快猫短视频

WOMEN only stay with men for security, and men only stay with women for sex.
It鈥檚 a cynical view of human relationships, but researchers now say it is the
driving force behind the evolution of monogamy鈥攁nd 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鈥檛 waste time on mates that
won鈥檛 get pregnant. Indeed, males usually give females no help in raising their
offspring. 鈥淭he male strategy is to stay with the female for as long as she is
fertile, and then to leave,鈥� 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. 鈥淭he male has no
cue,鈥� says Enquist. 鈥淎ll he can see is the behaviour of the female.鈥�

Once males are blind to a female鈥檚 condition, he says, it鈥檚 no longer worth
their while chasing lots of partners, because the one they鈥檙e with is as likely
to be fertile as any other. 鈥淭here is a search cost. It takes some time to find
a female.鈥�

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.

鈥淐lassical explanations of sexual behaviour always focus on the male,鈥� says
Enquist. 鈥淏ut this gives stronger focus on the woman.鈥� Evolutionary biologist
Anders M酶ller from the CNRS, France鈥檚 centre for scientific research in Paris,
agrees. 鈥淭his is driven by females,鈥� he says. 鈥淲hen ovulation becomes concealed,
the males stay with the females longer.鈥�

But animal behaviour expert Mike Siva-Jothy of Sheffield University argues
that tricking males into being monogamous isn鈥檛 the only reason for females鈥�
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鈥檛 fit with the traditional view of monogamous
societies, Siva-Jothy points out that even in species where pairs bond for life,
the females cheat. 鈥淲hen avian biologists went out and looked at the DNA
profiles of the offspring, they found that everyone was having a romping time,鈥�
he says. But so long as females can fool males into thinking they are being
faithful, their strategy of hidden fertility will still work. 鈥淭hey have to be
cryptic because they don鈥檛 want their partner to find out,鈥� says Siva-Jothy.