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Laughing together is an important way for people to connect and bond. And though the causes of laughter can vary widely across individuals and groups, the sound of a laugh is usually recognizable between people belonging to different cultures.

But what about animals? Do they "laugh"? And are the causes of animal and human laughter alike? In humans, people may laugh when they hear a joke, or when they see something that they think is funny, though it's unknown if animals' intelligence includes what humans would call a sense of humor.

However, many animals produce sounds during play that are unique to that pleasant social interaction. Researchers consider such vocalizations to be similar to human laughter. Recently, scientists investigated play vocalization to see how common it was among animals. The team identified 65 species that "laughed"while playing-most were mammals（哺乳动物）， but a few bird species demonstrated playful laughter too. Reports of playful laughter were notably absent in studies describing fish, perhaps because there is some question as to whether or not play exists at all in that animal group. This new study could help scientists to analyze the origins of human laughter.

But how can we identify play? Unlike fighting, play is usually repetitive and happens independently of other social behaviors, said lead study author Sasha Winkler, a doctor of biological anthropology at the University of California. When it comes to identifying it, "you know it when you see it," Winkler told Live Science. One sign is that primates-our closest relatives-have a "play face" that is similar to the expressions of humans who are playing.

When Winkler previously worked with rhesus macaques, she had noticed that the monkeys panted（喘气）quietly while playing. Many other primates are also known to vocalize during play, she said, so a hypothesis (laughter in humans is thought to have originated during play) supported by the play-related panting laughter of many primate species was put forward.

People now still laugh during play, but we also integrate laughter into language and non-play behaviors, using laughter in diverse ways to express a range of emotions that may be positive or negative. Human laughter notably differs from other animals' laughter in another important way: its volume. People broadcast their laughter loudly, often as a way of establishing inclusion. By comparison, when most animals laugh, the sound is very quiet-just loud enough to be heard by the laugher's partner.

"It's really fascinating that so many animals have a similar function of vocalization during play," Winkler told Live Science. "But we do have these unique parts of human laughter that are also an important area for future study."