London, November 14: Even monkeys become green with envy when they see other monkeys receiving better rewards for performing the same task, say researchers at the Yerkes National Primate Research Center in Atlanta, Georgia, US.

The findings may be helpful in understanding why humans have such a keen sense of fairness, they add.

In previous studied, it was found that monkeys would reduce putting in efforts in a task when they saw cage-mates receiving tastier treats for completing the same task. However, the reason behind the poorly rewarded animals’ turning into slackers was not revealed.

In order to unearth the reason as to why monkeys started showing reluctance to participate in the task, Frans de Waal and his colleagues introduced several variations in their experiments.

They trained 13 capuchin monkeys to retrieve a small rock and place it in the experimenter's hands. The animals were given a reward for completing this task.

In one of the experiments, the monkeys were given the same sized cucumber as reward for their efforts, and 90 per cent of them completed the task within five seconds.

Later, the researchers gave one of the subjects a grape, which monkeys prefer to cucumber.

Seeing their partners receiving grapes, the other monkeys invested less effort in future repetitions of the task and completed it within 5 seconds, only 80 per cent of the time.

In another experiment, wherein the monkeys were given the same cucumber reward and could see a bowl of grapes just beyond their reach, the animals performed the task with the same willingness as when the grapes were hidden.

According to the researchers, this finding contradicts the suggestion that the primates alter their behaviour due to greed.

The researchers also ruled out the suggestion that the monkey stopped performing the task because they had received large rewards in the past and felt frustrated by the measly amounts offered in later trials, as the animals exhibited the same patterns of behaviour regardless of whether they had received a grape or cucumber in preceding experiments.

"(The study) confirms that capuchin monkeys react negatively to situations in which they receive a less favourable reward than their partner for the same task. Our control procedures suggest that this response was due solely to the discrepancy between the monkey's own and the other's rewards and not to individual factors such as greed or frustration, " New Scientist magazine quoted them as saying. (ANI)