What chimps can teach us about changing human friendships

It is one of the hard truths of aging: The older you get, the smaller your social circle becomes.

Compared to teens, older adults' social groups tend to be tightly knit. As we age, we also lose our appetite for drama, instead preferring positive relationships. So the fact that you lost touch with the people in your life who were actually just a bit much is perhaps nothing to be sad about. By our very nature, we whittle our social group down to the folks who are really the most important for our well-being.

This gradual tuning to positive vibes only is inherently human, but it may not be so unique to our species as we think.

In a landmark new study published Thursday in the journal Science, a team of researchers reveal these same trends play out among one of our closest relatives: chimpanzees. The unprecedented insight into the social lives of our closest animal relatives stems from 78,000 hours-worth of video footage, collected over the span of 21 years.

BFFS — Chimpanzees are famous for their similarities to humans, like their ability to use tools. But while social interactions between chimps have been documented before, the researchers here were curious about what the changing dynamics of friendships over time said about chimps' socioemotional selectivity.

"In humans, old age is characterized by increasing selectivity for positive, meaningful social interactions," explain the authors in the study. "[A prevailing theory] argues that the central process generating life-span shifts in sociality is an explicit sense of future personal time and mortality."

Essentially, as we humans age (so the theory goes), we become preoccupied with making the most of the time we have left. And to do that, we cultivate only the social interactions that matter to us most.

By watching chimps' friendships play out over the decades, the researchers hoped to discover clues to this human phenomenon's evolutionary history.

The researchers looked at 78,000 hours of video of 21 male chimpanzees aged between 15 and 58 years old living in Kibale National Park, Uganda. For chimps in the wild, 58 represents a very old age.

On average, the researchers reviewed 10.6 years-worth of data per individual chimpanzee.

The study homes in on a couple of key social actions:

1) Did the chimps' preference for close friends change as they aged?

2) Did the chimps prioritize positive behaviors as they aged?

Because chimpanzees don't express friendship the same way humans do, researchers relied on analogous actions, like being close to the same individual on multiple occasions (within 16.5 feet), or choosing to sit near particular individuals. Positive behavior translated to grooming activities, while negative behaviors included acts of aggression, like hitting or chasing.

The researchers do not account for kin relationships between the chimpanzees. They say they chose not to because the adult males tended to have very few maternal brothers, if any. They also excluded female chimps, because they tend to be less social than males.

What they found — When they compared how these social actions were performed by both younger and older chimps, the researchers saw a striking similarity to what scientists have already observed in humans.

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They found that the number of mutual friends a chimp had increased with age, while one-sided relationships declined. Teenage chimps, for example, had on average 2.1 one-sided friends and 0.9 mutual friends at age 15, while 40-year-old chimps had 0.6 one-sided friends and 3.0 mutual friends on average.

The findings also suggest older chimps are more selective about their friends, choosing to socialize with male chimps their own age as opposed to juvenile or female chimps.

Interestingly, the researchers did not see an overall increase in grooming activity between friends over time, but they did observe declines in aggressive behaviors. The older chimps seemed to prefer calmer, mutually beneficial activities, they say.

What about us — The study suggests chimps exhibit similar trends in socioemotional selectivity to humans. But the reasons why may be quite different.

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"[S]ocial relationships are flexible, can occur outside of kinship, and last many years in long-lived humans and chimpanzees," the study authors write. "Thus, strongly established relationships may be more reliable for older chimpanzees than for other primates."

This is an important distinction. Previous studies suggest other primates display more social withdrawal in old age. The reason why may be because, unlike humans and chimps, their closest relationships are largely based on kinship. After family members die, it may be hard for these species to make new social connections.

To explain this, the researchers behind this study speculate accumulated social knowledge may make older chimpanzees more attractive social partners, or that increased emotional regulation may make them less likely to be aggressive as they age. Essentially: They may have good companionship skills, honed over time.

Ultimately, the findings offer important insights into our own social and emotional bonds, and how these may have evolved among humans and other animals.

"Our findings demonstrate how data from long-lived, socially flexible animals are crucial for disentangling the proximate and ultimate causes of human social aging patterns."

Abstract: 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. 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.