The social structure of chimpanzee communities is highly complex: there are alpha males and mother-daughter pairs, coalitions that form and fracture, and a strong hierarchical dominance structure. Clans ranging anywhere from a dozen to closer to 100 members, with chimps drifting in and out of these larger communities.
Among male chimpanzees especially, says the Jane Goodall Institute of Canada, a linear, ordered hierarchy determines who's the boss of who. Yet cutting through some of the confusion, says Ewen Callaway for New Scientist, is the undying power of the bromance: “A decade-long study shows that nearly all adult male chimps form enduring social bonds with other males, exchanging back scratches, sharing meat, and generally chumming around.”
Exactly why chimpanzees form these stable bonds is unknown, Mitani says. It could be that having a best friend boosts reproductive success or survival somehow. But this will require "staying out there to see who does what with whom, and how often, and counting up the babies," he says.
...As with human friendship, the strongest bonds seemed to be based on mutual respect. Chimpanzees that groomed each other for roughly equal amounts of times tended to stay friends longer.
The power of chimp friendship isn't quite enough to sever the otherwise strict social orders, though, says Mitani in his study: chimps born to the same mother, or those of the same standing on the social ladder, were more likely to become friends.