Coevolution of facial expression and social tolerance in macaques.

The purpose of this study is to test the hypothesis that social tolerance drives the evolution of facial expression in macaques. Macaque species exhibit a range of social styles that reflect a continuum of social tolerance. Social interactions in more tolerant taxa tend to be less constrained by rank and kinship than in less-tolerant macaques. I predicted that macaques that are more tolerant would exhibit a wider range of facial displays than less-tolerant species because interactions that are open to negotiation are characterized by greater uncertainty than interactions that are constrained by rank or kinship. To test this hypothesis, I conducted a phylogenetically informed regression analysis (N = 11) using previously published data on repertoire size and two quantitative measures of social tolerance (conciliatory tendency and counter-aggression). As predicted, macaques with more tolerant social styles tended to have larger repertoires than less-tolerant species. These results support the hypothesis that increased social tolerance favors the elaboration of communication to mitigate uncertainty.