Flipping adorable! Bottlenose dolphins 'smile' as they play

They're widely considered some of the most charismatic animals in the ocean.

Now, it turns out bottlenose dolphins actually 'smile' at each other while surfing, chasing and playfighting, according to a new study.

Until now, little has been known about how the species - famed for their intelligence and playfulness - interact during playtime.

But experts have finally revealed that these dolphins use the 'open mouth' facial expression – comparable to a smile – to communicate during social play.

The dolphins almost always use the facial expression when they are in their playmate's field of view, the researchers found, and playmates responded by 'smiling' back a third of the time.

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'We've uncovered the presence of a distinct facial display, the open mouth, in bottlenose dolphins, and we showed that dolphins are also able to mirror others' facial expression,' senior author and evolutionary biologist Elisabetta Palagi, from the University of Pisa, said.

'Open-mouth signals and rapid mimicry appear repeatedly across the mammal family tree, which suggests that visual communication has played a crucial role in shaping complex social interactions, not only in dolphins but in many species over time.'

Dolphin play can include acrobatics, surfing, playing with objects, chasing and playfighting.

To investigate whether dolphins visually communicate playfulness, the researchers recorded captive bottlenose dolphins while they were playing in pairs and while they were playing freely with their human trainers.

They showed that dolphins frequently use the open mouth expression when playing with other dolphins, but they don't seem to use it when playing with humans or when they're playing by themselves.

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While only one open mouth event was recorded during solitary play, the researchers recorded a total of 1,288 'smiles' during social play sessions, and 92 per cent of these events occurred during dolphin-dolphin play sessions.

Dolphins were also more likely to assume the open mouth expression when their faces were in the field of view of their playmate, and the team witnessed the playmate 'smiling' back 33 per cent of the time.

Dr Palagi said that while some may argue that dolphins are merely mimicking each other's mouth expression by chance, it 'doesn't explain why the probability of mimicking another dolphin's open mouth within one second is 13 times higher when the receiver actually sees the original expression.'

'This rate of mimicry in dolphins is consistent with what's been observed in certain carnivores, such as meerkats and sun bears,' she added.

'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.'

The researchers didn't record the dolphins' acoustic signals during playtime, and they say that future studies should investigate the possible role of vocalizations and tactile signals during playful interactions.

'Future research should dive into eye-tracking to explore how dolphins see their world and utilize acoustic signals in their multimodal communication during play,' said corresponding author and zoologist Livio Favaro.

'Dolphins have developed one of the most intricate vocal systems in the animal world, but sound can also expose them to predators or eavesdroppers.

'When dolphins play together, a mix of whistling and visual cues helps them cooperate and achieve goals, a strategy particularly useful during social play when they're less on guard for predators.'

The findings were published in the journal iScience.