HUDSON 



complex interrelationship between availability of food and water and 

 the prevailing temperature. However, some of the pocket mice, which 

 can maintain themselves on a dry diet ( Perognathus longimembris, 

 P. xanthonotus, P. formosus , P. penicillatus and P. fallax) become 

 torpid when food is withheld (Bartholomew and Cade, 19 57); 

 furthermore, P. californicus has a daily cycle of torpidity which is 

 related to the degree of deprivation of food (Tucker, 1961). 



Adaptation of two species of ground squirrels, C. mohavensis 

 and C. tereticaudus, to the desert environment depends in part on 

 their capacity to become torpid. The mohave ground squirrel ( Citel - 

 lus mohavensis) readily becomes torpid at laboratory temperatures 

 throughout the year, despite the continuous availability of food. Epi- 

 sodes of torpor are less frequent from March to August, which is 

 their period of activity under natural conditions. When entering 

 torpor at ambient temperatures between 22 C and 26 C, they as- 

 sume the usual sleeping posture, their oxygen consumption declines 

 rapidly, and body temperature approximates environmental tem- 

 perature within 3 or 4 hours. During torpor , oxygen consumption is 

 less than 0.2 cc/gm/hr, and the animal breathes irregularly, with 

 marked periods of apnea. Following the onset of arousal, oxygen 

 consumption increases 10- to 20-fold, and it usually peaks within 

 20 minutes. Body temperature increases more slowly, and the levels 

 of body temperature characteristic of normal activity are usually 

 attained in 45 to 60 minutes. Typically, rectal and oral temperatures 

 are within 0.5 C of each other during arousal. This pattern for the 

 onset of torpor, torpor itself , and arousal from torpor in the mohave 

 ground squirrel is typical of the classical picture of hibernation and 

 occurs at ambient temperatures between 10 C and 27 C (the high- 

 est measured). Under natural conditions, this species is torpid 

 during part of the hot, dry periods and continues this pattern 

 throughout the winter at a time when food and water are relatively 

 scarce. Thus, the physiological mechanisms for torpidity appear to 

 be the same during both summer and winter, although the level of 

 body temperature may differ. 



In contrast to Citellus mohavensis , C. tereticaudus kept in the 

 laboratory throughout the year with food and water available demon- 

 strated intermittent periods of torpidity from June to October only. 

 Animals with body temperatures within a degree of room tempera- 



430 



