For adult chimps, playing may be more important than previously thought

You are never too old to play — a maxim that even adult chimpanzees seem to follow.

Young chimpanzees are already known to get many benefits from playing. But it seems to also be more important among adult chimps than previously thought. A multiyear study of dozens of adult chimpanzees in Ivory Coast suggests that play helps adults reduce tension and boost cooperation among individuals, researchers report November 21 in Current Biology.

“Every time we think we have something that’s like, ‘This is the thing that makes humans different,’ eventually we knock it down,” says Kris Sabbi, a primatologist at Harvard University who was not involved in the study. “We used to think that playing into adulthood was something that humans did and, as it turns out, it’s something that chimps do, too.”

Play among chimps “is a joyful, mutual and synchronous act,” says behavioral ecologist Liran Samuni of the German Primate Center in Göttingen. It involves actions usually seen during aggression — such as biting, slapping and chasing — and therefore also involves trust between the participants. For young chimps, playing helps them develop social and physical skills. But what role playing has for adults has remained understudied.

So Samuni and colleagues monitored wild chimpanzees (Pan troglodytes) living in Ivory Coast’s Taï Forest, in a population where adults are known to play regularly. From 2012 to 2018, the researchers documented almost 5,000 play sessions involving at least one adult chimp (defined as those older than 12 years) among 57 adults belonging to three different communities.

The team found that play was quite common among these adult chimps, coming in many forms and situations. Overall, adult chimps played on about 40 percent of the days the researchers observed them, and mostly when food was more available. Some sessions involved objects, although most did not. Usually, an adult played with just one partner, but sometimes more joined in; the largest documented play session was among five individuals.

Adults who had a stronger social relationship were more likely to play with each other, the team found. The grown-ups were also more likely to play together on days when the group collaborated to hunt monkeys or defend their territory against other groups of chimps, mostly before those activities took place. And if adults did play together before those activities, they were more likely to defend their territory or hunt together.

“These coordinated behaviors require the participation of multiple individuals to be successful,” Samuni says, “thus a mechanism like adult-adult social play, which may foster trust, reduce anxiety and motivate collaborative engagement, is particularly beneficial.”

The team also found clues that play may help adult chimps in other ways. For example, adults were more likely to play with someone they recently fought with compared with anyone else — suggesting, Samuni says, that social play “might serve as a mechanism for conflict reconciliation and tension reduction.”

Adults also played more when the tension was higher. For example, on tense days when a female was ready to mate, females were nearly 50 percent more likely to play with other adults rather than at other times, “potentially as a means to reduce stress and social tension,” Samuni says.

The new work complements other findings, published November 20 in PLOS ONE, that chimpanzees living at a sanctuary in Zambia were more likely to engage in play and grooming if they saw other individuals doing so.

“In humans, we know that sharing positive experiences, such as playing, laughing and engaging in a bit of pampering are important for our social relationships, as well as for our well-being,” says Zanna Clay, a psychologist at Durham University in England and coauthor of the Zambia study. “It’s likely these functions are evolutionarily ancient and shared with our other primate relatives.”