86 G. A. BARTHOLOMEW 



remarkable powers of thermoregulation. Their capacity to tolerate 

 severe hyperthermia allows them to operate effectively at remark- 

 ably high environmental temperatures (Bartholomew and Dawson, 

 1958), and their high rate of heat production, effective insulation, 

 and peripheral vasomotor control allow them to operate at extremely 

 low environmental temperatures (Scholander et al., 1950a; Wallgren, 

 1954; Irving et al., 1955). With regard to water, however, birds have 

 less independence of the environment than do many mammals. 



In the birds that have been measured, evaporative water loss 

 exceeds metabolic water production even under resting conditions. 

 (Bartholomew and Dawson, 1953). This unfavorable relationship 

 necessitates a high intake of water either through drinking or the 

 eating of succulent foods, which places birds, particularly small 

 ones, at a disadvantage with regard to the occupancy of arid re- 

 gions (Bartholomew and Cade, 1956). 



Documented records of both daily and seasonal torpidity are now 

 available for adult birds in four different orders (see Bartholomew, 

 et al., 1957, for a summary). Although the advantages of torpidity 

 with regard to energy conservation are obvious for birds of extremely 

 high metabolism, such as humming birds (Pearson, 1954b), or for 

 birds dependent on periodically unavailable food (swifts, poor-wills, 

 and nighthawks) , the limited number of species and the fragmentary 

 nature of the available data do not warrant rigorous distributional 

 inferences. 



Mammals. One of the most impressive attributes of mammals is 

 the excellence of the physiological homeostasis that they have at- 

 tained ; one form or another is able to meet head on the most severe 

 naturally occurring environmental conditions of heat, cold, or 

 aridity. Hence, any taxonomically extensive generalizations concern- 

 ing physiologically determined distributional limits are apt to be 

 particularly unsatisfactory in this group. Large arctic mammals by 

 the excellence of their insulation and vasomotor control can main- 

 tain a difference of as much as 70° C between air and deep body 

 temperatures without increasing metabolism above basal level or 

 decreasing the deep body temperature (Scholander et al., 1950b; 

 Scholander, 1955). 



Most medium to small-sized mammals living in areas of very high 

 environmental temperatures avoid heat stress by being fossorial, 



