What if women who knew when they were fertile were less likely to have sex at those times? (image, but not the hypothesis, from Wikipedia)
Ready or not, here comes my final speculative post exploring the evolutionary conundrum of why our species conceals ovulation.
Maybe the ultimate evolutionary benefit from concealing ovulation had nothing to do with males, but was strictly a womanly matter, selected for as a way of concealing information from other females. Thus, even though biologists have mostly been concerned with identifying male-male competition, in which bull elk, dramatically maned lions, or silverback male gorillas tussle with each other for access to females, the reality is that females compete, too.
They just do so with greater subtlety. So perhaps prehistoric women who obscured their ovulation gained an advantage because by doing so, they obscured the fact that they were sexually mature and therefore made themselves less liable to being attacked or otherwise hassled by other, more dominant females. Not very persuasive, especially since socially dominant women are no less likely to conceal their ovulation than are their less dominant “sisters.”
Finally, a particularly intriguing explanation, one that speaks to an especially perplexing aspect of women’s ovulation: The fact that it is concealed not only from others (whether male or female), but from the ovulating women themselves. It’s one thing to keep others in the dark. Given that ovulation is such a key aspect of every woman’s biology, however, why doesn’t evolution let women in on their own secret?
(Actually, a small proportion of women can detect their ovulation, being sensitive to mittelschmerz, literally “middle pain,” a twinge of abdominal discomfort felt when the egg is released from the ovary.)
There is at least one way that natural selection could have favored women who didn’t know when they themselves ovulated. It’s a bit convoluted, but worth your attention. What if women who knew when they were most fertile actually had fewer children as a result? Seems counter-intuitive, but not necessarily. Consider the fact that in advanced, modern societies as well as those that are non-technological (and thus, presumably closer to our ancestral condition), women consistently want fewer children than do their husbands. Makes sense, since from a strictly biological point of view, the evolutionary payoff of reproduction falls equally to both fathers and mothers, whereas most of the costs are born by the mothers, all the more so during most of our evolutionary childhood when we lacked the option of anesthesia and Cesareans if necessary.
Until quite recently, even in the Western world, women often died giving birth, whereupon their husbands simply remarried. And the likelihood is that even early in our evolutionary history, women were aware of the morbidity and mortality associated with pregnancy and especially childbirth. All this makes it possible that our great-great, etc., grandmothers who detected their “time of the month” might well have made special efforts at those times to fend off the romantic advances of our great-great, etc., grandfathers.
Call it the Headache Hypothesis.
It’s not far-fetched. In fact, it seems especially appropriate to explain a uniquely human trait-concealed ovulation-by invoking another uniquely human trait: Conscious awareness of the down-sides of reproduction itself, something that other species presuably lack. The Headache Hypothesis offers the unique, added twist that women who knew when they were ovulating (as well as those who broadcast their status) would have been selected against. Who, then, would have inherited the earth? The descendants of those who did not know when they were ovulating, i.e., those from whom this information was concealed.
It should also be acknowledged that ovulation in our species isn’t an entirely covert operation. A number of research studies have shown, for example, that men can distinguish between the odor emanating from women who are ovulating and those who aren’t. Moreover, women themselves show a number of subtle behavioral differences during the peak of their ovulatory cycle, including a preference for more “masculine” seeming men.
Most striking, perhaps, is research conducted upon lap-dancers in New Mexico, in which eighteen women recorded their menstrual cycles and tips received during a 60-day period. Normally cycling women earned an average of $335 for a 5-hour shift during their fertile phase, $260 per hour during luteal (largely infertile) phase and a paltry $185 per shift during menstruation (infertile). As a kind of clincher, women using contraceptive pills showed no earnings peak, just as they had no fertility peaks. It isn’t known whether ovulating women sent out pheromones or whether they danced differently—presumably, more seductively—but in either case, there was something about them that men detected and responded to. (The Headache Hypothesis also must confront the inconvenient fact that modern women, at least, are also somewhat more libidinous when at peak fertility … just not necessarily with their current social partners.)
Nonetheless, there seems little doubt that compared to other animals, human ovulation is concealed, and remarkably well at that. If women looked, smelled, or behaved in ways that were dramatically different during ovulation, everyone would have known it long ago, and we wouldn’t have needed detailed biological research to tease out the differences.
As it is, our own biology presents us with yet another fascinating evolutionary mystery, one that biologically-based insights enable us to identify and some day, presumably, to solve.
