Experimental assessment of winter conditions on turtle nesting behaviour

Background: Many reptiles have temperature-dependent sex determination (TSD), and thus are potentially directly influenced by climate change. Where and when a reptile nests can influence nest temperature and offspring characteristics (including sex). Understanding how nesting behaviours are affected by climate is important, particularly in these temperaturesensitive species. Aim: Long-term field research in painted turtles (Chrysemys picta) has identified several nesting behaviours that correlate with air temperature during the preceding winter. Since painted turtles hibernate in water, we sought to observationally and experimentally test whether this correlation is mediated through differences in the duration of ice cover, specifically, the spring ice-off date. Methods: We performed a 25-year observational study of painted turtle nesting, and explored whether the timing of nesting (phenology) was related to ice-off dates recorded in the region. With a complementary experiment, we exposed female painted turtles to conditions simulating different ice-off dates and monitored nesting phenology, nest-site choice, and nest depth. Results and conclusions: Our study identified a significant, positive correlation between ice-off dates and nesting phenology of turtles, with turtles initiating nesting later in years where winter conditions persisted for longer. Contrary to expectations, however, turtles did not differ in nesting behaviours between the 'early' and 'late' ice-off treatments in the experiment. Thus, variation in these traits in the wild likely is driven by other factors. These results highlight the complexity of understanding how animal behaviours will be altered as climate changes.