The first observations of lethal attacks by chimpanzees against gorillas have been reported. Although this proves our closest relatives can pose a danger to those just a little further genetically removed, a paper in Scientific Reports indicates the ultimate cause may be us.
Loango National Park, Gabon, is home to both chimpanzees and western lowland gorillas. For 16 years it has also included the Loango Chimpanzee Project, a study of the relationships within and between chimp troops.
Chimpanzees are definitely the most warlike of the great apes (humans aside) but for most of the time the project has been observing them they took out their violent side on others of their species, or smaller animals. Longer-term observation efforts have seen the same thing elsewhere.
"Interactions between chimpanzees and gorillas have so far been considered as relatively relaxed," said Professor Simone Pika of Osnabrück University in a statement. "We have regularly observed both species interacting peacefully in foraging trees. Our colleagues from Congo even witnessed playful interactions between the two great ape species."
All that changed two years ago when Osnabrück PhD student Lara Southern and colleagues heard the sort of screams usually associated with an unfriendly encounter between two chimpanzee troops. “Then, we heard chest beats, a display characteristic for gorillas, and realized that the chimpanzees had encountered a group of five gorillas,” Southern said.
The fight lasted 52 minutes. Members of the project witnessed a longer conflict 10 months later. In both cases, the chimpanzees used weight of numbers (27 to 5 and 7, respectively) to counteract the gorillas' considerable size advantage. The first battle involved some aggression from both sides, but the second saw chimpanzees chasing gorillas up and down trees, and then attacking them as they attempted to escape through the canopy and on the ground.
Each time the adult gorillas eventually got away, but an infant became separated from its mother and was killed. Three chimpanzees were injured in the first battle.
West Africa's chimpanzees have been seen as less violent towards members of their own species than their East African counterparts, but Southern told IFLScience her team recently showed
Loango's; "Annual intercommunity killing rate was among the highest across all sites."
Naturally, the paper's authors are keen both to learn why the attacks occurred, and also why only now. Both events were during seasons when the chimpanzee and gorilla diets overlap heavily, while the friendly interactions were at times of the year when their diets diverge.
The authors suspect it's no coincidence Gabon's forests have started producing less fruit in response to climate change. The second dead baby gorilla was eaten by a chimpanzee, but the first wasn't treated as food, at least initially, making it more likely the violence was about competition for resources than direct predation.
“We are only at the beginning to understand the effects of competition on interactions between the two great ape species in Loango," said Pika. "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."