We studied the effects of timing and placement of eggs by gravid desert tortoises, Gopherus agassizii, on sex ratios and potential survival of hatchlings. We monitored nest placement by female tortoises under seminatural conditions at the Fort Irwin Study Site (FISS) in the Mojave Desert, San Bernardino County, California, United States, and under wild conditions in habitat surrounding FISS. We located 16 nests and found no significant difference between nest placement in the seminatural enclosure at FISS and nest placement in the wild. Gravid females deposited their eggs down the tunnel a mean distance of 0.7 m and buried their eggs at a depth of 8–10 cm from the soil surface. Utilizing this preliminary nesting data, we set up a manipulative experiment to further characterize the effect of nest site and date on sex ratios and survivorship of hatchlings. We constructed 14 egg nests and 9 pseudo-nests in the FISS enclosure and monitored incubation temperatures and resulting hatchling sex ratios. Forty-seven hatchlings emerged (79% survivorship) and 33 were positively sexed. Nests placed early in the reproductive season produced 6 all-female nests and nests placed late in the season produced 4 all-male nests. The mean incubation period was 90 days, which we divided into 3 time periods. Early nests were significantly cooler than late nests during the first and second time period (days 0–30 and days 31–60) and significantly warmer during the third time period (day 61–90). The shallowest pseudo-nest, located 0.2 m down the burrow tunnel, spent a significantly greater time above the critical temperature of 35.3°C than did the 0.4 m pseudo-nest or the egg nest. We hypothesize from these findings that females at the FISS site and surrounding area are selecting distances down the burrow tunnel to lay their eggs that increase the embryos' chances of survival. We found that the proportion of temperature observations above the pivotal temperature (35.3°C) during days 15–45 was a better predictor of hatchling sex than the proportion measured during days 30–61. Using nest date or the proportion of temperature observations above the pivotal temperature as predictors of hatchling sex ratios may be possible for desert tortoises in the Central Mojave Desert.
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Vol. 7 • No. 1