Lipsmack
The lips are pursed and the lower jaw is moved up and down rapidly and rhythmically. The jaw may be thrust upward. The mouth may be slightly open with the tongue moving back and forth. The lips often produce an audible sound. Alternatively, the mouth may be closed, sometimes the teeth knock together. Eyelids are generally half-lowered. The scalp may be retracted and the ears flattened. The display is used
during affiliative interaction. It may also end a conflict, and acts as an appeasement or reassurance signal (Thierry et al. 2000).
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.