The Strange Case of Concealed Ovulation: Another Evo-Mystery

I love science, and you should, too, if only because it provides us with the best (perhaps the only) way of genuinely knowing the world. But as a scientist, I also love what we don’t (yet) know. Indeed, I often think that in our enthusiasm for what has been discovered, scientists give insufficient attention—at least in their public utterances—to our unknowns. After all, it’ s what we don’t know that drives research and other forms of inquiry. As Richard Dawkins once put it, mystics love mystery for its own sake; scientists love it because it gives us something to do.

In several earlier posts, I looked at one such mystery: why women, alone in the mammal world, have prominent nonlactating breasts. Continuing now in the spirit of celebrating the unknown—not abracadabra, fancy-schmancy, mystical mystery, but genuine, hard-headed, scientific puzzles worth exploring and solving—here’s another one: Why are we among the very few species in which ovulation is hidden?

Go to a zoo and contemplate the chimpanzees or nearly any other primate. There is simply no question when the females are ovulating; it’s as clear as the bright pink cauliflower on their behinds, sometimes so dramatic that worried zoo-goers ask whether the poor dears have a tumor. Or consider almost any unspayed dog: There’s no doubt when she is “in season.” But Homo sapiens are another kettle of biological fish.

In fact, it is a remarkable fact that even now, in our techno-advanced, biomedically sophisticated 21st century, and despite intense interest and genuine consequence, there is still no simple, reliable “rhythm method” of birth control; that is, no guaranteed way for women to know with confidence when they are not fertile because, conversely, there is no easy way to know when they are. At the same time, pharmaceutical companies make huge sums of money marketing test kits that provide women with precisely the same information that most mammals get for free.

And this, in turn, raises the immediate question, Why the deep, dark secret? After all, the time of ovulation—when fertilization is most likely—is one of the most important things about every woman’s biology. Given that other animal species shout it from the rooftops, why do we keep it under wraps? Or, better put: What could be the reproductive payoff that gave women who obscured their time of maximum fertilizability a reproductive advantage over those who went public? In short, concealed ovulation in Homo sapiens is not just a recurring monthly mystery but an intriguing biological one that cries out for an evolutionary explanation.

At least as intriguing is this fact (at least for those of us who cherish knowing what we don’t know every bit as much as reveling in what we do): The mystery is as yet unsolved. Not that there aren’t explanations. In fact, there are lots of them, reminiscent of Mark Twain’s observation that it was easy to stop smoking: he had done it dozens of times! Here and in subsequent posts, I’ll review some of the most prominent hypotheses. The likelihood is that one of them will eventually be proven correct. Or not. (For additional details, if you’re really interested, see How Women Got Their Curves and Other Just-So Stories, by D. P. Barash and J. E. Lipton. Columbia University Press, 2009).

One possibility can be called the Keep Him Guessing Hypothesis, the idea being that by keeping her cards (in this case, her eggs) close to her chest (actually, a different part of her anatomy), our great-great-grandmother kept our great-great-grandfather in the dark as to when sexual intercourse was most likely to yield a parental jackpot. The possible payoff in this case would be that so long as the male is unable to determine when copulation with the female is going to “succeed” reproductively, he would be obliged to stick around all the time.

If ancestral women broadcast a clear message when they were ovulating, à la chimpanzees, then it is likely that ancestral men would have found them sexually appetizing at those times, but pretty much only then, being inclined to look for other sexual partners when their mates were obviously not open for reproductive business. Any such male tendencies would have been facilitated by the fact that other females would be broadcasting their maximum fertility, so a male with a wandering eye wouldn’t have to worry about wasting his reproductive efforts. Male philandering would have been all too easy, at least from the perspective of the male’s identified mate, who presumably preferred that he stay home to guard the cave, bring home the mastodon meat, and help care for the kids.

In addition, many species—including our own—engage in what biologists term “mate guarding,” whereby males in particular stay home and alert to defend “their” females’ honor—actually, their own paternity—from the threat posed by other sexually active males. But were ovulation to be clearly signaled, males would be free to abandon their mates, without fear that while gallivanting, they might be cuckolded by other males following the same strategy.

In short, the Keep Him Guessing Hypothesis suggests that prehistoric women played a sexual shell game on prehistoric men in order to keep them around, 24/7, something likely to be especially useful for a mammal that gives birth to helpless young, who, moreover, take a ridiculously long time to achieve independence. But the Keep Him Guessing Hypothesis isn’t the only conceivably viable explanation for concealed ovulation in Homo sapiens. Stay tuned: More to follow.

[Image from PZ Myers’ “Pharyngula” blog]