Data from: The role of fecundity and sexual selection in the evolution of size and sexual size dimorphism in New World and Old World voles (Rodentia: Arvicolinae)

Evolutionary ecologists dating back to Darwin (1871) have sought to understand why males are larger than females in some species, and why females are the larger sex in others. Although the former is widespread in mammals, rodents and other small mammals usually exhibit low levels of sexual size dimorphism (SSD). Here, we investigate patterns of sexual dimorphism in 34 vole species belonging to the subfamily Arvicolinae in a phylogenetic comparative framework. We address the potential role of sexual selection and fecundity selection in creating sex differences in body size. No support was found for hyperallometric scaling of male body size to female body size. We observed a marginally significant relationship between SSD and the ratio of male to female home range size, with the latter being positively related to the level of intrasexual competition for mates. This suggests that sexual selection favours larger males. Interestingly, we also found that habitat type, but not mating system, constitutes a strong predictor of SSD. Species inhabiting open habitats – where males have extensive home ranges in order to gain access to as many females as possible – exhibit a higher mean dimorphism than species inhabiting closed habitats, where females show strong territoriality and an uniform distribution preventing males to adopt a territorial strategy for gaining copulations. Nonetheless, variation in the strength of sexual selection is not the only selective force shaping SSD in voles; we also found a positive association between female size and litter size across lineages. Assuming this relationship also exists within lineages (i.e. fecundity selection on female size), this suggests an additional role for variation in the strength of fecundity selection shaping interspecific differences in female size, and indirectly in SSD. Therefore our results suggest that different selective processes act on the sizes of males and females, but because larger size is favoured in both sexes, SSD is on average relatively small.

自达尔文（1871年）以来的进化生态学家便一直致力于探究为何部分物种中雄性体型大于雌性，而另一些物种中雌性体型则为更大的一方。尽管雄性体型大于雌性的现象在哺乳动物中较为普遍，但啮齿类及其他小型哺乳动物的性别大小二态性（sexual size dimorphism, SSD）水平通常较低。本研究以系统发育比较框架为基础，对隶属于田鼠亚科（Arvicolinae）的34种田鼠的性别二态性模式进行了探究。本研究旨在探讨性选择与繁殖力选择在塑造体型性别差异中的潜在作用。未发现雄性体型相对于雌性体型存在超异速缩放的证据。我们观察到，SSD与雄性家域面积/雌性家域面积的比值之间存在边际显著的相关性，而该比值与同性配偶竞争的强度呈正相关。这表明性选择更青睐体型更大的雄性。值得注意的是，本研究还发现，生境类型而非交配系统，是SSD的强有力预测因子。栖息于开放生境的物种，其雄性需要拥有广阔的家域以尽可能多地接触雌性，这类物种的平均性别二态性水平高于栖息于封闭生境的物种；在封闭生境中，雌性具有较强的领地性且分布均匀，使得雄性无法通过领地策略获取交配机会。尽管如此，性选择强度的变异并非塑造田鼠SSD的唯一选择压力；我们还发现，不同演化支间雌性体型与窝仔数呈正相关。若假设该关系在演化支内部同样成立（即对雌性体型的繁殖力选择），则表明繁殖力选择强度的变异，除了直接影响雌性体型的种间差异外，还会间接作用于SSD。因此，本研究结果表明，不同的选择过程分别作用于雄性与雌性的体型，但由于两性均更青睐更大的体型，平均而言SSD的水平相对较低。