ADAPTATIONS TO DESEETS 



ture demonstrated the tjqjical arousal pattern when disturbed (Fig. 

 5). The difference between oral and rectal temperatures during 

 arousal was never more than 3 C, and arousal was accompanied 

 by strong visible shivering. Animals attained a normal body tem- 

 perature within 45 to 60 minutes after the onset of arousal at room 

 temperatures. No instance of torpor was observed between November 

 and May in a captive round-tailed ground squirrel. Furthermore, 

 between November and May, the body temperatures of animals were 

 much less variable and averaged higher than those found between 

 June and October (Fig. 6). It appears from the laboratory perfor- 

 mance of C. tereticaudus that this species may aestivate, but not 

 hibernate. This suggestion is supported by collection records (Donald 

 R. Dickey collection) , which indicate that this species has been 

 trapped in December, January, and February in the Coachella Valley, 

 California. Since C. tereticaudus has been readily trapped during 

 the summer and early fall, aestivation under natural conditions must 

 occur on either a daily or an intermittent basis. 



It is striking that aestivation is characteristic of one member of 

 a sympatric pair of desert ground squirrels. The ranges of C. 

 tereticaudus and C. mohavensis are overlapped by C. leucurus, but 

 C. tereticaudus and C. mohavensis do not occur in the same area. 

 C. leucurus neither aestivates nor hibernates, but remains active 

 above ground at all times of the year. Thus, in the area of sympatry 

 for these desert ground squirrels, only C. leucurus is active during 

 the more demanding anddifficultpartsof theyear. It seems reason- 

 able, therefore, to postulate that between these sympatric ground 

 squirrels competition, in the sense of utilization of a common re- 

 source which is in short supply (Birch, 1957, p. 6) , perhaps is 

 reduced, except in verypooryears,becauseof the differences in the 

 seasonal patterns of their metabolism. 



In contrast to the diurnal ground squirrels which aestivate or 

 to the nocturnal rodents which are fossorial, C. leucurus must cope 

 with muchof the rigor of the desert environment throughout the year. 

 The antelope ground squirrel depends on some of the types of 

 physiological mechanisms similar to those utilized by the Poor- will, 

 the kangaroo rat, and the camel in adapting to desert conditions 

 (Hudson, 1962). Like the Poor- will, the antelope ground squirrel 

 has abroad thermal neutral zone with a relatively high lower critical 



431 



