This report is interesting (for those who like this sort of thing – Rich, I’m speaking to you), and this point might ring a bell, so to speak, with Dick Grasso:

“The [research] team taught brown capuchin monkeys (Cebus apella) to swap tokens for food. Normally, capuchins were happy to exchange their tokens for cucumber. But if they saw their partner getting a grape – which is more coveted by capuchins – they took offence.”