Chimpanzees and bonobos are our closest relatives, and gorillas are only a little further away. But when it comes to violence, there are clear differences between the species. While bonobos are known to live in fairly harmonious societies, the same cannot be said either for us or for chimps.

Humans and chimps are both known to actively seek out and attack others. Our propensity for violence is believed to be part of our evolutionary story.

But what of the final member of the group? The conventional view is that gorillas, like bonobos, rarely turn violent. A male gorilla will commit the occasional act of infanticide, especially if he is overthrowing another male. But these rare attacks aside, gorillas are thought to be peaceable.

Now a surprising series of observations of mountain gorillas in Africa's Virunga Mountains, published in the journal Scientific Reports, has overturned this view of gorillas.

Groups of males and females have been observed attacking other gorillas on three separate occasions in 2004, 2010 and 2013. The attacks took place in the same population first made famous by primatologist Dian Fossey, who introduced gorillas to the world and revealed their seemingly peaceable nature.

In the first observed attack, a solitary male named Inshuti approached a group of 26 individuals, but was chased off by three males. When the males caught up with Inshuti, they pinned him to the ground and started to attack. The rest of the group soon followed and the mob started to bite, kick and pull out his hair.

"The alpha male repeatedly sank his teeth into his body and shook his head back and forth, similar to a canid shaking prey," the authors report.

I'm not sure I would have been that much more surprised if they'd suddenly decided to fly

Stacy Rosenbaum of the University of Chicago in Illinois witnessed the attack. She says she was stunned.

"I had seen several reasonably aggressive intergroup interactions, as well as the more typical type where no one gets hurt," she says. "This incident started out that way, so I assumed I was recording just another species-typical interaction."

Then, Rosenbaum says, it was as if a switch was flipped. "They became these other entities that I didn't recognise at all. I'm not sure I would have been that much more surprised if they'd suddenly decided to fly."

The attack stopped about four minutes later and Inshuti, though injured, escaped. Rosenbaum says there is no doubt he could have been killed, but the intention behind the attack was unclear. "Maybe his survival just speaks to his incredible resilience," she says: Inshuti has survived plenty over the years.

In 2010 a separate and even larger group of 42 gorillas attacked an unknown lone male. Again he was hit, kicked and dragged. But this time the attack lasted much longer: it went on for 18 minutes until the victim managed to flee.

The entire group participated in the attacks, including females and juveniles

Almost two weeks later the body of an adult male was discovered dead, with injury marks consistent with such an attack.

The final attack, in 2013, was by a small group of nine individuals who again attacked Inshuti, the same male attacked in 2004. The group also included some of the same gorillas, who had gone on to form their own group. This time Inshuti managed to escape after about a minute. "Because the same male was a victim twice, we cannot rule out the possibility that perhaps aberrant behaviour by this individual encouraged the groups' behaviour," the authors report.

In all three cases the entire group participated in the attacks, including females and juveniles. The question the researchers are now asking is why these attacks occurred.

There may be several things at play.

In particular, the group situation of these gorillas has been changing. In gorilla groups it is unusual for there to be more than one grown male. Gorillas tend to live in groups with one male and multiple females. Lone males usually disperse, and eventually form their own groups.

Group sizes have also been increasing since the 1990s

But these groups are different. There are now several with three or more males. "In groups like that, the costs of engaging in this kind of behaviour are pretty low, because the victim is so badly outnumbered," says Rosenbaum.

Lone males outside the group also pose a danger, both to infants and adult males as they defend their group. "So some of these animals have very good reasons to permanently drive off or kill an out-group male," she says.

Group sizes have also been increasing since the 1990s. One group was found to contain 65 individuals with nine males. It was only after this "remarkable demographic shift" in their social structures that the aggressive behaviour was observed, the researchers say. If there were fewer males in a group, it would likely be too risky to attack others.

Nobody knows whether or not groups were as large as they are now before gorillas were widely studied. Dian Fossey's research started when the gorillas were facing significant threats and their population numbers were lower than today.

The gorillas' behaviour appeared remarkably coordinated

"It's possible that the things [she] saw then were actually less representative of 'normal'," says Rosenbaum. "But that's pure speculation, and it's unclear exactly how long we would need to watch them to come up with a meaningful picture of what 'normal' looks like."

Kelly Stewart, who worked with this group of gorillas alongside Fossey in the 1970s, says the new observations have led her to question why these gorillas evolved a one-male polygynous mating system. "Having multiple males to defend the group against outside rivals (who pose a threat to infants) is clearly advantageous," she says.

The change to their group structures – with more males present – has made today's gorilla group structures more similar to humans and chimpanzees. In chimps, violent aggression is often attributed to an "imbalance of power". Something similar appears to be going on with these gorillas.

However, in chimpanzees and humans, it is usually only males that do the attacking, not the entire group. Another difference is that these gorilla attacks did not appear premeditated, whereas humans and chimps are known to actively seek out victims.

That said, "the gorillas' behaviour appeared remarkably coordinated, clearly had direct benefits for some individuals, and bore important hallmarks of classic descriptions of coalitionary intergroup aggression in chimpanzees," the team reports.

Sonya Hill of the University of Chester in the UK says the observations are an "exciting surprise" that highlight the important of long-term studies. "Animal behaviour isn't static, there are so many different influences," she says. "This study shows that, even though people have been studying mountain gorillas for 50 years, we are still finding out new things about them."

It's possible that the things [Dian Fossey] saw then were actually less representative of 'normal'

The gorillas appear to be changing their behaviour, says Shelly Masi of the National Museum of Natural History in Paris, France. "Behavioural innovation is the base pathway for developing culture, a feature that we share with our closest relatives."

Cyril Grueter of the University of Western Australia is less surprised, because other apes often turn to violence when females are at stake. He witnessed the second attack in 2010. "Solitary silverbacks may spend years wandering the forests alone in search of reproductive opportunities," he says. "When an opportunity to attract a female presents itself, the potential gains are often so attractive that they are ready for an all-out physical confrontation."

While these attacks are still believed to be rare, tracking staff have given anecdotal evidence of other such instances. The reality is that we do not have a clear answer as to whether this behaviour is unusual.

Melissa Hogenboom is BBC Earth's feature writer. She is @melissasuzanneh on Twitter.

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