Data from: Both sexual selection and fecundity selection drive 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 male-biased sexual size dimorphism (SSD) is widespread in mammals, rodents and other small mammals usually exhibit low levels of 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 as selective pressures in creating sex differences in body size. We observed a 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 extensively overlapping 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 low territoriality and males don’t need to patrol large distances to monopolize several females. Nonetheless, sexual selection is not the only selective force directing SSD in voles; we also found a positive association between female size and litter size, providing evidence for fecundity selection on female size across lineages. No support was found for hyperallometric scaling of male body size to female body size. 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. We conclude that SSD can be used as an indicator of the strength of sexual selection in this group.