Chimpanzees and bonobos structure their social relationships in similar ways to humans, according to a new international study led by researchers from Utrecht University and Universidad Carlos III de Madrid. By analyzing social grooming, the team found that both species have human-like social circles.
The researchers were also able to detect differences between the two great ape species: while chimpanzees turn out to be more selective, and increasingly so with age, bonobos are more egalitarian. The study is published in the journal iScience.
Humans tend to invest their time and energy in predictable, increasingly wide "circles of friends." Generally, humans have an inner circle of close friends and family, followed by more peripheral circles of friends, distant friends, acquaintances and (almost) strangers. Whether this pattern is uniquely human or shared with other primates has long remained unclear.
Social grooming
Lead author Edwin van Leeuwen and colleagues analyzed social grooming, one of the primary social currencies in great apes, across 24 groups of chimpanzees and bonobos. The team then used a mathematical model to investigate how apes distribute their limited social resources across group members.
The findings show that most apes concentrated intense grooming on a few partners while maintaining looser ties with many others, mirroring human friendship layers. And like in human social networks, apes were more selective toward group members in larger groups.
Bonobos were the more egalitarian of the two species, as they divided their grooming time more evenly across group members. Chimpanzees, on the other hand, invested more time in fewer partners.
More selective with age
When people get older, their inner circle becomes smaller, as they interact more intensely with fewer other people. The new results show that chimpanzees also become more selective with age. Bonobos, however, did not show this age-related narrowing of their inner circle.
"Possibly, this is due to their more egalitarian social systems. Bonobos appear to live together in more fluid relationships, with social bonds that transcend group boundaries, something we rarely see in chimpanzees," says Van Leeuwen.
Evolutionary pathways
"Our findings suggest that the fundamental rules that guide how individuals allocate social effort apply across multiple species," said Van Leeuwen. "This reveals deep evolutionary continuity in how complex societies are organized."
At the same time, the differences between chimpanzees and bonobos reveal that there are multiple evolutionary pathways to managing social relationships.
Van Leeuwen added, "Understanding these patterns may reveal crucial insights for studying cooperation, social learning, and emotional well-being in both humans and other animals."
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