Survival during early life stages of the desert tortoise (Gopherus agassizii) in the south-central Mojave desert

Early age classes of the Desert Tortoise (Gopherus agassizii) are particularly vulnerable to predation by several mammal and bird species. We studied tortoise survival at the Sand Hill Training Area of the Marine Corps Air Ground Combat Center, Twentynine Palms, California, from 1998 to 1999. We radiographed and thread-spooled 25 females to determine reproductive and nesting ecology. Predators consumed 11 of 42 monitored nests during the first 70 days of incubation. Ninety-one of 132 eggs in nests fenced after 70 days developed into healthy neonates. We recorded 0.84 and 0.91 neonate survival probability during dispersal in 1998 and 1999, respectively. Applying neonatal survival to egg success, we predict that 40% of individuals survived from oviposition to hibernation. Our findings support other chelonian life-history models indicating relatively high mortality early in life. Our data also suggest that neonatal Desert Tortoises are less susceptible to predation than was previously thought, perhaps because of their cryptic coloration and secretive habits. The Common Raven (Corvus corax) was not found to be a source of neonate mortality during this study.