Everything you know about sex is true.

Monkeys pay for sex too

We already knew that monkeys would pay for porn, now a research reveals that male macaques exchange grooming for the right to mate with females whose fur they have cleaned.

"I found that the amount of grooming a male performs on a female during a sexual interaction is related to the supply/demand ratio of females per male around the male-female pair at the time of the grooming," says research leader Michael Gumert.

Put another way, male monkeys - especially lower status ones - have to groom more to get more action when fewer females are around. Grooming in macaques involves using the teeth and hands to pick through the fur of the recipient to remove dirt, tangles and parasites. The activity often sexually excites the monkeys, particularly the males, so many scientists suspect it evolved into foreplay in humans.

The "grooming before sex" bouts lasted anywhere from a few seconds to a half hour or more, with the durations frequently linked to either the number of potential other partners or to the status of the groomer or recipient.

According to Gumert, "rank does not remove the market, it only skews it."

"Powerful individuals can take more and give less than low-ranked individuals can," he says, suggesting that such corruption of the fair trade ideal appears to be an inherent facet of primate social life that can apply to everything from monkey sex to human politics.

High-ranking females can also skew the system because, in the case of macaques, they demand more attention before they agree to mate.