Humans are not the only species to have a sense of fairness, according to a new U.S. study.

Capuchin monkeys also demand their equal share of food or rewards for tasks they've done, won't settle for an injustice and are miffed when they think they have been cheated.

Research by Sarah Brosnan and Frans de Waal of Emory University in Atlanta, Georgia report their findings in today's issue of the journal Nature.

"It's the first time a sense of fairness has been found in any non-human, at least to our knowledge," said Brosnan.

The researchers uncovered the sense of fair play in a study of the small brown primates from central and South America. They gave pairs of monkeys, who knew each other well, jobs to perform. The monkeys received food in exchange for doing a certain task but each partner did not always get the same quantity or quality of food for equal amounts of effort.

"We showed the subjects compared their rewards with those of their partners and refused to accept a lower-value reward if their partners received a high-value reward," said Brosnan.

If both members of the pair did not get the same reward, the monkey that was short-changed refused to accept it or threw it away, in a reaction similar to that of humans.

"That active response towards reward is really unusual. They were clearly not pleased with the way things were going," Brosnan added. "In humans, it is proposed that this sense of fairness is actually what makes co-operation work well."

She suspects other animals also have a sense of fairness but chose to study capuchins because they are known from field studies to be a co-operative species and to have a very tolerant society: "They have a reasonable expectation of getting a food reward if a food bonanza is found or if they help in a co-operative hunt," said Brosnan.

She believes the findings settle the question of whether a sense of fairness is something that is taught or an evolved behaviour: "Finding this in capuchin monkeys does indicate that a sense of fairness has evolved. Clearly it is an extremely beneficial behaviour," she said.