Male underarms relax women and may make them more fertile. Credit: © GettyImages

Ladies! Looking for a way to relax? Then try sniffing a man's underarm. New research shows that armpit sweat calms female volunteers.

It also shifts menstrual cycles, so the discovery could give rise to perspiration-derived drugs to manipulate female fertility.

"The underarm contains physiologically active pheromones," says chemist George Preti of the Monell Chemical Senses Center in Philadelpha, Pennsylvania, who led the study. These behaviour-altering chemicals - which are common throughout the animal world - can affect the brain, and hence our bodies, without our even realizing it.

For six hours, female volunteers were exposed to pheromones concentrated from manly armpit odour - mercifully masked by fragrance. Levels of luteinizing hormone, a key player in the menstrual cycle, were monitored.

Luteinizing hormone is released from the brain in pulses - these become larger and more frequent as a woman approaches ovulation. Exposure to the male odour accelerated the arrival of the next hormone rush, Preti's team found1.

The volunteers also reported feeling less tense and more relaxed as they sniffed. Both effects may be a throwback to our impulse-primed past, suggests Preti.

Busy hunter-gather cavemen and cavewomen may have had limited time to enjoy each other's company. A cavewoman's reproductive system may have evolved to be ready for her beau by shifting hormonal levels in response to his scent, agrees pheromone researcher Charles Wysocki, also at the Monell centre. "Women may respond to signals in the environment that facilitate successful reproduction," he says.

Again, this is speculative, but the reported relaxing effect of male armpit odour may also increase the chances of reproduction. A relaxed woman is more likely to be receptive to a man, suggests Wysocki.

"This presents a dichotomy," says Ivanka Savic from the Karolinska Institute in Stockholm, Sweden, who studies the effects of pheromones on the brain. The perceived free will of modern-day dating may be compromised by our ancestor's needs. "Our biology is primitive but we're living in a sophisticated world," she says.

The findings may be good news for couples wanting to maximize their chances of conception. Once the active pheromones have been identified and isolated, researchers may be able to modify them to produce a new generation of potent fertility drugs, Preti speculates. In the search for new drugs, "people have looked in the rainforests and under the seas", he says. "But there are some very interesting physiologically active chemicals under the arm."