Monkey envy

Great piece in New Scientist about Capuchin monkeys and their sense of fairness (or of envy - depends which way you frame it).

Researchers trained some monkeys to pick up a rock and hand it over to the experimenter for a reward. They then sat two monkeys side by side and repeated the experiment. When each monkey received the same reward - a cucumber each - for handing over their rock - they performed the task within 5 second 90% of the time. But when one monkey got a much better reward (a grape instead of a cucumber - you've got to understand monkey economics here and not be put off by the comparative size of the grape - but rather look at its sweetness) the "ripped off" cucumber-receiving monkey started to slack off. That monkey only delivered the rock within 5 second 80% of the time.

A sense of fairness? Or monkey envy?