26th Annual Meeting and Symposium of the
Desert Tortoise Council, March 16-18, 2001
Abstracts

STUDENT PAPER

Predation and Survival During Early Life Stages of the Desert Tortoise in the South-Central Mojave Desert

Curtis Bjurlin
Palm Springs Desert Museum, Palm Springs, CA 92263

Despite extensive research on desert tortoise population trends very little is currently known about survival during early life stages and the impacts of native and introduced predators. To address this issue, I monitored tortoise nests, eggs, and young for survival while simultaneously surveying predator communities at the Sand Hill Training Area (Sand Hill) of the Marine Corps Air Ground Combat Center, Twentynine Palms, California (1997-1999). I used x-radiography and thread-trailing devices to locate 42 tortoise nests. I fenced nests at the end of incubation to capture neonate tortoises, 32 of which I radio-tagged to monitor movements and survival during fall dispersal. Mortality during each life stage was quite low: 74% of nests survived incubation; 69% of 132 eggs in those nests developed and produced healthy hatchlings; and 88% of dispersing young survived until winter hibernation in late October. However, these three stages account for less than one year, or approximately 3% of the life of a tortoise that reaches 35 years or older before senescence. I estimate that less than half of the eggs produced by Sand Hill females successfully hibernated as neonates and therefore conclude that desert tortoises underwent high mortality early in life, as suggested by life history data for other chelonian species. I recorded kit fox visitation to scent stations throughout the training area and found fox tracks at many depredated nests. Raven abundance was considerably lower at Sand Hill than in surrounding residential areas and few tortoise carcasses were found beneath raven perch and nest sites, suggesting that ravens were not important local predators. I encountered 31 free-ranging dogs in 11 packs, witnessed dog harassment of a tortoise, had numerous radio-transmitters forcibly removed from study females, and believe that dogs may have been responsible for the high frequency (47%) of non-lethal shell damage to the 76 adult and immature tortoises examined during the study period.