Scaling and foraging behavior drive the evolution of humeral shape in hummingbirds

Abstract

Understanding how locomotion-related skeletal elements evolve under biomechanical and ecological constraints is central to animal evolutionary biology. In hummingbirds (Trochilidae), the humerus plays a key role in force transmission during hovering and flapping flight, yet the drivers of its shape evolution have not been examined. We combined geometric morphometrics with phylogenetic comparative analyses to examine humeral shape variation, evolutionary rates, and phenotypic integration in male hummingbirds from 78 species. Our analyses identified humerus allometry as the dominant predictor of shape, revealing a pattern in which larger humeri show broader proximal epiphyses, increased shaft robustness, and reduced curvature. In addition to this scaling pattern, we find that male humerus shape differs subtly among hummingbird species that differ in the use of aggression during nectar foraging. Evolutionary rates of humeral shape were heterogeneous and decoupled from ecological predictors. We also find phenotypic integration between the proximal and distal regions of the humerus, indicating coordinated evolution. Together, these results show that humeral evolution in hummingbirds is governed primarily by biomechanical scaling and internal integration, with foraging ecology introducing secondary, size-dependent modifications. This work highlights the importance of considering scaling and internal integration when interpreting morphological evolution in locomotor systems across vertebrates.