Chimpanzees and Bonobos Reveal Clues to Reciprocity in Humans

Using chimpanzees and bonobos as models, researchers from the University of California, Santa Barbara, are studying the nature of reciprocity in humans.

This study is led by Adrian Jaeggi, a postdoctoral researcher in anthropology at UC Santa Barbara and a research fellow at the campus's SAGE Center for the Study of the Mind.

"The article focuses on the question of whether individuals do favors because they expect them to be reciprocated at some other time, and, more specifically, whether such exchanges have to happen immediately, or can take place over longer time spans," Jaeggi explained. "We studied the question in chimpanzees and bonobos -- our two closest living relatives -- and looked at the exchanges of grooming and food sharing, which are two common types of favors among these apes."

Jaeggi noticed that the calculated exchanges, in which individuals keep a detailed score of past interactions, are much less common than the more loosely balanced exchanges that take place in stable relationships.

"In the chimp group we studied, we knew there was a lot of this long-term exchange," said Jaeggi. "We didn't find any evidence for a short-term effect." 

"In the wild, for example, chimps hunt for smaller monkeys, and they commonly share the meat. It's similar to what hunters and gatherers do," Jaeggi said. "Our experiment is meant to mimic the situation in which you have a large monopolized food item." 

With grooming as the favor, they tried see whether or not a chimp that had just been groomed was more likely to share food with the other who had groomed him. 

 "That would provide evidence for keeping track of who has done a favor," Jaeggi said. 

"We found that sharing was predicted by who the chimps' long-term friends and partners were," he said. "Grooming just before didn't play a role. Food owners didn't share specifically with their groomers. Nor did the groomers act in return. They didn't pay for the food, and they didn't reward the food owner's generosity afterward."

Whereas in Bonobos, an entirely opposite reaction was observed.  In chimps, no matter what the status of a chimp in the hierarchy stakes, every member receives food. But such hierarchy is absent in bonobos.  Also, they don't form coalitions as much as chimpanzees do.

"The food sharing situation sort of freaked them out," said Jaeggi. "All of a sudden there's all this food that's owned by one individual, and they don't really know what to do about it. They want to get it, but they don't dare, because they don't know what the consequence will be."

In addition to this, grooming was more in bonobos.

"And there we did see an effect of grooming on sharing," he said. "Chimps would go and take food pretty confidently, but Bonobos were more reticent. They'd reach out and then groom. It seemed to be that they'd groom to release tension, and then there would be these short-term reciprocal exchanges."

He noted that the exchanges were a byproduct in order to reduce tension. Jaeggi said that this evidence showed long term reciprocity in animals.

"It's really not qualitatively different from what people do," he said. "They establish these lasting relationships, and within them, services are exchanged without the participants keeping close track of who's doing what for whom."

However, humans also have the capacity for more contingent reciprocity, which raises questions about its purpose, and how it developed.

"Maybe that's something that's more culturally learned," said Jaeggi.

The findings appear online in the journal Evolution & Human Behavior.