27th Annual Meeting and Symposium of the
Desert Tortoise Council, March 22-24, 2002
Abstracts

Desert Tortoise Nest Site Selection and Embryogenesis in a Changing Environment

Curtis D. Bjurlin¹ and John A. Bissonette^2
1Endangered Species Recovery Program, Fresno, CA, 93727; 
²USGS-Utah Cooperative Fish and Wildlife Research Unit, Department of Fisheries and Wildlife, Utah State University, Logan, UT 84322;

Chelonian embryogenesis and sex determination is strongly affected by soil temperature and humidity, putting this group at risk to changes in temperature and precipitation associated with global warming. During 1998-1999, we investigated nesting behavior and the influence of nest site characteristics on embryology for the desert tortoise (Gopherus agassizii) in the Mojave Desert of southern California. Most females chose nest sites near resting burrows (88.1%), altering their choices in favor subterranean and thermally insulated locations as the season progressed. Nests laid deep underground had lower mean temperatures, smaller daily temperature fluctuations, and spent fewer hours above the laboratory-identified threshold for female sex determination. In addition, date of oviposition and year both affected incubation time. Low ambient temperature and high nest humidity during 1999 also may have caused the increased neonate mass for similar-sized eggs during that year. These data indicate that both female nest site selection and ambient weather conditions significantly alter nest microclimate and desert tortoise embryology, and could reduce individual fitness and skew population sex ratios. These interactions likely will be complicated by global climate change, with uncertain outcome.