88 G. A. BARTHOLOMEW 



ever, a deficiency in critical trace elements in the soil apparently 

 represents an unbeatable physiological problem even to ruminants 

 (see Underwood, 1956, for a comprehensive review). 



In several groups of mammals there is an additional capacity in 

 the repertory of environmentally relevant physiology, namely hiber- 

 nation, which allows smaller mammals to avoid for weeks or even 

 months climatic conditions too severe for them to cope with other- 

 wise (see Lyman and Chatfield, 1955, and Kayser, 1955, for recent 

 reviews). Prolonged periods of dormancy may occur at any season, 

 and the limited data presently available indicate that there is no 

 clear-cut difference between hibernation and estivation except the 

 environmental temperatures at which they occur (Bartholomew and 

 Cade, 1957). Daily periods of torpor are known among mammals 

 only in bats (Hock, 1951). Since hibernating mammals can arouse 

 spontaneously from their torpor, this capacity significantly extends 

 the range of environmental conditions which they can occupy, by 

 allowing them to confine their activity to the periods of the day or 

 the year when environmental conditions are favorable. 



In addition to the capacities summarized above, many animals 

 including members of all classes of vertebrates have the capacity to 

 acclimate to environmental changes. This process of acclimation 

 allows the organism to accommodate its own range of control to a 

 w^ide range of physical conditions (see Bullock, 1955, for a review of 

 temperature compensation in poikilotherms). 



Now that some of the relevant physiological capacities of verte- 

 brates have been surveyed very briefly, we may consider some ideas 

 concerning the relation of physiological tolerance to distribution. An 

 environmental factor that exceeds the limits of an animal's physio- 

 logical tolerance will control its distribution, but only at irregular 

 intervals in time and only on that perimeter of its range where the 

 factor is becoming extreme. On most of the boundaries of the 

 animal's range, the distributional limits are set by factors other than 

 simple physiological tolerance to the given environmental factor. 

 Familiar examples demonstrating that physiological incapacity to 

 meet environmental extremes is a factor in distributional control 

 only at isolated points in time and space are ofTered by the distribu- 

 tion of many species of vertebrates the ranges of which impinge on 

 the deserts of southwestern United States and northern Mexico. The 

 heat and aridity of the desert may actually limit the occurrence of 



