Though some research suggests that women unconsciously advertise their fertility, a new study shows that their voices are misleading to would-be mates.

Just before ovulation, when women are at their most fertile, their voices become higher-pitched — but the same happens after ovulation, when they’re less fertile.

“Other species have very obvious indicators of fertility, like the sexual swellings of Barbary macaques,” said Julia Fischer, a vocalization researcher at the German Primate Center in Göttingen, Germany. “We’d like to know if there is some comparable signal in humans.”

Evolutionary biologists long assumed that women hide ovulation to encourage faithfulness in their partners. If they can encourage males to mate with them regularly, rather than allowing men to cherry-pick the best times, the pair’s bond will be stronger. Women and their offspring can continually benefit from food and protection the male may provide.

During the past decade, however, hidden female fertility has been increasingly questioned. Several studies have found that men seem to subconsciously detect it. Lap dancers earn higher tips just before ovulation, and women in relationships report their mates are more jealous and attentive to their whereabouts.

Among the possible cues are scent, appearance and behavior: Fertile women seemingly smell better to men, look prettier, dress more alluringly and feel more confident. Another proposed signal is voice, which some studies suggest changes subtly but detectably during high- and low-risk fertile periods.

In a study published Sept. 21 in PLoS One, Fischer and her colleagues characterized the voices of 23 German-speaking women’s voices daily for a menstrual cycle. They analyzed variation in pitch, hoarseness, harmonics and breathiness (think Marilyn Monroe). The women also gave daily urine samples, so their hormones and the precise day of ovulation could be tracked.

Fischer’s team found that voices were highest three days before ovulation, lowered during ovulation, then rose again. The pattern makes it unlikely men can use a woman’s voice as a reliable mating cue.

‘There is no clear, broadcast signal of fertility.’

Playing the recordings to a group of heterosexual non-German speaking males, the researchers found only a slight preference for the higher-pitched pre-ovulation voices. No clear pattern of male preference emerged. Different men liked different things.

“The one consistent variation we did find was during menstruation,” Fischer said. During this time women’s voices were heavier and less harmonic, probably because having more water in their bodies changes the form of the vocal chords.

“It is important that [Fischer's team] measured the women daily, it allows us to see the complexities that our method didn’t,”  said Gregory Bryant, a researcher at the University of California, Los Angeles who has studied the vocal effects of ovulation.

Two years ago, Bryant co-authored a paper that found a clear male preference for women’s voices in the high-fertility phase of their cycle. They only sampled at two points during a month, however.

Bryant does wonder if some effects were obscured or missed in Fischer’s 23-woman sample size. “I don’t think the results rule out a vocal cue of ovulation that men could unconsciously perceive,” Bryant said. “But I agree that there is no clear, broadcast signal of fertility.”

According to standard evolutionary psychology, men have an incentive to detect when women are most likely to become mothers, allowing them the best chance of continuing their genetic lineage. To attract the best mate, women are motivated to enhance their femininity around the time of highest fertility, but perhaps only to a point.

Among Barbary macaques, for example, female monkeys try to lure males to mate, but because they hide the precise date of fertility, males can’t be certain who fathered each child. As a result, all males in a group contribute to raising its young.

Each reproductive system leaves its mark on a species. Since humans are neither exclusively monogamous nor as multi-partnered as macaques, a variety of forces are at play.

“It makes sense for human women to conceal fertility,” Fischer said, “probably to stabilize the pair bond.” But, at the same time, being a little extra attractive when she’s fertile might win a female the best father for her children, if she’s inclined to shop around.

Both Fischer and Bryant agree that differences a woman’s voice shows during ovulation are, if detectable at all, very slight and noticed only by those who know her well.

“People ask me if I can tell when a woman’s ovulating,” Bryant said. “I say, ‘Of course not.’ But if I know a woman, and am familiar with her voice, maybe I just like her a little more that day, and I don’t know why.”

Citation: “Do Women’s Voices Provide Cues of the Likelihood of Ovulation?” By Julia Fischer, Stuart Semple, Gisela Fickenscher, Rebecca Jürgens, Eberhard Kruse, Michael Heistermann, Ofer Amir. PLoS One. Sept. 21.