Data from: Female-biased gape and body-size dimorphism in the New World watersnakes (tribe: Thamnophiini) oppose predictions from Rensch’s Rule

Sexual size dimorphism (SSD) is ubiquitous across animals with female bias
most prominent in snakes and other ectothermic organisms. To understand
how SSD evolves across species, Rensch’s Rule predicts that in taxa where
males are larger, SSD increases with body size. In contrast, where females
are larger, SSD decreases with body size. While this rule holds for many
taxa, it may be ambiguous for others, particularly ectothermic
vertebrates. Importantly, this rule suggests that the outcomes of SSD over
phylogenetic time scales depends on the direction of dimorphism predicated
on the difference in reproductive efforts between males and females. Here
we examine SSD in the context of Rensch’s Rule in Thamnophiini, the garter
and waternsakes, a prominent group composing the North American snake
biota. Using a dated phylogeny, measurements of gape, body and tail size,
we show that these snakes do not follow Rensch’s Rule, but rather
female-biased SSD increases with body size. We in turn find that this
allometry is most pronounced with gape and is correlated with both neonate
and litter size, suggesting that acquiring prey of increased size may be
directly related to fecundity selection. These changes in SSD are not
constrained to any particular clade; we find no evidence of phylogenetic
shifts in those traits showing SSD. We suggest several ways forward to
better understand the anatomical units of selection for SSD and modularity

性体型二态性（Sexual size dimorphism, SSD）广泛存在于动物界，其中以蛇类及其他外温动物的雌性偏向型性体型二态性最为显著。为探究不同物种中性体型二态性的演化规律，伦施法则（Rensch’s Rule）提出：在雄性体型更大的类群中，性体型二态性程度会随躯体尺寸增大而增强；与之相反，在雌性体型更大的类群中，性体型二态性程度则随躯体尺寸增大而减弱。尽管该法则在诸多类群中得到验证，但在部分类群——尤其是外温脊椎动物中，其适用性仍不明确。尤为关键的是，该法则表明：在系统发育时间尺度上，性体型二态性的演化结果，取决于基于雌雄繁殖投入差异所定义的二态性方向。
本研究以束带蛇族（Thamnophiini）——包含袜带蛇与水蛇的北美蛇类区系优势类群——为研究对象，在伦施法则的框架下探究其性体型二态性特征。本研究借助定年系统发育树、口裂、躯体及尾部尺寸测量数据，证实该类蛇并未遵循伦施法则：其雌性偏向型性体型二态性程度反而随躯体尺寸增大而增强。进一步分析发现，该异速生长特征在口裂尺寸上表现最为显著，且与新生幼体体型及窝仔数均存在相关性，这表明获取更大体型猎物的需求或与繁殖力选择直接相关。此类性体型二态性的变化并未局限于某一特定演化支；研究未发现展现性体型二态性的相关性状存在系统发育转换的证据。本研究最后提出若干研究方向，以期进一步阐明性体型二态性相关选择作用的解剖学基础及演化模块化机制。