DISTRIBUTION OF TERRESTRIAL VERTEBRATES 87 



nocturnal, or both. Such patterns of behavior are often impossible 

 for large mammals, but if they can sweat and if drinking water is 

 available, they are able to cope with any high air temperatures that 

 occur naturally. In at least one large mammal, the dromedary camel 

 {Camelus dro^nedarius) , tolerance of hyperthermia contributes 

 significantly to effectiveness of adaptation to high ambient tempera- 

 tures by allowing heat storage rather than by requiring heat dissi- 

 pation through evaporation of water (Schmidt-Nielsen et al., 1957). 



The availability of surface water is of no importance in the distri- 

 bution of many desert rodents. Several species in the family Hetero- 

 myidae lose so little water through evaporation, excretion, and 

 defecation, that as long as they are not forced to resort to evaporative 

 cooling, they are able to produce all the water they need through 

 their own metabolism even while subsisting on a dry diet (see 

 Schmidt-Nielsen and Schmidt-Nielsen, 1952, for an extensive 

 review). It has yet to be demonstrated, however, that large herbiv- 

 orous mammals can produce enough metabolic water while on a dry 

 diet to compensate for water losses. Unlike a rodent, the larger 

 mammals cannot escape the heat of the day by burrowing and must 

 therefore depend in part on evaporative cooling to prevent harmful 

 hyperthermia. Nevertheless, the capacity of some large herbivores 

 to go without water is impressive. A dromedary camel exposed to the 

 full heat load of radiation from sun and ground during the summer 

 at a Saharan oasis survived a 17-day period on a dry diet without 

 water (Schmidt-Nielsen et al., 1956). Its performance is attributed to 

 its capacity to tolerate a loss in body water equivalent to 30 per cent 

 of its body weight. (Most mammals cannot tolerate more than 12 

 per cent dehydration.) 



One may summarize by saying that by a combination of behavior 

 and physiology mammals can successfully occupy all but the most 

 extreme environments on earth without anything more than quanti- 

 tative shifts in the basic physiological pattern common to all. With 

 regard to dietary limits their performance is almost as impressive. 

 For example, ruminant artiodactyls can subsist largely on cellulose 

 because of the synthetic capacity of the bacterial flora of the rumen 

 (see Blaxter, 1954, for a discussion). (The existence of a fauna of 

 ruminant ungulates of course is an essential feature for the survival 

 of populations of many of the larger mammalian carnivores.) How- 



