Chimpanzees ‘smile’ out of fear.

Chimpanzees have a surprisingly expressive range of facial movements — and when interpreting them, context matters. During play, they may use a relaxed, open-mouthed expression known as a “play face,” often with the teeth covered by the lips or only partly visible. This expression can signal excitement and playful intent rather than fear.

A toothy grin, however, can mean something very different. A chimpanzee flashing its teeth may look like it’s smiling at you, but it may actually be scared. When frightened, anxious, or confronted by a more dominant chimpanzee, a chimpanzee may pull back the corners of its mouth and expose its teeth in what researchers call a “fear grin.” Far from expressing happiness, an expression that looks like a smile can signal fear, submission, or an attempt to appease a threatening individual.