According to a new study all a healthy heterosexual woman needs to give their hormone levels a kick start is the smell of a man's sweat.
Scientists at the University of California, Berkeley have found that a mere hint of male sweat in the air raises the amount of the stress hormone cortisol in women's saliva.
Cortisol is secreted by the body to help maintain proper arousal and sense of well-being, respond to stress and other functions.
They believe their study represents the first direct evidence that shows that people, just like rats, moths and butterflies, secrete a scent that affects the physiology of the opposite sex.
Lead author Claire Wyart, a post-doctoral fellow at Berkeley says this is the first time it has been shown that a sniff of an identified compound of male sweat, affects women's hormonal levels as opposed to applying a chemical to the upper lip.
The study was prompted by previous studies by Wyart's colleague Noam Sobel, an associate professor of psychology and director of the Berkeley Olfactory Research Program.
Sobel discovered that the chemical androstadienone, thought to be a male chemical signal which is found in male sweat and is an additive in perfumes and colognes, changed mood, sexual arousal, physiological arousal and brain activation in women.
Wyart says there is no definitive evidence that humans respond to the smell of androstadienone or any other chemical in a subliminal or instinctual way similar to the way many mammals and some insects respond to pheromones.
She says though some humans develop a small patch inside their nose resembling the vomeronasal organ in rats that detects pheromones, it appears to be vestigial, with no nerve connection to the brain.
Pheromones are chemical molecules expressed by a species aimed at other members of the species to induce stereotyped behaviour or hormonal changes.
Wyart says there is debate over whether human pheromones exist, because humans do not exhibit stereotyped behaviour.
However this male chemical signal, androstadienone, does cause hormonal as well as physiological and psychological changes in women and Wyart says studies need to be done to understand how androstadienone affects female cognitive functions.
The authors say the finding suggests that there may possibly be better ways to raise cortisol levels in patients with diseases such as Addison's disease, which is characterized by low cortisol rather than giving the hormone in pill form, which has side effects such as peptic ulcers, osteoporosis, weight gain and mood disorder.
They say merely smelling synthesized or purified human chemosignals may be used to modify endocrine balance.
The main focus of research on human pheromones has been sweat, and male underarm sweat has been shown to improve women's moods and affect their secretion of luteinizing hormone, which is normally involved in stimulating ovulation.
Other studies have shown that when female sweat is applied to the upper lip of other women, these women respond by shifting their menstrual cycles toward synchrony with the cycle of the woman from whom the sweat was obtained.
Androstadienone is a derivative of testosterone that is found in high concentration in male sweat, as well as in saliva and semen.
However, though its effect on a woman's mood, physiological arousal and brain activity suggests that the chemical is a possible pheromone-like signal in humans, its effect on hormone levels was unknown.
Wyart and Sobel wanted to test whether androstadienone affected hormone levels as well and they focused on the study on the hormone cortisol.
Two trials were carried out with a total of 48 undergraduate women, average age of 21, at UC Berkeley.
The women were asked to take 20 sniffs from a bottle containing androstadienone, which smells vaguely musky, then over a period of two hours, the volunteers provided five saliva samples from which cortisol levels were determined.
The women had no skin contact with androstadienone.
The researchers found that compared to their response when sniffing a control odour (yeast), the women who sniffed androstadienone reported an improved mood and significantly higher sexual arousal, while their physiological response, including blood pressure, heart rate and breathing, also increased.
This is consistent with previous research.
The researchers also found that cortisol levels rose within about 15 minutes of sniffing androstadienone, and remained elevated for more than an hour.
Wyart says though this is the first time a specific component of male sweat has been shown to affect women's hormones, other constituents of male sweat may have a similar effect.
She says the question is which comes first - the change in cortisol level, which may induce a change in mood or arousal; or a mood change that increases cortisol levels.
Wyart says they now need to look at other hormones that could explain the diversity of effects of androstadienone.
The study is published in the current issue of The Journal of Neuroscience.