It comes with age more than graying and wrinkled skin. When humans reach their final years, they prefer more established friends and their social circle shrinks.
Now, in what appears to be the first time, scientists have seen the same behavior in other species. More than two decades of observations of chimpanzees revealed that older males choose to hang out with their long-term friends at the expense of other relationships.
“What we have shown is that chimpanzees and humans share the same pattern of social aging,” said Zarine Machanda, a primatologist at Tufts University in Massachusetts. “We know that as humans age, their social networks shrink but their social bonds get stronger, and we see the same thing here in chimpanzees.”
The researchers relied on 78,000 hours of observations made between 1995 and 2016 that followed the social interactions of 21 male chimpanzees between the ages of 15 and 58 in Kibale National Park in Uganda. Focus on males because they have stronger social bonds than females and interact a lot.
Working with Alexandra Rosati, of the University of Michigan in Ann Arbor, and others, Machanda classified chimpanzee relationships based on the amount of time they sat with others and groomed them. Then they classified the different pairs as mutual friendships, as both chimps seemed to enjoy the relationship; One-sided friendships, as one chimpanzee was more keen to be a friend than the other; And change friendships, as chimpanzees show no interest in others.
When scientists looked at friendship patterns, they found that older chimpanzees had more mutual friendships and fewer one-sided friendships compared to younger chimps. For example, a 40-year-old man had three times as many mutual friendships and one-sided friendships a third less than a 15-year-old man.
Another trait has also been seen in older humans in chimpanzees. As males get older, their levels of aggression have decreased, which means they start fewer fights and tend to intimidate others in their group less. “They showed a shift towards more positive behavior,” Machanda said.
The observations left the researchers baffled. According to an idea in psychology known as the socio-emotional selective theory, or SST, older adults prefer more positive relationships because they realize that time is running out. But many primatologists argue that chimpanzees lack a human sense of annihilation, indicating that something else is driving the behavior.
According to Machanda, the results – published in the journal Science – show that a sense of future time is not necessary for social circles to shrink with age, and that other factors may support behavior in humans and chimpanzees.
Robin Dunbar, professor of developmental psychology at the University of Oxford, was somewhat dismissive of the theory of social and emotional selectivity, saying it appeared to be a “naive psychic predisposition”. “In humans, the decline [in social circles with age] This is due to the decreased social drive to go out and meet people, accompanied by a lack of opportunities.
Looking at similar behavior between chimpanzees and humans, Professor Joan Silk, an anthropologist at Arizona State University, said it may have originated in humans before they developed modern cognitive skills and the ability to think about future events.
Because older males compete for mates, she said, they may focus on close and mutual relationships with trusted partners.
Or as Machanda said: “They can be very happy together.”