HUDSON 



temperature and no marked upper critical temperature (Fig. 7). 

 Unlike the Poor-will, the antelope ground squirrel has a basal 

 metabolic rate conforming to the predicted value cc 02/gm/hr 

 = 3.8 W * ). C. leucurus can tolerate ambient temperatures of 

 42.6 C for periods of at least 2 hours, whereas many of the noc- 

 turnal rodentsof similar size cannot withstand ambient temperatures 

 above 40 C for equivalent periods of time (Dawson, 19 55; Lee, 

 1960; unpublished observations, Carpenter, 1961; and Tucker, 1961). 

 The body temperature of C. leucurus increases linearly with ambient 



temperature when the ambient temperature increases from room 



o 

 temperature to 40 C (Fig. 8). Thus, the antelope ground squirrel 



depends on hyperthermia both to minimize heat gain from the envir- 

 onment at high ambient temperatures and to maximize loss of 

 metabolic heat by radiation, convection, and conduction. When the 

 difference between T and T is inadequate for dissipation of meta- 

 bolic heat by radiation-convection-conduction and pulmonary evap- 

 oration of water (Fig. 9) , C . leucurus drools copious amounts of sal- 

 iva, which it actively spreads over parts of the body. 



Under natural conditions, C. leucurus probably avoids prolonged 

 exposures to very high ambient temperatures, which would be ex- 

 pensive to the water economy, by periodically returning to the cooler 



burrow. An animal requires only 3 minutes to reduce its body tem- 



00 o 



perature from 42 C to 38 C when taken from a T . of 42 C to 



o A 



25 C. In this way, a hyperthermic animal can unload accumulated 



heat within the burrow and then return above ground. From this, 

 it is apparent that behavior can be an important factor in relating 

 the thermoregulatory capacity of this species to the prevailing en- 

 vironmental temperatures. 



Any consideration of the problem of thermoregulation at the 

 high ambient temperatures of the desert must take into account 

 the availability of water and the capacity of a species to conserve 

 water. The ability of the antelope ground squirrel to maintain a 

 positive water balance under desert conditions is a complex inter- 

 relationship between several factors: its type of food, its level of 

 pulmocutaneous water loss, and its capacity to conserve water 

 incidental to excretion and defecation. While each of the above 

 factors may be studied separately under laboratory conditions, 

 their synthesis in relation to natural conditions is extremely dif- 



434 



