In the world of capuchins grapes are luxury goods（and much preferable to cucumbers）So when one monkey was handed a grape in exchange for her token, the second was reluctant to hand hers over for a mere piece of cucumber. And if one received a grape without having to provide her token in exchange at all, the other either tossed her own token at the researcher or out of the chamber, or refused to；accept the slice of cucumber Indeed, the mere presence of a grape in the other chamber（without an actual monkey to eat it）was enough to reduce resentment in a female capuchin.
The researches suggest that capuchin monkeys, like humans, are guided by social emotions, in the wild, they are a co-operative, groupliving species, Such co-operation is likely to be stable only when each animal feels it is not being cheated. Feelings of righteous indignation, it seems, are not the preserve of people alone, Refusing a lesser reward completely makes these feelings abundantly clear to other members of the group. However, whether such a sense of fairness evolved independently in capuchins and humans, or whether it stems form the common ancestor that the species had 35 million years ago, is, as yet, an unanswered question.
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