Homo Mysterious: Evolutionary Puzzles of Human Nature (3 page)

For one, it could be a signal by which a woman’s body tells her brain that she isn’t pregnant. After all, that is how menstruation is “used” today, just as, conversely, a failure to “get one’s period” is an early signal that a woman may be pregnant. It would clearly be advantageous for a woman to know her reproductive status, and in a world before pregnancy test kits, it is quite likely that by its presence or absence, menstruation gave the first clue.

Fair enough, but logically unlikely. Why should natural selection have favored such an expensive “all-clear signal,” especially one that had to be broadcast each month? Wouldn’t it have been far more efficient—and not at all beyond the reach of evolution—to endow pregnant women with a distinctive smell, a unique sneeze, or the rapid blinking of their left eye? It seems like blatant overkill to lose much of your physical self—your uterine lining—just to send the message that you are not pregnant. And even if menstruation serves such a signaling function, why isn’t it simply a bit of spotting, as in dogs?

The copiousness of human menstrual bleeding nonetheless suggests that maybe it’s a social signal, a message intended for others. In many human societies today, the onset of menses indicates the beginning of adulthood. So maybe menstruation—for all its drawbacks—was selected as a way of informing one’s surrounding social network that a girl is becoming a woman and should be taken seriously.

One problem with this hypothesis is that such signaling appears more likely to be
disadvantageous
to the signaler. Even though biologists are agreed that male–male competition is typically more vigorous—even violent—than its female–female counterpart, it is increasingly clear that female–female competition also occurs. More subtle than the male–male version, it nonetheless involves situations in which dominant adult females of a number of species actually attack, intimidate, and often inhibit the breeding of subordinates. Assuming that something comparable occurs in our own species (admittedly, an unproven assumption at present),
it would seem to behoove a young woman to, if anything, hide the fact that she is entering reproductive competence, especially since such a signaler would necessarily be young and thus liable to be particularly vulnerable.

The possibility still exists—although a slim one—that menstrual bleeding is not targeted at other women generally, but more specifically aimed at close relatives and others who might be primed to cooperate rather than compete, to render useful assistance to a younger kinswoman once they know of her change in reproductive status. It would be interesting to see if menstrual flow is in any way correlated with presence or absence of potential competitors on the one hand or helpers on the other. At present, however, it is hard to see how menstruation could have evolved as a social signal, especially given that in nearly all human societies today, it is considered awkward, embarrassing, even shameful—the body conveying information that one’s mind would prefer to keep under wraps.

A signaling hypothesis for menstruation could still be salvaged, however, if it were directed toward men. But if so, the earlier question repeats itself: Why use such an expensive signaling system instead of, say, employing pheromones? After all, males are sperm makers by definition, and because sperm are cheap and abundant, sperm makers are selected to be highly attuned to indications of fertility on the part of prospective partners.

It is interesting, nonetheless, that in the modern Western world at least, substantial effort is expended to hide the fact of menstruation—which itself paradoxically suggests that in the absence of the “feminine hygiene” industry and its constant efforts to provide women with “protection” from their own bodies, enabling them to hide, suppress, tip-toe around, or otherwise obscure the reality of menstruation, maybe it really did evolve as a signal. In the past it was certainly widely noticed, although generally misunderstood. “Nothing could easily be found that is more remarkable than the monthly flux of women,” wrote Pliny the Elder, two millennia ago:

Contact with it turns new wine sour, crops touched by it become barren, grafts die, seeds in gardens are dried up, the fruits of trees fall off, the bright surface of mirrors in which it is merely reflected is dimmed, the edge of steel and the gleam of ivory are dulled, hives of
bees die, even bronze and iron are at once seized by rust, and a horrible smell fills the air.
¹

Demeaning and inaccurate as such attention has been, if nothing else it encourages us to look for other explanations.

Here is one. What if menstruation serves a cleansing function, using periodic blood flow to sluice away potentially dangerous pathogens, including—but not limited to—those introduced during copulation?
²
This “explanation” has pros and cons, like nearly all the perplexities to be considered in the present book, and, like most, it has generated fruitful debate. Moreover, it’s a nifty idea, not least because it reverses the old canard that menstruating women are somehow unclean or that the phenomenon is a reflection of female weakness, emphasizing instead that the exact opposite is more likely the case: Sperm plus semen are actually more likely to be unclean (not to mention possible pathogens introduced during coitus itself), with menstruation conceivably being a hygienic and even heroic countermeasure.

The reality, of course, is that ovaries and the uterus are internal organs, abundantly outfitted with delicate tissue that is vulnerable to infection. And sexual intercourse necessarily involves introducing foreign material deep inside a woman’s body, bypassing most of her traditional defenses. Certain pathogens such as
Chlamydia
—a common cause of pelvic inflammatory disease—hitchhike on the tails of sperm, and bacteria such as
Staphylococcus
and
Streptococcus
, which cause no harm when inhabiting the vagina, can be serious troublemakers when transported via an enthusiastic penis into a woman’s upper reproductive tract.

Margie Profet, who developed the “cleansing hypothesis,” emphasized that the uterus is “designed to bleed,” via its specialized spiral-shaped arteries and arterioles. When these constrict, the uterine lining dies and is sloughed off, presumably taking any unwanted pathogenic invaders along with it. In addition, the resulting copious blood flow essentially “hoses down” the underlying uterine wall. Profet also pointed out that menstrual blood contains a concentration of leukocytes that is about three times higher
than normal blood; these white blood cells, brought directly into contact with the uterus, are thus made available to fight any infections.

There are practical implications of the cleansing hypothesis. If, as the hypothesis holds, menstrual bleeding is an adaptive response to infection or potential infection, then it is clearly normal. What about unusually heavy bleeding (menorrhagia) or intracyclic bleeding (metrorrhagia)? These could be symptoms of endometritis, inflammation of the uterine lining, or they could be part of a body’s attempt to fight off such infections. Profet, arguing for menstruation’s cleansing role, points out that assuming the former, and therefore using medications to inhibit such bleeding, would be equivalent to “blaming firemen for a fire.”

Convincing as it might be at first blush, the cleansing hypothesis also has problems.
³
Menstrual blood contains nutrients—especially iron—that might actually
encourage
pathogen growth. And in fact, many pathogens are specifically iron deprived, such that surrounding them with blood might provide them with just the nutrients they need! There is also no evidence that menstrual intensity—either in human beings or animals—correlates with pathogen load, which would be predicted. Under the cleansing hypothesis, an already “clean” uterus would be expected to correlate with less tissue loss and reduced blood flow. Were this the case, it would doubtless have been trumpeted by devotees of the cleansing hypothesis as evidence in its favor, so the opposite finding must be acknowledged as evidence against.

Suppose, alternatively, that pathogen invasion is difficult for the uterus to detect, which might in turn prevent adaptive modulation of the cleansing response (in fact, insofar as bodies are able to respond to pathogens by killing them, selection would have favored discreet invaders that gave minimal indications of their presence). Even then, however, another correlation could be expected: between sexual activity—either number of partners or frequency of coitus with the same partner—and menstruation. But the reality, once again, is not encouraging: Although some women have relatively heavy menstrual flows and some much lighter, menstrual intensity also does not vary with the number of different sexual partners or with the nature and frequency of sexual encounters. Another strike against the cleansing hypothesis.

Also, if menstruation serves to diminish the uterus’s pathogen load, either by flushing them away or zapping them with white blood cells, then there should be fewer bacteria present after menstruation than before. It is notoriously difficult to assess total bacterial load in a given human organ. However, it is relatively easy to determine how many different kinds of bacteria are present, and when this is done, the evidence points the opposite way: more bacterial diversity
after
menstruation than before.

There is yet more evidence against the cleansing hypothesis. It is well established that women in traditional, nontechnological societies enter menarche later and spend more time pregnant and nursing than do Western women. As a result, they experience far fewer menstrual cycles than do Western women, and therefore, according to the cleansing hypothesis, they should be more susceptible to uterine infections. There is no evidence for this. The cleansing hypothesis would also expect that women with an especially heavy menstrual flow would have a lower frequency of pelvic inflammatory disease; this has not (yet) been evaluated.
ⁱⁱ
It is also problematic that oral contraceptives, which substantially decrease menstrual blood volume, do not increase the risk of uterine infection, even though it could be argued that such contraceptive use is balanced by a possible increase in heterosexual activity … which is typically why the contraceptives are used in the first place.

T
here is another possible explanation for why menstruation evolved, based on energy efficiency: Calculations suggest that it is metabolically cheaper to slough off the uterine lining (which is energetically expensive to maintain) and then regrow it in preparation for the next cycle of ovulation than to maintain it in a high level of vascularization. In support of this idea, anthropologist Beverly Strassman noted that a postmenstrual uterus consumes only about 14% of the oxygen required by a fully prepared endometrium. Not only that, but a woman’s overall metabolic rate during the preovulatory (“follicular”) phase of her cycle, when the
uterine lining is regressed, is about 7% lower than during the secretory (“luteal”) phase, when it is actively growing.

Efficiency is a less-than-exciting notion, but after all, energy is the basic currency of life, making it not unreasonable that a woman’s metabolic economy alternately revs up and down, economizing on the costs of remaining ready to reproduce. The idea is that unlike, say, the lungs or heart, a woman can afford to down-regulate the activity within her uterus, keeping it at a slow idle during those times between ovulations when reproduction is not an option.

On the other hand, the metabolic efficiency hypothesis is not entirely convincing. Why, for instance, isn’t the uterus simply kept in a more efficient, low-energy, less vascularized state until needed for nourishing an embryo? That is, why build up a fancy and expensive uterine lining, ready to receive an implanting embryo, only to tear it down every month? Why not just tamp down the endometrium and keep it quiescent until an embryo comes along? That would be more efficient yet.

Part of menstruation’s enigma, and the need for all these hypotheses, is that it poses a kind of Sisyphean dilemma. According to Greek mythology, Sisyphus was condemned to spend eternity pushing a heavy rock up a steep hill, only to have it roll back down each time. A menstruating woman finds herself constructing a snazzy, energetically expensive endometrium each month, only to dislodge it again and again. Sisyphus had no choice; ditto for most women. But presumably, evolution did.

An alternative explanation is what I have dubbed the “evaluation hypothesis,” which derives from the fact that human beings are unique among mammals in how much they invest in each offspring. This makes it especially important that any embryo that is brought to term be an especially capable one. After pregnancy and childbirth comes lactation, followed by years—even decades—of continuing expenditure on behalf of human offspring: spending time and energy, running risks, and so forth. It may therefore be significant that for every successful pregnancy, there are many
“spontaneous abortions,” caused by the failure of an early embryo to implant successfully.

For anyone with a scientific mind set and who therefore believes that all phenomena have causes, the very word “spontaneous” should set off alarm bells. Maybe in this case a “spontaneous” abortion really means that the embryo or fetus was tried and found wanting or, at least, not deserving of further maternal investment. And maybe the events surrounding menstruation are how a woman’s body evaluates her would-be offspring and does so early in the “investment” cycle, thereby minimizing wasted investment in case of a thumbs down.

A key aspect of early pregnancy takes place when an embryo (really, just a fertilized zygote) begins burrowing into the uterine lining and starts secreting a hormone—human chorionic gonadotropin or HCG—that inhibits menstruation. Early in a woman’s menstrual cycle, luteinizing hormone, produced by the brain, not only kick-starts ovulation; it also keeps a woman’s ovaries making its own hormone, progesterone, which in turn keeps the uterine lining in place. If no pregnancy occurs, luteinizing hormone levels decline, which in turn causes a precipitous drop-off in progesterone, which results in the breakdown of the uterine lining and, shortly thereafter, menstruation.