Male production in green sea turtles under a feminizing climate: A validated model of sex ratios with temperature-dependent sex determination.

Green sea turtles (Chelonia mydas) are globally distributed and use tropical and subtropical beaches for nesting. Green turtles exhibit temperature-dependent sex determination (TSD), a mechanism by which incubation temperature during a critical developmental window determines offspring sex. As global temperatures rise, many nesting populations could be producing increasingly female-biased sex ratios, raising concerns about long-term population viability. Our ability to assess demographic risks has been hindered by the absence of validated models that accurately convert field-recorded nest temperatures into sex ratios. In this study, we combine high-resolution temperature data from 62 monitored nests with known hatchling sex ratios (n = 594 individuals) to develop and validate a predictive model for sex ratio in green turtles. We developed several methodological innovations to study the impact of temperature-dependent sex determination (TSD) under natural conditions. First, we developed a novel approach to quantify spatial temperature heterogeneity (H, range between the 2.5th and 97.5th percentiles of the pairwise differences of nest temperatures recorded at the same time) on a nesting beach. The median heterogeneity of the beach is 1.58°C (95% credible interval: 1.52-1.64°C). Second, using a Bayesian mixed-model framework and experimental data from 1480 embryos incubated at constant temperatures under different moisture conditions, we detected genetic and/or maternal differences among clutches in the thermal sensitivity of sex ratio estimates, but no effect of Regional Management Units (RMU). Third, we fitted nine models to estimate field sex ratios; the best-performing model combined a thermal reaction norm of sexualization with a sensitivity function across the thermosensitive period (TSP), showing that both the timing and magnitude of temperature exposure influence sex determination. Our results indicate that males can be produced even under apparent female-producing conditions, due to temperature heterogeneity, temporal autocorrelation of nest temperatures that increased sex ratio variance and non-linear sensitivity to male-producing temperatures. This study establishes a new standard for predicting sex ratios in reptiles with TSD, offering critical insights for population assessments and adaptive conservation planning under changing climatic conditions.