Nature

Scientists find a striking similarity between the friendships of gorillas and humans

Gorillas living in larger groups might have more friends to choose from, but new research suggests at a certain point, they max out on close relationships.

That’s eerily similar to what is thought to happen in our own species, where a link between brain size and group size has led to the famous idea that we can only maintain about 150 stable friendships – this is known as ‘Dunbar’s number’, after British evolutionary psychologist Robin Dunbar.

Still, just because a group gets larger, doesn’t mean the relationships within it become stronger or grow more complex.

Judging by a mix of brain size and time available each day to maintain close social bonding, non-human primates are thought to sustain about 50 stable friendships, and yet a new study on gorillas in Rwanda finds that beyond a typical group of 12 to 20 individuals, social lives don’t become more complex.

While scientists aren’t sure why this pattern exists, they think it might have to do with the time and effort it takes primates to maintain a strong social circle.

“[O]ur study suggests that social diversity is lower in very large groups where gorillas must maintain a larger number of relationships – with most relationships falling into the weakest category,” says anthropologist Robin Morrison, who works at the Fossey Fund and the University of Exeter in the UK.

It’s often assumed that the bigger a population, the more diverse and complex an animal’s social life will be. In fact, according to Dunbar’s social brain hypothesis, this is why primate brains are so big – historically, we needed more mental power to develop and maintain our growing social circles.

But the link between a group’s size and its complexity may not be so straightforward.

Drawing on a dozen years of data from 13 gorilla groups in Rwanda, including over 150 individuals, researchers tracked how much time these gorillas spent moving, feeding, and nesting with one another.

“In many primates, social interaction can be measured by how much time individuals spend grooming each other,” explains Morrison.

“However, gorillas spend less time grooming than most other primates. Instead, a lot of gorilla society is about who individuals choose to sit next to, and who they move away from.”

This is known as ‘proximity data’, and collecting it over several years, the authors found group size was a poor proxy for relationship diversity.

In short, they explain, “social complexity measured at the group level may not represent the social complexity experienced by individuals in those groups.”

Similar to humans, this suggests that gorillas have a finite number of close friends they can manage, maintaining only weak ties with the rest. What’s more, just like our own species, some gorillas are better at socialising than others.

“Not only were groups above a certain size not more socially diverse, but individuals living in the same group had variable levels of social complexity – some gorillas had a greater diversity of social relationships than others,” says biologist Lauren Brent who studies the evolution of sociality at the University of Exeter.

“This adds to a rich body of evidence that shows that, whether you are a human, gorilla or another type of social animal, not everyone experiences their social world in the same way.”

Growing up, for instance, male and female gorillas maintained similar diverse relationships, but as they got older, the two sexes began to split.

On the one hand, female gorillas were found to maintain a relatively constant diversity of relationships throughout their lives, while males tended to cut ties with more of their peers in adolescence (potentially as a way to distance themselves ahead of setting off on their own), before returning to a more social role later in life.

If researchers were just examining group size, however, these subtle complexities would have never come to light. While the number of gorillas in a group can tell us roughly how many encounters happen on a regular basis, that’s just one small factor of a gorilla’s social life.

“Studies relying solely on group size as a measure of social complexity may therefore be limited, especially when it comes to understanding the cognitive demands experienced by individuals,” the authors conclude.

“Conversely, this also implies that the diversity of social relationships alone may not fully describe social complexity.” 

If we truly want to understand the complex scope of primate relationships, we need to start using a more comprehensive approach. Simply comparing brain size and group size isn’t enough.

“Long-term monitoring and protection of endangered mountain gorillas is crucial,” argues Tara Stoinski, President and CEO of the Dian Fossey Gorilla Fund, “not only for their conservation but also for what we can learn from this intelligent and highly social species about how complex social behaviour, such as our own, has evolved.”

The study was published in Proceedings of the Royal Society B.

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