Dubner and Levitt then continue with what is almost certainly the most illuminating anecdote with respect to the capuchins’ understanding of money. “Something else happened during that chaotic scene [of the bank heist], 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 monkeys’ 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 for a grape.” Prudishly, and perhaps incuriously, Chen has taken measures to assure no repetition of the incident. “It wouldn’t reflect well on anyone involved if the money turned the lab into a brothel,” write Dubner and Levitt.
The author might be leaping to conclusions. Either that, or he's a monkey mind-reader.