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Booty Call: How to Spot a Fertile Woman

Ask the average person how the menstrual cycle affects women's moods, and you're likely to get an earful about PMS, or premenstrual syndrome. While it's true that PMS symptoms are common (although not as stereotypical as usually believed), new research is finding women's behavior shifts at another point in the reproductive cycle: ovulation.

Two new studies in the November issue of the journal Evolution and Human Behavior find that women get a little wilder during their most fertile days of the month. One study found that fertile women are more open to the idea of hooking up with a stranger or acquaintance, while the second found that women with less masculine-looking partners are more likely to lust after strong-jawed men during fertile days than women with partners with manly mugs.

The new studies are two of more than 20 that have examined the effects of ovulation on the way women dress, talk and think. While it's not yet clear if or how these temporary changes affect women's relationships in the real world, they may by a key to humanity's past.

"The idea is you see the preferences that would have evolved ancestrally still show up," Steven Gangestad, a University of New Mexico evolutionary psychologist and co-author on both new papers, told LiveScience.

Ovulation and lust

Until about a decade ago, most research on the menstrual cycle focused on PMS, which occurs after ovulation and before menstruation. Few people had investigated whether ovulation affected women, Martie Haselton, a psychology professor at the University of California, Los Angeles, told LiveScience. (Ovulation occurs when a woman’s body releases an egg down the fallopian tube where it can meet up with Mr. Sperm.)

"Nobody was asking what happens on fertile or non-fertile days," Haselton said.

One reason no one had tackled the question is that researchers in human evolution had long assumed that ovulation didn't matter much in humans. Unlike other mammals, which go into "heat" during fertile periods, women can be up for sex any time of the month. Evolutionary theorists have tried at several explanations for the loss of this estrus cycle in humans, one of the most common being that humans lost the "heat" phase to conceal ovulation. If a man didn't know which sexual encounter would pass his genes along, the theory went, he'd be more likely to stick around and help raise offspring.

The problem is that men do seem to be able to tell when their partner is ovulating, albeit imperfectly. No one knows how men can tell (smell is one hypothesis), but several studies have found women report that men become more attentive and jealous around fertile days. One 2006 study published in Evolution and Human Behavior found that when women were fertile, their male partners saw other men as a greater threat to their dominance.

Another problem is that women do seem to have a "heat" phase, although it works very differently than in other mammals. It's not that women become more receptive to sex in the five or so days around ovulation, Gangestad said. It's that they want sex for different reasons.

"It may be experienced more as a kind of lust mid-cycle," Gangestad said. "Outside of mid-cycle, women may be more interested in sex for intimacy."

Multiple studies have found that women go for more masculine-looking men when they're ovulating. According to a 2008 review of research by Gangestad and his colleagues published in the journal Proceedings of the Royal Society B, some of the masculine traits fertile women prefer are strong-jawed faces, muscular bodies, dominant behaviors, deep voices and tallness.

Because most studies on the topic have been done on heterosexual women, little is known about how ovulation affects lesbians or bisexual women. One study, published online in May 2010 in the journal Archives of Sexual Behavior, found that ovulation boosted lesbian women's motivation to act on their desires for other women, while bisexual women showed smaller increases in motivation. While women's sexuality is more fluid than men's, study coauthor Lisa Diamond, a psychologist at the University of Utah, told LiveScience, the findings suggest that ovulation could be a window into the biologically-based component of female sexual orientation.

Girls gone fertile

Back in the more-studied world of heterosexual couples, Gangestad's most recently published study on female preferences finds that if a woman is partnered with a man without a masculine face, her eye is more likely to wander during her fertile days. 

This wandering eye doesn't necessarily translate into a cheating heart. Most women probably notice nothing more than a "funny little feeling" of lust during their fertile days, Haselton said. But other studies have shown that the hormonal fluctuations surrounding ovulation do change women's attitudes and behavior.

For instance, fertile women seem to be more open than non-fertile women to the idea of taking "sexual pleasures where [they] find them," in the words of one question asked to participants in the study by Gangestad and his colleagues published in the November issue of Evolution and Human Behavior. They're also more likely to express interest in sleeping with an attractive stranger or someone they don't care about.

A 2006 study in the journal Hormones and Behavior found that women who had their pictures taken during fertile and non-fertile stints were judged to be trying harder to look nice on fertile days, suggesting the hormonal boost of ovulation may translate to real-life decision-making. The women's faces were blacked out, so judges could only go on hairstyles and clothes to make their assessments, said Haselton, who co-authored the paper. To avoid any menstruation effects, none of the women were photographed right before or during their periods.

"We saw that the effect was strongest for women who were photographed closest to the day of ovulation within the high-fertility window," Haselton said.

An online shopping experiment reveals fertile women are also more likely to choose sexy clothes and accessories than women who weren't fertile, according to the study published online in the Journal of Consumer Research in August. For those who aren’t as interested in sexy clothes, ovulation can mean more money. In a widely publicized study published in Evolution and Human Behavior in 2007, researchers found that ovulating strippers made an average of $30 more an hour than menstruating strippers and $15 more an hour than non-ovulating, non-menstruating strippers.

Do fertility fluctuations matter?

The big question, of course, is how much ovulation affects the real lives of the non-strippers among us. Do women give in to the temptation to cheat more often when fertile? (No one has found any evidence suggesting they do, Gangestad said.) And what about the massive number of women on the birth control pill? The pill halts ovulation and smoothes out monthly hormonal cycles, so do women on hormonal birth control act differently than those who aren't on the pill?

The answer to that question depends on whether women base any long-term relationship decisions on the changes that happen around ovulation, Haselton said. So far, none of the studies on the topic have shown that ovulation changes women's preferences for long-term mates, just short-term flings.  

"Being on the pill does remove a lot of the variation in the changes in hormones across the cycle, so we certainly would expect it to have an impact on a woman's behavior," Haselton said. "But if it's only having an impact within that narrow range of fertility, then maybe it's not that big a deal."

From an evolutionary perspective, unraveling the question of ovulation could reveal a lot about how human reproduction evolved. It's possible that women really are trying to signal their fertility with sexy clothes, Haselton said, but it's more likely that the changes are side effects of the hormonal tides women experience throughout the month.

Perhaps women really did evolve to conceal ovulation but can't quite cover up all the side effects, Haselton said. "There may be a co-evolution arms race between women concealing and men detecting any cues that leak out."

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