By FIONA MACRAE

Last updated at 21:47 12 November 2007

If you felt you were being unfairly treated at work, you might well throw a sulking fit or even go on strike.

And it turns out that monkeys share that militant tendency.

Scientists have found that capuchin monkeys down tools when they feel they have been treated unfairly.

Researchers trained the animals to exchange pieces of rock for a reward of a slice of cucumber or their favourite treat - a grape.

They then looked at how pairs of monkeys reacted if one was given a tastier morsel than the other.

When the first monkey was rewarded with a grape and the second was given less-appealing cucumber, the latter would start to sulk.

Upset by the apparent unfairness of being given an inferior reward for completing the same task, it would either slack off or refuse to participate any further in the experiment.

Making slight alterations to the experiment appeared to show that the animals were not acting out of greed or even frustration. Instead, they were upset at not being rewarded or "paid" as well as others for their work.

Writing in the journal Proceedings of the National Academy of Scientists, the U.S. researchers said the monkeys' behaviour could point to the evolutionary origin of our own resentment at being treated unfairly.

The study, carried out at Emory University in Atlanta, is not the first to highlight human-like traits in capuchin monkeys.

Earlier this year, Italian researchers showed that they display a shrewd business sense.

Taught how to use "coins", the crafty creatures did simple sums aimed at ensuring they got the most for their money.

They quickly grasped that a big pile of low-value "coins" did not necessarily translate into as many treats as a smaller stack of higher-value currency.

Captive capuchins have also been taught how to shop for bargains.

The monkeys were trained to use silver discs as coins to buy pieces of apple and cucumber from researchers at Yale University in the U.S.

When the apple slices were made "cheaper" than cucumber - meaning more apple was offered for the same amount of money - they opted for the better-value apple.

The monkeys even resorted to underhand tactics to hold on to their cash - by hiding the real coins and offering up "counterfeit" coins made of cucumber to the researchers.