Why chimpanzees and gorillas become friends in the wild

A small band of anthropoid monkeys were feeding on a tree in the Congolese forest, permeated by fog. Adult chimpanzees were eating fruit in the treetops, while two young monkeys played nearby. But one of the apes was a gorilla, not a chimpanzee.

“What we’ve been told about the interactions between these two species is primarily that the individuals are competitive or that one species avoids the other,” commented Crickette Sanz, an anthropologist at Washington University in St. Louis who first witnessed such a scene in 2000. But over two decades of observations in the Nouabalé-Ndoki National Park in the Democratic Republic of Congo, she and her colleagues recorded relationships that lasted years and other forms of social interaction. between individual chimpanzees and gorillas. Her research was published last month in iScience.

While chimpanzee populations in East and West Africa have been studied in recent decades, the flocks present in Congo are not as well known, Sanz said. Its territory partially coincides with the region where most of the remaining gorillas live, especially in the remote Goualougo Triangle.

Starting in 1999, Sanz and his colleagues began a long-term study of a flock of chimpanzees from Goualougo. Over the course of daily excursions in which they followed the monkeys in the forest, the team documented 285 interactions between the two species in encounters that lasted between one minute and more than eight hours.

According to Sanz, in many cases the interactions occurred when a flock of chimpanzees located an attractive meal, such as a ficus with fruit or a kapok. The sound of excited chimpanzees attracted a family group of gorillas. In 34% of the encounters, the two groups of monkeys ended up eating at the same time, from the same tree, or looking for other food side by side.

Sanz said the interactions the team witnessed were generally tolerant and at times overtly friendly. Larger gorillas tended to approach chimpanzee mothers more than male or childless female chimpanzees. From this, different individuals formed pairs to run after each other, fight and play together. These relationships tended to last for years, the team found: When they encountered a flock of the other species, the individual apes would sometimes look closely at them and then go directly to individuals they already knew.

These interactions were not random. Every now and then, the monkeys that were interacting would irritate each other, and the team observed moments of friction. But the aggressive interactions rarely went beyond a few shouted warnings and never reached the level of the lethal attacks between monkeys of different species witnessed in Gabon.

“They don’t spend all their time together, but they do get together more regularly and more often than we anticipated would happen,” Sanz said. “These social bonds are not what we would have expected if they were just random interactions in a foraging scenario.”

Sanz and his colleagues found that this kind of bonding between monkeys of different species doesn’t seem to help prevent predation. Instead, maintaining friendly bonds appears to open up new food opportunities, with monkeys of different species sometimes warning each other about harder-to-find fruit. Feeding together, in turn, gives the monkeys the opportunity to form lasting relationships.

“Five or ten years on, these individuals in the landscape know each other. They grew up together, interacted every week or so, in the face of different types of food resources,” Sanz said.

Interestingly, these bonds often begin with play involving individuals of two similar species, said primatologist Frans de Waal of Emory University, who was not involved in the study. “It should be just as fun for them to play together as it is for us to play with a dog, for example, or another companion animal. This broadens our view of primate social systems, which has traditionally been limited to relationships within each species.”

The presence of peaceful interactions between two ape species has interesting implications for our own evolutionary history. According to Sanz, anthropologists have often assumed that the various hominin species actively compete with one another. But if we can judge from chimpanzees and gorillas, humanity’s ancestors may also have banded together to share resources in a landscape. This is a possibility suggested by the amount of crosses between different hominin species.

Unfortunately, this kind of food sharing can provide opportunities for the transmission of diseases like Ebola, waves of which have killed thousands of chimpanzees and gorillas in the past two decades. The interspecies tolerance that the team documented suggests that disease outbreaks may be transmitted between endangered ape populations more easily than previously thought.

“It’s crucial that we’re all committed to conservation and to the effort to protect these species,” Sanz said, both for themselves and because we still have a lot to learn about them. “As primatologists, I think we still have a long way to go in understanding behavioral variety or diversity. We tend to understand a specific group or model and think that applies to everyone. It seems to me that there is a lot more variation than we thought.”

^{Translation by Clara Allain}