Some monkeys have a sense of fairness and complain if they're getting a bum deal, primatologists say.

Researchers trained brown capuchin monkeys (Cebus apella) to exchange a granite token for a cucumber treat.

If a monkey saw another capuchin received a better payoff, such as a grape, she would often show her disapproval by throwing a tantrum.

Capuchin monkeys share a piece of food (Courtesy: Frans de Wall, Emory University)

Sarah Brosnan and psychology Prof. Frans de Waal of the Yerkes National Primate Research Center at Emory University in Atlanta studied 10 small primates from the forests of central and South America.

In field studies, scientists have found capuchins to be cooperative animals living in a tolerant society where they often share their food.

The researchers set up the captive female monkeys in pairs and took turns rewarding each one during 25 trials.

Monkeys won't stand for monkey business

They found if both monkeys received a piece of cucumber as a reward, the primates' behavior stayed the same.

But if one monkey received a grape, considered a more valuable prize, the other often refused to accept cucumber, threw it away or gave it to the other monkey.

"This effect is amplified when the partner does not have to work for the reward," Brosnan said in a release.

She said the findings may help explain why people often forgo a reward if they don't perceive it as fair. The seemingly irrational behaviour has baffled scientists and economists.

The researchers are now testing chimpanzees to see if they have the same sense of fair play.

The study appears in Thursday's issue of the journal Nature.