Like humans, some bonobos cooperate with members of other social groups, even when they don’t receive immediate benefits in return. This finding, published Thursday in the journal Science, may offer insights into the evolutionary history of cooperation among humans.
Bonobos (Pan paniscus) are slender apes that inhabit the lowland rainforests of the Democratic Republic of the Congo. Both bonobos and chimpanzees are humans’ closest living relatives, so scientists often study these creatures in hopes of learning more about our own origins.
Most of this research has focused on chimpanzees, however, in part because they are logistically easier to study. Over time, scientists have learned that chimps have male-dominated societies and that the animals can be extremely hostile and territorial. Chimps regularly patrol the borders of their territory for possible intruders, which they may attack or kill. The primates will even climb to higher ground to scope out their surroundings and avoid other groups, since fights would be costly. Males also sometimes kill baby chimps.
Chimpanzees’ propensity for violence has raised questions about human nature: If one of our closest living relatives is inclined to fight and kill, are humans innately violent, too?
The less-studied bonobo offers a possible counterpoint to this narrative. Bonobos, which have female-dominated societies, tend to be more peaceful overall than chimpanzees. Scientists have also never observed infanticide among bonobos, per the New York Times’ Carl Zimmer.
To get a better understanding of bonobos’ behavior, researchers spent two years observing the animals in the Democratic Republic of the Congo. They watched two separate social groups that together included 31 adults.
The team recorded 95 encounters between the groups—some interactions lasted less than an hour, while others stretched for more than two weeks. All told, interactions between the groups made up 20 percent of the researchers’ total observation time of the bonobos.
Bonobos within the same social group groomed each other, shared food and developed alliances with each other, as the researchers expected. But the team was surprised to see these same behaviors play out between members of the two different factions. Roughly 10 percent of all grooming behaviors they observed and 6 percent of food sharing occurred between chimpanzees that were not in the same group.
And while grooming between in-group and out-group members typically had an immediate reciprocal benefit—the groomer typically got groomed in return—food sharing appeared to be more selfless. Just 14 percent of animals that got food from an animal in the other group returned the favor.
This aligns with what staffers at bonobo rescue organizations have long observed among their animals. For bonobos, “the most important thing is to be stable, to be in good understanding—more than everything,” says Suzy Kwetuenda, manager at the bonobo rescue and rehabilitation sanctuary Lola ya Bonobo in the Democratic Republic of the Congo who was not involved in the research, to NPR’s Ari Daniel.
“For bonobos, cooperation is important, because it is proof of love,” she tells the publication.
Drilling down even deeper, the scientists noticed that some bonobos were more cooperative than others—and that those individuals from each group sought each other out. The friendliest bonobos from each faction forged connections with one another, which in turn promoted cooperation among the groups more broadly, the researchers posit.
“Scholars used to believe that in-group ‘love’ goes together with out-group ‘hate,’ but recent research suggests that often in-group cooperators are also out-group cooperators,” says Kasper Otten, a sociologist at the Netherlands’ Research and Documentation Center who was not involved in the study, to New Scientist’s Christa Lesté-Lasserre.
This study only looked at two groups of wild bonobos, so the scientists are hopeful that additional research could shed light on how widespread this cooperation really is among the creatures.
But in the meantime, what should humans take away from the findings? Do we naturally tend more toward cooperation, like bonobos, or are we inherently more violent, like chimpanzees?
That remains an open question—but scientists say it’s likely some mix of both.
“I wouldn’t say it’s either-or,” says study co-author Liran Samuni, an evolutionary biologist at Harvard University and the German Primate Center, to the New York Times. “They are jointly teaching us about our past.”