A woman's voice becomes more attractive to both men and women at the point in her monthly cycle when she is at her most fertile, according to a study of vocal changes during ovulation.
If the findings can be replicated, they will add further weight to the theory that women give off subtle and unwitting cues about their fertility as part of an evolutionary battle of the sexes centred on attractiveness and fidelity.
The research also found that men tend to find higher-pitched female voices more attractive: so the idea is that a woman's voice would sound less like Mariella Frostrup and more like Marilyn Monroe at a certain time of the month.
Unlike most mammals, human females show no obvious signs of oestrus – the point when they ovulate and are at their most fertile – but some scientists believe there may be hidden signals that men, and other women, can subconsciously identify.
Nathan Pipitone and Gordon Gallup, of the State University of New York in Albany, investigated vocal attractiveness by recording female volunteers counting from one to 10 at four different points in their menstrual cycle, which they categorised as low to high risk of conception.
They then asked a panel of men and women to listen to the recorded voices and give their unbiased assessment as to whether they sounded attractive or not. Both sexes judged the women's voices to be sexiest when they were recorded at the times during their menstrual cycle when they were at their most fertile.
"The results showed a significant increase in voice attractiveness ratings as the risk of conception increased across the menstrual cycle in naturally cycling women. There was no effect for women using hormonal contraceptives," the scientists said. "More work is needed to identify the biological mechanisms that underlie these perceptual differences, but growing evidence points to the impact of hormones on the larynx as being the source of these changes," they say in the study, to be published in the Journal of Evolution and Human Behaviour.
A separate study by Gregory Bryant and Martie Haselton, of the University of California in Los Angeles, investigates the pitch of 33 women's voices at various points in the menstrual cycle. They found that the women tended to become higher pitched at the most fertile point of the cycle, during ovulation.
Previous work on female voice pitch had established that men tend to find higher-pitched voices more attractive. Taken together, the findings of all three studies suggest that the female voice increases in pitch at the most fertile moment of the monthly cycle.
The fact that men notice differences in vocal attractiveness suggests there is a subtle evolutionary battle of the sexes going on, Dr Gallup told New Scientist magazine. As women evolve ever more efficient ways to conceal fertility, men become increasingly sensitive to the small and subtle changes that take place.
Other scientists have documented changes to body scents, flirtatious behaviour and style of dress that they believe are also used as clues to a woman's fertility status. For instance, Geoffrey Millar, of the University of New Mexico, has shown that lap dancers tend to be given bigger tips by male customers when the women are ovulating. "The voice changes might explain some of the shift in lap dancer tip earnings. Dancers certainly chat with their customers," Dr Millar told New Scientist.