SEX has probably been a commodity for as long as human society has existed, and perhaps even longer. The 鈥渙ldest profession鈥� seemingly has pre-human evolutionary roots. 鈥淲hen the opportunity arises, male macaque monkeys groom females to 鈥榩ay鈥� for sex,鈥� says Michael Gumert of Nanyang Technological University, Singapore.
Gumert looked at research on a 50-strong group of long-tailed macaques in Kalimantan Tengah, Indonesia, that covered a 20-month period. He found there was an increase in sexual activity after bouts of male-to-female grooming. On average, females had sex 1.5 times per hour, but immediately after being groomed by a male partner, this rate jumped to 3.5 times per hour. After grooming, the female was also less likely to offer herself to males other than her grooming partner (Animal Behaviour, ).
鈥淢y interest in this study stemmed from Trivers鈥檚 theory of reciprocal altruism,鈥� says Gumert. In the early 1970s, Robert Trivers suggested that an organism will provide a service benefiting another, as long as it gets something back at a future date. But Gumert suspected that if the payback involved sex, the value would vary depending on the context 鈥� like all commodities in economics 鈥� a finding not predicted by reciprocal altruism. Sure enough, if there were several females in the area, the value of sex would drop 鈥� a male could 鈥渂uy鈥� a female for just 8 minutes of grooming. But if there were fewer females than males in the area, a male would have to groom his partner for up to 16 minutes before sex was offered.
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鈥淚f there were fewer females than males, a male would have to groom his partner for 16 minutes before sex was offered鈥�
A two-player interaction, such as is usually considered in reciprocal altruism studies, doesn鈥檛 make sense in this 鈥済eneral mating market鈥�, says Ronald No毛 of the University of Strasbourg, France. No毛 and Peter Hammerstein of Humboldt University in Berlin, Germany, formulated biological market theory to better explain the kind of social behaviour Gumert identified in macaques. Market forces have a powerful influence on behaviour, says No毛. 鈥淭here is a very well-known mix of economic and mating markets in the human species itself,鈥� he says. 鈥淭here are many examples of rich old men getting young attractive ladies.鈥�
Yet prior to Gumert鈥檚 study, the evidence that market forces influence mating in nature has been scant 鈥� the only other clear example was in wood mice. 鈥淢any studies that fail to find biological market effects were performed in captivity,鈥� says Gumert. 鈥淚t is quite possible that the confinements of captivity alter or remove the effects of a social market.鈥� For instance, there is no migration within captive communities, so the value of commodities such as sex remains stable, which makes market forces difficult to identify, he says.
Gumert says macaque males are very 鈥渟hort-termist鈥� in their thinking. 鈥淪ome work is showing that monkeys really don鈥檛 have the capacity to wait for long-term trades and therefore trades probably only occur in the immediate sense,鈥� he says.
