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Do you laugh or smile when playing around with your friends? It turns out bottlenose dolphins do too! Scientists have found that dolphins “smile” to communicate during playtime. And they probably do it to make sure their pals don’t mistake playfulness for aggression.

Like us, dolphins are playful animals. You can spot them doing under water acrobatics, surfing waves, playing with objects, chasing each other and even playfighting. But until now, scientists didn’t know whether marine mammals like dolphins also smile to show playfulness. It seems a dolphin smile looks like a relaxed open mouth.

“The relaxed open mouth, seen in social carnivores, monkeys’ play faces, and even human laughter, is a universal sign of playfulness, helping animals – and us – signal fun and avoid conflict,” says biologist Elisabetta Palagi.

To study how dolphins signal fun, Elisabetta and her team recorded captive bottlenose dolphins while they played in pairs or with their human trainers. They found that dolphins were much more likely to smile when they were playing with another dolphin as opposed to a human. The dolphins were also more likely to smile when their dolphin playmate could see them. This tells the team that dolphin smiles are a social signal.

When humans smile, it’s polite to smile back. The team found that dolphin playmates also smiled back about a third of the time. While one third might seem small, Elisabetta says this is a similar rate of “smiling back” seen in social carnivores, like dogs, meerkats and sea lions. The researchers didn’t record sound in this study, but they say that future studies should explore whether dolphins also use sounds, like whistling, to communicate during play.