Personality and facial morphology: Links to assertiveness and neuroticism in capuchins (Sapajus [Cebus] apella)

• We examined facial morphology and personality in capuchins, Sapajus apella.
• Two metrics of facial morphology associated with ratings of assertiveness.
• A third metric was associated with ratings of neuroticism and attentiveness.
• We examine differences of dimorphism in these three metrics.
• We suggest that facial morphology may act as a signal of status in capuchins.

Personality has important links to health, social status, and life history outcomes (e.g. longevity and reproductive success). Human facial morphology appears to signal aspects of one’s personality to others, raising questions about the evolutionary origins of such associations (e.g. signals of mate quality). Studies in non-human primates may help to achieve this goal: for instance, facial width-to-height ratio (fWHR) in the male face has been associated with dominance not only in humans but also in capuchin monkeys. Here we test the association of personality (assertiveness, openness, attentiveness, neuroticism, and sociability) with fWHR, face width/lower-face height, and lower face/face height ratio in 64 capuchins (Sapajus apella). In a structural model of personality and facial metrics, fWHR was associated with assertiveness, while lower face/face height ratio was associated with neuroticism (erratic vs. stable behaviour) and attentiveness (helpfulness vs. distractibility). Facial morphology thus appears to associate with three personality domains, which may act as a signal of status in capuchins.