Selling sex is said to be humankind's oldest profession but it may have deep evolutionary roots, according to a study into our primate cousins which found that male macaques pay for intercourse by using grooming as a currency.

Michael Gumert of Nanyang Technological University in Singapore made the discovery in a 20-month investigation into 50 long-tailed macaques in Kalimantan Tengah, Indonesia, New Scientist reports on Saturday.

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.

The work supports the theory that biological market forces can explain social behaviour, the British weekly says.

"There is a very well-known mix of economic and mating markets in the human species itself," said Ronald Noe of France's University of Strasbourg.

"There are many examples of rich old men getting young attractive ladies."

If only that worked for us in Patts I would gladly comb a t.g.'s hair for 20 minutes for a short time