From Wikipedia's Lapdance Entry

"Menstrual cycle and earning power

In 2007, based on statistics from 18 dancers over 60 days, it was noted that female lap dancers earned the highest tips around the time of ovulation, during the most fertile period of their menstrual cycle, and the lowest tips during menstruation; the average difference in earning between these two times amounted to about $30 per hour. Women on the pill earned overall less than those not on the pill. The results were interpreted as evidence of estrus in humans: females apparently advertise their fertility status to males in some manner.[20] This find earned its authors the 2008 Ig Nobel Prize in Economics.[21]"