A new study done in Rwanda's Volcanoes National Park has found that female mountain gorillas often choose to join groups where they know someone, especially other females they lived with in the past, even if they haven’t seen them for many years.

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Published in the UK Royal Society Journal Proceedings B on August 6, the study shows that gorillas, one of the human's closest relatives, have deep emotional connections and long memories.

The study based on over 20 years of research. Scientists followed 56 female gorillas and recorded 152 times when they left one group to join another.

The study reports that nearly 34 per cent of the moves were into groups where at least one familiar female from their birth group was present, and around 39 per cent involved a familiar female they had lived with before, even if not from the same birth group.

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"Females prioritized moving into groups containing familiar females, especially those from their natal group,” wrote lead researcher Victoire Martignac from the University of Zurich and her co-authors, who include two Rwandan researchers, Jean Pierre Mucyo, Felix Ndagijimana, who work with the Dian Fossey Gorilla Fund in Musanze District.

Inbreeding avoidance

Interestingly, the female gorillas avoided groups with males from their birth group. Only 15 per cent of the moves were into such groups.

The researchers used two different methods to test what influenced the females’ decisions. Both methods showed that having familiar females in a group, especially from their birth group, was a big reason to move there.

"Numerous driving forces have been identified, including kin and resource competition, environmental stochasticity and inbreeding avoidance,” reads the study report.

One of the models found that even when they considered how close the groups were, how many males or females were in them, or how big they were, females still preferred groups with familiar females. On the other hand, if a group had males from their birth group, it made the gorillas less likely to join.

The study also looked at how often gorillas move together. In about 34 per cent of the moves, two or more females left at the same time. Of those, 84 per cent moved with familiar females from other groups, while 28 per cent moved with familiar females from their birth group.

This means gorillas might plan to move with friends to stay close to them.

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Time also mattered. If two gorillas had lived together in the past two years or had lived together for five to ten years in the past, they were much more likely to reunite.

As the researchers put it, "co‑residency within 2 years positively affected dispersal, with a stronger influence from natal females.”

"These results show that females not only remember their past social companions, but also preferentially relocate toward them—even after substantial time apart, ” Martignac explained, adding this likely helps them adjust more easily to new groups and avoid stress.

This is important because moving into new groups helps gorillas spread genes, share behaviors, and build strong populations. Having familiar faces around may make this easier and safer.

The study also gives us clues about early human society. Gorillas have some of the same social patterns that our ancestors did, like males and females both moving between groups and groups living near each other.

Friendships across groups may have helped early humans share knowledge, cooperate, and move around safely.

There are more than 1,000 in the Virunga Massif, which is shared by Rwanda, DR Congo and Uganda. About a third of the gorillas live in the Volcanoes National Park on the Rwandan side.