Selling sex is said to be humankind's oldest profession but it may have deep evolutionary¹ roots, according to a study.
 
Selling sex is said to be humankind's oldest profession but it may have deep evolutionary roots, according to a study into our primate² cousins which found that male macaques pay for intercourse³ by using grooming⁵ as a currency.

Michael Gumert of Nanyang Technological⁶ University in Singapore made the discovery in a 20-month investigation⁷ 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 groomed⁸ 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 groom⁴ 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 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."

“性交易”据说是人类最早的职业，而一项研究表明，这可能具有很深的进化渊源。该项对人类亲缘的研究发现，雄性猕猴会通过给雌性猕猴梳理毛发的手段来换取交配机会。这种手段好比人类使用的货币。

据《新科学家》周刊上周六报道，新加坡南洋理工大学的麦克尔•古莫尔特通过对印度尼西亚加里曼丹地区的50只长尾猕猴进行为期20个月的研究，得出以上结论。

研究发现，雌性猕猴的交配频率为平均每小时1.5次。

但在一只雄猴给她梳理完毛发后，这一频率会立刻提高至每小时3.5次。此外，雌猴很可能会选择为她梳理毛发的那只雄猴作为自己的“性伴侣”。

此外，市场力量也会影响交易价格。

如果某一区域内有几只雌猴，那么“性交易”的价格会大大下跌，一只雄猴只需为一只雌猴抓虱八分钟就能“换取”交配机会。

但在周围没有雌猴的情况下，他得付出16分钟的努力。

英国《新科学家》周刊称，该研究为“生物市场力量可以解释社会行为”的理论提供了支持。

法国斯特拉斯堡大学的罗纳尔德•诺伊说：“人类物种自身存在着一种经济市场与交配市场的结合。”

“当今社会不乏有钱的老男人与年轻貌美的女人结合的实例。”