Thermal reaction norm for sexualization: The missing link between temperature and sex ratio for temperature-dependent sex determination

Species with temperature-dependent sex determination (TSD) can experience biased sex ratios in natural conditions, which raises questions the vulnerability of populations in the face of climate change. Studies addressing the adaptive significance TSD have been hampered by the difficulty of accurately estimating sex ratios under natural incubation conditions. Here we introduce the thermal reaction norm for sexualization, a novel concept measuring the strength of masculinization or feminization of temperatures, to model the effect of temperature for sex determination in TSD species. We use hatchling sex ratio data and field incubation temperatures collected between 2002 and 2018 at a globally important loggerhead turtle (Caretta caretta) nesting rookery. The new parametrization makes possible the understanding of how temperature-sensitive sex determination works. We show that the temperature could influence the sexualization of the gonad earlier than what is currently recognized. Additionally, we explore the results of several easy to implement proxies that have been used in literature. Our approach greatly outperforms previous ones in sex ratio prediction. Our results should help further studies to refine population-wide primary sex ratio estimates of reptiles with TSD to adapt current conservation strategies and develop them in the future.

Automated summary

Species with temperature-dependent sex determination (TSD) can experience biased sex ratios in natural conditions. This study introduces a novel concept called thermal reaction norm for sexualization to model the effect of temperature on sex determination in TSD species. The results show that temperature can influence the sexualization of the gonad earlier than previously recognized. The new approach greatly outperforms previous ones in sex ratio prediction.