chimpanzee
© John Lower/Harvard University/PA Science editor
Three male chimpanzees grooming together.
There is more that comes with older age than greying hair and wrinkled skin. When humans reach their later years, they favour more established friends and their social circle is pared down.

Now, for what appears to be the first time, scientists have seen the same behaviour in another species. More than two decades of observations of chimpanzees reveal that older males choose to hang out with their long-term friends at the expense of other relationships.

"What we've shown is that chimpanzees and humans share the same pattern of social ageing," said Zarin Machanda, a primatologist at Tufts University in Massachusetts. "We know that as humans age, their social networks shrink but their social bonds grow stronger, 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 years old in Kibale national park in Uganda. They focused on males because they have stronger social bonds than females and interact more often.

Working with Alexandra Rosati, of the University of Michigan in Ann Arbor, and others, Machanda classified the chimps' relationships depending on the amount of time they sat with others and groomed them. They then rated the various pairings as mutual friendships, where both chimps seemed to enjoy the relationship; one-sided friendships, where one chimp was more keen to be friends than the other; and non-friendships, where neither chimp showed interest in the other.

When the scientists looked at the patterns of friendships, they found that the older chimps had more mutual friendships and fewer one-sided friendships than younger chimps. For example, one 40-year-old male had three times as many mutual friendships and a third fewer one-sided friendships than a 15-year-old male.

Another trait seen in older humans was also spotted in the chimps. As the males got older, their levels of aggression tailed off, meaning they started fewer fights and tended to intimidate others in their group less often. "They show a shift towards more positive behaviour," Machanda said.

The observations have left the researchers puzzled. According to an idea in psychology known as socioemotional selectivity theory, or SST, older humans prefer more positive relationships because they are aware time is running out. But many primatologists argue that chimpanzees lack the human sense of mortality, suggesting something else is driving the behaviour.

According to Machanda, the findings - published in Science - show that a sense of future time is not necessary for social circles to shrink with age, and other factors may underpin the behaviour in humans and chimps.

Robin Dunbar, a professor of evolutionary psychology at the University of Oxford, was rather dismissive of socioemotional selectivity theory, saying it sounded like "naive psychobabble". "In humans, the decline [in social circles with age] is due to declining social motivation to get out and meet people combined by lack of opportunity," he said.

Given the similar behaviour between chimps and humans, Prof Joan Silk, an anthropologist at Arizona State University, said it may have arisen in humans before they evolved modern cognitive skills and the ability to mull over future events.

As older males compete less for mates, they may focus on close, reciprocal relationships with trusted partners, she said.

Or as Machanda put it: "They could just be very happy together."