Mountain Gorillas Are Friendly with Some Neighbors

Mountain gorillas live in small, close groups. They sleep, forage, and hang out together in a core home range and a larger peripheral range. They are sociable and polite to their neighbors – as long as they stay out of their closest territory, according to a new study.

The research, by the Dian Fossey Gorilla Fund and the University of Exeter, found that these groups sometimes break up, splitting up gorillas that may have lived together for many years and might be closely related. If these gorillas meet up again, they’re four times more likely to be friendly to each other, even if it’s been a decade since they split up, the researchers found.

But that friendliness ends if other gorillas venture into their core territory, even if the interlopers are familiar. Outside the core territory in the periphery, the gorillas act aggressively only with intruders they don’t know. They’re much more tolerant of familiar neighbors in those areas.

“When gorillas come across another group these encounters are usually quite tense to begin with, often involving the dominant males of each group chest beating, hitting the ground or pushing over branches to display their strength. After this initial period of wariness the two groups may part ways or the encounter could become affiliative with groups intermingling and youngsters playing with each other or the encounter could escalate to violence,” lead author Robin Morrison, of the Gorilla Fund and Exeter's Centre for Research in Animal Behaviour, tells Treehugger.

“When groups become violent this can involve pushing, hitting, biting and often a lot of screaming from different group members. Wounds inflicted during these encounters can even be life threatening.”

In the study, researchers found that whether or not these encounters became violent depended on where the encounter took place and the familiarity between the groups. In core regions of the home range, about 40% of the encounters became violent. 

Similarly, in broader-based peripheral regions, around 40% became violent when the groups were unfamiliar with each other. However, in groups that had grown up together but then split apart, only about 20% of those meetings became violent.

“This suggests that gorilla groups may be using physical aggression, defending the entirety of their home range against unfamiliar groups, but only the core area of their home range against familiar groups with whom they are more tolerant,” Morrison says.

For the study, researchers monitored 17 groups of mountain gorillas between 2003 and 2018 in the Volcanoes National Park in Rwanda. They observed 443 encounters during that time. The results of their research were published in the Journal of Animal Ecology.

Cooperation and Relationships

Gorillas live in groups of about eight, Morrison says, although some groups can be as large as 65 or as small as just two. Most groups have a single dominant adult male, several adult females, and their offspring. However, about half of mountain gorilla groups have more than one adult male. In those groups, one male sires most of the offspring.

About half of offspring leave the group when they reach sexual maturity. Males stay solo until they’re able to attract females to create a group, while females either directly join another group or join a lone male to establish a new group.

“Past research has shown that if a group encounters a solitary male they are very likely to become aggressive, more so than if they encountered another group,” Morrison says. “Our paper also suggests that if they encounter another group that is unfamiliar they are more likely to become aggressive than groups with which they are more familiar.”

The researchers point out that humans have the ability to cooperate based on friendship beyond our closest groups. The study tests the theory that shared access to resources and space benefits these friendships and reduces the risk of competition and aggression.

“The key parallel here is that these social relationships are maintained over many years even when gorillas are no longer living in the same group. These long-term relationships are a core component of human society, so investigating the benefits they provide in a closely related species can help us understand how they may have evolved,” Morrison says.

“In humans we know that our social relationships can have really important consequences for how we share space. We tolerate a stranger on the street but not within our house and we might happily have a friend over for dinner but be offended if they start poking around our bedroom. We see similar patterns going on here within the gorillas where familiar groups are ‘allowed’ within the peripheral home range but not within the core.”