SOCIAL STRUCTURING. “Visual communication among conspecifics within Primates is part of maintaining social groups, social bonds, reproduction, and many aspects of daily life, especially so among the diurnal species. Primates generate visual communication signals in the face and these signals include skin coloration/patterning and facial expressions/displays.”
http://onlinelibrary.wiley.com/doi/10.1111/joa.12440/full