Sex-specific survival to maturity and the evolution of environmental sex determination.

Four decades ago, it was proposed that environmental sex determination (ESD) evolves when individual fitness depends on the environment in a sex-specific fashion--a form of condition-dependent sex allocation. Many biological processes have been hypothesized to drive this sex asymmetry, yet a general explanation for the evolution of sex-determining mechanisms remains elusive. Here, we develop a mathematical model for a novel hypothesis of the evolution of ESD, and provide a first empirical test using data across turtles. ESD is favored when the sex-determining environment affects annual survival rates equivalently in males and females, and males and females mature at different ages. We compare this hypothesis to alternative hypotheses, and demonstrate how it captures a crucially different process. This maturation process arises naturally from common life histories and applies more broadly to condition-dependent sex allocation. Therefore, it has widespread implications for animal taxa. Across turtle species, ESD is associated with greater sex differences in the age at maturity compared to species without ESD, as predicted by our hypothesis. However, the effect is not statistically significant and will require expanded empirical investigation. Given variation among taxa in sex-specific age at maturity, our survival-to-maturity hypothesis may capture common selective forces on sex-determining mechanisms.