Studying the behavior of our closest living relatives may help scientists better understand the human impulse for generosity.

Share story

How generous is an ape? It is a hard question for scientists to tackle, but the answer could tell us a lot about ourselves.

People in every culture can be generous, whether they are lending a cellphone to an office mate or sharing an antelope haunch with a hungry family.

While it is easy to dwell on our capacity for war and violence, scientists see our generosity as a remarkable feature of our species. “One of the things that stands out about humans is how helpful we are,” said Christopher Krupenye, a primate behavior researcher at the University of St. Andrews in Scotland.

This generosity may have been crucial to the survival of our early ancestors who lived in small bands of hunter-gatherers.

“When our own attempts to find food are unsuccessful, we rely on others to share food with us — otherwise we starve,” said Jan Engelmann, a researcher at Göttingen University.

To understand the origin of this impulse — known as prosociality — a number of researchers have turned to our closest living relatives. For example, a new study involving bonobo apes suggests that the roots of human generosity run deep, but only came into full flower over the course of the evolution of our species.

Roughly 7 million years ago, our lineage split from the ancestors of chimpanzees and their cousin species, bonobos. Chimpanzees and bonobos share a common ancestor that lived about 2 million years ago.

These two closely related species of apes look almost identical to the untrained eye. But they have evolved some intriguing differences in their behavior, including which objects — food or tools — prompt them to behave with generosity.

Recently, Krupenye and his colleagues tested the generosity of bonobos that live in the Lola Ya Bonobo sanctuary in the Democratic Republic of Congo.

They proved to be generous — to a point.

The researchers designed an experiment that could provide strong evidence that bonobos could give things to each other simply out of generosity — rather than being pressured into doing so, or expecting some sort of immediate payback.

“Would they do it if there was no benefit to them?” asked Brian Hare, a primatologist at Duke University who helped run the study.

For their experiment, the researchers took advantage of the fact that the Lola Ya Bonobo apes have learned to crack open palm nuts with rocks. Without a rock, they have to gnaw on the nuts for a long time to get them out of their shell.

The scientists put one bonobo in a cage with five nuts. In an adjacent cage, a second bonobo — a stranger to the first one — had two rocks, but no nuts. The cages were connected by a window.

The bonobos were free to bring gifts to the window to give to each other — or to ignore their neighbor.

The researchers found that the bonobos with the nuts proved generous. In 18 percent of the trials, the bonobos with the nuts handed one through the window to their neighbor, a rate that showed their willingness to give food to others.

But the bonobos in the other cage almost never returned the favor. They refused to pass one of their rocks through the window.

In another experiment, Krupenye got to experience their lack of generosity firsthand.

Each bonobo would sit in a cage, with a mesh wall hanging in front of the door to the hallway. A colleague would slip a stick into the cage near the bonobo and leave.

Then Krupenye would come to the doorway and beg for the stick. He would reach out his arm, plaintively calling the bonobo’s name.

The bonobos seldom handed Krupenye the stick. In fact, sometimes they seemed to tease him.

“They will put it through the mesh a little bit and then pull it back when I’m trying to reach for it,” said Krupenye.

Last month, Krupenye and Hare published their results with their co-author, Jingzhi Tan of the University of California, San Diego, in the Proceedings of the Royal Society.

“It’s a really striking result,” said Felix Warneken, a University of Michigan psychologist who was not involved in the study. What makes it surprising is that in studies involving chimpanzees in the same situations, they will do the opposite.

“Chimps are really reluctant to give food away,” Warneken said.

On the other hand, when it comes to tools, chimpanzees turn out to be generous. They will give stones to other chimpanzees. In the stick-begging experiment, they will help humans out.

“The same species that will not help you get food will help you get an object,” said Hare.

It is possible that the separate evolutionary paths of bonobos and chimpanzees have shaped their generosity. Chimpanzees live in habitats where food is often scarce. They have to compete for food, and groups of chimpanzees will sometimes even engage in warlike conflicts over territory.

Chimpanzees have also learned a lot of clever strategies for using tools to get food. In addition to cracking nuts with rocks, some chimpanzees kill monkeys with wooden spears. Others fish for termites with carefully fashioned poles.

Bonobos, by contrast, live in forests where food is far more abundant. “It’s paradise — the stuff just falls off the trees,” said Warneken.

Adapting to this ecosystem, bonobos may have become more tolerant of each other. They recognize the value of food to others, and do not feel an urge to hoard it for themselves.

But bonobos also seem to be less adept with tools. In the wild, they have never been observed to crack nuts with a rock or fish termites with a stick.

“They may just not have a deep-seated understanding of tools,” said Warneken.

Chimpanzees may be unable to override their selfish tendencies about food. On the other hand, Warneken said, they may recognize the importance of tools for other chimpanzees.

Warneken and other researchers have carried out similar studies on children. They have found that even babies will spontaneously offer both food and objects to adults.

The work of Krupenye and others makes it clear that humans aren’t unique in their generosity. It is possible that our common ancestors with bonobos and chimpanzees were already prosocial, at least to a limited extent. And now our generosity expands beyond what he and other scientists observe in our closest relatives.

“We’re really good at realizing when other individuals could benefit from something,” said Hare.

This versatility may have evolved early in our lineage, producing traits that encouraged more sharing. It leads toddlers to have generous inclinations without any coaching.

Warneken notes that around 5 years old, children become more aware of their prosocial actions. They know that being generous is good for their reputation.

It is possible that after our ancestors evolved the tendency to be generous, they then evolved a brain capable of understanding norms. In turn, humans came to see the benefits of being generous.

“It’s no longer the same kind of motivation that we would find in other animals,” he said. “Now there is some kind of obligation to share with others.”