The thermal sensitivity of growth and survival in a wild reptile with temperature-dependent sex determination

The Charnov-Bull hypothesis is the leading explanation for the evolution of environmental sex determination (ESD), which includes temperature-dependent sex determination (TSD), the most common form of ESD. Charnov-Bull predicts a sex-by-incubation temperature interaction for fitness, matching offspring sex with thermal conditions that increase parental fitness. However, there is no general explanation for how the sex-by-temperature interaction arises. Two competing explanations for the interaction lie in the Survival to Maturity hypothesis (SM) and the Trivers-Willard extension (TW). Under SM, the sex that matures later is produced under optimal incubation regimes as the late-maturing sex accrues more mortality by maturation, while TW suggests that males are always produced under optimal incubation regimes as male mating success is more sensitive to condition (general health, vigor) than female fecundity. In a system where females mature later than males, as in the painted turtle (Chrysemys picta), SM and TW generate opposite predictions for the effect of incubation temperature on juvenile survival. We incubated C. picta eggs under either female-promoting temperatures (28±3°C) or male-promoting temperatures (25±3°C), then released the hatchlings into their natal pond. We used a Bayesian capture-mark-recapture approach to follow their survival over two growing seasons. We found a 2% depression of biweekly survival in individuals incubated under the cooler temperature, providing subtle support to SM. Incubation treatments did not influence growth. Large-scale field experiments such as this one will be necessary for understanding TSD evolution, and we underline general principles to execute such experiments successfully.

Methods
Painted turtle eggs were collected from Algonquin Provincial Park and incubated under two temperature regimes. The following year, individuals were released into their natal pond with unique identifiers painted on their back. Researchers returned every two weeks to the pond and sampled for individuals following a capture-mark-recapture methodology.

查诺夫-布尔假说（Charnov-Bull hypothesis）是解释环境性别决定（environmental sex determination, ESD）演化的主流理论，其中最常见的ESD形式为温度依赖型性别决定（temperature-dependent sex determination, TSD）。该假说预测适合度存在孵化温度-性别互作效应，即让后代性别与能提升亲本适合度的热环境相匹配。然而，学界尚未就这种温度-性别互作的形成机制达成通用解释。目前针对该互作机制存在两种对立假说：成熟存活率假说（Survival to Maturity, SM）与特里弗斯-威拉德拓展假说（Trivers-Willard extension, TW）。根据SM假说，发育较慢的性别会在最优孵化条件下产生——因为该晚成熟性别在成熟前会承受更高的死亡率；而TW假说则认为，雄性会在最优孵化条件下产生，原因是雄性的交配成功率相较于雌性的繁殖力对个体状态（整体健康水平、活力）更为敏感。在雌性成熟晚于雄性的体系中（如锦龟*Chrysemys picta*），SM与TW假说对孵化温度对幼体存活率的影响给出了完全相反的预测。本研究将锦龟卵分别置于促雌性孵化温度（28±3℃）与促雄性孵化温度（25±3℃）下进行孵化，随后将幼龟放归其原生池塘。我们采用贝叶斯标记重捕法（Bayesian capture-mark-recapture）追踪了两个生长季内的幼体存活率。结果显示，在较低温度下孵化的个体的双周存活率下降了2%，为SM假说提供了有限的支持证据。孵化处理对幼体生长无显著影响。此类大规模野外实验对于理解TSD的演化机制至关重要，本研究同时总结了成功开展此类实验的通用原则。

方法
研究人员从阿尔贡金省立公园（Algonquin Provincial Park）采集锦龟卵，设置两种温度条件进行孵化。次年，研究人员将在背甲上带有专属识别标记的幼龟放归其原生池塘。每两周研究者返回该池塘，按照标记重捕法对个体进行采样监测。