Next time you smile politely at a stranger, think of a gorilla baring his big pointy fangs while looking like he's just heard the best joke of his life. The grin doesn't mean what you might think—it's actually the great ape's equivalent of politely smiling.
Researchers have long believed that gorillas use facial cues to communicate with one another the same way humans do, but they have only recently started to uncover the meaning of the expressions. Their two main facial expressions are baring teeth and opening the mouth to smile without showing any teeth.
While it runs counter to what you might expect, the bared teeth expression is a sign of appeasement or submission. Gorillas use the open mouth, no teeth smile during playtime to show that they have no intention of biting. (Scientists call this the "play face.") "[During play, gorillas] open their mouths and cover their teeth as if to say, 'I could bite you but I'm not going to,'" researcher Bridget Waller told the BBC.
Researchers also discovered that the Western lowland gorilla has one extra facial expression that seems to combine the other two. When these gorillas are having quite a bit of fun during playtime, they will grin and bare only their top teeth. When this happens, playtime tends to last a lot longer than it would if they only used their toothless smile to show their intentions.
Researchers believe these findings could help us better understand the evolution of human facial expressions—Waller believes the play face, for example, is a foundation of human laughter—which tend to be hard-programmed into our DNA no matter how we were raised.