Chimpanzees and gorillas are not known for violence towards each other, and the two hominid species live peacefully together in certain areas – so it's surprising and saddening that researchers have witnessed lethal fights between them for the first time.

Two separate encounters were observed in the Loango National Park in Gabon in 2019. Both times the chimpanzees outnumbered the gorillas and instigated the attacks, and both times an infant gorilla was killed.

In a new study documenting the fighting, researchers are hoping to shed some light on what might be behind the unusual aggression – whether it's to do with territorial battles, competition for resources, or something else.

"Our observations provide the first evidence that the presence of chimpanzees can have a lethal impact on gorillas," says primatologist Tobias Deschner from the Max Planck Institute for Evolutionary Anthropology in Germany.

"We now want to investigate the factors triggering these surprisingly aggressive interactions."

The first interaction in February 2019 involved 18 chimpanzees and five gorillas (one silverback, three adult females, and one infant), and lasted 52 minutes. The chimps encountered the gorillas on the way back from an excursion into neighboring territories.

The second interaction in December 2019 involved 27 chimpanzees (some of which were involved in the first incident) and seven gorillas (one silverback, three adult females, one juvenile and two infants), and lasted 79 minutes. Here, the chimps came across the gorillas at the start of a territory border patrol.

In both cases the chimpanzees were able to separate an infant gorilla from its mother and kill it – in the second incident the baby gorilla was eaten by the chimps. The other gorillas escaped, while some chimpanzees were injured in the fighting in the first clash.

With these rare battles now on record, the next question is what could have caused it. The researchers think that the chimpanzees may have seen the infant gorillas as prey, or that they were competing for food, or that the fights were over territory.

"It could be that sharing of food resources by chimpanzees, gorillas, and forest elephants in the Loango National Park results in increased competition and sometimes even in lethal interactions between the two great ape species," says Deschner.

Observing interactions between chimps and gorillas isn't easy – because of the need to give them space and respect, the large areas involved, the rarity of gorilla populations, the nature of the habitat and so on.

With that in mind, it's possible that killings between these two species are actually more common than the records show, but it is interesting to note that the lethal encounters happened during parts of the year when foods such as fruit would have been less readily available.

The work at the Loango National Park goes on, and the researchers are hoping that they can learn more about the attitudes of chimpanzees and gorillas to each other, and the rest of the hominid species.

"We are only at the beginning to understand the effects of competition on interactions between the two great ape species in Loango," says cognitive biologist Simone Pika from the University of Osnabrück in Germany.

"Our study shows that there is still a lot to explore and discover about our closest living relatives, and that Loango National Park with its unique mosaic habitat is a unique place to do so."

The research has been published in Scientific Reports.