- The Washington Times - Tuesday, May 1, 2001

At the heart of all long-term relationships lies a fundamental deception, say scientists. Women stay with men for security, and men remain with women for sex.
Researchers in Holland and Sweden say in the journal New Scientist that the deception is the driving force behind the evolution of monogamy.
By offering sex all the time, they say, females in monogamous species, such as humans, birds and porcupines, disguise whether they are fertile and trick males into staying.
The result is that men haplessly become monogamous, unable to move on to a new partner because they cannot determine whether their most recent coupling has resulted in pregnancy.
Although it intuitively makes sense, scientists had no explanation why men were mainly monogamous, whereas males in most species seek to mate with as many females as possible to maximize the number of children they father.
Usually, a male choosing a stable relationship over a philandering lifestyle would have fewer offspring, putting him at an evolutionary disadvantage.
Now, Magnus Enquist, a zoologist at Stockholm University, and Miguel Girones of the Netherlands Institute of Ecology have made a mathematical model proving that under certain conditions monogamy makes more sense than sleeping around.
In most species, females only had sexual relations when they were fertile because it took energy and risked disease, said Mr. Enquist. "The male strategy is to stay with the female for as long as she is fertile, then leave." This meant that males could easily tell which females were fertile.
But humans, birds and porcupines were more sophisticated, Mr. Enquist said, because the females did not use visual or chemical clues to indicate when they were fertile. Instead, women had sex at any time to prevent men from telling when they might be fertile.
Once males were blind to a females condition it was no longer worth their while chasing lots of partners, because the one they were with was as likely to be fertile as any other.

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