For the first time ever, a deadly chimpanzee attack on gorillas in the wild has been documented by German scientists. Altercations between great apes are thought to happen only within the same species, and the animals involved have only shown signs of peaceful coexistence up to this point. So, what could have triggered this deadly encounter?

These two great apes are unlikely foes. Credits: Charles Patrick Ewing & tiswango

Scientists of the Loango Chimpanzee Project have been keeping a close eye on the great apes of Loango National Park for many years. They are observing and analyzing the lives and behavior of about 45 chimpanzees, gathering knowledge on their social relationships, inter-group relations and hunting habits.

The team observed nine encounters in which chimpanzees and gorillas were peacefully hanging out. Sometimes these occasions even included playful interactions and co-feeding in trees.

Since there is no documented case of a clash resulting in death between the two species, the scientists were quite surprised when they witnessed not one but two cases that have cost the lives of two gorillas. Chimpanzees are known to defend their territories aggressively, but so far, this kind of defensive behavior has only been observed towards other chimps.

Two adult male chimps patrolling at Loango National Park. Image credits: LCP, Lara M. Southern

The incidents took place on the outskirts of the chimpanzees’ territory and lasted about 59 and 72 minutes, respectively. The first clash involved 27 chimpanzees and 5 gorillas, the second 27 chimpanzees and 7 gorillas. In both cases, the adult male chimpanzees were the main aggressors.

At the first conflict, the screams of chimpanzees were the first thing the researchers noticed, but they thought they were just the sounds of a normal encounter between members of a neighboring group. Then they soon heard the sound of chest-beating, characteristic of gorillas, and it turned out that the chimps had bumped into gorillas.

In both attacks, the silverback males and adult females protected themselves and the young from the chimpanzees that were attacking in groups. The adult gorillas were able to fight off the chimp troops and sustained no injuries during the attacks, but a baby gorilla was separated from its mother and killed by the chimps in the course of both fights.

Silverback gorillas are known for their ferocious strength. Image credits: J.G. in S.F.

Simone Pika, a co-author of the study and a cognitive biologist at Osnabrück University, told Live Science that a male gorilla was seen literally throwing chimpanzees in the air, which isn’t surprising when we consider that silverback gorillas usually weigh three times as much as a chimpanzee.

On the other hand, when chimpanzees form a group, they are a force to be reckoned with. And although they are smaller than gorillas, their power and speed play a huge role when they attack in groups. Nevertheless, chimps are about 1.5 times as strong as humans relative to their body mass.

The video of the first attack can be seen below:

It isn’t entirely clear why the chimpanzees attacked the gorillas, although scientists believe that the competition for food between the two species could be a possible explanation.

According to Lara Southern, a PhD student at Max Planck Institute for Evolutionary Anthropology and the first author of the study, chimpanzees see gorillas as strong competitors for both space and food use.

“…at certain times of the year when the favorite fruits of chimpanzees and gorillas are at their ripest, there are super high levels of competition between the two apes. If this competition gets intense enough, it may even lead to the kind of violence we observed.” This is supported by the fact that the attacks happened in February and December when indeed, the food is more scarce.

The scientists think that this kind of competition may be increased by climate change, which has led to a reduction in the amount of fruits to feed on, which has already been observed in other forests in Gabon. However, it’s necessary to conduct more research on the subject to determine the exact source of the conflicts.

One of the adult male chimpanzees of the Loango National Park in Gabon. Image credits: LCP, Tobias Deschner

Even if the reason for these attacks is unknown, one thing is for sure; researchers are only just beginning to understand the relationship and competition between the two primates in the diverse habitat of Loango National Park.

Sources: 1, 2