This smile ... human

Facial expression, body postures, and gestures are all forms of non-vocal communication among rhesus macaques and are important in interactions between individuals at short distances (Partan 2002). One facial expression that is seen throughout macaque species and is one of the most common expressions in rhesus macaques is the "silent bared teeth" face (Maestripieri 1999). Among rhesus macaques this is seen between individuals of differing rank with the lower-ranking or submissive animal performing the "silent bared teeth" face to the dominant animal (Flack et al. 2000).