Kevin MacDonald, Ph.D.
Monogamy among animals
Sex, lies and monogamy
At the heart of all long-term relationships lies a fundamental deception
From New Scientist magazine, April 2001
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 Mˆ½ller 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.
More at: Animal Behaviour (vol 61, p 695)