Phenological shift mitigates predicted impacts of climate change on sea turtle offspring

Many studies have documented the impacts of climate change on species and ecosystems, but few have shown how species may respond to mitigate these effects. For species with temperature-dependent sex determination, rising temperatures directly impact offspring sex ratios. If species do not change their geographic ranges, phenology of breeding or thermal restrictions for incubation, sex ratios will become skewed towards that produced at higher temperatures and hatching success may be impacted. Using nearly 3 decades of empirical data (1993-2021) and a heuristic model, we show that if the seasonality of nesting remains unchanged, by 2100, loggerhead turtles Caretta caretta at our study site will produce almost no offspring. Modelling the advancement of nesting by 0.5 d yr -1 stabilised offspring sex ratios at their current rate, but advancement of 0.7 d yr -1 was required to stabilise hatching success. This population, however, has responded to rising temperatures, with advancements in both the mean day of the nesting season and onset of nesting (5 th percentile ordinal day) by 0.23 and 0.43 d yr -1 , respectively, since 1993. However, returning females that have higher fidelity to the site have advanced the mean day of nesting and the onset of nesting by 0.54 and 0.78 d yr -1 , respectively, which is within range of those predicted by our heuristic models to stabilise offspring sex ratios. Our study suggests that loggerhead turtles at this site are currently compensating for the predicted negative impacts of rising temperatures on offspring sex ratios through a change in the phenology of breeding.