One morning 10 years ago, psychologist Jaak Panksepp walked into his lab and made an unusual proposition to a research assistant: “Come tickle some rats with me!” Panksepp wasn’t just trying to entertain himself, or his lab rats. He was trying to solve one of the great questions of human evolution: Why do we laugh?

“Laughter is one of those human mysteries about which we know essentially nothing,” Panksepp cheerfully admits. To solve this mystery, scientists have probed the brain circuits involved in human laughter, but this research has faced a number of limitations. For one, though researchers have used sophisticated equipment to observe people’s brain activity while they read cartoons or jokes, this equipment is quite sensitive to movement—a major drawback when you’re trying to induce a hearty belly laugh.

"Laughter is one of those human mysteries about which we know essentially nothing," says psychologist Jaak Panksepp. © Becky Phillips

So researchers have turned instead to some of our furry cousins in order to learn more about laughter. Over the past few years, several different labs have begun to uncover different forms of laughter in species ranging from chimpanzees and bonobos to dogs and even rats. In addition to helping identify the biological machinery involved in laughter, these animal studies have also shed light on laughter’s evolutionary significance. Laughter, this research suggests, isn’t merely a way to signal joy. It may also be a vital, age-old tool used to promote social bonding and to help individuals improve their standing within a group.

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Laughter does sound different across species, of course. Chimpanzee laughter sounds more like rhythmic panting, and rat laughter is so high-pitched as to be undetectable without specialized equipment. And, as we all know, it even varies among individuals within a species (think Janis on Friends). However, the basic vocal pattern to laughter is remarkably familiar across the animal kingdom, and it is often expressed in similar social situations. Indeed, our tendency to laugh appears to be hardwired. Human babies, even those born both deaf and blind, will smile, gurgle, and laugh by the age of four months.

It is precisely this more childlike, instinctive form of laughter that scientists believe they have uncovered in rats. Panksepp, the Baily Endowed Chair of Animal Well-Being Science at Washington State University, has found that rats emit their high-pitched chirps when tickled, especially in areas, such as the nape of the neck, that are targeted by fellow rats during playful bouts. They make the same sounds when playing or anticipating playtime with one another, as well as when anticipating a reward. Other research has found that this chirping is common when rats enter new environments or encounter new animals; Panksepp likens this to nervous laughter in humans.

As rats age, their chirping generally becomes less frequent. (A similar phenomenon seems to occur in humans: Research done by William Fry, a professor emeritus at Stanford University School of Medicine, has found that kindergarteners laugh 300 times a day, whereas adults laugh just 17 times.) However, rats who were tickled often when young usually retain their tendency to chirp later in life.

Studies with rats have also suggested several reasons why laughter may be innate to so many different animal species. Panksepp and others have noticed that chirping occurs when rats are sizing each other up in the moments before a potential fight, which may mean that laughter can help diffuse situations that might otherwise escalate into physical conflict.

Similar behavior has been documented among chimpanzees. A few years ago, Takahisa Matsusaka, then a graduate student at Kyoto University in Japan, observed that when young chimps were tickled or chased, they made vocal sounds that sounded like laughs, which he called “play-panting.” Matsusaka suggests that play-panting signals to their partners that they want to keep playing, especially during interactions that contain mock-aggressive acts.

A recent paper in the journal Behavioral Brain Research builds upon this observation. The authors of this paper suggest that when primates like chimps engage in “fragile interactions”—such as fake fights that border on actual violence—laughter may serve as a strategy to keep it light. In an interview, Martin Meyer, a researcher at the University of Zurich who was the lead author of the paper, says he believes primates use laugh-like vocalizations primarily when they’re in a subordinate position, trying to appease a potential assailant.

In light of these findings, it’s not hard to understand how, in any highly social species, natural selection might favor those who laugh. Laughter may help some animals avert an attack, and according to Panksepp, animals may be attracted to others who laugh, seeing their laughter as a sign that they have a positive temperament and can get along well with others—and some of those positive interactions could lead to reproduction. “Laughter indicates emotional health, just as a peacock’s tail indicates physical health,” he says. Indeed, just as humans like to spend time with their funny friends, Panksepp has found that rats gravitate towards those who “chirp” the most.

Though animals can illuminate our own human tendency to laugh, no study of rats will ever explain why we chuckle at The Daily Show, and researchers are under no illusion that animal laughter is identical to our own. As Panksepp points out, rat arms and human arms have all the same joints, and one can be a good model for the other, but they’re not interchangeable. Still, research so far has yielded insights that may help us better understand both why we laugh and why we sometimes don’t, such as during bouts of depression or anxiety. In the meantime, Panksepp and his colleagues are busy tickling away.

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