to remain in damp soil where 

 rapid recovery from seasonal dessication is not critical (Bentley, 

 Lee, and Main, 1958). 



The behavioral and physiological virtuosity of birds and mammals 

 makes the assignment of distributional control to environmental 

 extremes particularly difficult even after detailed studies of ecology 

 and local distribution, although some documented instances are 

 available in the literature. Opossums {Didelphis marsupialis) on the 

 northern limits of the species' distribution frequently suffer frostbite 

 of ears and tail and it may be that low temperatures per se are limit- 

 ing the northward spread of this species (see Hamilton, 1958, for a 

 discussion). At the opposite extreme, Alaska fur seals {Callorhinus 

 ursinus) become overheated at air temperatures only a few degrees 

 above 0° C, and death from heat prostration is frequent during the 

 commercial seal drives in the Pribilof Islands (Bartholomew and 



