Abstract Mandibular shape in primates can reflect more than just dietary function; it carries signals of ecological adaptations and evolutionary constraints. We investigated mandibular variation in Capuchin monkeys (Sapajus spp.) across contrasting South American biomes, ranging from humid forests to dry, seasonal savannas. Using geometric morphometrics and phylogenetic comparative methods, we tested how shape is structured by intrinsic and extrinsic predictors. Our results revealed a consistent allometric relationship with mandibular shape. However, accounting for shared ancestry revealed a functional decoupling: while mandibular size diverged in savanna lineages (S. cay, S. libidinosus) due to energetic restrictions (Resource Rule), a ‘robust’ mandibular shape was shared by both Amazonian and savanna populations. This shared robusticity confounded categorical analyses, where broad ecological groups failed to predict shape and environmental variables explained only a small unique fraction of shape variation. Nevertheless, continuous climatic analyses confirmed that robust phenotypes are linked to specific thermal drivers independent of phylogeny. Consequently, savanna populations retain this ancestral robust morphological apparatus not by phylogenetic constraints, but as a necessary biomechanical compensation for fallback foods despite their reduced body size. Moderate phylogenetic signals suggest that developmental plasticity facilitates this fine-scale optimization, allowing ecological pressures to modulate the mandible shape.
Chuma et al. (Sun,) studied this question.