A dog’s invitation to play is unmistakable: Bowing with his front legs and wagging his tail, he’s basically saying to another dog, Let’s have fun! The two animals then chase and leap and box each other, matching each other’s moves, often with expressions we humans interpret as smiles.

Now scientists report for the first time that dogs and horses play together in a similar manner, with open mouths and synchronously matched behaviors.

Perhaps most remarkably, the two species also rapidly mirror the expression on the other’s face, a behavior called rapid facial mimicry. This phenomenon occurs in primates, domestic dogs, meerkats, and sun bears, but has never been documented between play partners of different species.

“It’s a wonderful study, and takes