Capuchin monkeys (Cebus apella) modify their own behaviors according to a conspecific's emotional expressions.

Understanding conspecifics' emotional states is important for managing social interactions. We examined whether capuchin monkeys modify their own behaviors in response to a conspecific's emotional expressions. Six monkeys saw a demonstrator monkey responding emotionally to an object in a container; the subject monkeys could not see the object. The subjects reached for the container more frequently when the demonstrator showed an emotionally positive expression than when she showed an emotionally neutral or negative expression. This is the first report that New World monkeys are sensitive to the emotional valence of conspecifics' emotional expressions. The finding is consistent with the hypothesis that monkeys can recognize emotional meanings in others' expressions, an ability previously attributed only to humans and great apes.