Can diet niche partitioning enhance sexual dimorphism?

Classic evolutionary theory suggests that sexual dimorphism evolves
primarily via sexual and fecundity selection. However, theory and evidence
is beginning to accumulate suggesting that resource competition can drive
the evolution of sexual dimorphism, via ecological character displacement
between sexes. A key prediction of this hypothesis is that the extent of
ecological divergence between sexes will be associated with the extent of
sexual dimorphism. As the stable isotope ratios of animal tissues provide
a quantitative measure of various aspects of ecology, we carried out a
meta-analysis examining associations between the extent of isotopic
divergence between sexes and the extent of body size dimorphism. Our
models demonstrate that large amounts of between-study variation in
isotopic (ecological) divergence between sexes is non-random and may be
associated with the traits of study subjects. We therefore completed
meta-regressions to examine whether the extent of isotopic divergence
between sexes is associated with the extent of sexual size dimorphism. We
found modest but significantly positive associations across species
between size dimorphism and ecological differences between sexes, that
increased in strength when the ecological opportunity for dietary
divergence between sexes was greatest. Our results therefore provide
further evidence that ecologically mediated selection, not directly
related to reproduction, can contribute to the evolution of sexual
dimorphism.