A woman's voice becomes more alluring when she is at her most fertile, according to US research.
Recordings of women taken at different points in their menstrual cycle were played to people of both sexes.
New Scientist magazine reports that the voices rated as most attractive belonged to women at peak fertility.
The study suggests sex hormones can alter the workings of the voice box, but the change may be too subtle to pick up in many situations.
Human reproduction differs from reproduction in other mammals in that there are no obvious signs that a woman is at her fertile phase.
However, scientists have suggested that very subtle changes caused by the rise and fall of different sex hormones can be detected by men, who then perhaps find a woman more attractive without necessarily even realising why.
The latest research, from the State University of New York at Albany and originally published in the journal Human Evolution and Behavior, involved taking recordings of women counting from one to 10 at four points during the menstrual cycle and then played them back to male and female students.
The recordings taken close to ovulation - the moment at which an egg is released and can be fertilised - were marked as more attractive than recordings of the same woman speaking earlier or later in her cycle.
There was no effect if the woman was taking oral contraceptives, which change the ratio of sex hormones during the cycle.
The researchers wrote: "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."
The researchers did not test the recordings to see if the woman was speaking with a higher or lower tone at different points in her cycle, and one leading voice-attraction researcher said that the subtle differences might not be practically useful in the real world.
Dr David Feinberg, from the McMaster University in Canada, said: "The missing link here is finding out how this works in plain conversation - in a bar, for example.
"While it's possible, the other issue is that women do have mood changes across their menstrual cycle, and people might just be attracted to a happy-sounding woman, rather than a fertile one."