Unlike baboons and cats, human females don’t advertise their fertility with obvious signals like swollen red bottoms and loud howling.
For years, though, evidence has been building that women become subtly different when they’re ovulating. By talking, dressing, acting and even smelling more alluring, studies have suggested, women might increase their chances of having sex when they are most likely to conceive. And even if men don’t realize what’s happening, they seem to find women more attractive at that time of the month.
A new study offers another view. Focusing purely on the way women talk over the course of a month, researchers found that the female voice is not a reliable predictor of ovulation.
Those results support the theory that, in our species, women tend to conceal the timing of their fertility so that men will be more likely to stick with them for the long haul. And while the conversation is far from over, the findings only add to the complexity and mystery of human sexual attraction.
"It's just really interesting to find out why we are the way we are," said Julia Fischer, a primatologist and communications researcher at the German Primate Center in Göttingen. “Why do men go for certain women? And why are humans generally monogamous whereas monkeys aren’t?”
Those questions have fueled a number of studies that have scrutinized women during their fertile windows -- and results have been tantalizing. Among other findings, studies have shown that women wear more provocative clothes when they are ovulating, that lap dancers take in more tip money at that time, and that men prefer the smell of T-shirts that have been worn by ovulating women.
There is reason to believe that women’s voices might change throughout their monthly cycles, too. Previous work has revealed that vocal tissues have receptors for sex hormones. Women who take oral contraceptives have more stable voices. And a study published last year, which collected voice and hormone samples from 69 women at two points during their cycles, found that their voices become slightly higher pitched just before ovulation.
To pursue the question in a slightly different way, Fischer and colleagues studied 23 German-speaking women over the course of a full month. Every day during the cycle, the researchers collected urine samples for hormone analysis, and they asked the women to record themselves talking freely about anything. Later, they presented pairs of voices to British men who did not understand German, and asked them to pick the voice they liked better.
Analyses, published in the journal PLoS One, showed that vowel sounds became slightly lower as ovulation approached for women, though the statistics were not significant and the change would likely be imperceptible to a listener, Fischer said. The new study also found a rise in the pitch of women’s free speech as ovulation approached, just like in the study published last year. But on the day of ovulation, their voices dipped again.
The only reliable trend was during menstruation, when voices generally became more hoarse. As for the attractiveness of their speech, 55 percent of men preferred higher-pitched voices, not enough to reveal an obvious trend.
While the new findings differ from what last year’s study on fertility and voice pitch found, the two studies don’t necessarily contradict each other, said UCLA evolutionary psychologist Greg Bryant, who co-authored the earlier paper. Both were lab studies, and it’s possible that neither captured women’s voices in the right context.
Instead, Bryant said, the research as a whole indicates that it’s generally not advantageous for women to advertise the status of their fertility. From an evolutionary perspective, offering such an obvious signal would make men more likely to want sex with a woman but less likely to stick around to take care of her and her babies.
Still, it is to a woman’s advantage to become ever so slightly more appealing during ovulation to up her chances of enticing her mate at that time. One side effect is that women might give off subtle cues that some men -- or at least, some scientists -- pick up on.
"In an ideal world, women would be able to control whether or not others can detect their fertility, but also get the benefits of acting more feminine at certain times," said Bryant, adding that these insights apply more to our understanding of human evolution than to our understanding of dating strategies and behaviors.
"People have been getting together for a long time without knowing anything about the underlying machinery," he said. "I don't think it’s necessary to know any of this stuff on a day to day basis."