Dolphins’ open-mouth behaviors during play are like smiles, a study claims

Dolphins are known for what appear to be big, contagious smiles. But do they actually, well, smile?

The answer, according to a new study of dolphin play, is a resounding “maybe.” Dolphins use their smile-like facial expression when interacting with their mates during playful times, researchers report October 2 in iScience. That, the team says, suggests the cetaceans are doing something akin to a human laugh. But other experts urge caution in attributing humanlike behavior to creatures whose intent we can only guess at.

Scientists have long known anecdotally that dolphins can display a behavior called “open mouth,” which people often associate with a smile. Other animals, like primates, can make a similar relaxed open mouth to communicate during playful contexts (SN: 6/10/15). “It’s a signal that communicates, ‘Look, I’m just playing!’,” says Elisabetta Palagi, a comparative ethologist at the University of Pisa in Italy. “Like when we put a smiley face on a cell phone message that could be misleading.”

However, such behavior has never been studied in-depth in dolphins. So Palagi and colleagues recorded nearly 900 play sessions among 22 captive bottlenosed dolphins (Tursiops truncatus), filming nearly 1,300 “smiles” from 17 animals. The researchers filmed dolphins playing alone, with other dolphins and with humans.

The dolphins mainly showed their open-mouth behavior while playing with a companion, usually another dolphin, the team found. When the animals were playing alone, they practically never did. Furthermore, about 90 percent of open-mouth expressions were performed when the dolphins were in their playmate’s field of view.

Also, when one of these “smiles” was seen by another dolphin, in about a third of cases, the receiver also “smiled” back in less than a second. That, Palagi says, is the same time that elapses between a human perceiving a facial expression and then mirroring it.  

The results suggest that the dolphin’s open-mouth expression is “a very sophisticated form of communication,” Palagi says, which might be used together with acoustic signals (SN: 12/7/17). While it’s hard to say whether it had the same evolutionary origin as humans’ smile, it probably has “the same function,” since the context and the way in which it occurs is the same as in humans, she says.

“Being able to see some evidence that’s demonstrating [dolphin’s open-mouth behavior] in a play context is really great,” says animal behavioralist Erin Frick, who was not involved with the study. But, she adds, dolphins use mouth opening in other contexts as well, such as displaying a threat in an aggressive context. “I don’t think open mouths are always communicating play. I think they do have a role in play,” says Frick, of Eckerd College in St. Petersburg, Fla.

But Palagi is convinced the open mouth is dedicated only to playful contexts: When the dolphins in her study were acting aggressively, her team didn’t see the animals performing this particular open-mouth behavior. “During the few aggressions we witnessed, we saw the opening of the mouth, but after this extremely rapid opening, either a bite or an attempted bite followed.”

Regardless, Frick is not ready to call the behavior a smile. “It’s not the same,” she says. “But it still has a very … functional form in how they communicate.”

Like Frick, comparative psychologist Heather Hill also urges caution in interpreting the apparent smiles. “I am not super comfortable calling it a ‘smile,’ given that delphinids and whales use the open mouth display in a variety of social contexts,” says Hill, of St. Mary’s University in San Antonio.

Palagi is cognizant of the hesitancy. “There is a strong debate if in nonhuman animals, the act of smiling or laughing … is driven by emotional arousal or intention,” she says. “We were extremely conservative and simply referred to facial display”.

Palagi adds, “It is not easy to say if [the open mouth] in dolphins conveys an emotional mood or is used to simply communicate to others, ‘Hey, don’t be scared, I am just playing!’, or both.” One thing the team wants to investigate is whether the presence of open-mouth behavior alters play sessions in any way. Perhaps “laughing together” — if that is indeed what it is — makes the dolphins play together longer.