Unlike their sisters in the animal kingdom, human females don't openly advertise their ovulation. But even without a human version of the baboon's bright pink behind, signs of fertility sneak out, according to several studies. Subconsciously, women dress more provocatively and men find them prettier when it's prime time for conception. And a report from the University of New Mexico demonstrates that the cyclic signs have economic consequences.

Psychologist Geoffrey Miller and colleagues tapped the talent at local gentlemen's clubs and counted tips made on lap dances. Dancers made about $70 an hour during their peak period of fertility, versus about $35 while menstruating and $50 in between.

Miller links the wage fluctuations to changes in body odor, waist-to-hip ratio, and facial features. Despite operating at the upper limits of flirtatiousness already, he says there may also be subtle shifts in their behavior—"how they talk and move when enticing a customer to buy a dance, and how they perform the dance itself."

Women on the pill averaged $37 (and had no performance peak) versus $53 for women off-pill. The contraceptive produces cues indicating early , not an enticing target for a would-be suitor. Birth control could lead to many thousands of dollars lost every year.

The researchers were surprised that almost no one in the business had noticed the pattern before. But if you're a woman in any service-industry job looking to maximize your tips, Miller suggests scheduling more shifts for the phase right before ovulation: "It might help to know about this so that you can exploit these effects."