THE OLDEST PROFESSION

I saw a rerun of the program QI recently and it told of a fascinating study done with Capuchin monkeys.  The monkeys were given little pieces of silver which they could exchange for foodstuffs, such as grapes.  They fully grasped the monetary potential of these pieces.  One of them gave a female monkey a piece of silver so she would have sex with him.  She subsequently used it to purchase a grape.

Capuchin monkey