Chimpanzees and bonobos, our closest living relatives, organize their social lives in ways strikingly similar to humans, according to a new international study led by scientists from Utrecht University and Universidad Carlos III de Madrid.
The research, published in iScience, found that both species have layered “circles of friends,” much like humans do.
People tend to divide their social connections into levels—an inner circle of close friends and family, followed by wider circles of acquaintances and more distant relationships.
Whether this kind of structured social life is unique to humans or shared with other primates has long been debated.
Now, researchers have found that great apes follow similar patterns, using social grooming as their version of human social bonding.
Social grooming—when one ape carefully cleans another’s fur—is one of the most important ways apes form and maintain friendships.
Lead author Edwin van Leeuwen and his colleagues analyzed grooming patterns in 24 groups of chimpanzees and bonobos living in zoos and sanctuaries around the world.
Using mathematical models, the team examined how each individual distributed its time and energy among other group members.
The results revealed a familiar structure.
Both chimpanzees and bonobos concentrated their most intense grooming efforts on a few close partners while keeping looser connections with many others—mirroring how humans maintain a few best friends and a broader network of casual relationships.
The researchers also noticed that apes living in larger groups were more selective about whom they groomed, just as people with many social contacts often prioritize time with their closest friends.
But there were clear differences between the two species. Bonobos were more egalitarian, spreading their grooming time more evenly across their groups.
Chimpanzees, by contrast, invested much more in a smaller number of partners, forming tighter inner circles.
Interestingly, the study also found that chimpanzees become more selective with age, focusing their social energy on a few long-term companions—just as humans often do later in life. Bonobos, however, did not show this age-related narrowing of friendships.
Van Leeuwen suggests this might reflect bonobos’ more fluid and inclusive social systems, in which bonds often extend beyond group boundaries—something rarely seen in chimpanzees.
“Our findings show that the same basic rules of social life—how individuals balance their time and energy between friends—apply not just to humans, but to our closest relatives,” Van Leeuwen said. “It highlights a deep evolutionary connection in how complex societies are organized.”
At the same time, the differences between chimpanzees and bonobos reveal that evolution can take multiple paths toward social success. Understanding these patterns, the researchers say, may help scientists uncover how cooperation, emotional health, and social intelligence evolved in both humans and other animals.
