I love the Bonobos, those crazy little apes. You know what I like best about them? Just like us, those little guys like to have sex all the time. I am not kidding; living in the primal horde is like having been 20 years old in the 1970s. Unlike other ape species, female bonobos engage in sex even when they are not fertile. They do it just for fun. They even like girl-ape on girl-ape action from time to time.
Human females also have sex when they are not fertile and, in fact, most women cannot tell you when they are at peak fertility. The evolutionarily dominant strategy for human females appears to have been to conceal their fertile phase from their mates. You see how this could improve the fitness, which is dependent on the survivability of their children, of one of our ancient ancestors. If her mate knew she was fertile, then she would have a pretty hard time sneaking out to get herself inseminated by a superior male. Even if she did, her mate might be suspicious and refuse to provide for any offspring she then has. So she conceals her fertility from her mate to prevent mate-guarding and she does so well that eventually she conceals it from herself.
So it is accepted that humans have concealed fertility, but maybe we let up little clues from time to time and swing our hips a little more when we walk, dress a little more provocatively, smile a little more, and become a little chattier. These little hints of fertility are called "estrus cues" and their existence has been tested in a laboratory. It turns out that, maybe, we don’t conceal our fertility as well as we think. There is a way of determining this without experiments that uses the theory of revealed preference that I wrote about in my last post, Do Women Value Ethnicity Over Income in a Mate? If a man reveals that he prefers a woman who is at peak fertility to an identical woman who is not, then we would have proven that he has picked up on subtle clues that the woman is fertile. The best way to measure just how much he prefers her is by seeing how much money he is prepared to spend to be close to her. So, who pays to be close to a woman? Well, patrons in lap dancing clubs of course.
Strippers don’t make their money dancing on the stage; they make it giving lap dances. If they are really lucky the gentleman they are performing for will decide to move the performance from the main area to a private room. It is in the private rooms that lap dancers really make their money. In fact, in a private room a lap dancer can make $400-$500 per hour. A patron often samples several dancers before finding one he prefers to take into the private room. One can imagine that he makes this decision based on his interactions with her: talking, touching, smelling and watching. The dancers have to compete with each other for the opportunity to move into the private room so they have an incentive to sell themselves. The question is, are they more successful at selling themselves when they are fertile? If they are, then we will have effectively shown that fertility is not completely concealed from the male of the species.
Clever researchers have done just this, collecting data from dancers in Albuquerque, New Mexico. Lap dancers there make $10 per 3 minute song in the main area or $20 in the private area and, as a point of information, they don’t remove their panties. In the trial, the dancers who volunteered to provide information recorded their mood, work hours, work location, tip earnings and the day that their period began. They did this for 60 days, basically two complete menstrual cycles. From that information the researchers estimated when the dancer reached peak fertility, which was between 9 and 15 days from the onset of their period. They also asked if the dancer was using birth control pills, in which case she would not have a fertile phase.
The results are actually quite remarkable. The lap dancers made $90 more per shift when they were ovulating than when they are in their luteal stage (the stage after ovulation but before menstruation) and $170 more than when they are in their menstrual phase. The news was very bad for lap dancers taking the pill. Those women made an average of $193 per shift compared to normally cycling women who averaged $276 per shift. Given the number of shifts the women perform, this difference translates into an almost $700 monthly penalty for women using oral contraceptives.
The theory of revealed preference shows that fertility may be hidden but it is not completely concealed either. Economic theory suggests that these women should work more shifts during their fertile stage but they did not. So while it may be the case that men prefer lap dancers when they are fertile, the dancers themselves are not aware that they have a phase in which they are more attractive to the customers. It is almost a shame, it would be an easy way for them to increase their income.
The authors summarize their finds better than I can: "For these reasons, we suspect that human estrus cues are likely to be very flexible and stealthy—subtle behavioral signals that fly below the radar of conscious intention or perception, adoptively hugging the cost-benefit contours of opportunistic infidelity.” (pg. 380)
I like to think that the bonobos would approve.
Miller, G., J. Tybur and B. Jordan (2007) "Ovulatory cycle effects on tip earnings by lap dancers: Economic evidence for human estrus?" Evolution and Human Behavior, Vol (28): pp 375–381.