The Role of Sexual Selection in the Evolution of Facial Displays in Male Non-human Primates and Men

Objective: Sexual selection theory provides a framework through which some facial displays of male non-human primates can be investigated and understood. Here, we explore how both intra- and inter-sexual selection may influence facial morphology, physiology, and the behavior of male primates inhabiting diverse social and ecological environments. Methods: First, we will review how elements of the ecological environment, such as the spatio-temporal distribution of food, interacting with the diet of different species, shapes the social and mating systems of primates. In turn, these dynamics then influence how facial expressions, colors, and shapes are utilized in both the competition for and attraction of mates. We will focus on sexually dimorphic facial features that exhibit variation in their expression among males and that can be linked to differences in proxies of reproductive success. Results: Facial displays can generally be divided into four types, each commonly associated with certain mating systems, social systems, and sexually selective pressures. Facial expressions, skin color, pelage, and variation in facial shape, can be involved in either mediating intra-sexual competition, mate choice, or a combination of both. Conclusions: Certain aspects of male non-human primate facial displays are likely to be shaped by processes of both intra- and inter-sexual selection. Accordingly, homologies and analogies between human and non-human primate facial displays can inform us of the processes of sexual selection that may have been operating throughout human evolution.