A lingering question about human evolution is what happened to female estrus. In other mammals, estrus is more commonly referred to as being "in heat," the brief window when females can be impregnated. Women, on the other hand, can get pregnant at any point during their menstrual cycles, and -- unlike some other animals, like baboons, which may experience temporarily engorged vulvas -- ovulation comes with no outright physical signs of fertility. At the same time, some evolutionary psychologists suppose that heterosexual men are subconsciously able to detect women's concealed ovulation, as long as they aren't on the pill [source: Pappas].
A trio of University of New Mexico researchers tested that hypothesis in a less-than-academic environment: a strip club. For 60 days, strippers recruited for the study logged their daily tips from customers, along with their menstrual cycle progress. Plotting those data points together revealed a fluctuation in earnings correlated to fertility. During ovulation, strippers took home $335 per shift, compared to $185 per shift during their low-fertility periods [source: Miller, Tybur and Jordan]. However, customers tipped women on hormonal birth control roughly the same amount each day, averaging $80 less than their naturally cycling cohorts [source: Miller, Tybur and Jordan].
Why the paltry pay? Researchers suspect that men instinctively found the naturally cycling, more immediately fertile dancers more sexually appealing [source: Miller, Tybur and Jordan].