Mammal species attack and kill other species often; it’s nothing new and has been observed for millennia.
The great apes, of which we are the most vicious and cruel, and specifically chimpanzees, can be quite violent to each other and kill members within their own community and neighboring groups, and other species of mammals. But it is rare for them to attack and kill other species of great apes.
Scientists recently witnessed a group of chimpanzees that they have been studying for years violently attack and kill western lowland gorillas. It’s never been reported before, and scientists worry that the behavior may result from our changing climate where food has become scarce and therefore competition for survival emerge as a threat. Of course, habitat destruction by humans leads to local extinction events.
Gorillas and chimpanzees historically feed together in trees where the fruit is plentiful. Researchers always found these interactions to be peaceful. They even play together at times.
In 2019 the research team observed two deadly attacks in Loango National Park in Gabon. The victims were both gorilla infants. The chimps formed a coalition comprised mostly of adult males that entered a neighboring chimpanzee territory. At first, the researchers expected a battle between two groups of chimps. But instead, they heard the unmistakable sounds of gorillas beating their chests.
Gizmodo reports on what came next.
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.
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.”
Gorillas are a hefty species; a female can be twice the size of an adult male chimp, while an adult male gorilla can be four to five times larger. For chimps to risk bites and broken bones is quite risky.
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.
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.”
In sum, the observed events show similarities to patterns reported in IGP’s, IK’s and intraspecies agonistic encounters. Ultimately, additional observations in combination with isochronous assessments of fruit availability and dietary overlap are needed to differentiate whether coalitionary attacks are indeed the output of interspecific predation spurred by opportunistic hunting, interspecies competition for food resources or whether these interactions are merely a non-adaptive by-product of the “xenophobic nature” of chimpanzees. Finally, analyses of long-term phenological data could aid in investigating if potential high levels of feeding competition may be a more recent phenomenon caused by a collapse in fruit availability as observed in other tropical forests in Gabon86.
Our observations provide the first evidence that the presence of chimpanzees can have a lethal impact on gorillas. Additionally, they may instigate future studies aiming to test whether lethal coalitionary interactions of chimpanzees against gorillas are the output of opportunistic hunting or interspecies competition. Future studies could try to simultaneously monitor movement patterns and monthly dietary overlap of the two species in relation to actual encounter rates and outcomes. Future investigations of interspecific interactions may also encourage more cross-fertilization between behavioural ecologists and palaeontologists87,88. Gorillas and chimpanzees have adapted to sympatry throughout their own evolutionary histories. However, up until now, their extensive population decrease and the threats of extinction have predominantly been driven by escalating anthropogenic pressures rather than by the presence of another non-human great ape species89. Ultimately, these living models, combined with continuous new insights from the fossil record, can aid in expanding our understanding of the ecological constraints and mechanisms governing the co-existence of not only these two ape species but other, now extinct, sympatric hominin taxa.
The study emphasizes that the observations and the cause of the attack would need additional study. However, the lack of food supply is a serious threat to all large apes as sapiens have heated the planet and leveled their habitat to the point where extirpation is likely.
Tropical rainforests are being decimated; if we have a chance of surviving the climate emergency, saving the rainforests is critical. Allowing indigenous peoples to care for these lands is also critical as they know the land and its lifeforms better than anyone.