Woman

by Natalie Angier

  We also have dwelled overmuch on the negatives of menstruation and premenstruation: the headaches, the weepiness, the sore breasts, the pimples. We have turned premenstrual syndrome into a distinct genus in the taxonomy of psychiatry, right up there with panic disorder and obsessive-compulsive behavior. We suspect that women may be slightly less competent right around the time of their periods' onset. And yet the opposite may be true. As Paula Nicholson has pointed out, empirical research suggests that "the premenstrual phase of the cycle is frequently accompanied by heightened activity, intellectual clarity, feelings of well-being, happiness and sexual desire." This is an empire I can vouch for. One of my most beautiful memories of college is of a day when my period was due but hadn't yet arrived. I was sitting in my living room, studying, and I felt an unaccountable surge of joy. I looked up from my book and was dazzled by the air. It was so clear, so purely transparent, that the objects in the room were sharply etched and proud against it, and yet it was as though I could see the air for the first time. It had become visible to me, molecule for molecule. My mind was focused and free of anxiety. I felt for a moment as though I had taken the perfect drug, the one that has yet to be invented; call it Liberitium or Creativil.

  My enthusiasm quickly vanished, and I couldn't recapture these sensations in subsequent periods. It was the 1970s, and feminists were trying to create a woman's-eye mythology and, among other things, to give menstruation a good name, but I couldn't help greeting their efforts with a sneer. They were, as I'm sure my daughter will say to me someday, so twentieth-century. An instructor in my women's studies class, for example, suggested that we students all trade in our tampons for napkins over the next few months, the better to feel the process of menstruation, to go with the flow, as it were. Phooey, I thought. Women have been wearing tampons for at least three thousand years; the ancient Egyptians wrote about what sound like early tampons, and so did our father of the tentacled uterus, Hippocrates. I didn't take my teacher's advice, then or since. I'd been delighted when my mother let me switch from pads to tampons—when she was assured by some doctor, I think, that tampons are safe for a young girl and her hymen—and I was not about to return to the awkwardness of a cotton football between the legs.

  Nonetheless, I believe there should be a woman-centered myth of menstruation, a construct of our shared feminine low-giene—something on a par with the male pissing ritual, perhaps. Men obviously find their upstanding approach to urination manly, amusing, and potentially seditious, or the public urinal scene would not be such a fixture of film and television. Emily Martin has described menstruation's potential to foment rebellion and solidarity, offering as it does an excuse for wage-earning women to retreat to the one place where their male managers cannot follow. "In early twentieth-century documents," she writes, "there are scattered references to groups of two or three girls frequently found in the washroom 'fussing over the universe'...a girl sobbing in the washroom over her stolen wages and girls reading union leaflets posted in the washroom during a difficult struggle to organize a clothing factory." Let us again retire to the chamber for some universal fussing and fomenting. Let us overthrow the lore, the idiocy, and the Paglian prissiness that surround menstruation and found a myth on reality. How and why do we bleed? Why have we evolved the cycle of endometrial death and renewal? Surprisingly enough, that question was not asked until recently, and the quest for an answer is still very much alive. In exploring origins, we may find new blood.

  Menstruation is the way that we first experience the uterus, and if we are Western women who have small families, we experience the uterus thusly 450 to 480 times in a lifetime. During the average period, we cast off a volume of material amounting to about six tablespoonfuls, or three fluid ounces, half of it blood, the other half the shed endometrial layers, along with vaginal and cervical secretions. Most of us think of menstruation as a passive business, decay aided by gravity. The uterine lining builds up and awaits the sacred blastocyst that would be a baby; if none appears, the lining disintegrates and falls away like so much mildewed wallpaper. The active process, we imagine, is the anticipatory phase of the menstrual cycle, a time of anabolism, the plumping up of the endometrium with tissue and nutrients that occurs in concert with the ripening of the egg. If nothing happens to keep anabolism alive, if conception and implantation do not happen and the lining is no longer needed to feed the baby, then activity ceases, the plug is pulled, and there goes the ruddy bathwater.

  That is not in fact what happens. Recall the lesson that contemporary biology teaches us: dying is as active as living. Eggs die by undergoing apoptosis; that is, they commit suicide. So too is menstruation a dynamic and directed affair. Margie Profet, an evolutionary biologist now at the University of Washington, has described menstruation as an adaptation: it is a product of design, the designer in this case being that greatest and most unpretentious of deities, evolution by natural selection. "The mechanisms that collectively constitute menstruation appear to manifest adaptive design in [their] precision, economy, efficiency and complexity," she has written. "If menstruation were merely a functionless byproduct of cyclic hormonal flux, there would be no mechanisms specifically designed to cause it."

  The first relevant mechanism is a specialized type of artery. Feeding into the two superficial layers of the endometrium, the ones that are disposed of each month, are three spiral arteries, so named because they look like corkscrews. During pregnancy, the spiral arteries serve as important conduits of blood for the placenta. Yet their purpose extends beyond fetal feeding. Several days before a woman's period begins, the tips of the spirals grow longer and more tightly coiled, like a Slinky that's being pulled and twisted at the same time. Circulation to the endometrium grows sluggish—the calm that presages calamity. Twenty-four hours before the onset of bleeding, the spirals constrict sharply. The faucets are twisted off, the blood flow ceases. It is a heart attack of the uterus. Deprived of blood and therefore of oxygen, the endometrial tissue dies. Then, as abruptly as the arteries squeezed shut, they temporarily open again, allowing blood to rush in. The blood pools in pockets beneath the dead endometrium, causing the lining to swell and burst, and the period begins. Their mortal work complete, the spiral arteries constrict once again. (Fibroids disturb the ritual of menstruation because their parasitic blood supply does not conform to the squeeze-relax-squeeze pattern of the spiral arteries.)

  A second outstanding feature of menstruation is the quality of the blood. Most blood is poised to clot. Unless you are a hemophiliac, when you cut yourself the blood flows briefly and then coagulates, for which you can thank your platelets and sticky blood proteins such as fibrin. Menstrual blood does not clot. It may seem goopy at times, and the dead tissue accompanying it may pass out in clots—our slimy medusas!—but the blood proper contains very few platelets and does not form the interlocking coagulatory mesh that characterizes blood released from a wound. The only reason that menstrual blood does not keep flowing is that the spiral arteries constrict in the wake of endometrial death.

  Corkscrew arteries and blood like wine: surely we are designed to menstruate. Yet this is not the whole story. Every problem in biology, as the evolutionary thinker Ernst Mayr has pointed out, comes in two parts: the how and the why, the proximate explanation and the ultimate one. There must be an ultimate rationale for menstruation, the reason that this precise and intricate system evolved to begin with. Here we run up against the limits of history. Until recently scientists have been almost exclusively male; men do not menstruate, and so scientists have not delved terribly deeply into the ultimate causes of this strictly female phenomenon. The physiology of menstruation, the how of it, was of sufficient interest to gynecologists to be explored in some detail. Not until the early 1990s, though, did anybody seriously ponder the why of menstruation, when Margie Profet presented in the Quarterly Review of Biology a theory too provocative to be ignored.

  Profet is a slender and beautiful woman in her late thirties, California velvet on the o
utside, iron maiden beneath. She has long blond hair and blue eyes, talks in a friendly singsong voice, and wears cute outfits like a black leather skirt with large, decorative zippers and a matching short jacket. She has won a MacArthur fellowship—a "goddamn genius award," as Roy Blount, Jr., described it—but she never bothered to earn a Ph.D., for fear the formal accreditation would tempt her toward the path of professional conformity. Politically she is something like a feminist libertarian, the kind of person who thinks Charles Murray of The Bell Curve fame is a good guy, the Food and Drug Administration a threat to American liberty. Intellectually she's a radical, a hell-raiser, which is another way of saying she asks annoying questions that are so obvious, nobody has asked them before.

  Like any good evolutionary thinker, Profet framed her question about menstruation in economic terms, as a cost-benefit analysis. Menstruation, she decided, is extraordinarily expensive. Shedding and replenishing endometrial tissue on a monthly basis burns a lot of calories, and for our Pleistocene ancestors, who likely spent most of their brief lives on the rim of malnutrition, every calorie counted. Moreover, when you lose blood, you lose iron, an essential micronutrient and another scarce commodity for our forebears. Finally, menstrual cycling makes women less efficient in reproduction. All that building up and tearing down of the uterine lining limits the time when a woman might conceive. If evolution is so keen on reproduction, why devote this much effort to counterproduction?

  A pricey feature demands extravagant justification, and Profet had her candidate. Menstruation, she suggested, is a defense mechanism, an extension of the body's immune system. We bleed to rid the uterus of potentially dangerous pathogens that might have hitched a ride inside on the backs of sperm. Think of it. The uterus is a luxurious city just waiting to be sacked, and sperm are the ideal Trojan horse. Bacteria, viruses, and parasites all can find passage to the womb by playing opportunistic gene jockey, and it so happens that scanning electron micrographs of sperm reveal a cartoonish mob scene, the tadpole cell at the center surrounded by a cluster of microbial hangers-on. If permitted to linger in the uterus indefinitely, the pathogens might run amok, sickening, scarring, or killing us. Our endometrium must die, Profet proposed, so that we might live.

  Profet also emphasized that menstruation is not the only sort of uterine bleeding that might act to expel pathogens from the uterus. Women bleed at ovulation, they bleed at conception, and they bleed heavily after giving birth. Bleeding in toto should be thought of as the uterus's solution to the perils of internal fertilization.

  The novel formulation of menses as defenses put several confounding features of the process into a sensible light. Why, for example, is the shedding of the endometrium accompanied by a river of blood? The body can discard dead tissue without the use of blood. We replace the lining of the stomach on a regular basis, for example, and blood has nothing to do with it. Profet suggested that we bleed because blood carries the body's immune cells, the T cells, B cells, and macrophages, and the immune cells participate in routing out whatever nasty pathogens have tried to infiltrate the uterus. Why shed the lining rather than resorbing it into the body, as a more logically parsimonious system might do? To avoid the risk of recycling diseased tissue. And why do we bleed so heavily compared to other female mammals, any of which, presumably, are at risk of unintended spermatic donations? We bleed by the pint because we are an amorous species. We do not limit intercourse to a defined season of estrus, and we use sex for many nonreproductive reasons—to bond, to barter, to appease, to distract. Therefore, we must bleed heavily to cleanse ourselves: call it the macrophages of sin. Despite the relative heaviness of the human period, though, Profet also predicted that most if not all mammals undergo some sort of protective uterine bleeding and that scientists would find many more instances of menstruation in the animal kingdom than are currently known, if only they would start looking. Most of the species known to bleed are our sister primates, but bats, cows, shrews, and hedgehogs, among others, have been observed on occasion to shed blood from the vagina.

  The response to Profet's radical suggestion was immediate, and, from professional quarters, overwhelmingly negative. Outlandish! squawked the gynecologists. Far from being a protective mechanism, they argued, menstruation is the time of month when women are at greatest risk for bacterial infections such as gonorrhea and chlamydia. That's when the cervical mucus thins, allowing microbes in the vagina easy access to the uterus. And forget about sperm as gift-bearing Greeks. Menstrual debris itself frequently backwashes, serving as an especially efficient means of transmitting pathogens from the upper genital tract to the delicate tissue of the uterine cavity and fallopian tubes. Using menstruation for uterine defense, the critics claimed, is like hiring a wolf to guard your flock of prize cloned sheep.

  Others pointed out that routine menstruation is a modern invention. Our Pleistocene ancestors didn't have to worry about losing nutrients and iron with their monthly flow; they were too busy bearing children or lactating to menstruate. Even today, in some underdeveloped countries, women may go several years without menstruating. One anthropologist said he had interviewed a thirty-five-year-old woman in India who not only had never menstruated, she had never heard of the concept. Married at age eleven, she'd conceived her first child before menarche and had been pregnant or nursing, and thus amenorrheic, ever since.

  In the end, what really bothered Profet's critics was being caught with their intellectual pants down. They had no counterhypothesis to explain menstruation. Only after the first sputterings of scorn and denial did some scientists have the decency to put the proposition to the test and offer a viable alternative should Profet's theory flunk the exam.

  Beverly Strassmann, of the University of Michigan, took up the challenge with spitfire enthusiasm, publishing a lengthy exegesis in the same journal that had presented Profet's theory. Strassmann noted that Profet's hypothesis led to several predictions: first, that the uterus should be more riddled with pathogens prior to menstruation than after it; second, that the timing of menstruation should bear some relation to the timing of a female's greatest risk of pathogenic infiltration; and finally, taking a cross-species comparison, that the heaviness of a primate's period should correspond to the relative promiscuity of the animal—in other words, the more sexually active the species, the heavier the bleeding.

  Strassmann concluded that none of these predictions were supported by the evidence. In various studies, specimens of uterine smears taken from women throughout their menstrual cycle showed no significant difference in the bacterial load from one phase to the next; if anything, the concentration of microbes was lowest, rather than highest, right before menstruation. In fact, blood is an excellent growth medium for many types of microbial flora, offering not only protein and sugar but iron, and we all know what the iron in spinach does for Popeye. Researchers have shown that they can expedite the proliferation of Staphylococcus aureus in culture by feeding it iron, which is probably why a tampon left in place too long is tempting territory for this agent of toxic shock syndrome.

  Strassmann also considered whether the timing of menstruation and other types of uterine bleeding corresponds to times when a female might logically need the laundering; whether, to look at it another way, women don't need protection as much when they're not bleeding—during pregnancy and breastfeeding, for example. Might our ancestors have refrained from sex for at least part of the lengthy gestation and postpartum periods? Evidence from contemporary hunter-gatherer tribes that supposedly mimic humanity's formative years indicates scant effort at abstinence. The Dogon of Mali, for example, have sex throughout the first two trimesters of pregnancy and then resume lovemaking a month after birth. Yet the women don't start menstruating again until an average of twenty months postpartum. And women in all cultures have sex after menopause, but there is no indication that risk of infection rises when cycling ends.

  Nor did Strassmann's phylogenetic analysis of other primates bolster the antipathogen hypothesis. She
found no connection between a species' heaviness of menstruation and the degree to which it monkeys around, if you will. Some types of baboons, for example, are highly licentious and shed little or no uterine blood; other species of baboon are sexually restrained, breed with just one male, and yet bleed heavily. Female gorillas are monogamous and menstruate covertly. Gibbons are monogamous and bleed overtly.

  So if not for defense against microbes, what of our bleeding? Why the extravagant, wasteful system of menstruation? Here Strassmann strikes at Profet's core assumption—that menstruation is so costly it demands evolutionary justification. Far from being expensive, Strassmann argues, periods are a steal. Calorie for calorie, the Shiva approach to reproduction, the perpetual death and rebirth of the uterine lining, is cheaper than maintaining the uterus in fertile form would be. Consider the endometrium at its peak, right after ovulation, when it is capable of receiving a blastocyst. It is thick, rich, and metabolically dynamic. It secretes hormones, proteins, fats, sugars, nucleic acids. This plump endometrium is the woman's equivalent of an egg yolk, and it is energetically dear. Strassmann calculated that the uterine lining at its ripest uses seven times more oxygen than it does at its thinnest, after menstruation. The need for more oxygen translates into a need for more calories. In addition, the secretory endometrium revs up the entire body, as the hormones it releases stimulate tissues from brain to bowel. Again, a higher metabolism demands more calories. It makes sense to restrict the luxuriant productivity to a time of month when conception is likely—that is, at ovulation. If no embryo arrives, the lining and its secretions become a burden to sustain, so get rid of the whole bundle. Kill it. We can start over again the next month. Strassmann has estimated that in four months of cycling, a woman saves an amount of energy equal to six days' worth of food over what she would have needed to stoke a perpetually active endometrium. Even in lizards, the oviducts shrivel up when breeding season ends.