Effects of climate change on reptiles with temperature-dependent sex determination and potential adaptation via maternal nest-site choice

Refsnider, Jeanine
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Reptiles with temperature-dependent sex determination may be particularly threatened by climate change, as increasing temperatures could lead to skewed sex ratios. A potential compensatory mechanism is nest-site choice, with females selecting nest sites to match incubation conditions to climatic conditions. I studied nest-site choice in painted turtles (Chrysemys picta) to determine the extent to which behavioral plasticity in nest-site choice can compensate for the effects of climate change. In a common-garden experiment, gravid females from five populations across the species' range were collected and nest-site choice was compared among populations to evaluate variation in nesting phenology, shade cover over the nest, nest depth, incubation regime, and offspring sex ratio. Populations differed in nesting phenology and nest depth, but not in shade cover over nests; thus, when exposed to novel climatic conditions, females from transplanted populations chose nest-sites with similar shade cover to those of local females, thereby producing similar offspring sex ratios. The performance of hatchlings produced in this experiment declined with decreasing mean temperature of the mother's site of origin, and nests with greater variation in daily temperature range produced hatchlings that performed faster and more readily than nests with less variable incubation temperatures. Therefore, selection of shadier nest-sites may be a mechanism by which female turtles could compensate for climatic warming, and the increase in temperature fluctuations predicted by climate change models may result in the production of faster hatchling turtles with enhanced righting ability. In an experiment manipulating nest depth, I found that nest depth affected the magnitude of daily temperature fluctuation, but neither mean nest temperature nor sex ratio were affected by depth. The adjustment in nest depth that would be required to affect sex ratio in this species is biologically unfeasible, and therefore female adjustment of nest depth is unlikely to compensate for climate change. Finally, I compared used vs. available nesting habitat between a southern and central population of painted turtles. Compared to the central population, turtles from southern population were more limited in the available range of accessible shade cover under which to nest. As shade cover is an easily-manipulated feature, land managers can adjust the range of shade cover available to nesting turtles in order to preserve populations in the face of climate change. Overall, this research is fundamental to understanding processes in evolutionary ecology in general, and to contemporary climate change biology specifically.

Chrysemys picta, conservation, incubation, painted turtle, sex ratio