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It turns out that one of humanity's oldest professions may be even older than we thought: In a recent study of macaque monkeys in Indonesia, researchers found that male primates "paid" for sexual access to females — and that the going rate for such access dwindled as the number of available females went up.

According to the paper, "Payment for Sex in a Macaque Mating Market," published in the December issue of Animal Behavior, males in a group of about 50 long-tailed macaques in Kalimantan Tengah, Indonesia, traded grooming services for sex with females; researchers, who studied the monkeys for some 20 months, found that males offered their payment up-front, as a kind of pre-sex ritual. It worked. After the females were groomed by male partners, female sexual activity more than doubled, from an average of 1.5 times an hour to 3.5 times. The study also showed that the number of minutes that males spent grooming hinged on the number of females available at the time: The better a male's odds of getting lucky, the less nit-picking time the females received. Though primates have been observed trading grooming for food sharing or infant care, this is the first time this kind of exchange has been observed between male and female primates in a sexual context, says lead researcher Michael Gumert of Singapore's Nanyang Technological University, demonstrating that the amount of time a male macaque "will invest in [its] partner" depends largely on how many options it has around.

We, more evolved primates, may be tempted to take a cynical view of these findings, but the study's author suggests a more favorable interpretation: The macaques' exchange of services simply illustrates a nifty system of cooperation that allows for successful mating. The basic premise, says Gumert, is called biological market theory, which follows the elementary principles of supply versus demand. When applied to the voluntary sex life of long-tailed macaques, it means that the price that one group is willing to pay for a commodity that the other group has depends on the scarcity or abundance of that commodity on the market. Scientists think female macaques may use grooming, too, to try to maintain social relationships within the group to benefit their offspring, or as a way to distract or appease males from getting aggressive after a sexual encounter. In fact, when female macaques groomed males, their services decreased sexual activity in males.

It's easy to draw parallels between the monkeys' mating dance and our own, but Gumert warns against reading too much into primate studies like this one. The paper draws no conclusions about what these observations in monkeys mean for the human world. In fact, whether and how scientists should extrapolate from primate behavior is a fairly "big debate," says Gumert. Certainly, our biology underpins much of what we do, but so does our culture and environment. Gumert asks, "Where do we draw the line?"

That inquiry is at the heart of primate studies like Gumert's. While science would do well to understand more about the long-tailed macaques' social world — especially as the animals are increasingly losing their natural habitat in Asia — Gumert says figuring out how this market concept can be applied to the social settings of other animals, including humans, will be its long-term value. In the meantime, it can at least make for some thought-provoking pillow talk.

• Krista Mahr