WHEN it comes to attracting a man, or deciding what sort of guy we want to have a relationship with, we girls are more at the mercy of our hormones than perhaps we like to think. That's according to two new pieces of research published in the journal Evolution and Human Behavior.
It seems that for a few days every month, the chemistry takes over.
Several studies have shown that women become more attractive around the time of ovulation when they're able to become pregnant; their faces become prettier; they smell sexier and they even appear more symmetrical.
Now evolutionary psychologist Geoffrey Miller and his colleagues at the University of New Mexico have demonstrated that these changes are marked enough to have economic effects in the real world. In a study of the earning patterns of lap dancers in gentlemen's clubs, they found that the dancers were tipped more during their fertile phase than at other times of month.
A lap dancer's earnings through tips increases with the number of "dances" she performs, in which she is required to get "up close and personal" with her client. The more attractive a dancer is, the more dances she will be requested to perform, thus her earnings are an accurate gauge of her appeal to the opposite sex.
The hormone oestrogen has been implicated in monthly increases in women's attractiveness, and sure enough, the researchers found that the patterns of lap dancers' tip earnings mapped on to the patterns of oestrogen levels across their cycles. Oral contraceptives wipe out these monthly hormonal effects and lap dancers who were taking them didn't experience the monthly increase in earnings. In fact, dancers who were on the pill earned, on average, $80 less per shift than those who were not.
The effect of fertility is not confined to a woman's attractiveness: Women who are ovulating show changes in their preferences in male partners, especially if they are looking for a short term relationship.
Meghan Provost of Mount Saint Vincent University, Nova Scotia and her colleagues showed that women in their fertile phase prefer men who walk with a masculine gait, while other studies have demonstrated that women prefer more masculine faces and deeper voices in potential partners during the fertile time of the month.
So why do we get these changes in relation to a woman's fertility?
If we look at our closest ape cousins, the chimpanzees, females advertise their fertile periods by having huge swollen bottoms - this induces a lot of mating competition among males and it is thought that this benefits the female by getting her the best males to father her offspring.
Clearly we don't see the same dramatic cyclic effects in human females and the conventional wisdom has it that humans have "concealed ovulation", that is, neither a woman nor her male partner is aware of when her fertile period is.
One theory as to why this is the case is that by not letting her partner know when she's fertile, a woman can keep him hanging around all the time to make sure no other man impregnates her - and so this promotes pair bonding and means that dad stays around to help look after the kids.
But the idea that ovulation is concealed and that such concealment has evolved to promote monogamy is steadily being squashed by a growing weight of evidence that "the time of the month" has a significant effect on a woman's attractiveness, her behaviour towards the opposite sex and her preferences in men, although there may be no conscious awareness of the time of ovulation.
So what's going on? There is an important difference between chimpanzee females and human females in their mating strategies which could give a clue: both will look for a male with good genes that can be passed on to the children, but while chimpanzee females expect no input from fathers after conception, human females may choose male partners on the basis of their ability to provide resources or help with the kids as well as good genes. Women may pursue a dual strategy that could potentially get them all of these benefits.
Prof Provost suggests that masculinity in men may be a good indicator of genetic quality because it indicates a good immune system that can be passed on to the children, and this could explain the increase in attraction to masculine men when at peak fertility.
But there could be a downside: "Women may not be attracted to high levels of masculinity across the cycle because being partnered with such men may have drawbacks, such as less parental support," she says. The implications here are that women may have a tendency to be attracted to men of high genetic quality to sire their children, but may be drawn to a more nurturing, supportive man to help raise those children.
That's the biology - what we actually do in our lives comes down to a whole host of factors. "Men and women both have to balance the costs versus benefits of their relationship with any given partner," says Provost. "Just like men, women may be motivated to cheat on a long-term partner, or try hard to maintain a relationship with a good partner. It is a complicated game."
Prof Miller and his colleagues suspect that human fertility cues are likely to be very flexible and stealthy, so that women can maximise their ability to attract high-quality partners just before ovulation, while minimising their primary partner's possessiveness and sexual jealousy, and this could explain why women are still sexually attractive, albeit less-so, at times when they are unable to conceive.
It seems then that although fertility in humans is not overtly advertised, it certainly has an effect on male and female behaviour and this may function subtly to get a woman the genetic and resource benefits she needs to rear children successfully.