Humans have long believed their ability to maintain a small circle of close friends while vaguely acknowledging the existence of everyone else was a uniquely human trait. A new study suggests we may have to share that distinction with our closest living relatives, who apparently have been doing the same thing with grooming instead of brunch.
Researchers from Utrecht University and Universidad Carlos III de Madrid observed social grooming in 24 groups of chimpanzees and bonobos and found that these great apes organize their relationships in layers, much like humans do. Using a mathematical model, the team analyzed how each ape distributed its limited social effort - grooming time, in this case - among group members. The results showed that most apes devoted the bulk of their grooming to a small number of preferred partners, while maintaining lighter relationships with many others. Apes in larger groups were pickier about who got their grooming attention, a trend also seen in human social networks.
But the two species didn't manage their social lives identically. Bonobos distributed grooming more evenly across their group, creating a more egalitarian social network, while chimpanzees concentrated their efforts on a smaller circle of favorites. The researchers also found that as chimpanzees age, they narrow their social circles - just like humans who decide they've had enough small talk. Bonobos, however, did not show this age-related narrowing, likely due to their more fluid social bonds that can cross group boundaries, something rarely seen in chimpanzees.
Lead author Edwin van Leeuwen noted that the findings suggest deep evolutionary continuity in how complex societies are organized, while the differences between the two species indicate there's more than one evolutionary strategy for managing social connections. Understanding these patterns, he added, could improve scientists' understanding of cooperation, social learning, and emotional well-being in both humans and other animals - essentially confirming that everyone, whether ape or human, has that one friend they'd rather groom than the rest.