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.