It turns out monkeys know when they get a raw deal —which, in this case, is a cucumber. In a study published in last Thursday’s issue of Nature, scientists reported that for the first time a species of nonhuman primate knows when it has been treated unfairly.
Scientists at Yerkes National Primate Research Center of Emory University in Atlanta studied capuchin monkeys, known for their social system built on cooperation. The social groups consist of one dominant male and three to six adult females and their offspring.
The study involved a simple exchange: Two female residents of a colony were each given a small token —a granite pebble. When they returned it to the researcher, they received a slice of cucumber. Those exchanges were completed 95 percent of the time.
But when a monkey got a grape instead, the rate of completed exchanges fell to 60 percent as the other monkey often refused to accept the cucumber slice, took it and threw it away or handed it to the other capuchin.
Finally, one monkey would get a grape without having to do any work —and the completion rate fell to 20 percent. When the other monkey recognized the disparity in effort and reward, she sometimes would even refuse to hand over the pebble.
The same findings were observed in each of five pairs. The researchers, Frans de Waal, an endowed professor of primate behavior and director of the Living Links Center at Yerkes, and Sarah Brosnan, a graduate student, did not find the same behavior in male monkeys. Brosnan said she suspects that is because adult males don’t live together cooperatively. “These female monkeys don’t like it when someone gets a better deal,” Brosnan said. “Just as we wouldn’t like it if someone got more money for the same amount of work or more money for less work.” The grape-deprived monkeys did not show any emotional reaction toward their partners —but they did ignore the scientist. “They knew I was the source of the inequality,” Brosnan said.
The researchers said their work is aimed at understanding the evolutionary development of social fairness. They are repeating the study in chimps, a species more closely related to humans.
Robert Frank, an endowed professor of economics at Johnson School of Management at Cornell University, said he had never seen an animal study “where someone turns down an offer because it wasn’t good enough” — a trait he said is common in human exchanges.
“People often pass on an available reward because it is not what they expect or think is fair,” Brosnan said. “Such irrational behavior has baffled scientists and economists, who traditionally have argued all economic decisions are rational. Our findings in nonhuman primates indicate the emotional sense of fairness plays a key role in such decision-making.”
— 24 September 2003
