Local Adaptation May Help Mitigate Feminisation of Sea Turtle Populations Globally

Climate warming currently threatens many species with extinction, particularly those with a limited capacity for adaptation. Sea turtles have temperature-dependent sex determination, whereby female hatchlings are produced at warmer incubation temperatures; hence, climate warming might cause the feminisation of populations. Recent evidence suggests that climate warming will outpace the ability of turtles to adapt through phenological shifts in nesting. Here, we examine 138 published estimates for hatchling sex ratios spanning the seven sea turtle species and all ocean basins. We evaluate whether turtles have the capacity to adapt to warming temperatures through local adaptations of the pivotal temperature at which they produce a balanced amount of male and female hatchlings. We show that at warmer sites, lower proportions of female hatchlings are produced than expected from generalised sex ratio versus incubation temperature relationships that have been previously used across all sea turtle species. This points to local adaptation of the pivotal temperature (i.e., the temperature at which a balanced hatchling sex ratio is produced) as evidenced by an analysis of 33 pivotal temperatures recorded at sites around the world that showed generally higher pivotal temperatures at warmer sites, confirming previous work. These findings point to local adaptation of the pivotal temperatures, which could help the production of male hatchlings at warmer sites and so assist with population viability. These results suggest that the sea turtle hatchling sex ratio is more resilient to climate change than previously thought.