Maternally derived sex steroid hormones impact sex ratios of loggerhead sea turtles

Abstract

Global warming could drive species with temperature-dependent sex determination to extinction by persistently skewing offspring sex ratios. Evolved mechanisms that buffer these biases are therefore paramount for their persistence. Here, we tested whether maternally-derived sex steroid hormones affect the sex-determination cascade and provide a physiological mechanism to buffer sex ratio bias in the endangered loggerhead sea turtle (Caretta caretta). We quantified estradiol and testosterone in nesting females and their egg yolks at oviposition, before incubating nests in situ at standardised temperatures. Upon hatchling emergence, we developed a new, non-lethal method to establish the sex of individuals. Despite standardised incubation temperatures, sex ratios varied widely among nests, correlating non-linearly with the estradiol:testosterone ratio in egg yolks. Males were produced at an equal ratio, with females produced either side of this optimum. This result provides evidence that maternal hormone transfer forms a physiological mechanism that impacts sex determination in this endangered species.