Physical constraints, not sociality, shape primate vocal repertoire size

Primate vocal richness and ultimately human language are widely thought to have co-evolved with social scale, as formalized by the social brain/complexity hypothesis, whereby managing increasing numbers of social relationships drives increasingly diverse vocal signals. However, this framework largely overlooks the physical constraints of sound production and transmission-specifically body size and habitat structure-which define the conditions under which communication unfolds on the social stage. Here, using the most comprehensive dataset of primate vocal repertoires assembled to date (77 sps) and phylogenetically controlled analyses, I find that neither brain size nor group size predicts repertoire size, providing no support for socially driven hypotheses. Instead, vocal repertoire size is explained by an interaction between body size and arboreality: across primates ranging from 40 g to 100 kg, each doubling of body size is associated with an approximately 28% increase in repertoire size in arboreal species. By contrast, terrestriality reduces repertoire size relative to expectation (approx. 3%), whereas arboreal (+6%) and mixed (+17%) habitat use increase it. Findings challenge the role of human uber-sociality as a driver of language evolution, highlight arboreality as a potential precondition for vocal richness in large-bodied ancestral hominids and show that primate vocal richness reflects physics more than politics.