Are badges of status adaptive in large complex primate groups?

Sexual dimorphism in ornamentation in primates may have been sexually selected as signals of rank and dominance to males or by augmenting attractiveness to females. While male primates display tremendous variation in secondary sexual traits, such as sexual skin, capes of hair, and beards, which are often attributed to sexual selection, their phylogenetic distribution remains to be fully understood. Here we investigate the hypothesis that sexual dimorphism in ornaments is more pronounced in larger more ‘anonymous’ social organizations where quick reliable assessment of male quality, social status, dominance, and aggressiveness are selective pressures. Multiple regression analyses, including phylogenetic correction, were performed on 154 species representing 45 genera of simian primates. We found a positive relationship between degree of ornamental dimorphism and group size, even after controlling for other independent variables such as habitat type (i.e. openness of terrain) and fission–fusion dynamics. Dimorphism was also significantly associated with social organization, so that males from species with multilevel social organizations had the highest ratings for ornamentation. In sum, our analysis suggests that among primates with larger group sizes and multilevel social organizations, males have more developed visually conspicuous secondary sexual traits. This may reflect selection for amplified signals of individual identity, rank, dominance, or attractiveness in large and complex social organizations wherein social and physical conflict may arise frequently and individual recognition is limited.