In the misty forests of the Congolese rainforest, a small band of apes fed in a tree. Adult chimpanzees dined on fruit in the canopy, while a pair of young apes played nearby. But one of the playing apes was not a chimpanzee: It was a gorilla.
“Most of what we’d been told about the interactions between these two species is that they’d be competitive or they would avoid each other,” said Crickette Sanz, an anthropologist at Washington University in St. Louis who witnessed such a scene for the first time in 2000. But over two decades of observations at Nouabalé-Ndoki National Park in the Republic of Congo, she and her colleagues recorded yearslong relationships and other forms of social interactions between individual chimpanzees and gorillas. Their research was published last month in iScience.
While the populations of chimpanzees in East and West Africa have been well studied over the past several decades, the bands present in Congo are less well known, Dr. Sanz said. Their range overlaps with the region containing a majority of remaining gorillas, particularly within the remote Goualougo Triangle.
From 1999, Dr. Sanz and her colleagues embarked on a long-term study of a band of Goualougo chimps. Over the course of daily excursions following them through the forest, the team documented 285 interactions between the two species, over encounters lasting anywhere from a minute to over eight hours.
Often, Dr. Sanz said, interactions occurred after a band of chimps located an exciting meal, such as a fruiting strangler fig or kapok. The sound of excited chimpanzees would draw in a family group of gorillas. In 34 percent of the encounters, the two ape bands went on to co-feed in the same tree, or forage alongside each other for different foods.
The interactions the team saw were “generally tolerant,” Dr. Sanz said, and occasionally actively friendly. The larger gorillas tended to approach chimpanzee mothers more often than males or childless females. From there, different individuals paired off to chase one another, wrestle and generally roughhouse. These relationships tended to last for years, the team found: Upon encountering a band of the other species, particular apes sometimes scanned it and then made a beeline for individuals they knew.
These interactions weren’t random. Social apes occasionally stepped on one another’s toes, and the team noted moments of friction. But aggressive interactions rarely got beyond yelled warnings, and never escalated to the kind of lethal interspecies attacks seen at sites in Gabon.
“They’re not spending all of their time together, but they’re definitely coming together more consistently and regularly than we’d anticipated,” Dr. Sanz said. “These social ties are not what we’d have been expecting if these were just chance interactions in a foraging landscape.”
These sorts of groupings don’t seem to help ward off predation, Dr. Sanz and her colleagues found. Instead, maintaining friendly relationships seems to open up new feeding opportunities, with apes of different species sometimes alerting one another to fruits that are harder to spot. Co-feeding, in turn, gives apes a chance to make lasting relationships.
“Five or 10 years down the road, these individuals on the landscape know each other — they grew up together, they interacted every week or so at different types of food resources,” Dr. Sanz said.
It’s notable that these connections often start with play between two similar species, said Frans de Waal, a primatologist at Emory University who was not involved with the study. “It must be as much fun for them to play together as it is between us and, say, a dog or other companion animal. It expands how we look at primate social systems, which traditionally is entirely within species.”
The presence of peaceable interactions between two species of great ape has intriguing implications for our own evolutionary history. Anthropologists have often assumed that various species of hominin actively competed with one another, Dr. Sanz said. But if chimps and gorillas are any indication, humanity’s ancestors may also have come together to share resources on the landscape — a possibility hinted at by the amount of interbreeding between different hominin species.
Unfortunately, such co-feeding can provide opportunities for the transmission of diseases like Ebola, waves of which have killed thousands of chimpanzees and gorillas over the past two decades. The interspecies tolerance the team documented suggests that outbreaks might be able to hop between populations of endangered apes more easily than previously guessed.
“It’s critical that we’re all engaged in conservation and engaged in trying to protect these species,” Dr. Sanz said, both for their own sake and for how much we still have to learn about them. “As primatologists, I think we have a long way to go in understanding variation or behavioral diversity. We tend to get one particular group or one model and run with that, and it seems to me there’s a lot more variation than we thought.”
