Rensch's Rule Crosses the Line: Convergent Allometry of Sexual Size Dimorphism in Hummingbirds and Flower Mites

Hummingbirds demonstrate the full scope of Rensch's rule (females larger than males in small species, males larger than females in large species). The phoretic flower mites that hummingbirds compete with for nectar show the same pattern. An analysis of phylogenetically independent contrasts for 154 hummingbird and 37 mite species confirmed that both small (female-larger) and large (male-larger) species contribute to this result. I propose a common cause for this convergent pattern of sexual size dimorphism, with support from ecological and behavioral data for both groups. When morphology or behavior constrains each species to a limited range of resource densities, natural selection tends to diversify body size among species. If mating is promiscuous or polygynous, sexual selection favors larger males when resources are sufficient. But if male reproductive behavior is energetically costly, smaller males may have an advantage when resources are limiting, producing a pattern of allometry for sexual size dimorphism that conforms with Rensch's rule. Stabilizing selection on female size, if it occurs, amplifies this pattern. When the resource-density spectrum is broad enough, both female-larger and male-larger size dimorphism should occur within the same clade.