Avalos et al. Rench´s rule in Costa Rican hummingbirds

We examined sexual size dimorphism (SSD) in body mass across 45 species of Costa Rican hummingbirds to test conformity with Rensch’s rule, which predicts increasing SSD with body size when males are larger. Using Bayesian phylogenetic regression, we found that the group conformed to Rensch’s rule, with a regression slope of male vs. female body mass significantly less than 1 (0.84), indicating male-biased allometry in 80% of species. SSD averaged 12% and was significantly associated only with male body mass. These findings suggest that physiological constraints—linked to high metabolic demands, energy-rich diets, and the costs of hovering flight—limit SSD variation in hummingbirds regardless of size, while also implying that selective pressures act differently on each sex, potentially driving sexual niche segregation.