Dolphins are known for being playful, but how they communicate is less understood. In a new study, biologists found that bottlenose dolphins (Tursiops truncatus) “smile” by opening their mouths during play. It was also known under what conditions they did this more often.

While playing, dolphins can slide among the waves, perform acrobatic tricks, throw objects, arrange chases and funny fights. At the same time, it is important not to confuse such actions with aggression.

Many mammals, including dogs and monkeys, show playful intent through their facial expressions. However, cetaceans’ abilities in this regard are severely limited, since the facial muscles are under a dense skin that resembles rubber and is not very elastic and mobile.

A group of researchers from the University of Pisa and other universities in Italy, together with colleagues from France, studied bottlenose dolphins (Tursiops truncatus) indicates this by comfortably opening its beak-like mouth, also called the rostrum.

According to scientists, this is an analogue of a smile – a universal sign of playfulness that can be observed during laughter in social predators, monkeys and humans. With its help, animals and people clarify their cheerful mood and avoid conflicts.

To test the hypothesis that dolphins have this form of visual communication, researchers tracked these marine mammals in captivity, videotaping their behavior during play alone, with each other, or with trainers. An article about this scientific study was published in the journal. iScience.

During the 60-day observation, scientists shot 80 hours of video. As a result, they found 837 episodes of play behavior involving 22 dolphins and recorded 1,288 cases of mouth opening. Analysis of the recordings showed that animals almost never use the smile analogue during solitary games and very rarely resort to it during communication with a person. In 92% of cases, dolphins “smiled” while playing with their kin, and they did this more often when they were in their partner’s line of sight. In one-third of the cases, they also opened their mouth in response.

According to the authors of the study, such movements of dolphins are not imitations of each other by chance. If the other dolphin could see its partner’s facial expression, it was 13 times more likely to open its mouth in response in the next second.

The dolphins’ acoustic signals during the games were not recorded this time; Therefore, experts plan to investigate the possible role of vocalizations and tactile contacts in game interaction in the future.

In a comment to the online journal Science, biologist Peter Tyack, who was not involved in the research, warned against interpreting the results too loosely. According to him, dolphins’ “smiles” need to be studied in more detail because open-mouthed expressions have also been recorded in other contexts related to sexual behavior and aggression.