Hibernaculum Use by a Population of Desert Tortoises (Gopherus agassizii) in the Sonoran Desert

Abstract

We quantified aspects of hibernaculum use by desert tortoises (Gopherus agassizii) in the San Pedro Valley, Arizona. Tortoises hibernated primarily 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 packrat (Neotoma albigula) nests. Male tortoises used longer hibernacula (x = 118.3 cm) than females (i = 24.4 cm). Maximum temperatures of hibernacula of females (x = 24.5 C) were consistently higher than maximum temperatures of hibernacula of males (k = 18.2 C), but the difference was not significant. Minimum temperatures of hibernacula of females (x = 4.3 C) were lower than minimum temperatures of hibernacula of males (x = 9.3 C). Temperatures in hibernacula of females fluctuated over a wider range than temperatures in hibernacula of males. Hibernacula used by males provided greater thermal buffering than those used by females. No tortoise (N = 8) used the same hibernaculum during both years of the study. Use of burrows and other shelters for thermoregulation and other purposes has been documented for all extant members of the genus Gopherus, and for other genera of land tortoises (Woodbury and Hardy, 1948; Mackay, 1964; Auffenberg, 1969; Rose and Judd, 1975; Douglas and Layne, 1978; Morafka, 1982; Geffen and Mendelssohn, 1989). Desert tortoises (Gopherus agassizii) use burrows throughout the year for thermoregulation, nesting, and protection from predators (Woodbury and Hardy, 1948; Auffenberg, 1969; Luckenbach, 1982; Vaughan, 1984; Barrett and Humphrey, 1986; Nagy and Medica, 1986; Barrett, 1990). Tortoises in the northern Mojave Desert remain below the surface in burrows approximately 98% of the year (Nagy and Medica, 1986). Although detailed data for Sonoran Desert populations are not yet available, those tortoises also spend considerable time within burrows (S. Bailey, unpubl. data; J. Snider, Arizona Game and Fish Dept., pers. comm.). Many reptiles hibernate and some species of turtles hibernate for more than half of their lives, but until recently this phenomenon remained poorly understood (Mayhew, 1965; Gregory, 1982; Ultsch, 1989). Little data concerning hibernation in desert tortoises is available, and quantitative data are generally limited to a few characteristics of hibernacula (e.g., dimensions) and duration of hibernation (Woodbury and Hardy, 1948; Auffenberg, 1969; Burge, 1977, 1978; Luckenbach, 1982; Vaughan, 1984; 2 Present Address: 3617 E. Third Street, Tucson, Arizona 85716, USA. Burge et al., 1989; Nagy and Medica, 1986; Lowe, 1990). During most years desert tortoises in the United States hibernate throughout the winter months (Nagy and Medica, 1986; pers. obs.). The majority of desert tortoises in the Mojave and Sonoran Deserts spend over 100 d (many over 150 and some over 200) per year in their hibernacula without exiting (Burge, 1977; Vaughan, 1984; Nagy and Medica, 1986). Male and female reptiles often select different thermal environments and differ in their use of habitat and shelter sites (Beauchat, 1986; Burger and Zappalorti, 1989; Sievert and Hutchison, 1989; Kaufmann, 1992; Brafia, 1993; Magnusson, 1993). In addition, some male and female reptiles are known to hibernate for different periods of time (Prestt, 1971; Gaffney and Fitzpatrick, 1973). Female desert tortoises in the Sonoran Desert seem to use shallow hibernacula more often than males and, thus, are observed more often in their shelters during the winter than males (C. Lowe and C. Schwalbe, unpubl. data). Our objectives were to quantify characteristics of hibernaculum use in a population of Sonoran desert tortoises to test the hypotheses that male and female desert tortoises differ in their use of overwintering sites and that thermal environments differ between hibernacula used by males and females. MATERIALS AND METHODS Study Area.-The study was conducted in the San Pedro River Valley (Lat. 32038'N, Long. 110?34'W), in Pinal County in southeastern Arizona. Vegetation was ecotonal between the Arizona Upland subdivision of the Sonoran DesThis content downloaded from 207.46.13.169 on Sat, 01 Oct 2016 05:31:35 UTC All use subject to http://about.jstor.org/terms