The role of between-group signaling in the evolution of primate ornamentation.

Gregarious mammals interact to varying degrees and in a variety of ways with neighboring groups. Since navigating this wider social environment via conventional means (social knowledge) may be challenging, we hypothesize that between-group socio-spatial dynamics have exerted strong selection on phenotypic markers of individual identity, quality, and competitive ability. Ornaments are sexually selected decorative traits with far-reaching signaling potential. Here, we examined the links between sexual dimorphism in ornamentation, home range use and encounter rates across 144 primate species in a Bayesian framework. We show that home range overlap (shared space among neighbors), an indicator of the complexity of between-group interactions (but not necessarily male-male competition), is positively associated with dimorphism in ornamentation. We find no clear effect for between-group encounter rates. We also find that inter-group interactions were less agonistic when there was greater home range overlap. Taken together, these findings indicate that ornaments play a hitherto underappreciated role in signaling to conspecifics outside the realms of their home groups.