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Authors: Natalie Angier

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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 outside, 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

 

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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

 

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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

 

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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

 

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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. 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.
The uterus, then, is like a deciduous tree, an oak or a maple, and the endometrium acts like the leaves. When the weather is warm, when

 

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sunlight sings, the tree awakes and invests in leaves. The branching pattern of the tree its trunk, its branches, its twigs is like the branching of the body's vascularization, parceling out water rather than blood. The homology of the pattern is no coincidence. Holy water, sacred blood, they are one and the same, and branching is the most hydraulically efficient means of pumping the fluid from a central source the heart, the trunk out to all extremities. Thus nourished, the leaves bud, unfurl, thicken, and darken. The leaves are photosynthetic factories, transforming sunlight into usable energy. That energy allows the tree to create seeds and nuts, the acorns that are embryonic trees. The leaves are expensive to maintain the tree must deliver them water, nitrogen, potassium, the nutrients from the soil but they repay the tree by spinning sunlight into gold. In the same way, the endometrium is metabolically expensive and yet generative as well. It has the potential to nourish an embryo. In both cases, too, the investment is worthwhile only at certain times. For a tree considering foliation, that time is spring and summer, when there is abundant sunlight, water that is not frozen, and soil that is soft enough to be mined for nutrients. Then and only then can a leaf repay its debt with interest. For the uterus, the time corresponds to the moment when there might be something worth nourishing, a ripened egg that has met its match. Interestingly, a leaf dies in fall as the endometrial lining dies at the end of a fallow cycle. The corpuscle at the tip of the twig constricts, shutting off water and killing its dependent leaf.
The putative cost-effectiveness of cyclic endometrial death does not, however, explain the need for menstrual blood. Can't we have retrenchment without seeing red? The blood, in Strassmann's view, is beside the point. It is a byproduct of the loss of a tissue that is by necessity highly vascularized. If you're going to lose that tissue, you're going to have to spill some blood. Those fancy spiral arteries that destroy the endometrium and so start the flow, the ones that Profet thought were evidence of the adaptiveness of menstruation? Those arteries are there for the sake of the placenta, Strassmann says. That is their reason to be and, come menstruation, not to be. The placenta is a spectacular thing, but it is vampirous. It needs blood, and the spiral arteries give it blood. Each month they spread their coiled fingers through the en-

 

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