“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.”
