Decades of data suggest climate change resilience in a reptile with temperature-dependent sex determination at its northern range limit

Jessica A. Leivesley and Njal Rollinson, Department of Ecology and Evolutionary Biology, University of Toronto, Toronto, ON, Canada, E. Graham Nancekivell and Ronald J. Brooks, Department of Integrative Biology, University of Guelph, Guelph, ON, Canada, Njal Rollinson, School of the Environment, University of Toronto, Toronto, ON, Canada

Background/Question/Methods
The threat of climate change is particularly acute in reptiles as many have temperature-dependent sex determination (TSD), where sex is irreversibly determined by temperatures experienced during incubation. In most TSD species, hot environments result in an overproduction of females, therefore, with climate change highly feminized nest sex ratios are predicted. However, theory and data suggest that climate change resilience could be realized in temperate populations that experience relatively cooler and more variable temperatures; and in species with ancestral TSD (females produced at low and high temperatures and males at intermediate temperatures; FMF) as both sexes are produced over a greater range of temperatures. Yet, no study has documented the response of FMF TSD species to climate change. Here, we use a decades-long study of the common snapping turtle in Algonquin Provincial Park, Canada to understand how the nest sex ratio has changed since 1983. We complement this by building a model to estimate within nest development from air temperature to understand how the developmental environment has also changed. Finally, we test common assumptions underlying predictions of TSD species’ response to climate change, as these are rarely tested with large datasets, and have not been tested in FMF TSD species.
Results/Conclusions
Neither the primary nest sex ratio nor the recruited sex ratio of the population has changed significantly between 1983 and 2020. Furthermore, we found that nesting date, the onset of the thermosensitive period (the period in incubation when sex is determined), and thermosensitive period length also has not changed. However, we did find that development rates in June and September have increased. Overall, we show that despite documented warming in the area and some changes to the developmental environment, the nest sex ratio of this population is displaying resilience to climate change. Interestingly, we note large intra- and inter-annual variation in sex ratio which likely contributes to long-term resilience in this population. We also show that common theoretical assumptions such as nest date predicting the onset and duration of the thermosensitive period are disrupted as the relationship changes over time. This suggests that models using these assumptions may not accurately predict responses of either temperate populations or FMF TSD species. Further work is necessary to determine if climate change resilience is realized in other species and populations in temperate regions and other FMF TSD species, and to adapt models to predict responses of these populations to climate change.