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There are more things that come with old age than graying hair and wrinkled skin. When humans reach their last years, they favor more established friends and their social circle is reduced.
Now, for what appears to be the first time, scientists have seen the same behavior in another species. More than two decades of chimpanzee observations reveal that older males choose to hang out with their longtime friends at the expense of other relationships.
“What we have shown is that chimpanzees and humans share the same pattern of social aging,” said Zarin Machanda, a primatologist at Tufts University in Massachusetts. “We know that as humans age, their social networks shrink, but their social ties strengthen and we see the same thing here in chimpanzees.”
The researchers drew 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 at Kibale National Park in Uganda. They focused on men because they have stronger social ties than women and interact more frequently.
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 and groomed others. They then rated the various pairs as mutual friends, where both chimps seemed to enjoy the relationship; one-sided friendships, where one chimpanzee was more interested in being friends than the other; and not friendships, where neither chimpanzee showed interest in the other.
When the scientists looked at the friendship patterns, they found that older chimps had more mutual friendships and fewer one-sided friendships than younger chimps. For example, a 40-year-old man had three times as many mutual friends and one-third fewer one-sided friendships than a 15-year-old man.
Another trait seen in older humans was also detected in chimpanzees. As the males got older, their levels of aggression decreased, meaning that they started fewer fights and they tended to bully others in their group less often. “They show a shift toward more positive behavior,” Machanda said.
The observations have left researchers baffled. According to an idea in psychology known as the socio-emotional selectivity theory, or SST, older human beings prefer more positive relationships because they are aware that time is running out. But many primatologists argue that chimpanzees lack the human sense of mortality, suggesting that something else is driving the behavior.
According to Machanda, the findings, published in Science, show that a future tense sense is not necessary for social circles to shrink with age, and other factors may support behavior in humans and chimpanzees.
Robin Dunbar, a professor of evolutionary psychology at the University of Oxford, was rather dismissive of the theory of socio-emotional selectivity, saying it sounded like a “naive psycho babble.” “In humans, the decline [in social circles with age] it is due to the decrease in social motivation to go out and meet people combined with the lack of opportunities, “he said.
Given the similar behavior between chimpanzees and humans, Professor Joan Silk, an anthropologist at Arizona State University, said it may have arisen in humans before they developed modern cognitive abilities and the ability to reflect on future events.
As older males compete less for partners, they may focus on close, reciprocal relationships with trusted partners, he said.
Or as Machanda said: “They could be very happy together.”
