A recent study, however, has shaken up one of those assumptions. While chimp groups have long been known to engage in territory-expanding raids, violent intra-species behavior had rarely been observed or even suspected in gorillas. But the new paper documents three occasions on which mountain gorilla groups have ganged up on isolated individuals for no immediately obvious reason.
Writing in the journal Nature, Stacy Rosenbaum, a biological anthropologist at the University of Chicago, and colleagues devote particular detail to one such attack, in the Virunga National Forest in 2004, in which a solitary male named Inshuti approached a group known as the Beetsme group:
"Fifty minutes after initial contact, observers heard loud screams but were unable to identify the screamer(s) due to dense vegetation. Within seconds of the screaming, Inshuti ran away from Beetsme group's primary direction of travel, followed by three unidentified group males. The three males caught Inshuti and held his arms and legs to the ground. The rest of the group ran toward them from multiple directions... The alpha male's actions were the most violent of the behaviors visible to observers. While many gorillas were pulling out chunks of Inshuti's hair, biting, kicking, and hitting him, the alpha male repeatedly sank his teeth into his body and shook his head back and forth, similar to a canid shaking prey. Inshuti attempted to escape and moved ~20 meters before being dragged down and held under the group again... Approximately 3–4 minutes after the attack began, it abruptly stopped. It was unclear to observers why, but all attackers stopped within seconds of each other. Inshuti fled into nearby vegetation."
While Inshuti survived that attack — and another one, in 2013 — another male died of his injuries after being subjected to an 18-minute assault in 2010.
The Virunga mountain gorillas have been studied since the 1960s, but Rosenbaum and her colleagues note that such behavior had never been recorded until a major demographic shift began occurring in the the mid-1990s. During that time, instead of previously observed groups of one adult male and multiple females, groups increasingly contained several mature males. The presence of multiple males, Rosenbaum told Seeker, likely made such attacks a more attractive proposition, because it spread the risk of injury among multiple members of the group rather than heaping it all on the shoulders of one silverback.
It's unclear why such demographic changes — the result of young males simply electing not to leave the group in which they were born, rather than setting out to start their own — should have begun happening so dramatically over the last 20 or so years.
"That is the million-dollar question," Rosenbaum told Seeker. "We don't have a convincing explanation for that. We always want nice, clean answers to things like this. But at least so far, that's not the case here. One possibility, of course, is increased population density: the population has grown quite remarkably. It's really a remarkable conservation success story. And one of the side effects of that is that population density is higher. We don't actually believe this forest is anywhere near its carrying capacity, so we're somewhat skeptical that that would account for this, because it seems that there's plenty of food and space for males to disperse if they so choose. There used to be very, very few gorillas because they were being extensively hunted by humans. So in many ways this density is probably more representative of what it used to be.
"Another possibility is that human pressures are causing this in some way; there's obviously a lot of human presence in the forest, and people are always tempted to try and attribute it to that. But again, because of the timing of human pressures on the gorillas, it isn't a terrible convincing explanation. They were under the most pressure in the 1960s and 1970s, and we weren't seeing that kind of thing then."
Which raises the possibility that the previously-observed social group structure was the anomaly and that, as human pressures have decreased, the gorillas are reverting to type. But that doesn't wash, either; the extreme difference in size between females and mature males is a classic sign, replicated throughout the natural world, of a species that has evolved to live in a one-male, multiple-female social grouping.
"Obviously, the correlation is not perfect, and it could be that gorillas are just some bizarre outlier and that this does represent a return to the norm, or that throughout their history, there have always been fluctuations," added Rosenbaum. "We can't say, because the bottom line is we've only been studying them for 50 years, and gorillas have a very slow life history: They live a long time. So it takes an equally long time to be able to see enough generations of them that we can say anything meaningful about any changes."
Aside from the issue of how the demographic changes have taken place, what could be the possible motivation behind such attacks? Neither food nor space seem likely to be the issue: Mountain gorillas are non-territorial herbivores that eat at least 55 species of plant, many of which are available year-round. "There are," Rosenbaum and colleagues write, "probably few wild primate populations on Earth with less food resource stress than Virunga mountain gorillas and solitary males are in no way a threat to a group's food supply."
"I think there are probably many factors," Rosenbaum explained. "For some of the individuals that participate, particularly the adult males and the females and infants, there are very clear motivations for driving off outsiders. Females are presumably trying to protect the infants; males are presumably trying to keep the females from leaving, because when they interact with other males is when females may make the decision to leave their current group. Why something so extreme happened in these cases is harder to explain, because gorilla groups interact with each other all the time, and these cases are extreme outliers. We don't know why these are so exceptional."
Also hard to explain are why so many other members of the group — in two of the three cases, every single member (fully 42 gorillas in one instance), and in the other all but one — join in. It prompted Rosenbaum and her colleagues to draw a parallel, not with chimpanzees' planned-out territorial raids, but with a behavior witnessed in another great ape species: human mob violence.
Read more at Discovery News