That is the title of a Time Magazine article which you can read here. The male monkeys pay for sex by grooming the female monkeys. The article discusses how this behavior conforms to our ideas about supply & demand and scarcity. "The study also showed that the number of minutes that males spent grooming hinged on the number of females available at the time: The better a male's odds of getting lucky, the less nit-picking time the females received."
Adam Smith, in The Wealth of Nations, talked about the human race's "propensity to truck, barter, and exchange one thing for another." Maybe evolution has built this into us. For more on how our attitudes, especially in relation to economics, might come from other primates, read Why people believe weird things about money. (I saw this latter link at the Freakonomics blog)
Update: To any of my new students checking this out for the first time, I hope to generate some discussion on these issues so feel free to leave comments. You can do so anonymously if you want. I usually post something about 3 times a week, like Sunday, Tuesday and Thursday. Sometimes I am a day late.