Most of us try to avoid people who are mean to others, but a new study by Duke scientists showed that one species may prefer jerks.
Published Thursday in the journal Current Biology, the research focused on bonobos—a species of African ape considered to be humans' closest relatives along with chimpanzees—living in the Lola ya Bonobo Sanctuary in the Democratic Republic of the Congo. The results provide evidence for the theory that humans differ from their related species in that they avoid those who mistreat others.
The study was conducted by Brian Hare, associate professor of evolutionary anthropology, along with Christopher Krupenye, Ph.D. '16 and now Marie Sklodowska-Curie Fellow at the University of St. Andrews in Scotland.
“Bonobos exhibit an array of cooperative behaviors, so it was surprising to see that they weren't particularly motivated to associate with helpers—at least not more so than associating with hinderers,” Krupenye wrote in an email.
Previous research has shown that bonobos will help strangers get food even when they don’t receive any immediate benefit from their generosity, adding to the scientists' surprise at the recent study’s results.
Krupenye explained that the researchers got the idea for the study from research showing that human infants can evaluate the social world around them and favor characters who are assisting others.
“We wanted to test whether this early-emergent motivation to prefer helpers was shared with our closest relatives or whether it might be a human-specific trait that partly explains why humans cooperate in sophisticated ways that we don't see in other species,” he noted.
Bonobos were the ideal animals to study because of their close relations to the human species, Hare explained. They are also known for being less aggressive than chimpanzees and for often lacking an alpha male in their communities.
“Given all we’ve seen, if you were going to put your money on who would be more like humans, you would put your money on bonobos,” he said.
In the study, Hare and Krupenye showed 24 bonobos videos of an animated character trying to climb a hill. In one version, another cartoon character helped the first reach the top, while in a second version, a different unhelpful character pushed the first down the hill.
Following the video, the scientists placed pieces of an apple underneath cutouts of the helpful character as well as the unhelpful character and watched to see which the bonobos reached for first.
They also had bonobos watch a skit in which a person dropped a stuffed animal. Another individual then tried to give the animal back to its owner, but a third person stole it away. Then the researchers had the bonobos choose whether to take a piece of apple from the helpful person or the thieving one.
In both cases, the bonobos could determine which individual was helpful and which was unhelpful, but they chose the unhelpful one. In fact, they liked the unhelpful people even more once the hinderers began their bad behavior.
Researchers also showed the bonobos another video in which one cartoon character hogged a desired spot from another. The bonobos preferred the characters who were dominant over the ones who gave up their spot.
Krupenye explained that the researchers think the bonobos' choices were primarily driven by an attraction to dominant individuals.
“In many cases, it may be more important for an ally to be strong than 'nice.' Bonobos also may not see 'niceness' or helpfulness as stable character traits,” he wrote. “In any case, we think they saw the hinderers as dominant since the hinderers won out over other characters in all of our social displays.”
Associating with dominant individuals may allow bonobos to get better access to food or mates and avoid being bullied.
Hare noted that although the results were surprising, the study doesn't refute previous research on bonobos. Instead, it shows that the animals are still in competition with each other and prefer competitive individuals, even if they aren’t aggressive.
“We learned something new, and if we knew everything, we wouldn't have to do the research,” he said. “I would rather know what bonobos are really like, not keep maintaining my incorrect idea about how they think.”
Studying bonobos is significant because they provide a window into human evolutionary history, Krupenye noted. Scientists can use bonobo research to determine which traits were likely already present six million years ago in humans' last common ancestor, as well as which traits evolved uniquely in the human lineage.
The study could also be used to inform future research on why human infants prefer more helpful individuals and how reputation affects this, Hare added.
“I think this all ties into how we monitor one another and how we maintain cooperation because of it, that’s really interesting,” he said.
Moving forward, the researchers hope to test chimpanzees to see if a preference for helpers is really unique to humans among the apes.
Hare said that he would like to test whether the results are the same when the bonobos watch videos of other bonobos, instead of animated characters or humans.
“We'd also like to see how these social preferences vary across contexts and what other kinds of social information might guide apes' preferences,” Krupenye wrote.
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