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http://www.time.com/time/printout/0,8816,1700821,00.html

Do Monkeys Pay for Sex?

By Krista Mahr

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.