Altitudinal variation in sexual dimorphism: a new pattern and alternative hypotheses

The colder climate and disjunct distribution of nesting and foraging habitats at high elevations increases the necessity of biparental care for successful breeding in birds. If differences in parental investment between the sexes correlate with intensity of sexual selection, the intensity of sexual selection should covary with ecological factors associated with elevation. I used sexual dimorphism as an indirect measure of intensity of sexual selection and examined variation in sexual dimorphism in 126 extant species of cardueline finches. I controlled for phylogeny and potential confounding factors and tested the prediction that the extent of sexual dimorphism negatively covaries with elevation of breeding. As predicted, interspecific variation in sexual dimorphism was more strongly associated with changes in elevation than with habitat, nest dispersion and placement, and migratory status. Species occupying lower elevations were more sexually dimorphic in plumage than species at higher elevations. This variation was largely due to increased brightness of male plumage at lower elevations. I address possible explanations of this trend, which may include increased opportunities for extrapair fertilizations at lower elevations, an increase in the cost of secondary sexual trait production (i.e., molt) and maintenance at high elevations, and elevational variation in predation pressure.