Like humans, male chimps mellow with age

For all its drawbacks, aging brings a benefit: Social relationships generally improve. Older individuals have fewer but closer friendships, avoid conflicts, and are more optimistic compared with younger adults. Now, 20 years of data on chimpanzees suggest they, too, develop more meaningful friendships as they age.

The finding challenges a long-standing assumption that humans mellow with age because we are aware of our approaching mortality. Simply put, “You don’t have time for all this negativity in your life, so you shift toward more positive thinking,” says Zarin Machanda, a primatologist at Tufts University and an author of the new study. But finding the same pattern in chimps suggests a simpler explanation: It could be an evolved trait found in a wider range of species. The new study “should make us think twice” about the roots of some human behaviors, says Ian Gilby, a behavioral ecologist at Arizona State University, Tempe, who was not involved in the work.

Machanda and colleagues gathered data from the Kibale Chimpanzee Project, which has tracked wild chimpanzee behavior in Uganda’s Kibale National Park since 1987. Because chimps are socially similar to humans—they live in large groups and engage in both cooperative and antagonistic relationships throughout their lives—they serve as an ideal test group for studying changes in social behavior. The researchers zeroed in on the males, who had more purely peer-to-peer relationships than females.

Combing through 21 years of behavioral logs on 21 chimps aged 15 through 58, the researchers found that older males (aged 35 and up) had more mutual friendships than younger ones, they report today in Science. Older “friends” would sit together and groom one another on a regular basis, whereas younger chimps were more likely to engage in one-sided relationships, in which they groomed preferred elders who rarely returned the favor.

That makes sense to Gilby, who suspects that younger males groom older, dominant ones to rise in the group hierarchy. But as males age and fall in rank, they stop competing for dominance and “tend to give up,” he says. Forming these cooperative relationships with peers could help older males maintain their status, helping them fend off challenges by younger and fitter chimps.

The researchers also found that older males had fewer aggressive interactions with other members of the group. “They’re not getting drawn into scuffles all the time, in the way a younger chimpanzee might be,” says Alexandra Rosati, a psychologist at the University of Michigan, Ann Arbor, and an author of the study.

The findings wouldn’t surprise most primatologists, says Gilby, who has observed these types of one-sided and mutual male relationships during field research. But the evidence that we and our closest relatives share a social aging pattern challenges the idea that these behaviors are uniquely human. Rather than being tied to our mortality, they could be an adaptive response that improves the mating success or group rank of older chimps.

Rosati is eager to see whether other chimpanzee groups—and female chimpanzees—also experience this mellowing with age. She says the theory could also be tested in other long-lived social species, like bonobos, elephants, and orcas. Next, however, she and Machanda will take a deeper look at how social bonds might benefit aging chimps—and whether the same mechanisms could be at work in humans. “There is a lot more to learn,” Gilby says.

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