Researchers at Yale did a study on teaching monkeys to use money to buy things like grapes, jello and other things. It turns out once the monkeys learned they could buy things with the money they started paying females monkeys for sex, who in turn immediately bought some sweets with it.

Regular readers will remember my entry "Monkeys Will Pay For Monkey Porn"

New York Times

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But do the capuchins actually understand money? Or is Chen simply exploiting their endless appetites to make them perform neat tricks?

Several facts suggest the former. During a recent capuchin experiment that used cucumbers as treats, a research assistant happened to slice the cucumber into discs instead of cubes, as was typical. One capuchin picked up a slice, started to eat it and then ran over to a researcher to see if he could "buy" something sweeter with it. To the capuchin, a round slice of cucumber bore enough resemblance to Chen's silver tokens to seem like another piece of currency.

Then there is the stealing. Santos has observed that the monkeys never deliberately save any money, but they do sometimes purloin a token or two during an experiment. All seven monkeys live in a communal main chamber of about 750 cubic feet. For experiments, one capuchin at a time is let into a smaller testing chamber next door. Once, a capuchin in the testing chamber picked up an entire tray of tokens, flung them into the main chamber and then scurried in after them -- a combination jailbreak and bank heist -- which led to a chaotic scene in which the human researchers had to rush into the main chamber and offer food bribes for the tokens, a reinforcement that in effect encouraged more stealing.

Something else happened during that chaotic scene, something that convinced Chen of the monkeys' true grasp of money. Perhaps the most distinguishing characteristic of money, after all, is its fungibility, the fact that it can be used to buy not just food but anything. During the chaos in the monkey cage, Chen saw something out of the corner of his eye that he would later try to play down but in his heart of hearts he knew to be true. What he witnessed was probably the first observed exchange of money for sex in the history of monkeykind. (Further proof that the monkeys truly understood money: the monkey who was paid for sex immediately traded the token in for a grape.)