Once a month, your favorite lap dancer will make you go broke. Scientists have discovered that lap dancers gain more when they are in the fertile period of their menstrual cycle. This would mean that women subtly show off when they are most fertile, even if the mechanism is not clearly known.

Women, unlike most mammal females, don't have an estrus period, when they display obvious fertility that lures potential mates. During the human evolution, the estrus was lost. "The notion is that women evolved "concealed ovulation" along with around-the-month sexual receptivity the better to manipulate males by keeping them in the dark," said Geoffrey Miller, an evolutionary psychologist at the University of New Mexico in Albuquerque.

Now, Miller's team has discovered that a woman's fertility state may not be so concealed. They collaborated with 18 lap dancers from local clubs, asked them to report on their work hours, tips, and the menstruating periods.

Lap dancers generally work in 5-hour shifts, when they perform an average of 18 or so 3-minute "dances", with an average of $14 per "dance". During the 60-day research, the team gathered information from 5300 lap dances assigned in the menstrual phase, the high-fertility estrous phase, or the luteal phase.

Of the 11 subjects with normal menstrual cycles, those in the fertility period gained about $70 an hour, while those in the luteal phase just $50, and just $35 an hour for menstruating subjects. The other 7 lap dancers were on birth control pills and had lower incomes overall, as they lacked the fertility peak money gain. "The numbers suggest that men can tell when a woman is most fertile, although the message seems to be conveyed by "subtle behavioral signals" that evade conscious detection", wrote the researchers.

The research did not detect if it is scent or other physical changes that attract the men, but it was clear that the dance moves in that period had something special that attracted men. "The result fits with my findings that it's possible to detect ovulation through the effect of raised levels of estrogens on the way women walk and dance. It is highly possible that estrogen modulates motion abilities. It seems to be most likely that body motion--and not pheromones--is the information carrier.", said evolutionary psychologist Karl Grammer of the Ludwig Boltzmann Institute for Urban Ethology in Vienna.