In his three-year study, Dr. de Waal and his colleagues at the Living Links Center modeled the natural hunting behavior of capuchin monkeys. In the wild, these primates pursue a prey as a group, but only one actually makes the capture. That individual ultimately shares the meat with the rest of the group.

Dr. de Waal examined how same-sex pairs of brown capuchins in a test chamber cooperatively work for food when separated by a mesh partition. Two transparent food bowls, one containing apple slices, were positioned in front of the monkeys on a weighted tray. An individual monkey could not pull the tray within reach on its own, but the monkeys could accomplish the task cooperatively. When successful in the effort, the monkey that secured the food consistently shared it with the helper.

"It was remarkable that the second monkey helped to pull the tray even though there was no guaranteed reward of food for him," explained Dr. de Waal. "His willingness to cooperate in the pull was enhanced if he received payment for his work."

Compared to tests of solitary effort in which an individual monkey could pull a tray of food on his own, the researchers found that food obtained through joint effort was shared substantially more than individually obtained rewards. The scientists also determined that the helping capuchin was two or three times more willing to assist again when it was rewarded for its efforts with food in the immediately preceding trial.

"The increased tendency to share after successful joint efforts to obtain food may be the result of psychological mechanisms as complex as displaying gratitude for receiving help," said Dr. de Waal, "or as simple as 'attitudinal reciprocity' - when the joint effort results in generally positive feelings."

In order for the monkeys to cooperate in pulling the tray, Dr. de Waal found that they had to be able to see one another. "When we put an opaque panel between the monkeys, they wouldn't cooperate," explained Dr. de Waal. "This showed that they weren't randomly pulling. Only if they could see one another, did they work together."

Previous studies conducted by Dr. de Waal have shown that capuchins spontaneously share certain foods even if separated by a mesh divider. In a process called "facilitated taking," the monkey who possesses the food approaches the divider and drops pieces which his partner then can collect. Dr. de Waal's most recent study differed in that the monkeys shared food more readily when they worked together to obtain it.

Capuchins and chimpanzees are the only primates that engage in cooperative hunting in the wild. According to Dr. de Waal, such a system in which three or four individuals pursue a prey, but only one makes the capture, carries with it the expectation of a payoff for those who helped in the hunt.

"Why would those who help with the hunt be willing to work with an individual who doesn't share the payoffs?" asked Dr. de Waal. Noting the survival value of cooperative behavior, Dr. de Waal believes that his research offers insight into the evolutionary basis for an essential element of human society.

"Society wouldn't exist without cooperative behavior," said Dr. de Waal. "Tit-for-tat is essential in our economies, and even our morality emphasizes how one good turn deserves another. Our lives depend on our ability to cooperate with one another and to reciprocate for the help of others."

The Living Links Center at Yerkes is an interdisciplinary program dedicated to the study of human and ape evolution. For additional information about the capuchin study, including a video of the experiment, visit the Living Links web page at http://www.emory.edu/LIVING_LINKS/capuchins.

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