The evolution of sexually dimorphic traits in ecological gradients: An interplay between natural and sexual selection in hummingbirds

Traits that exhibit differences between the sexes have been of special
interest in the study of phenotypic evolution. Classic hypotheses explain
sexually dimorphic traits via intrasexual competition and mate selection,
yet natural selection may also act differentially on the sexes to produce
dimorphism. Natural selection can act either through physiological and
ecological constraints on one of the sexes or by modulating the strength
of sexual/social selection. This predicts an association between the
degree of dimorphism and variation in ecological environments. Here, we
characterise the variation in hummingbird dimorphism across ecological
gradients using rich databases of morphology, colouration, and song. We
show that morphological dimorphism decreases with elevation in the
understorey and increases with elevation in mixed habitats, that
dichromatism increases at high altitudes in open and mixed habitats, and
that song is less complex in mixed habitats. Our results are consistent
with flight constraints, lower predation pressure at high elevations, and
with habitat effects on song transmission. We also show that dichromatism
and song complexity are positively associated, while tail dimorphism and
song complexity are negatively associated. Our results demonstrate that
key ecological factors shape sexually dimorphic traits and that different
communication modalities do not always evolve in tandem.