HIBERNACULUM USE BY A POPULATION OF DESERT TORTOISES (GOPHERUS-AGASSIZII) IN THE SONORAN DESERT

We quantified aspects of hibernaculum use by desert tortoises (Gopheru s agassizii) in the San Pedro Valley, Arizona. Tortoises hibernated pr imarily on steep south-facing slopes. Hibernacula included burrows in silt, silt with loose gravel, diatomite and/or diatomaceous marl, and beneath layers of well-lithified volcanic ash. Burrows were often also associated with live vegetation, dead and downed vegetation, and/or p ackrat (Neotoma albigula) nests. Male tortoises used longer hibernacul a (($) over bar x = 118.3 cm) than females (($) over bar x = 24.4 cm). Maximum temperatures of hibernacula of females (($) over bar x = 24.5 C) were consistently higher than maximum temperatures of hibernacula of males (($) over bar x = 18.2 C), but the difference was not signifi cant. Minimum temperatures of hibernacula of females (($) over bar x = 4.3 C) were lower than minimum temperatures of hibernacula of males ( ($) over bar x = 9.3 C). Temperatures in hibernacula of females fluctu ated over a wider range than temperatures in hibernacula of males. Hib ernacula used by males provided greater thermal buffering than those u sed by females. No tortoise (N = 8) used the same hibernaculum during both years of the study.