Michael Gumert of Nanyang Technological University of Singapore discovered that male macaques "pay" to have sex by grooming the females. Interestingly, market forces apply even to monkey sex:
On average, females had sex 1.5 times per hour.
But this rate jumped to 3.5 times per hour immediately after the female had been groomed by a male -- and her partner of choice was likely to be the hunky monkey that did the grooming.
Market forces also acted on the value of the transaction.
If there were several females in the area, the cost of buying sex would drop dramatically -- a male could "buy" a female for just eight minutes of nit-picking.
But if there were no females around, he would have to groom for up to 16 minutes before sex was offered.
http://afp.google.com/article/ALeqM5gUTyYpPmEHzLP5a_rDsPDkGv8aXA - Thanks casey!