Each of us is a single node in a branching web of family, friends, and acquaintances. As our ancient ancestors transitioned from small, autonomous groups to a complex network of associations, scientists theorize that we humans, boosted by our social brains, became the only animals capable of creating a multi-tiered society. However, new evidence in Proceedings of the Royal Society B suggests that a close relative of ours exists in a similar system: the gorilla.
In a paper published July 10, an international team of scientists made the case for the existence of hierarchical social modularity (HSM) in gorilla groups. In humans, HSM looks like small units — a family, for instance — nesting within increasingly larger units, like a town, a city, and so on. Until now, researchers argued that this way of living evolved after the chimpanzee-human split and arose uniquely among humans as we evolved to have brains that support advanced social cognition.
But careful field observations of western lowland gorillas, combined with statistical algorithms, suggest that gorilla society contains social layers as well. This, the researchers argue, demonstrates that the ability to form tiered societies stems from before our species became uniquely human.
The study argues that gorillas interact within groups that act as an analogy to early human tribes. Within these groups, there are families and un-related groups who spend a lot of time together. From the lens of human experience, it would be easy (and likely accurate) to call the latter group friends.
Stacy Rosenbaum, Ph.D., is a biological anthropologist at Northwestern University who studies wild populations of a different subspecies called mountain gorillas. Rosenbaum, who was not a part of this research, tells Inverse that this study adds to a growing realization that gorilla social structures are more complex than we previously realized.
"It would almost be more surprising if these were simply random interactions."
“The traditional story about gorillas — that their societies are simply made up of one-male, multi-female units who don’t really have anything to do with one another unless they are actively competing — seems overly simplistic,” Rosenbaum explains.
“Given what we know about how the spatial dynamics of how they disperse from the groups they were born in, as well as how important social relationships and social knowledge are to them, it would almost be more surprising if these were simply random interactions.
This study works off pre-existing datasets from two gorilla research sites in the Republic of the Congo. For six years, scientists documented hundreds of social exchanges between western lowland gorillas at locations known locally as bais — swampy clearings where gorillas gather to feed on mineral- and protein-rich aquatic vegetation.
The simplest way of describing the societal set-up of gorillas is to say there are small family units, consisting of one male and several females, as well as solitary male “bachelors.” However, years spent at the bais revealed previously unrecognized patterns of interactions between groups and individuals. In turn, an analysis of the frequency and length of these interactions revealed the social layers they were looking for.
For example, the researchers saw that beyond a gorilla’s family, there is also a tier of extended family — a group of about 13 gorillas that consists of aunts, grandparents, and cousins. After that is a broader tier of association that includes about 39 gorillas. These were gorillas that spent time together but weren’t necessarily related. Over 80 percent of these “close associations” were between unrelated or distantly related male silverbacks.
Lead author Robin Morrison, Ph.D., a biological anthropologist from the University of Cambridge, says that this tier of gorillas — the 39 member aggregated group — could serve as an analogy to early human populations who existed within a “tribe or small settlement, like a village.”
Some scientists, like Kim Hill, Ph.D., an evolutionary anthropologist at Arizona State, have taken issue with the study’s central claim that “these results indicate that the hierarchal social organization observed in humans may have evolved far earlier than previously asserted and may not have be a product of the social brain evolution to the hominin lineage.”
Hill tells Science that “the extreme social brain hypothesis doesn’t claim other primates don’t form hierarchically increasing groupings,” but instead focuses on the human capability of creating large thousand-person plus groups.
In response, Morrison agreed that ape societies aren’t comparable to humans at the highest social tiers — but she and her team maintain that their discovery shows that multi-tiered social groups existed before humans became uniquely human:
Given the likely presence of HSM in both humans and gorillas, and its relatively rare occurrence across mammalian species, our results suggest it is more parsimonious to assume that HSM evolved in a common ancestor of gorillas and humans and was lost in chimpanzees, rather than evolving independently in both lineages.
And while evolving social capabilities have long been seen as a driver of social complexity, the team offers a possible alternative mechanism: collaborative foraging. Going to the swamps and seeking out the same food likely led to the formation and maintenance of communicative relationships between gorillas. Ultimately, they theorize, the cooperation and collective memory it takes to forage successfully led to the relationships they saw in the wild.
It also stands to reason that the gorillas like to be around these other gorillas when meeting up for some grub.
“Wouldn’t you rather go hang out at the bar or the park down the street when you know a lot of your acquaintances are going to be around, than when a bunch of strangers are?” asks Rosenbaum. “They probably would too! [The study] has interesting implications for the way we think about gorillas’ social lives, and for understanding how social dynamics work on different scales.”
Modern human societies show hierarchical social modularity (HSM) in which lower-order social units like nuclear families are nested inside increasingly larger units. It has been argued that this HSM evolved independently and after the chimpanzee–human split due to greater recognition of, and bonding between, dispersed kin. We used network modularity analysis and hierarchical clustering to quantify community structure within two western lowland gorilla populations. In both communities, we detected two hierarchically nested tiers of social structure which have not been previously quantified. Both tiers map closely to human social tiers. Genetic data from one population suggested that, as in humans, social unit membership was kin structured. The sizes of gorilla social units also showed the kind of consistent scaling ratio between social tiers observed in humans, baboons, toothed whales, and elephants. These results indicate that the hierarchical social organization observed in humans may have evolved far earlier than previously asserted and may not be a product of the social brain evolution unique to the hominin lineage.