BBC News Online: Sci/Tech

To love, honour and deceive

Long-term relationships are fundamentally dishonest. And it's all the fault of females.

Scientists claim that monogamous relationships among many species, including humans, only persist because females have found a way to disguise whether they are fertile.

The researchers claim that by offering sex anytime, but no clues as to whether they will conceive, females trick males into hanging round for a long time.

The evolution of this deception is the only reason that monogamy has developed.

Sexual deception

New Scientist reports that two zoologists claim to have uncovered the dishonest heart of every faithful relationship.

In many species, females often advertise their fertility with visual or chemical cues and, as a result, only suffer the amorous attentions of males when they are likely to conceive.

They typically avoid too much sex because it takes energy and leaves them at the risk of contracting disease. This behaviour also helps males decide with whom to mate, and when they can move on and try to spread their genes elsewhere.

Roguish behaviour

In many other species, such as humans, birds and porcupines, females hide their fertility and so keep males hanging round on the off-chance that they will conceive.

This deception also discourages males from seeking other mates because they are just as likely to be fertile as the female the male has shacked up with claims Magnus Enquist.

The pair tested their theory in a mathematical model and found that males only stopped their roguish bachelor lifestyle when females start hiding their fertility.

"Classical explanations of sexual behaviour always focus on the male," Magnus Enquist told New Scientist. "But this gives stronger focus on the woman."