When we humans smile at each other it usually means “Hi!” or “I’m happy to see you!” or “I’m not about to viciously attack you!” However, for other members of the primate family, it can mean the exact opposite.

Chimpanzees bare their teeth to express fear, and when they’re backed into a corner, they’ll protect themselves from a perceived threat the only way they know how: by completely mutilating it.

Even a strong grown man is no match for the primal fury of a great ape, so if you see a monkey smiling and reaching out its hand, don’t go for a handshake. At least, if you want to keep that arm attached to your body.