Observations on a newly described usage of the primate play face

Previously, the use of the relaxed open-mouth face (play face) in primates has been considered to be solely associated with play, both solitary and social. In a captive group of long-tailed macaques (Macaca fascicularis) one variant of this facial signal (mime open- mouth face) was found to occur in apparently non-play social interactions. The nature of these interactions is compared and contrasted with social play, revealing that they differ in many respects. A general qualitative description of the non-play interactions is provided, including prominent behavioral elements involved, age-sex class and relative status of participating individuals, and some speculations concerning potential functions.