Female gorillas rely on old friends to choose new homes

Anyone who has ever scanned a party guest list for a familiar face will sympathize with female mountain gorillas. When it comes time to leave home and join another troop, they too prefer a little social security.

A new study shows that dispersing females routinely pick groups that contain females they have lived with before, while steering clear of males from their childhood homes.

The findings emerged from more than two decades of near-daily monitoring by the Dian Fossey Gorilla Fund in Rwanda’s Volcanoes National Park.

By piecing together the life histories of dozens of animals across many overlapping groups, researchers from the University of Zurich, the Fossey Fund, and collaborating institutions were able to ask a deceptively simple question: How do females decide where to go?

Female gorillas avoid familiar males

Early analyses revealed what did not matter. For instance, widespread group traits such as group size or sex ratio did not seem to matter.

Instead, past social experience loomed large. Females almost never joined a group that housed males they had grown up with.

“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 at the University of Zurich.

That rule is context-dependent, not merely a function of familiarity. Female gorillas may encounter many males on the forested slopes as they move between troops, yet they single out those childhood companions for avoidance.

“This really tells us that it’s not just who they know that matters but how they know them,” Martignac said.

Old friends as social insurance

Far more attractive than familiar males were familiar females. The study found that the presence of even one former denmate dramatically increased the odds that a female would choose a particular destination.

The effect was strongest when the pair had shared at least five years in the same troop and had seen each other within the last two. Those long-standing bonds appear to buffer the risks of immigration.

“Going into a new group could feel pretty scary, with individuals usually entering at the bottom of the social hierarchy,” said Robin Morrison, the senior author on the study. “A familiar female might help reduce this, providing a social ally.”

“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.”

These results challenge the assumption that multiple dispersals undermine the value of social investment. Instead, they reveal a network of relationships that stretches well beyond any single troop. Females may spend years apart, yet the memory of past cooperation still shapes their future moves.

Gorilla behavior mirrors humans

The parallels with our own species are hard to miss. “This mirrors a key aspect of human societies: the existence of strong ties between different social groups,” Martignac noted.

Humans hop between schools, jobs and cities, leaning on old friends for introductions and support. Mountain gorillas, it seems, deploy a comparable strategy as they navigate a landscape of fluid fission-fusion societies.

Such flexibility brings evolutionary payoffs. By avoiding males from their natal group, females limit the risk of inbreeding and spread genetic diversity across the Virunga Massif.

By following trusted female acquaintances, they ease the social costs of joining a new hierarchy, increasing their chances of reproductive success. In turn, those decisions influence the genetic and social architecture of the entire population.

Tracking lives over decades

Untangling these threads required patience. “Being 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,” said Tara Stoinski, the president and chief scientific officer of the Dian Fossey Gorilla Fund.

“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 Fossey Fund’s continuous presence allowed researchers to code every male and female gorilla ever met, how long they co-resided, and when they last saw each other.

The resulting dataset revealed the subtle calculations behind each female’s leap into the unknown.

Gorilla behavior guides protection

Understanding how gorillas choose new homes matters for their conservation. As habitat fragments and human pressures mount, facilitating safe dispersal corridors could help maintain the social and genetic fabric that keeps populations viable.

More broadly, the study invites scientists to rethink dispersal as a deeply social process – one that both dissolves and forges bonds, shaping animal societies in ways that resonate with our own.

In other words, when a female gorilla trudges across a lava flow to join strangers in the next valley, she is not rolling the dice.

She is following the whisper of memory, looking for an old friend in an unfamiliar crowd – proof that friendship can span forests, decades, and even species.

The study is published in the journal Proceedings of the Royal Society B.

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