Judgments of Monkey's (Macaca mulatta) Facial Expressions by Humans: Does Housing Condition "Affect" Countenance?

The ability to interpret facial expressions of others is one of the more important abilities possessed by humans. However, is it possible for humans to accurately interpret the facial expressions of another species of primate, namely rhesus monkeys (Macaca mulatta)? We investigated this possibility by taking digital photos of four rhesus monkeys housed either singly or socially and allowing thirty-one participants to judge these photographs as representing either a happy, sad, or neutral monkey. Results indicated that the photographs of monkeys that were socially housed were more likely to be rated as happy or neutral than were photographs of singly housed monkeys. We suggest that these results imply important parallels between the perception of human and nonhuman primate facial expressions as well as introduce a potential new method for assessing nonhuman primate well-being.