Aging chimps show social selectivity

No new friends and no drama.

When humans age, they tend to favor small circles of meaningful, established friendships rather than seek new ones, and to lean toward positive relationships rather than ones that bring tension or conflict. These behaviors were thought to be unique to humans but it turns out chimpanzees, one of our closest living relatives, have these traits, too. Understanding why can help scientists gain a better picture of what healthy aging should look like and what triggers this social change.

The work is described in the Oct. 23 issue of the journal Science and is authored by a team of psychologists and primatologists, including current and former researchers from the Harvard Department of Human Evolutionary Biology.

The study draws on 78,000 hours of observations, made between 1995 and 2016. It looked at the social interactions of 21 male chimpanzees between 15 and 58 years old in the Kibale National Park in Uganda. It shows what’s believed to be the first evidence of nonhuman animals deliberately selecting who they socialize with during aging.

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The researchers looked only at male chimpanzees because they show stronger social bonds and have more frequent social interactions than female chimps. Analyzing a trove of data, the researchers saw that the chimpanzees displayed much of the same behavior aging humans exhibit.

The older chimpanzees they studied, for instance, had on average more mutual friendships while younger chimps had more one-sided relationships. Mutual friendships are characterized by behavior such as reciprocated grooming whereas in lopsided friendships grooming isn’t always returned.

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Older males were also more likely to spend more time alone and showed a preference for interacting with — and grooming — chimps they deemed to be more important social partners, like other aging chimps or their mutual friends. And like older humans looking for some peace and quiet, the chimpanzees showed a shift from negative to more positive interactions as they reached their twilight years. The preference is known as a positivity bias.

Harvard Gazette

Journal Reference

Social selectivity in aging wild chimpanzees

Humans prioritize close, positive relationships during aging, and socioemotional selectivity theory proposes that this shift causally depends on capacities for thinking about personal future time horizons. To examine this theory, we tested for key elements of human social aging in longitudinal data on wild chimpanzees.

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Aging male chimpanzees have more mutual friendships characterized by high, equitable investment, whereas younger males have more one-sided relationships. Older males are more likely to be alone, but they also socialize more with important social partners.

Further, males show a relative shift from more agonistic interactions to more positive, affiliative interactions over their life span. Our findings indicate that social selectivity can emerge in the absence of complex future-oriented cognition, and they provide an evolutionary context for patterns of social aging in humans.

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