In a lot of animals, females make it abundantly clear when they're ovulating, or when they can get pregnant. But in humans the signs aren't as obvious. It was thought to be concealed, but this researcher and a group of international colleagues wondered if women might give off subtle clues when they're most fertile, specifically in their voices. So they recruited young German woman for an unusual study. Over one menstrual cycle, the women repeatedly recorded a series of vowel sounds. Meanwhile the women had to follow a strict protocol and collect their urine every morning. The urine helped the team monitor the women's hormones, and when they compared the hormone data to the voice recording they found the pitch of the vowels got a bit lower over the course of a month. Now the shift is subtle but it is audible. The researcher suspects the shift in pitch is due to sex hormones which cause the vocal chords to swell. She says this might also be evolutionary because it signals when a woman is ready to procreate, and this may make a man more amorous. This conclusion was supported by another recent study which discovered that men and women rated women's voices as more attractive when they were in fertile stages. A third study has recently muddied the water a bit. In their research, the women's voices went up in pitch as they got closer to their ovulation.