Who's Looking At You, Babe?

A monkey grinning, or pulling their lip up to show the teeth, may appear as a friendly smile to humans.

But for them, it is not a welcoming social nicety but instead it's a sign of aggression or anger.

Biting is one way that monkeys fight and defend themselves.

Other signs of aggression include head bobbing, yawning- again, to show the teeth - and jerking the head and shoulders forward.