The monkey smiles

The bared-teeth grin is not to be confused with a wide-open mouth and intense staring eyes. That fierce face, which looks like an intention to bite, acts as a threat. In a grin, the mouth is closed, but the lips are retracted to expose the teeth and gums. The row of bright white teeth makes it a conspicuous signal, visible from far away, yet its meaning is the exact opposite of a threat.