Homo Mysterious: Evolutionary Puzzles of Human Nature (4 page)

Let’s consider, therefore, that menstruation, rather than signaling “no pregnancy,” is a way of ensuring its absence. But of course, evolution shouldn’t promote nonreproduction … except perhaps in a species such as
Homo sapiens
, whose investment in offspring is so great that it pays to establish a kind of competence test, making sure that any would-be fetus and eventual child is sufficiently sturdy to warrant all that expenditure of time and energy and running of risks that are to come.

To ward off menstruation, the newly implanted embryo has to substitute its own HCG for the luteinizing hormone produced by the mother. Molecule for molecule, HCG is more potent than luteinizing hormone, and it actually causes an increase in progesterone levels, which in turn prevents menstruation and maintains the uterus as a rich and warmly receptive receiving blanket for the embryo.

The foregoing leads toward a hypothesis whereby menstruation is part of a regularly repeating competence test. Because HCG
is a very large molecule, it cannot pass directly into the mother’s body by crossing her cell membranes; it must be secreted directly into her blood. As a result, a human embryo cannot guarantee its survival by simply secreting HCG: It has to get to the endometrium and dig itself in. This Big Dig isn’t easy, which might be exactly the point. The process of implantation in human beings is more invasive—and thus more difficult—than in other mammals, consisting of a delicate dance between receptive maternal tissues and a capable embryo. In the earliest stages of pregnancy, it’s the embryo that does nearly all the work, struggling to get itself deeply enmeshed in uterine tissue so that it can eventually get nourishment—but first, so that it can secrete HCG to prevent menstruation. If so, then menstruation is a sword held over the head of the as-yet headless embryo.

Implantation itself is a kind of Rubicon. Once crossed, the mother is committed to ongoing investment, and lots of it. This, in turn, may have selected for the mother ensuring that any inadequate early embryos can be weeded out quickly and painlessly. To summarize, perhaps menstruation is essentially a regularly repeating competence test, whereby evolution selects against embryos whose burrowing and secretory abilities are inadequate. Unfortunately for this hypothesis, however, it makes a prediction identical to the cleansing hypothesis, and one that is not supported by reality: Menstruation should be tied to sexual activity. Also, if people have been selected to menstruate as a means of subjecting their embryos to competence testing, then why isn’t this the case for other species for whom each offspring also represents a major commitment? Why don’t elephants menstruate? Or blue whales, or manatees?

It’s a mystery. Period.

Most female mammals are altogether above board when their eggs are ripe and ready to encounter a suitable sperm. In addition to signs of genital swelling, they typically emit characteristic pheromones and their behavior changes as well. Not so for our own species. (Actually, there is growing evidence that women do in fact
exhibit subtle behavioral cues as to their ovulatory status, but the key here is
subtle
; it is only in recent years that these indications have been discovered. If they constituted what scientists call a “robust phenomenon,” everyone would have known about them long ago.)

The surprising reality is that very few people can tell—and no one with certainty—when their neighbor, friend, relative, lover, or wife is about to ovulate. Not only that, but most women cannot even tell when, or if, they will do so themselves. To some degree, ovulation can be detected by a very small rise in body temperature as well as changes in the consistency of the vaginal mucus, but both assessments are difficult and unreliable. Indeed, the fact that such careful ascertainment must be exercised only further italicizes the extent to which it is not obvious! Even now, we have no reliable “rhythm method” of noncontraceptive birth control, which is to say, no easy way to know when women are ovulating. Pharmaceutical companies make huge amounts of money marketing test kits that provide anxious women the same information that most mammals get for free.

Concealed ovulation, therefore, is a mystery squared: Not only is the timing of human ovulation hidden, and thus a mystery in itself, but furthermore, it is a mystery why it is such a mystery!

It is, of course, possible that concealed human ovulation hasn’t been actively selected for, but rather that shout-out-loud, Technicolor ovulation, á la chimpanzees, is the derived condition—and thus the one that needs explaining—with inconspicuousness, as found among
Homo sapiens
, being the evolutionarily irrelevant default state. This is unlikely, for several reasons. Start with the fact that nearly all mammals (including our closest relatives, the chimps and bonobos) announce their ovulation, which itself is strong presumptive evidence that our ancestors, too, were relatively uninhibited about drawing attention to their ovulatory status.

Beyond this, there is essentially no variability with regard to concealed ovulation in our species. If natural selection were indifferent to whether human ovulation was hidden or advertised, then we would expect substantial variability since public ovulators, concealed ovulators, and in-betweeners would all be pretty much equally fit and thus equally abundant. There is, for example, substantial
variability in human skin color, eye color, blood type, and so forth, all traits about which natural selection is evidently more or less indifferent. But there are no women whose ovulation is even remotely like a chimpanzee’s.

The likelihood, therefore, is that human ovulation isn’t just neutral or subtle but that it is actively hidden. Yet a moment’s thought suggests that if nothing else, any woman who knows when she is fertile (whether or not she informs others) should be better equipped to become pregnant, or avoid pregnancy, or choose her offspring’s father than would someone who hasn’t a clue and doesn’t give any.

Earlier, when considering menstruation, we considered and for the most part rejected the idea that it might have evolved as a social signal. Could the same be true, but reversed, for concealed ovulation? What of the prospect that human ovulation is concealed as a way of
suppressing
a social signal? It is—pardon the expression—conceivable.

Thus, it could be argued that by concealing ovulation, our early hominid ancestors obscured their reproductive status, thereby limiting possible aggressive competition from other, more dominant women. Consistent with this idea, there is growing evidence that—contrary to the generalizations still popular in evolutionary biology about the exclusive maleness of same-sex competition—females generally and women in particular do in fact compete, albeit more subtly than via the chest-beating, fangs-bared style more characteristic of males. Hence, it might well have contributed to a woman’s ultimate evolutionary success if she kept her reproductive status under wraps. Almost literally.

This seems a plausible hypothesis, except that it would be stronger if ovulation were more concealed among women living in more densely interactive social environments and comparatively unobscured when the woman in question was the only show in town. This isn’t the case. Similarly, this hypothesis would be more convincing if younger, less dominant women concealed their ovulation, while older, more socially and physically secure women flaunted theirs. But they don’t.

It is reasonable to hypothesize that concealed ovulation is essentially an evolutionary shell game whereby women who hid their time of maximum fertility kept “their” men in a kind of
sexual thrall. Among species in which ovulation is clearly signaled, males are free to copulate at this time, then essentially abandon their inamorata, often in search of other short-term partners. Such, for example, is the notorious sex life of chimps and bonobos. But in
Homo sapiens
, in which ovulation is concealed, men wanting confidence of paternity are obliged to remain in attendance throughout the female’s cycle, engaging in regular sexual relations during the month. A study examining 68 different primate species, looking for correlations between mating system and visible signs of estrus, found that not a single monogamous species was a conspicuous ovulator.
⁴

Human beings are also unusual among living things—not just primates—in the extent to which they copulate without much regard to ovulation or the details of a woman’s hormonal condition. This further suggests that concealed ovulation may have evolved as a tactic whereby our great-, great-, great-grandmothers made sure that our great-, great-, great-grandfathers kept close tabs on them, instead of (or in spite of) lusting after someone else. One of
Homo sapiens’
signature characteristics is our long period of infancy and childhood dependency, which is why even today, single parenting is difficult. It therefore makes sense that having a devoted mate would enhance the fitness of the woman in question, even if said devotion is purchased via a kind of sexual hostage taking, playing to his unconscious uncertainty rather than his love.

It is certainly possible that if women were more chimplike, men would be, too: copulating avidly with a given partner while she is fertile, but then seeking other and equally alluring “partnerships,” bolstered by an unconscious confidence that his prior mate would not cuckold him in the meanwhile, since she obviously is incapable of conceiving. In addition, it is interesting to speculate that by inducing men to keep close company with a given woman, concealed ovulation contributed to making them fathers and not just sperm donors, since one result of all that “mate guarding,” as biologists term it, would be greater male confidence that their off-spring—or rather, their female partner’s offspring—are in fact theirs too.

But such speculation—compelling as it may be—doesn’t prove anything. For instance, the “keep him close by keeping him uncertain” hypothesis assumes that regular sex is a prerequisite for social
bonding and biparental behavior, yet there are numerous bird species and even some primates (e.g., gibbons) that commonly go long periods without sexual intercourse and are nonetheless socially monogamous, and some that copulate rarely yet demonstrate notably shared parental duties (e.g., pigmy marmosets, in which fathers carry infants and even reportedly assist with the birthing process). Most of us know people who stay devoted to each other even though the sexual spark may be only intermittent, if not altogether extinguished. And on the other side, human marriages can fail despite intense and satisfying sexual chemistry; sometimes, indeed, this is the only thing that works in a relationship, and if so, it’s rarely enough.

In fact, there is another possible explanation for concealed ovulation that goes precisely against the grain of the “shell game” or “keep him guessing to keep him close” explanations. Rather than promoting social and sexual bonding (cynics might say “bondage”), ovulation might be hidden in our species because such obscurantism makes it easier for women to have sex with men other than their designated partner. After all, given that women don’t identify their precise time of ovulation, even the most dedicated man would likely have a hard time guarding “his” woman so closely as to be able to monopolize her sex life—something that would presumably be more possible if her fertile times were clearly signaled as in so many other primates. By obscuring their exact ovulation, ancestral women might therefore have actually given themselves more leeway to mate with other, more attractive males when and if they chose.

In addition to providing women with greater potential choice of mates, concealed ovulation may have yielded a counterintuitive evolutionary payoff, by enabling them to mate with males who might otherwise be potential murderers of their offspring. It is now well established that among many social species—including, presumably, our own ancestors—strange adult males were a major threat to the survival of infants. This is because after taking over a social unit, male langur monkeys, chimpanzees, gorillas, lions, and
so forth often kill lactating infants, which had been sired by their predecessor. Why such carnage? Two reasons: First, these infants had been sired by the
previous
male, so the new harem tyrant has no genetic interest in preserving them. And second, by killing their suckling babes, a newly ascendant male induces nursing mothers to begin cycling once again, thereby making these females potential recipients of the murderous male’s sexual attention and, eventually, contributing to his reproductive success.

Not a pretty picture, especially for the victimized infants and their mothers.
ⁱⁱⁱ
But the latter might have a few tricks of their own, including perhaps one inherited by our ancestors: Typically, newly ascendant males spare the offspring of females with whom they had previously copulated, as though they say to themselves, “Isn’t that my old flame from several months ago? And just look at that cute little baby, he’s got my chin!” It has been suggested, in short, that female choice of multiple male sexual partners—itself facilitated by concealed ovulation—may be a means whereby our great-, great-, great-grandmothers fooled the men in their lives, inducing several to think that each might be the father and thereby taking out a kind of “infanticide insurance.”

Here is a related but more cheery hypothesis for why human ovulation is concealed, focusing on benefits to the woman, ultimately via payoffs to her offspring.
^iv
It is clearly advantageous to every woman to be fertilized by the best available sperm, which unfortunately might not be provided by her mate/husband. Sadly, the real world of potential sexual and social partners is not like Lake Wobegon, “where all the women are strong, all the men are good-looking and all the children are above average.” The average mate of the average woman is, well, average! Since a would-be mother is more likely to be reproductively successful mated to a less-than-perfect male specimen than if she were unmated, she might therefore be predisposed (which is to say, favored by natural selection) to increase her fitness by sticking with her partner—who
presumably is the best she is able to obtain—while also trying to have sex with more attractive men on the sly.