Variation in sexual size dimorphism and fit to rensch’s rule in Costa Rican hummingbirds

Abstract

Rensch’s rule predicts that sexual size dimorphism (SSD) increases with body mass in species where males are larger but decreases when females are larger. To analyze patterns of SSD in body mass and test Rensch’s rule in 45 species of Costa Rican hummingbirds. We hypothesize that the physiological capacity to buffer environmental variation increases with body mass, creating a size-dependent tolerance for SSD that explains interspecific differences in SSD. Larger species were expected to exhibit greater SSD variation than smaller, more energetically constrained species. We evaluated SSD in body mass across 45 hummingbird species to test for conformity with Rensch’s rule using Bayesian phylogenetic regression. Hummingbirds exhibited mixed allometry and conformed to Rensch’s rule, with a phylogenetic regression slope of male vs. female body mass significantly less than 1.00 (0.84). Male-biased allometry was observed in 80% of species. On average, SSD variation in body mass was 12% and was significantly associated only with male size. In hummingbirds, the predominant polygynous mating systems generated the male-biased SSD pattern across species through sexual selection, while physiological constraints associated with extreme metabolic demands and hovering flight limit variation around this sexually selected baseline. Our findings are consistent with sexual selection representing the primary evolutionary force shaping SSD in this clade, with energetic limitations as critical modulating factors.

Marcelo Araya-Salas

Research Associate

My research interests include evolutionary behavioral ecology, cultural evolution, scientific programming and all possible combinations of them.