The results are in line with evidence that the female voice box, or larynx, is under the influence of sex hormones, says Gallup. He says the changes in the female voice during peak fertility support the view that women are "different" at that point in the menstrual cycle - in other words, that they experience oestrus.

The theory of human oestrus remains controversial because its effects are subtle; human females show none of the distinct genital swellings seen in other female mammals "in heat". But there is increasing evidence of more subtle changes. "Other differences include changes in sexual receptivity and odour sensitivity," Gallup says.

Martie Haselton and Greg Bryant at the University of California, Los Angeles, say that vocal pitch plays an important role in judging fertility. "We have found that voices are higher in pitch on high-fertility days of the cycle," says Haselton.