If a group of baboons encounters another group in the savanna, they may keep a respectful distance or engage in combat. But human groups often do something else: they cooperate.
Hunter-gatherer tribes regularly gather for communal hunting or to form large-scale alliances. Villages and cities give rise to nations. Trade networks span the entire planet.
Human cooperation is so striking that anthropologists have long considered it a hallmark of our species. They speculate that it came about thanks to the evolution of our powerful brains, which allow us to use language, establish cultural traditions, and perform other complex behaviors.
But a new study published in Science on Thursday casts doubt on that uniqueness. It turns out that two groups of monkeys in Africa have been regularly mixing and cooperating with each other for years.
“To have extended, friendly cooperative relationships between members of other groups who are not related is really extraordinary,” said Joan Silk, a primatologist at Arizona State University who was not involved in the study.
The new research comes from long-term observations of bonobos, a type of monkey that lives in the forests of the Democratic Republic of Congo. A century ago, primatologists thought that the bonobo was a slender subspecies of the chimpanzee. But the two species are genetically different and behave in some remarkably different ways.
Among chimpanzees, males occupy a dominant position in society. They can be extremely cruel, even killing babies. However, bonobo groups are female-dominated, and males have never been observed to commit infanticide. Bonobos often mitigate conflict with sex, a strategy that primatologists have not observed among chimpanzees.
Scientists made most of their early observations of bonobos in zoos. But in recent years, they have conducted long-term studies of monkeys in the wild.
Martin Surbeck, a behavioral ecologist at Harvard, in 2016 established a new observation site in the Kokolopori Bonobo Reserve in the Democratic Republic of Congo. Working with the Mongandu people who live in the neighboring villages, he goes on treks through the forests in search of bonobos.
On their first scouting trip, Dr. Surbek was shocked to see what happened when the group of bonobos they were following encountered another. After some excited shouting, the monkeys settled down for a friendly meeting.
The meeting could not be more different from what happens between groups of chimpanzees. Male chimpanzees usually patrol the boundaries of their ranges, ready to fight with males from other groups. They will even climb hilltops to scan the horizon for other groups.
“I felt very privileged to witness this meeting,” Dr. Surbeck recalled.
Dr. Surbeck and his colleagues then got to know the two groups of bonobos very well. They named one group of 11 adults Ekalakala. The other group, with 20 adults, became known as Cocoalongo.
He and his colleagues observed 95 encounters between the two groups over two years. Some lasted less than an hour, but others lasted days. The Ekalakala and Kokoalongo groups once lingered for two weeks before splitting up.
During these mixes, the bonobos behaved much as they would in a group. They cut each other’s hair, shared food, and cooperated to drive away the snakes.
Yet the two groups remained distinct. Scientists have found no evidence of any offspring from Ekalakala and Kokoalongo monkeys. The two groups even maintained their own cultures. Although their ranges overlapped, they hunted different types of game. Ekalakala bonobos hunted small deer-like mammals called duikers. Cocoalongo bonobos caught squirrels.
Liran Samouni, a chimpanzee expert at the German Primate Center in Göttingen who joined Kokolopori’s research, said the cooperation between the groups was not simply a result of the bonobos being friendly in general. “It’s not just a coincidence,” she said.
Dr Samuni and her colleagues found that individual monkeys from different groups gradually formed bonds as they offered favors and gifts back and forth. In some cases, two monkeys from different groups even form an alliance to harass a third bonobo.
Dr Silk hoped the new research would encourage similar studies elsewhere to see how widespread this cooperation among bonobos really is. “You always want to see how things happen over and over again in different populations before you’re really convinced how important this function is,” she said.
These sightings may not appear anytime soon. Bonobo research sites are hard to come by, and not just because the apes live deep in the rainforest. Scientists also have to contend with internal conflicts in the Democratic Republic of the Congo. And bonobos, which may number as few as 15,000 individuals, are threatened by logging and poaching.
Dr. Samuni noted that chimpanzees, with their hostile encounters, are just as closely related to us as bonobos. Our species resembles both lineages in various ways. While human groups can cooperate in remarkable ways, they can also organize to fight.
“I wouldn’t say it’s either-or,” Dr. Samuni said. “Together they teach us about our past.”