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.