Gorillas have complex relationships and social tiers, a new study reports. The system bears striking similarities to human society.

Gorillas resemble us in more than one way.

The way human society is arranged is pretty neat: it starts with a nuclear group of our closest family and friends, which is nested in increasingly larger units. We don’t exactly know when and how humans transitioned from small and autonomous groups to increasingly larger and tiered social systems, but it is a key part of what enabled us to thrive as a species.

But this system might not be unique to us among primates: a new study also reports that gorillas share a similar system.

Gorillas are not easy to study. Not only do they live in inaccessible areas, but they also tend to avoid humans without previous habituation. This study used over six years of data from two research sites in the Republic of Congo, where scientists documented the social exchanges of hundreds of western lowland gorillas.

“Studying the social lives of gorillas can be tricky,” said lead author Dr Robin Morrison, from the Department of Archaeology, University of Cambridge. “Gorillas spend most of their time in dense forest, and it can take years for them to habituate to humans.”

“Where forests open up into swampy clearings, gorillas gather to feed on the aquatic vegetation. Research teams set up monitoring platforms by these clearings and record the lives of gorillas from dawn to dusk over many years.”

Gorillas live in family groups, but these groups are very different from what we humans have. Typically, a group consists of a dominant male, a few females, and offspring. Meanwhile, the other males live as solitary “bachelors”. But there’s more to the story than that.

After the immediate family, there’s an extended group with which gorillas interact regularly — this group features 13 gorillas on average. Beyond this, there’s a further tier, which averages 39 gorillas and also features regular (though rarer) interactions. There’s also a different type of group formed by male gorillas who are old enough to leave their group but not old enough to fully care for themselves. They form an all-male group to help them cope.

Does all this sound familiar? That’s because it’s a lot like what we humans do.

“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 said.

The similarities run even deeper. Not only did the team find permanent relationships, they also found periodic interactions, similar to annual gatherings or festivals. For gorillas, these seem to be based around fruiting events (although they are a bit too infrequent to draw definite conclusions from them).

This could also offer new insight regarding the evolution of this type of behavior. Humans (and primates, for that matter) are not the only ones to employ this type of hierarchy.  A small number of mammal species have been found to have similar structures, and these are typically the species relying on “idiosyncratic” food sources — such as elephants looking for irregular fruitings or dolphins hunting for mercurial fish schools. Furthermore, all of them have well-developed spatial memory centers, much like humans do.

However, our closest relatives, chimpanzees, have a very different social structure: they live in small territorial groups with fluctuating and aggressive alliances. The findings suggest that either the behavior evolved independently in humans and gorillas or, more likely, it stretches down to the common ancestors of humans and gorillas

The findings suggest that the origins of our own social systems stretch back to the common ancestor of humans and gorillas, rather than arising from the “social brain” of hominins after diverging from other primates, say researchers.

“While primate societies vary a lot between species, we can now see an underlying structure in gorillas that was likely present before our species diverged, one that fits surprisingly well as a model for human social evolution.”

“Our findings provide yet more evidence that these endangered animals are deeply intelligent and sophisticated, and that we humans are perhaps not quite as special as we might like to think,” concludes Morrison.

The study “Hierarchical social modularity in gorillas” was published in Proceedings of Royal Society B.