When it comes to friendships, people are known to become more selective with age. It turns out that the same is true of male chimpanzees, who have fewer yet more genuine pals to ape around with as they get older, a study published in the journal Science on Thursday showed.
The research, which was led by animal psychologist Alexandra Rosati, an assistant professor at the University of Michigan, was said to provide the first evidence that animals exhibit age-related social selectivity and could help researchers understand more about why humans behave as they do.
The authors drew on a huge dataset of 78,000 hours of observations performed at the Kibale National Park in Uganda between 1995 and 2016. They reported on the social interactions among 21 wild male chimpanzees between the age of 15 and 58.
Males were chosen because they remain in the communities in which they are born, whereas the majority of females disperse to new groups when they become sexually mature.
Aside from being our closest animal relatives, chimpanzees are an ideal species to compare to humans because they live long lives — sometimes until their 60s — and have a wide degree of choice in whom they befriend.
Male friends groom one another, hunt and share meat, collectively patrol the boundaries of their territories, and form alliances to attain and keep high rank in their groups, which in turn leads to more individual reproductive success.
To study their affinities for one another, the team developed an “association index” that was based on how often a male within a party was in close proximity (under 5m) to another, relative to how much they associated with all group members.
The team produced three categories: mutual friends, who both showed a preference for sitting with each other; one-sided friends where only one party showed a preference, but the other did not; and non-friends where neither side preferred each other.
Older males had significantly more genuine friendships than their younger counterparts, whose attempts at bromance were more often lopsided, the study found. For example, a 40-year-old male had on average three times as many high-quality friendships and one-third as many asymmetric friendships as a 15-year-old.
Males over the age of 35 were also found to selectively groom their genuine friends, while the chimpanzees engaged in less aggressive behavior toward others in their party — such as hitting, biting, chasing or hostile displays — as they got older.
Finally, while the older chimpanzees spent more time by themselves than the younger ones, when they did choose to mingle, it was with important social partners.
In a related editorial, Joan Silk, a professor at Arizona State University, said that the paper had provided “convincing evidence that male chimpanzees behave much like humans do as we age, and this pattern might exist in other primates as well.”
It had been theorized that, as people age, they focus on their oldest and most important friends rather than looking for new ones because of their approaching sense of mortality.
However, as chimps are probably not capable of such perspective, Rosati and her colleagues suggested that other factors might be at play, like a reduction in emotional reactivity.
Strong friendships might also help aging chimps continue to thrive despite a decline in their health and social status, they added.
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