A bold claim about gorilla societies is drawing mixed reviews. Great apes, humans’ closest evolutionary relatives, were thought to lack our social complexity. Chimpanzees, for example, form only small bands that are aggressive toward strangers. But based on years of watching gorillas gather in food-rich forest clearings, a team of scientists has concluded the apes have hierarchical societies similar to those of humans, perhaps to help them exploit rich troves of food.
The finding, reported in the current issue of the Proceedings of the Royal Society B, challenges the prevailing notion that such sophisticated societies evolved relatively recently, after humans split from chimpanzees. Instead, these researchers say, the origins of such social systems extend at least as far back as the common ancestor of humans and gorillas, but were lost in chimpanzees.
The group has presented “a pretty convincing case for a hierarchical social structure in gorillas,” says Richard Connor, a cetacean biologist and expert on dolphin society at the University of Massachusetts in Dartmouth. But because other primates that are not great apes—notably baboons, geladas, and colobine monkeys—show similar hierarchies, he’s not surprised they have turned up in gorillas, too.
Gorillas spend most of their time in dense forests, travel great distances to a new home spot daily, and are slow to get used to observers, making their social lives hard to study. But western gorillas in the Republic of Congo gather periodically at swampy clearings in the forests to feed primarily on the highly abundant vegetation, but also on favorite and rare foods such as certain fig trees that produce massive amounts of fruit only every 3 to 5 years, says Robin Morrison, a zoologist at the University of Cambridge in the United Kingdom and the study’s lead author.
By stationing themselves near the Mbeli Bai clearing in the Republic of Congo’s Nouabalé-Ndoki National Park, she and her colleagues gained an intimate view of gorilla social connections from 2010 to 2015. They added to their observations similar data collected by others in 2001–02 at the Lokoué clearing in Odzala-Kokoua National Park in the Republic of Congo. By analyzing the frequency and duration of social interactions among the hundreds of gorillas that gathered at each site, the scientists discovered a multitiered hierarchy. Family units were nested inside larger social units in a pattern strikingly similar to modern human societies. At both sites, individual gorillas spent time not only with their immediate families, but also with an average of 13 extended family members—for example, cousins, aunts, and grandparents.
Even more surprising, each ape interacted with some 39 other gorillas to whom they weren’t related. Sometimes, younger males gathered in “all-male bachelor groups,” Morrison said in a press statement, comparing the overall gatherings to dynamics in a village. Her team’s analysis revealed that more than 80% of the close associations were between more distantly related—or even unrelated—silverbacks, as dominant male gorillas are called. Gorillas “clearly had preferences,” she said.
“If we think of these associations in a human-centric way, the time spent in each other’s company might be analogous to an old friendship,” she added. The ability to form friendships and cooperate with unrelated individuals is considered integral to the evolution of humans’ “social brains.”
Kim Hill, an evolutionary anthropologist at Arizona State University in Tempe, rejects such parallels to humans. “[T]he extreme social brain hypothesis doesn’t claim other primates don’t form hierarchically increasing groupings,” Hill wrote in an email to Science. “It focuses on the size of the largest human groupings.” Humans recognize and remember details about more than 1000 individuals, he notes, whereas the “highest level groups in the gorillas are not even as big as large chimp communities.” Morrison agrees that ape societies are not comparable to those of humans at the highest social tiers, but she says her group’s discovery reveals that some elements of our multitiered systems are older than previously believed.
Connor, for his part, doubts that foraging drove the emergence of these complex social associations. More likely, he says, these “are based on cooperative defense”—as they are in other primate societies and in dolphins. Morrison says she’ll watch for evidence of that as she continues to monitor the gorilla gatherings.