by Dolphins don’t just look like they’re smiling when they open their mouths. Some scientists think they are actually grinning at each other, and the expression seems contagious.
A relaxed, open mouth in a dolphin is open to human interpretation, and not everyone agrees that this is a ‘smile’ as such. But researchers in Europe have now emerged evidence suggesting that bottlenose dolphins live in captivity (Tursiops truncatus) use the expressions as friendly gestures.
During play fights, researchers suspect that when one dolphin mimics another’s open mouth, the playmates agree to a friendly interaction that might otherwise be misinterpreted as aggressive.
If the team is right, it suggests that “smiley faces” are playful expressions that have deep roots in mammalian biology, not just on land but also in the sea.
“We discovered the presence of a distinct facial expression, the open mouth, in bottlenose dolphins, and we showed that dolphins are also able to mirror the facial expression of others,” said evolutionary biologist Elisabetta Palagi of the University of Pisa in Italy.
“The relaxed open mouth seen in social carnivores, the playful faces of monkeys, and even the laughter of humans, is a universal sign of playfulness, allowing animals (and us) to signal fun and avoid conflict.”
Previous evidence suggests that primates, horses, and wild and domestic dogs also share relaxed, open-mouthed expressions during play, but this is the first study to suggest that marine mammals do the same.
Bottlenose dolphins are known to be extremely playful at all ages, and while research shows that language is a crucial part of their social interactions, their visual communication is relatively understudied.
For their research, Palagi and her colleagues analyzed 80 hours of footage, showing 22 captive dolphins showing free play within four social groups. Ultimately, only one dolphin used an open mouth during solitary play. The vast majority of open mouths were used during social play, such as hunting or playing.
These are not aggressive encounters, because each dolphin gets a turn to chase, flee, bite, or tap its nose, and neither intends to actually hurt the other.
If an open mouth is used to prevent a play bite, the expectation would be that a smile would often lead to a bite in retaliation. But that’s not what the scientists discovered.
Of all the open mouths in the footage, almost 90 percent were used when the dolphin’s face could be seen by its playmate, and in a third of the cases the playmate grinned back within a second.
Even if a dolphin playmate did not make an open-mouth gesture, the open-mouthed dolphin never proceeded to bite.
Furthermore, when dolphins showed actual aggression, the authors did not count open mouths in the footage.
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The findings cannot completely rule out that dolphins use an open mouth in other contexts, but the results are consistent with previous hypotheses suggesting that smiling faces are friendly, social signals resulting from biting.
“The open-mouth gesture likely evolved from the biting action, which breaks the biting sequence and leaves only the ‘intention to bite’ without contact,” Palagi explains.
“This degree of mimicry in dolphins is consistent with what has been observed in certain carnivores, such as meerkats and sun bears.”
The study was only conducted on dolphins in captivity, so it’s difficult to say how the species might use open mouths in the wild.
However, from an adaptive perspective, switching from acoustic to visual communication could be an effective strategy during social play, when dolphins tend to be less vigilant against predators.
‘Although we are far from understanding the evolutionary origins of play and the ability of animals to fine-tune their playful sessions, the ubiquitous presence of open-mouth signals and rapid mimicry in the mammalian phylogenetic tree indicates the relevance of such visual mechanisms in shape complex communication,” argues the team of researchers.
The research was published in iScience.
