Regional variability in skull sexual dimorphism: Olive baboons, blue monkeys, and vervets confirm the posterior region of the skull and the snout as opposite extremes of sex differences in Old World monkeys.

Abstract

This study, alongside prior work on crab-eating macaques with O'Higgins, investigates regional variation in skull sexual dimorphism across different species of Old World monkeys. Findings reveal consistent patterns: the posterior skull regions (mandibular vertical ramus, chondrocranium and, possibly, the cranial vault) exhibit lower dimorphism, which may be related to developmental constraints such as the early formation of the cranial base. In contrast, the snout (including the horizontal ramus of the mandible and oro-facial regions of the cranium) shows the most pronounced dimorphism, aligned with functional and ecological demands, such as canine size differences and masticatory adaptations. Regional dimorphism patterns, if demonstrated, may aid taxonomic identification (focusing on less dimorphic regions) or sex discrimination (using snout measurements). However, these are, for now, hypotheses and potential contradictions in primate studies (e.g., great apes) indicate the need for standardized measurements and methods, as well as an expanded taxonomic sampling. Nonetheless, exploring regional anatomical variation in relation to sex, and potentially other factors such as age and taxon, is a promising avenue of research that can contribute to elucidating evolutionary patterns in morphological diversity of living and extinct species.