Being exposed to the smell of a sweaty male armpit can make a woman feel calmer, according to a new study by a U.S. research team.

Male sweat contains one or more chemical signalling compounds, known as pheromones, that can affect both a woman's state of mind and her reproductive hormone levels, according to a study by Dr George Preti, of the Monell Chemical Senses Centre in Philadelphia, and colleagues.

In the study, reported in the journal Biology of Reproduction, Preti and team placed pads under the armpits of male donors to collect sweat. They then put concentrated compounds extracted from the sweat, but masked by a fragrance, under the noses of female volunteers. After being exposed to the smells for six hours the women reported feeling less tense and more relaxed.

Preti and team also measured the womens' levels of luteinising hormone, which plays varied roles in triggering ovulation and in sex drive. Pulses of this hormone released from the brain increase in size and frequency as women approach ovulation. In the experiment, the researchers found when women smelt the sweat extract it induced a surge of the hormone.

"The underarm contains physiologically active pheromones," Preti says. He also took part in an earlier study investigating why women who have heterosexual sex weekly have more normal menstrual cycles of regular length than those who do not. In that study, an armpit extract from male donors was also used to test whether exposure to it had any effect in its own right.

The menstrual cycle lengths of two groups of women subjects were evaluated over several months: one group was exposed to regular applications of the extract while a control group was exposed to a compound that had no smell. The women who received the extract were less likely to have variable cycle lengths and fewer of them had abnormal cycles.

In a report in Nature News Service on the latest findings, another Monell Centre pheromone researcher, Dr Charles Wysocki, speculates that women may have evolved to respond to signals that make successful reproduction more likely - the scent of a male partner might not only help to trigger ovulation at the right time but may make women more relaxed and receptive to sex as well.

Researchers at the University of Texas reported two years ago that men can tell by smell alone when women are at their most fertile. The had male subjects sniff T-shirts worn by women during fertile and infertile stages of their menstrual cycles and found the men overwhelmingly rated the smells from the shirts to be most pleasing or "sexy" when they had been worn during a fertile phase.

The latest findings also suggest that if the active pheromones in male sweat can be isolated they may prove to be the basis of new fertility drugs or relaxant perfumes for women.

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