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WOMEN only stay with men for security, and men only stay with women for sex.
It’s a cynical view of human relationships, but researchers now say it is the
driving force behind the evolution of monogamy—and women started it. By
offering sex all the time, females in monogamous species disguise whether they
are fertile and trick males into sticking around.

In most species, females only have sex when they are fertile. This is because
sex takes energy, and carries the risk of disease. But it also means males can
easily tell which females are fertile, so they don’t waste time on mates that
won’t get pregnant. Indeed, males usually give females no help in raising their
offspring. “The male strategy is to stay with the female for as long as she is
fertile, and then to leave,” says zoologist Magnus Enquist of Stockholm
University.

But in some species, including birds, porcupines and humans, the girls have
wised up. By cutting down on visual and chemical cues, and by having sex all the
time, they stop males from telling whether they are fertile. “The male has no
cue,” says Enquist. “All he can see is the behaviour of the female.”

Once males are blind to a female’s condition, he says, it’s no longer worth
their while chasing lots of partners, because the one they’re with is as likely
to be fertile as any other. “There is a search cost. It takes some time to find
a female.”

Although this idea makes intuitive sense, until now it was a mystery whether
the trick works. Normally, a male choosing a stable relationship over a
philandering lifestyle would have fewer offspring, putting him at an
evolutionary disadvantage. Would women hiding their fertility by offering sex
continuously be enough to tip the balance?

Enquist and his colleague Miguel Girones from the Netherlands Institute of
Ecology in Nieuwersluis decided to investigate. They created a mathematical
model to test the theory, and found that under certain conditions, monogamy is
the preferred option. Even in a population where males were used to having many
partners, if females started to conceal their fertility, the males settled down
into long-term partnerships.

“Classical explanations of sexual behaviour always focus on the male,” says
Enquist. “But this gives stronger focus on the woman.” Evolutionary biologist
Anders Møller from the CNRS, France’s centre for scientific research in Paris,
agrees. “This is driven by females,” he says. “When ovulation becomes concealed,
the males stay with the females longer.”

But animal behaviour expert Mike Siva-Jothy of Sheffield University argues
that tricking males into being monogamous isn’t the only reason for females’
high sex drive. Having lots of sex with lots of different males might ensure
that at least some of their offspring were fathered by good-quality mates.

Although this idea doesn’t fit with the traditional view of monogamous
societies, Siva-Jothy points out that even in species where pairs bond for life,
the females cheat. “When avian biologists went out and looked at the DNA
profiles of the offspring, they found that everyone was having a romping time,”
he says. But so long as females can fool males into thinking they are being
faithful, their strategy of hidden fertility will still work. “They have to be
cryptic because they don’t want their partner to find out,” says Siva-Jothy.