A common reptile conservation strategy involves artificial incubation of embryos and release of hatchlings or juveniles into wild populations. Temperature-dependent sex determination (TSD) occurs in most chelonians, permitting conservation managers to bias sex ratios towards females by incubating embryos at high temperatures, ultimately allowing the introduction of more egg-bearing individuals into populations. Here, we revisit classic sex allocation theory and hypothesize that TSD evolved in some reptile groups (specifically, chelonians and crocodilians) because male fitness is more sensitive to condition (general health, vigor) than female fitness. It follows that males benefit more than females from incubation environments that confer high-quality phenotypes, and hence high-condition individuals. We predict that female-producing temperatures, which comprise relatively high incubation temperatures in chelonians and crocodilians, are relatively stressful for embryos and subsequent life stages. We synthesize data from 28 studies to investigate how constant temperature incubation affects embryonic mortality in chelonians with TSD. We find several lines of evidence suggesting that warm, female-producing temperatures are more stressful than cool, male-producing temperatures. Further, we find some evidence that pivotal temperatures (TPiv, the temperature that produces a 1:1 sex ratio) may exhibit a correlated evolution with embryonic thermal tolerance. If patterns of temperature-sensitive embryonic mortality are also indicative of chronic thermal stress that occurs post-hatching, then conservation programs may benefit from incubating eggs close to species-specific TPivs, thus avoiding high-temperature incubation. Indeed, our models predict that, on average, a sex ratio of >75% females can generally be achieved by incubating eggs only 1°C above TPiv. Of equal importance, we provide insight into the enigmatic evolution of TSD in chelonians, by providing support to the hypothesis that TSD evolution is related to the quality of the phenotype conferred by incubation temperature, with males produced in high-quality incubation environments.

中文翻译：

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温度依赖性性别决定进化的简单模型解释了濒危爬行动物胚胎死亡率的温度敏感性

一种常见的爬行动物保护策略包括人工孵化胚胎并将幼体或幼体释放到野生种群中。大多数龟类都发生温度依赖性性别决定（TSD），这使得保护管理者可以通过在高温下孵化胚胎来使性别比例偏向雌性，最终允许将更多的卵子个体引入种群。在这里，我们重新审视经典的性别分配理论，并假设 TSD 在一些爬行动物群体（特别是龟类和鳄鱼）中进化，因为男性健康对状况（一般健康、活力）比女性健康更敏感。由此可见，雄性比雌性更能从赋予高质量表型的孵化环境中受益，从而带来高条件个体。我们预测产生女性的温度，龟类和鳄鱼的孵化温度相对较高，对胚胎和随后的生命阶段造成的压力相对较大。我们综合了 28 项研究的数据，以调查恒温孵化如何影响患有 TSD 龟类动物的胚胎死亡率。我们发现有几条证据表明温暖的、产生女性的温度比凉爽的、产生男性的温度更有压力。此外，我们发现一些证据表明关键温度（TPiv，产生 1:1 性别比的温度）可能表现出与胚胎耐热性相关的进化。如果温度敏感的胚胎死亡率模式也表明孵化后发生的慢性热应激，那么保护计划可能会受益于在特定物种的 TPivs 附近孵化鸡蛋，从而避免高温孵化。事实上，我们的模型预测，平均而言，通常可以通过仅在 TPiv 以上 1°C 的温度下孵化卵来实现＞75% 的雌性性别比。同样重要的是，我们通过提供对 TSD 进化与孵化温度赋予的表型质量相关的假设提供支持，从而深入了解龟类 TSD 的神秘进化，雄性在高质量的孵化环境中产生。