Data from: Patterns of sexual size dimorphism in Chelonia

Macroevolutionary patterns of sexual size dimorphism (SSD) indicate how sexual selection, natural selection, and genetic and developmental constraints mold sex differences in body size. One putative pattern, known as Rensch’s Rule, posits that among species with female-larger SSD, the relative degree of SSD declines with species’ body size, whereas among male-larger SSD species, relative SSD increases with size. Using a dataset of 196 chelonian species from all fourteen families, we investigated the correlation in body size evolution between male and female Chelonia and the validity of Rensch’s Rule for the taxon and within its major clades. We conclude that male-female correlations in body size evolution are high, but these correlations differ among chelonian families. Overall, SSD scales isometrically with body size; Rensch’s Rule is valid for only one family, Testudinidae (tortoises). Since macroevolutionary patterns of SSD can vary markedly among clades, even in a taxon as morphologically conservative as Testudines, one must guard against inappropriately pooling clades in comparative studies of SSD. Our results also indicate that regression models that assume the x - variable (e.g. male body size) is measured without statistical error, although frequently reported, will result in erroneous conclusions about phylogenetic trends in sexual size dimorphism.

性大小二态性（sexual size dimorphism, SSD）的宏观演化模式，阐明了性选择、自然选择以及遗传与发育约束如何塑造生物体型的性别差异。其中一个被广泛探讨的假说模式即伦施法则（Rensch’s Rule），其核心论断为：在雌性体型大于雄性的SSD类群中，相对SSD程度会随类群体型增大而降低；而在雄性体型大于雌性的SSD类群中，相对SSD程度则随体型增大而升高。本研究基于涵盖龟鳖目（Chelonia）全部14个科的196个物种数据集，分析了龟类雌雄个体体型演化的相关性，并检验了伦施法则在该类群整体以及其主要演化支中的适用性。研究结果表明，龟类雌雄个体的体型演化相关性整体较高，但该相关性在不同龟鳖科之间存在显著差异。整体而言，SSD随体型呈现等比例缩放；而伦施法则仅在陆龟科（Testudinidae, tortoises）这一科中成立。鉴于SSD的宏观演化模式在不同演化支间可能存在显著差异，即便在形态保守性极强的龟鳖目（Testudines）类群中亦是如此，因此在开展SSD的比较研究时，应避免随意将不同演化支合并分析。此外，本研究结果还显示，尽管这类方法常被使用，但假设自变量（例如雄性体型）无测量误差的回归模型，会导致关于性大小二态性的系统发育演化趋势得出错误结论。