Variation in sexual size dimorphism and fit to Rensch's rule in Costa Rican hummingbirds

Introduction : 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. Objective : To analyze patterns of SSD in body mass and test Rensch’s rule in 45 species of Costa Rican hummingbirds. Methods : We evaluated SSD in body mass across 45 hummingbird species using Bayesian phylogenetic regression to test for conformity with Rensch’s rule. We hypothesized that physiological constraints would limit SSD variation due to the group’s high metabolic rates, dependence on energy-rich food, and the energetic costs of small size and hovering flight, all of which scale with body mass and elevation. Larger species were expected to tolerate greater SSD variation than smaller, more energetically constrained species. Results : 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 (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. These results suggest that physiological constraints limit the magnitude of SSD variation across the clade, regardless of overall body size. Conclusions : SSD in body mass influences ecological performance, mating displays, resource access, and foraging strategies. The presence of mixed allometry and the positive association between male body mass and SSD support the idea that selective pressures act differently on each sex, potentially promoting sexual niche segregation. Future studies should assess intersexual morphological variation, quantify habitat use and niche partitioning between sexes, and evaluate interspecific and intraspecific competition — particularly in species located at the extremes of the Rensch’s rule continuum.