THERMAL ECOLOGY OF DESERT TORTOISES IN THE EASTERN MOJAVE DESERT: SEASONAL PATTERNS OF OPERATIVE AND BODY TEMPERATURES, AND MICROHABITAT UTILIZATION

We monitored meteorological variables, daily and seasonal patterns of body tem- perature, corresponding operative temperatures, and microhabitat utilization by desert tortoises (Gopherus agassizii) during the 1991 and 1992 activity seasons of tortoises in the eastern Mojave desert. We studied tortoises in enclosures of natural habitat at the Desert Tortoise Conservation Center (DTCC) near Las Vegas, Nevada and a population of free-ranging tortoises in a field site adjacent to the DTCC. Air, ground and operative temperatures coincided with daily and monthly patterns of incident solar radiation. Variation in body temperature was primarily a consequence of microhabitat selection, principally use of burrows. During July-October, in the morning, body temperatures of tortoises in burrows were cooler than those of individuals on the surface. During midday, tortoises remained in burrows where body temperatures were cooler than extreme surface operative temperatures. While tortoises remained in burrows during much of the day, tortoises typically did not sleep in burrows at night. Microhabitat utilization was dictated by avoidance of extreme temperatures during midday, and microhabitat selection corresponded qualitatively to maintenance of energy and water balances. Effective conservation efforts to preserve habitat of desert tortoises will focus upon managing variables associated with integrity of burrows. ulation