Variable grouping patterns in primates: A test of the better competitor hypothesis in male Verreaux's sifaka...

Intraspecific variation in social organization (IVSO) within primate populations can arise due to male mating competition. Verreaux's sifaka (Propithecus verreauxi) populations exhibit both single-male and multi-male groups, providing an opportunity to examine the relationship between male competitive ability and social organization. We predicted that males in single-male groups have traits for excluding competitors compared with dominant males in multi-male groups. We analyzed skeletal and muscular measurements, along with body mass and canine length, from 18 adult male sifaka studied at the Ankoatsifaka Research Station in Kirindy Mitea National Park, Madagascar, between 2007 and 2024. Multivariate patterns of limb morphology associated with fighting and chasing ability were summarized using principal component analysis (PCA), and the first two PCA-derived axes, together with body mass and canine length, were used as predictors in Bayesian logistic regression models of group membership. No morphological trait or PCA-derived component showed a consistent association with group type, with posterior credible intervals overlapping zero. Consequently, variation in male skeletal and muscular morphology alone is insufficient to explain IVSO. Together, these results indicate that although PCA-derived axes capture structured variation in overall limb configuration and proximal-distal functional trade-offs related to fighting and chasing ability, social organization is not determined by male morphology alone, underscoring the importance of female influence and dynamic behavioral and physiological pathways in shaping intraspecific variation in social organization.