Effects of Incubation Conditions on Sex Determination, Hatching Success, and Growth of Hatchling Desert Tortoises, Gopherus agassizii

Abstract

Incubation temperature has a direct effect on sex determination of the desert tortoise. Low temperatures (26.0-30.6 C) produce males and high temperatures (32.8-35.3 C) produce females. Pivotal temperature is approximately 31.8 C. Macroscopic and microscopic anatomy of the gonads is similar to that of other turtles. Hatching success and survival is very good between 28.1 and 32.8 C in dry sand (-5000 kPa). Incubation at 35.3 C is lethal for 72% of the eggs and produces weak hatchlings that die within 45 days. Wet sand (-5 kPa) is lethal for desert tortoise eggs. Hatchling size was dependent upon egg size and incubation condition. Hatchlings from eggs incubated at 32.8 and 35.3 C were significantly smaller than hatchlings from eggs incubated at 28.1 and 30.6 C. Hatching mass had no effect on growth rate of hatchlings. Thus, large eggs produced large hatchlings that were larger than their siblings at 120 days of age. Hatchlings from eggs incubated at 30.6 C grew significantly more than hatchlings incubated at 28.1 and 32.8 C. Hatchlings incubated at 35.3 C lost mass. Incubation condition did not affect temperature selected in a substrate thermal gradient when hatchlings were tested within one week (x = 29.2 C) or 40 days of hatching (x = 26.6 C). Because of temperature-dependent sex determination and the effect of incubation conditions on hatching success and later growth, management strategies for the desert tortoise should be very conservative. To ensure normal sex ratios of desert tortoises, natural vegetation communities and native soil composition and structure must be preserved or restored. Long term recovery and survival of desert tortoises can only be assured when we have information on pivotal temperatures and nesting ecology for its various populations.