Silent bared-teeth (affiliative)
The upper lip or both lips are vertically retracted, exposing the teeth and sometimes the gums. The corners of the mouth may be drawn back. The jaw may be either closed or opened to various degrees. The scalp is often raised and the ears flattened. This is an affiliative display, it is commonly observed during affiliative interactions and social play. It may be used to initiate a positive interaction and is frequently associated to an approach. In exceptional occasions, it may be seen in individuals retreating in response to a threat.
Thierry, B., EL, B., Baker, S., Matsumura, S., Muroyama, Y., TG O, B., ... & Watanabe, K. (2000). The social repertoire of Sulawesi macaques. Primate Research, 16(3), 203-226.