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Current Herpetology
Abstract
Abstract:
Agassiz's desert tortoise (Gopherus agassizii) reaches the southern edge of its range in the Sonoran Desert of California. The reproductive ecology of this wideranging species is understudied here compared to populations in the adjacent Mojave Desert. Understanding potential geographic variation in reproductive ecology is important for effective management of conservation-reliant species like G. agassizii. We studied the fecundity and clutch phenology of female G. agassizii at two study sites in the Sonoran Desert region of Joshua Tree National Park over five years (1997–1999, 2015–2016) spanning two of the strongest El Niño events on record and an epic drought. Across all years, mean clutch size was 4.3±1.5 eggs, mean clutch frequency was 1.78 clutches/female/year, and mean X-ray egg width was 36.51±1.56 mm, all of which are comparable to other published studies both in the Sonoran and Mojave deserts of California. Our results generally support earlier published findings that G. agassizii utilize a bet-hedging strategy of consistently producing small clutches almost every year, even during times of low annual primary productivity. A regionally warmer climate in the Sonoran Desert of California appears to have an effect on the timing of egg production, as the earliest dates of egg visibility in our study (April 6) were approximately two weeks earlier than the earliest dates reported for G. agassizii in the Mojave Desert. Shelled eggs were no longer visible in tortoises after mid-June in all years but the El Niño year 1998, when eggs were visible until mid-July.
Many people assisted us during the course of the present study. Field support for this project was provided by interns from the Student Conservation Association, EarthWatch, the National Park Service, and volunteers too numerous to list. We especially thank Ramona Daniels, Gillian Bowser, and Mark Massar for their commitment to the support of our project. Al Muth kindly provided accommodations at the Philip L. Boyd Deep Canyon Research Center of the University of California, Riverside, during our research. Mart Westbrooks allowed us to use his veterinary facility to process X-radiographs. Charlie Porter and Annette Burvick of the Flagstaff Medical Center also provided access to processing facilities. Research was supported by the Coachella Valley Conservation Commission, California Desert District Office of the Bureau of Land Management (BLM), the Palm Springs−South Coast Field Office of BLM, and Joshua Tree National Park. Research was conducted under permits from the U.S. Fish and Wildlife Service, the National Park Service, and the California Department of Fish and Wildlife. We are grateful to the Institutional Animal Care and Use Committee of Northern Arizona University for reviewing and approving our research procedures. Earlier versions of the manuscript benefitted greatly from comments offered by Roy Averill-Murray. We thank Sarah Sweat for preparing our map. Any use of trade, product or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
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