Dolphins are incredibly playful, but little is known about how they—and other marine mammals—communicate during play. A new investigation published in the journal Cell Press iScience shows that bottlenose dolphins (Tursiops truncates) use the facial expression “open mouth” — analogous to a smile — to communicate during social play.
In almost all cases, dolphins use the facial expression when they are in the field of vision of their playmates, and when these notice a “smile,” they respond in the same way 33% of the time.
“We discovered the presence of a distinct facial expression, the open mouth, in bottlenose dolphins, and showed that dolphins are also capable of mirroring others’ facial expression,” states senior author and evolutionary biologist Elisabetta Palagi (@bettapalagi) from the University of Pisa.
“Open-mouth signals and rapid mimicry appear repeatedly across the mammalian family tree, suggesting that visual communication played a crucial role in the formation of complex social interactions—not only in dolphins, but in many species over time,” she adds.
Dolphin play can include acrobatics, surfing, playing with objects, chasing and roughhousing, and it is important that these activities are not misinterpreted as aggression. Other mammals use facial expressions to communicate playfulness, but it has not yet been explored whether marine mammals also use facial expressions to mark the moment of play.
“The open-mouthed gesture probably evolved from the act of biting, breaking the sequence of biting to leave only the ‘intention to bite’ without contact,” says Palagi.
“The open and relaxed mouth, observed in social carnivores, in the play faces of monkeys and even in human laughter, is a universal signal of play, helping animals—and us—to signal fun and to avoid conflicts,” she notes.
To investigate whether dolphins visually communicate play, researchers recorded bottlenose dolphins in captivity while they played in pairs and while they played freely with their human trainers.
They showed that dolphins frequently use the open-mouth expression when they play with other dolphins, but do not appear to use it when they play with humans or when they are playing alone.
Although only a single open-mouth event was recorded during solitary play, the researchers logged a total of 1,288 open-mouth events during social play sessions, and 92% of these events occurred during dolphin-dolphin play sessions.
Dolphins were also more likely to take on the open-mouth expression when their faces were in the sightline of their play partner — 89% of open-mouth expressions recorded were emitted in this context — and when this “smile” was perceived, the play partner smiled back 33% of the time.
“Some may argue that dolphins are merely mimicking open-mouth expressions from one another by chance, since they are often involved in the same activity or context, but that does not explain why the probability of mimicking another dolphin’s open mouth within 1 second is 13 times higher when the recipient actually sees the original expression,” says Palagi. “This rate of mimicry in dolphins is consistent with what has been observed in certain carnivores, such as meerkats and sun bears.”
The researchers did not record the dolphins’ acoustic signals during play, and they state 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 use acoustic signals in their multimodal communication during play,” says the corresponding author and zoologist Livio Favaro (@LivioF_80).
“Dolphins have developed one of the most complex vocal systems in the animal kingdom, but sound can also expose them to predators or nosy onlookers. When dolphins play together, a mix of whistles and visual cues helps them cooperate and achieve goals, a strategy particularly useful during social play when they are less attentive to predators,” he concludes.
