''It's not fair!'' is a common call from the playground and, in subtler form, from more adult assemblies. It now seems that monkeys, too, have a sense of fairness, a conclusion suggesting that this feeling may be part of the genetically programmed social glue that holds primate societies together, monkeys as well as humans.
Two researchers at Emory University, Dr. Sarah F. Brosnan and Dr. Frans B. M. de Waal, report today in the journal Nature that they taught female capuchin monkeys to trade pebbles for pieces of food. The capuchins were caged in pairs, so that each member of a pair could see the other. If one monkey got a grape in return for her pebble but the other only a less desired piece of cucumber, the shortchanged monkey would often refuse to hand over the pebble in exchange or might decline to eat the cucumber -- both very unusual behaviors.
These refusals were often accompanied by emphatic body language, like dashing the pebble or the cucumber on the floor, Dr. Brosnan said. The expressions of exasperation were twice as common if the monkey offered a cucumber saw her companion being given a grape without even having to hand over a pebble.
The behavior suggests that the monkeys have a sense of fair treatment and respond negatively when their expectations are violated, the researchers say.
Monkey Business
To test whether or not such behavior is found in other species, Brosnan designed an experiment for brown capuchin monkeys, a species well-known for strong social bonds and relatively cooperative behavior, particularly in shared food-gathering activities like hunting squirrels and locating fruit trees.
Individuals were drawn from two large, well-established social groups of captive brown capuchins from colonies at the Yerkes National Primate Research Center and paired with a partner. Pairs were placed next to each other and trained to exchange with human handlers a small granite rock within 60 seconds to receive a reward, in most cases, a piece of cucumber.
"That may actually sound simple, but not very many species are willing to relinquish things, especially intentionally," Brosnan said in a telephone interview. (Think of trying to pry a large bone from a dog's mouth.)
Only female capuchins were tested because they most closely monitor equity, or fair treatment, among their peers, Brosnan said.
