The Effects of Nest Thermal Stratification on Predictive Models for Temperature-dependent Sex Ratios and Survivorship of the Leatherback Turtle, Dermochelys coriacea, In Pacuare Reserve, Costa Rica

The sex of leatherback sea turtle hatchlings, Dermochelys coriacea, is determined by the nest temperature during the middle third (approx. 3 weeks) of the 9-week incubation interval. The pivotal temperature (Tp) is 29.5 °C. Above 30 °C, all hatchlings are female. Below 29 °C, all hatchlings are male. The transitional range temperature (TRT) falls between these temperatures, at which sex ratios follow an S-curve. Past research has estimated sex ratios for nest hatchlings by examining the temperature at a single location in the center of the nest. There is emerging evidence that many nesting beaches may be producing female-predominant sex ratios, which could cause a decrease in fertility in highly skewed populations of adults. This research examined thermal stratification and variation within the nest in order to predict sex ratios more accurately and investigate factors that influence hatchling survival. Multiple temperature data loggers were used in nests to analyze the uniformity of temperature within nests to more precisely model and predict sex ratios of hatchlings. The results helped evaluate predictive models for temperature-dependent sex ratios of the leatherback turtle by examining the effects of thermal stratification within the nest and found modeling based on the center of the nest alone would underestimate the proportion of male hatchlings. Results indicated that because of more variable temperatures, the bottom of the nest had greater potential to produce higher male ratios than the middle or top of the nest, and the top had the highest temperature variability. The impact of meteorological factors during the middle third of incubation had a significant effect on nest temperature, particularly at the bottom of nests. Metabolic heating played a role in the second half of incubation, but data showed marginal effect on nest temperatures during the critical middle third of incubation. Pacuare Reserve nesting beach produced highly skewed sex ratios, with a greater than 90% female bias during the 2008 season