Spencer R-J, Janzen F J
Wildlife and Water Ecology Group, Native and Pest Animal Unit, School of Science and Health, University of Western Sydney, Locked Bag 1797, Penrith South, DC New South Wales 1797, Australia Department of Ecology, Evolution, and Organismal Biology, Iowa State University, Ames, IA 50011-1020, USA
Department of Ecology, Evolution, and Organismal Biology, Iowa State University, Ames, IA 50011-1020, USA.
Proc Biol Sci. 2014 Aug 22;281(1789):20140831. doi: 10.1098/rspb.2014.0831.
Temperature-dependent sex determination (TSD) is widespread in reptiles, yet its adaptive significance and mechanisms for its maintenance remain obscure and controversial. Comparative analyses identify an ancient origin of TSD in turtles, crocodiles and tuatara, suggesting that this trait should be advantageous in order to persist. Based on this assumption, researchers primarily, and with minimal success, have employed a model to examine sex-specific variation in hatchling phenotypes and fitness generated by different incubation conditions. The unwavering focus on different incubation conditions may be misplaced at least in the many turtle species in which hatchlings overwinter in the natal nest. If overwintering temperatures differentially affect fitness of male and female hatchlings, TSD might be maintained adaptively by enabling embryos to develop as the sex best suited to those overwintering conditions. We test this novel hypothesis using the painted turtle (Chrysemys picta), a species with TSD in which eggs hatch in late summer and hatchlings remain within nests until the following spring. We used a split-clutch design to expose field-incubated hatchlings to warm and cool overwintering (autumn-winter-spring) regimes in the laboratory and measured metabolic rates, energy use, body size and mortality of male and female hatchlings. While overall mortality rates were low, males exposed to warmer overwintering regimes had significantly higher metabolic rates and used more residual yolk than females, whereas the reverse occurred in the cool temperature regime. Hatchlings from mixed-sex nests exhibited similar sex-specific trends and, crucially, they were less energy efficient and grew less than same-sex hatchlings that originated from single-sex clutches. Such sex- and incubation-specific physiological adaptation to winter temperatures may enhance fitness and even extend the northern range of many species that overwinter terrestrially.
温度依赖型性别决定（TSD）在爬行动物中广泛存在，但其适应意义及其维持机制仍不明确且存在争议。比较分析表明，TSD在龟类、鳄鱼和喙头蜥中起源古老，这表明该特征若要持续存在应具有优势。基于这一假设，研究人员主要采用一种模型来研究不同孵化条件下幼体表型和适合度的性别特异性差异，但成效甚微。至少在许多幼体在出生巢穴中越冬的龟类物种中，对不同孵化条件的持续关注可能放错了重点。如果越冬温度对雄性和雌性幼体的适合度有不同影响，TSD可能通过使胚胎发育成最适合这些越冬条件的性别来适应性地维持。我们使用彩龟（Chrysemys picta）来检验这一新假设，彩龟是一种具有TSD的物种，其卵在夏末孵化，幼体在巢穴中一直待到次年春天。我们采用分窝设计，将野外孵化的幼体在实验室中暴露于温暖和凉爽的越冬（秋冬春）环境中，并测量了雄性和雌性幼体的代谢率、能量利用、体型和死亡率。虽然总体死亡率较低，但暴露于较温暖越冬环境的雄性幼体的代谢率显著高于雌性幼体，且消耗的剩余卵黄更多，而在凉爽温度环境下情况则相反。来自混合性别的巢穴的幼体表现出类似的性别特异性趋势，关键的是，它们的能量效率较低，生长速度也比来自单性别窝的同性幼体慢。这种针对冬季温度的性别和孵化特异性生理适应可能会提高适合度，甚至扩展许多陆地越冬物种的分布范围。
