Monkeys learn to budget, exchange money for sex

My only complaint about this article by Stephen Dubner and Steven Levitt is that it could have been a bit more serious. They describe two psychologists at Yale who have trained capuchin monkeys to use money, despite Adam Smith's belief that the ability to understand exchange value sets people apart from all other animals:

The essential idea was to give a monkey a dollar and see what it did with it. The currency Chen settled on was a silver disc, one inch in diameter, with a hole in the middle -- ''kind of like Chinese money,'' he says. It took several months of rudimentary repetition to teach the monkeys that these tokens were valuable as a means of exchange for a treat and would be similarly valuable the next day. Having gained that understanding, a capuchin would then be presented with 12 tokens on a tray and have to decide how many to surrender for, say, Jell-O cubes versus grapes. This first step allowed each capuchin to reveal its preferences and to grasp the concept of budgeting.
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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.)