We know that female monkeys in heat are very obvious for males, as their buttocks swell enormously and turn to a vivid red.

But human females have lost their bottom callosities.

So, how do they signal heat period?

Psychologists believe that women do this by dressing. Literally "Dress to conceive".

They found that women take greater care over their appearance when they are at peak levels of fertility.

A team at the University of California, led by Martie Haselton, made an experience using a group of 30 women aged 18 to 37.

They presented two full-body photographs of each woman, one close to ovulation when the woman was highly fertile, and one at a point of low fertility in the menstrual cycle to a group of male volunteers.

They were then asked to decide which of the two photos showed the woman trying to look more attractive. They chose the woman in her "high fertility" photo some 59.5 % of the time more often than would be expected by chance.

Even if what women do is not so obvious like the bottom callosities of the monkeys and apes, this is the first proof that women look to advertise their fertility.

Paradoxically at first sight, all the women in the study group described themselves as being in committed relationships with men. So why would they signal their fertility status? Perhaps to attract other men. "Women with high fertility tend to feel attracted to men other than their primary partners," says Haselton.

Other studies have shown a double standard on the way women regard men. Men with very masculine traits are seen as the best progenitors for their offspring: their healthy strong aspect depicts healthy genes.

But usually these men - which possess high levels of testosterone - are not very devoted to raising kids and caring for a family. That's why females prefer as stable partners males that are less attractive, with lower testosterone levels, which are seen as more careful.

They make good husbands, but when it's about reproduction...