The surprising observation could yield new insights into early human evolution.
By George Dvorsky
New research details two fatal encounters in which wild chimpanzees attacked and killed gorillas. It’s a rare example of one great ape species attacking another—and scientists are worried that climate change might have something to do with it.
Chimps and gorillas can be violent and territorial, but their squabbles—which can be fatal at times—happen almost exclusively within their own species. As for lethal conflicts involving two different great ape species (at least those not involving humans), that’s virtually unheard of. Hence the importance of new research published in Scientific Reports, in which scientists document two fatal clashes involving chimps and gorillas at Loango National Park in Gabon.
The reason for these seemingly unprovoked attacks is unknown, but the fatal encounters may be linked to diminished access to food. As the scientists speculate, increased food competition in Loango National Park and possibly elsewhere might be the result of climate change, though more research is needed to be sure. If this turns out to be the case, however, it’s yet another example of the natural world being turned upside down by human-instigated climate change.
“It was only when we heard the first chest beat, a sound which only gorillas make, that we knew something different was about to happen.”
Scientists with the Loango Chimpanzee Project have been observing great apes at the park for several years, and they’re learning much about their social relationships, group dynamics, hunting behavior, and communicative abilities. From 2014 to 2018, the team documented nine occasions in which chimpanzees and gorillas hung out together, which they often do in this park and elsewhere in eastern and central Africa. As the scientists write in their study, these encounters “were always peaceful, and occasionally involved co-feeding in fruiting trees.” And as Osnabrück University cognitive scientist Simone Pika notes in a press release, the team’s colleagues from Congo have even witnessed “playful interactions between the two great ape species.”
So imagine their surprise when, in 2019, the team witnessed not one but two violent encounters, each ending in fatalities. In both cases, chimpanzees formed coalitions, attacked the gorillas, and used their greater numbers to their advantage. Both incidents took place on the outer boundaries of the chimps’ territory, and the main aggressors were adult male chimpanzees. The researchers were able to observe the attacks from about 100 feet away, and they describe them in detail in their new report.
“Our observations provide the first evidence that the presence of chimpanzees can have a lethal impact on gorillas,” Tobias Deschner, a primatologist at the Max Planck Institute for Evolutionary Anthropology and a co-author of the study, explained in a release from the institute. “We now want to investigate the factors triggering these surprisingly aggressive interactions,” said Deschner, who leads the Loango Chimpanzee Project alongside Pika.
The first encounter, lasting for 52 minutes, happened on February 6, 2019, and it “occurred after a territorial patrol during which the males made a deep incursion into a neighbouring chimpanzee territory,” according to the study.
“At the first encounter, when we heard the initial chimpanzee screams, we actually thought our chimps had bumped into another group of chimpanzees,” Lara Southern, a PhD student at Max Planck Institute for Evolutionary Anthropology and first author of the study, explained in an email. “It was only when we heard the first chest beat, a sound which only gorillas make, that we knew something different was about to happen.”
A group of 27 chimps attacked five gorillas—two male silverbacks, two adult females, and one infant. The gorillas tried to defend themselves with physical force, intimidating body postures, and threatening gestures, but to no avail. The four adults managed to escape, but the infant, separated from its mother, did not survive. Several chimps were wounded during the battle, including a serious injury endured by an adolescent female.
The second lethal encounter, on December 11, 2019, lasted nearly 80 minutes and was very similar to the first, involving chimps from the same community. In this attack, 27 chimps attacked a group of seven gorillas, leaving yet another infant gorilla dead. In the first encounter, the killed infant was left alone, but the “infant in the second encounter was almost entirely consumed by one adult chimpanzee female,” the study noted.
“In both cases, once the first chimpanzee who saw the gorillas let out an alarm bark or scream, the majority of other group members reacted immediately and joined in, all barking together,” noted Southern. “The chimpanzees then worked together to single out certain gorillas, and in both events they were able to separate the baby gorillas from their mother.”
Jessica Mayhew, a biological anthropologist at Central Washington University, said primates adopt different strategies to navigate both intragroup and intergroup conflict and that chimps and gorillas exhibit very different approaches in this regard.
“If you study chimpanzees, you come to expect that any squabble can quickly turn lethal, which is a testament to their excitability but also their incredible speed and power,” Mayhew, who wasn’t involved in the study, explained in an email. “However, having this expectation doesn’t make a lethal outcome any easier to witness. Life for a young gorilla is quite dangerous—infant mortality is high—and this study again highlights their vulnerability within a group even with a formidable silverback as a dad.”
Large silverbacks can weigh as much as 590 pounds (270 kg), but chimps have ferocious strength. Research from 2017 showed that chimpanzees are 1.5 times stronger than humans at pulling and jumping tasks.
“Considering that female western gorillas can be almost twice the weight of a typical 100-pound male chimpanzee, while male gorillas can be three to four times as heavy as a male chimpanzee, the fact that chimpanzees can steal an infant gorilla from its mother is remarkable,” Richard Wrangham, an evolutionary biologist at Harvard University, said in an email. “As the researchers note, chimpanzees had the advantages of a larger group, like hyenas when they occasionally kill lions. Their agility and ability to cooperate give them extra force,” explained Wrangham, who wasn’t involved in the study.
Both Mayhew and Wrangham said the new observations highlight the importance of long-term primate studies in the field.
As noted, chimpanzees conduct territorial boundary patrols to search for signs of other chimps or to invade neighboring communities. Scientists believe these incursions are linked to fission-fusion social systems, in which individuals will leave one group to join another. This behavior in chimps suggests “functional parallels and evolutionary continuities between chimpanzee violence and lethal intergroup raiding in humans,” according to the study. As such, modern observations of great apes represent a living model that scientists can study in real time, as Southern explained.
“By looking at the current pressures faced by these two species, both in their environment and in the way they interact socially, we may learn a little more about how we as humans, so to speak, ‘rose to the top,’” she wrote. “It is crucial, now more than ever, that we work to protect these endangered species who provide a window into our past and deserve a place in this future.”
As to why the chimps attacked gorillas in these two instances, that’s not entirely clear. Gorillas are as distantly related to chimps as they are to humans. For Wrangham, however, the chimpanzee attack on the gorillas was not very surprising, given their interest in killing. As he wrote to me in an email:
Chimpanzees clearly revel in hunting and killing other primates, from monkeys to chimpanzees and even humans (mostly infants). Bonobos too kill various other species for meat, and there are even a few observations of their stealing infant monkeys away from their distraught mothers and then carrying them around, apparently to play with, until they died. Gorillas, by contrast, show very little interest in killing other species, whether in the wild or captivity.
But the gorillas were not merely gentle giants, because one silverback severely wounded a female chimpanzee. That shows that it can be risky for chimpanzees to attack gorillas, which makes their being so aggressive into a fascinating puzzle. As Southern et al. note, more observations are needed, ideally with gorillas that do not run from humans, to understand whether chimpanzees get any benefit from gorillicide beyond the thrill of killing.
As for other possibilities, Southern said they can “only really guess as to why this happened,” but they have some theories. One possibility is that the chimps wanted to hunt gorilla infants as prey, but seeing that only one chimp expressed any interest in this, and given the risks involved, it doesn’t really add up.
“It also could be possible that at certain times of the year when the favourite fruits of chimpanzees and gorillas are at their ripest, there are super high levels of competition between the two apes,” Southern explained. “If this competition gets intense enough, it may even lead to the kind of violence we observed.”
To which she added: “We think that at Loango, gorillas are perceived as strong competitors by chimpanzees, for both space and food use, much in the way that our group [at Loango] see other enemy chimpanzees.”
Which is a very good point. If this is the case, the chimps aren’t so much looking at the gorillas as member of another species as they are assessing them as a threat to their access to food.
As the Max Planck Institute release points out, fruits in the tropical forests of Gabon are not as abundant as they used to be, and human-caused climate change might have something to do with that. In turn, this could be causing the observed conflict between the two great ape species. More research will be needed, especially sightings of repeat conflicts between chimps and gorillas (both at Loango and elsewhere) and investigations showing the effects of deforestation, climate change, and other factors that could be changing the way these apes use their forest space and interact with one another. As Mayhew explained, these types of pressures can push ape populations closer together, resulting in more frequent encounters and increased competition over food.