Networks of long-distance female friends help gorillas move between groups

Networks of long-distance female friends help gorillas move between groups

“I’m not going if I don't know anyone” -- sound all too familiar? Well it’s not just humans. Socialising in a new group can be tricky. We often rely on word of mouth and friends-of-friends connections. New research on gorillas suggests they may be using a similar strategy: when moving to a new social group, female gorillas seek out groups containing females they have lived with in the past. 

In many animal societies, individuals of one sex, or sometimes both, eventually leave their birth group to join another. In a smaller number of species, including humans and gorillas, individuals can change between groups multiple times. This process, known as dispersal, plays a key role in avoiding inbreeding, spreading gene diversity and shaping social relationships. But how do individuals choose where to go? 

A new study, published in Proceedings of the Royal Society B, cracks this mystery, thanks to more than 20 years of data on multiple groups of wild mountain gorillas, continuously monitored by the Dian Fossey Gorilla Fund in Rwanda. 

The study reveals that females do not disperse randomly. Broad group traits such as group size or sex ratio did not seem to matter, but past social experiences did, with females avoiding males they grew up with, and seeking out females they already knew. 

"Because female mountain gorillas do not know with certainty who their fathers are, they might rely on a simple rule like ‘avoid any group with males I grew up with’ as the likelihood of them being related will be higher than with males they did not grow up with,"said lead author Victoire Martignac, a Ph.D. student from the University of Zurich. 

And this rule is not just based on familiarity but also on its context. "Because females can disperse multiple times, they will become familiar with many males from different groups," explains Martignac, "yet, when choosing their next group, they only avoid males they grew up with. This really tells us that it’s not just who they know that matters but how they know them."

But what seemed to matter even more, was the presence of females they had lived with before, suggesting that these relationships matter even after years apart. 

"Going into a new group could feel pretty scary, with individuals usually entering at the bottom of the social hierarchy. A familiar female might help reduce this, providing a social ally," adds Dr. Robin Morrison, senior author on the study. "It could also act like a recommendation from a friend -- if a female they know has chosen to stay in this group it could indicate positive things about the group as a whole or the dominant male leading that group."

The study also found that it’s not just any familiar individuals, but females that had spent at least 5 years together and those that they had seen in the last two years which were the most influential.

The researchers argue that it is often wrongly assumed that individuals dispersing multiple times gain few benefits from being social as any member of the group might leave at any time.

But investing in these relationships clearly matters. Spatial separation can be ephemeral with individuals being reunited in the future, easing the difficult process of starting over in a new social group.

These findings highlight the deeply social nature of dispersal in gorillas, not only enabling the formation of new relationships, but also facilitating the maintenance of pre-existing ones. 

Crucially, these dispersal patterns, with individuals moving multiple times, combined with the fact that groups often interact and share overlapping ranges, all show that relationships can extend beyond group boundaries.

"This mirrors a key aspect of human societies: the existence of strong ties between different social groups," adds Martignac. "As humans, we’re constantly moving across jobs, cities and social groups. We do it so effortlessly that we forget how unusual this flexibility actually is within the animal kingdom."

"This is a reminder of the meaningfulness of social relationships kept across boundaries and how this extended network of relationships might have played a key role in the evolution of larger and more cooperative societies."

Finally, Dr. Tara Stoinski, president and CEO/chief scientific officer of the Dian Fossey Gorilla Fund and a co-author on the study emphasises that such discoveries are really only possible thanks to long-term data. "Being able to study dispersal, to track not only where individuals are from but also where they go, and to construct their whole social history in such detail, is only possible because of decades of data collection. With just a few years and a few groups, all of these inter-group ties and extended networks would be invisible to us. This really highlights the value of long-term observations on multiple groups in better understanding the evolution of sociality."

The research, conducted by researchers from the University of Zurich and the Dian Fossey Gorilla Fund, was funded by the Swiss National Science Foundation and Dian Fossey Gorilla Fund.