28th Annual Meeting and Symposium of the
Desert Tortoise Council, February 21-23, 2003
Abstracts

Temperature-dependent Sex Determination in North American Tortoises, Gopherus agassizii and Gopherus polyphemus

David C. Rostal¹, Thane Wibbels², Roy C. Averill-Murray³, Eric W. Stitt⁴, and J. Daren Riedle^3
1Department of Biology, Georgia Southern University
²Department of Biology, University of Alabama at Birmingham
³Arizona Game and Fish Department, Nongame Branch
⁴School of Renewable Natural Resources, University of Arizona, Tucson

Two of the four North American tortoise species (Gopherus agassizii and Gopherus polyphemus) show temperature-dependent sex determination (TSD). Both species have extensive latitudinal ranges varying in xeric habitat structure. Gopherus agassizii occurs in the Sonoran and Mojave Deserts, from Southwestern Utah to Alamos, Mexico, while G. polyphemus occurs in the southeastern U.S, from southern Florida to southern South Carolina and west to Louisiana. Gopherus agassizii displays one of the highest pivotal temperatures (31.3 C) for a turtle species while G. polyphemus displays a pivotal temperature similar to other turtle species in its range (29.3 C). Temperature dataloggers were set in latitudinal arrays throughout both species U.S. ranges to measure ground temperatures in suitable nest site areas including one site which extended beyond the northern-most range for both species. All other sites had tortoises present or exhibited suitable vegetation characteristics. Variation in ground temperature of nest site choices by females was compared to laboratory based pivotal temperatures for both species. Timing of nesting and nest site locations by females in different regions provided temperature conditions near laboratory pivotal temperatures during the critical period of incubation. Relationships between timing of nesting, nest site choice and northern range extent for these species will be discussed.