A Yale economist/psychologist taught seven monkeys how to use currency. It started as an experiment to teach Capuchin monkeys the concept of money by allowing them to earn tokens which later could be traded in for food.

After a few months, the monkeys learned how to exchange the money for fruit. Later they experimented with Jell-O as an available product at a lower price to see if they would buy more Jell-O instead of grapes. Over time this is exactly what ended up happening.

Later they taught them gambling and ultimately things lead to a monkey exchanging a token for sex. After they finished, the monkey traded the token in for a grape.

More about the experiment can be found here