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Why Do Women Menstruate?Author(s): John TravisReviewed work(s):Source: Science News, Vol. 151, No. 15 (Apr. 12, 1997), pp. 230-231Published by: Society for Science & the PublicStable URL: http://www.jstor.org/stable/4018480 .

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Mnst ruat e

cientists s k r e s o n o r t i s

f minin ph nom non

By JOHN TRAVIS

yths, taboos, and jokes concern-ing menstruation-the periodicshedding of the uterine lining

and consequent vaginal bleeding-arelegion and go back centuries. At leastone female comedian, for example, hasoffered the whimsical thought that thisphenomenon is compelling evidencethat God is indeed a man.

Ancient philosophers thought men-strual blood was the source of new life:Aristotle speculated that it harbors a sub-stance, the materia prima, that a man'ssperm shapes into an embryo. This theo-ry persisted for almost 2,000 years, but itcertainly hasn't been the only false beliefabout menstruation. Hippocrates arguedthat men cleanse their blood by sweatingbut that women menstruate to removeimpurities. Another theory held thatwomen generate more blood than theycan handle and that men-struation allows them to

expel the excess.Menstruation has alsooften been used to castwomen in a threateninglight. Parts of the Biblecontend that menstruat-ing women are pollutedand dangerous to men.The Roman historianPliny wrote that menstru-ating women cause wineto sour, vines to wither,grass to die, and fruit tofall. As recently as 1974, LANCET pub-lished a letter speculating why flowerswilt if held by menstruating women.

Menstrual taboos, usually based on thenotion that menstruating women areunclean, persist in many societies. Prohi-bition of sex during menstruation is com-mon, and in some cultures menstruatingwives aren't allowed to cook meals fortheir husbands. At one time, the CatholicChurch didn't allow menstruating womento receive communion. Among the Dogonpeople of West Africa, menstruatingwomen must sleep in special huts.

Even in the United States today, men-struation essentially remains a tabootopic. It's a conversation-ender, saysHarry Finley, who several years ago

founded the Museum of Menstruation nNew Carrolton, Md., n part to demystifythe phenomenon.

In truth, there are few mysteries leftabout menstruation. The details of howit happens are well understood. Perhapsthe one puzzle that remains s menstrua-tion's biological ignificance. ust why dowomen menstruate? n he last few years,a debate has erupted over that funda-mental question.

To many biologists and physicians, asufficient explanation or menstrua-tion is that it marks a women's fail-

ure to become pregnant during her repro-ductive cycle. They would argue hat if anovulated egg is not fertilized and implant-ed in the uterine ining, or endometrium,a women simply sheds the complex tis-

sue that has readied itself tonourish an embryo and starts

building t anew.Yet some scientists press theissue further and seek an evo-lutionary explanation or men-struation. To these biologists,bodily features or phenomenanecessarily serve a functionalrole. According o the Darwin-ian idea of natural selection,they wouldn't have persisted ifthey didn't offer an advantage.

Fever, or example, has beendismissed as a mere side effect

of an infection. Yet evolutionary biolo-gists have suggested that fevers mayinstead represent an integral part of thebody's attempt o eliminate he infection.

Can menstruation be subjected to asimilar analysis? My assumption s thatif menstruation wasn't advantageous,there are ways over long periods of time,in large populations, or natural electionto abolish it. And since it's still there, ourjob as good evolutionary biologists is tofigure out what are the advantages main-taining t, ays Kim Hill of the Universityof New Mexico n Albuquerque.

Of the serious attempts to explainmenstruation rom this perspective, thefirst that gained widespread scientificand public recognition was made several

years ago by Margie Profet, a self-taughtevolutionary biologist who has a historyof arguing ontroversial heories.

Profet, who lacks a Ph.D. but won a so-called genius grant from the MacArthurFoundation n 1993, everses he historicalperception hat menstruating women areunclean by proposing hat the phenome-non defends women from pathogens inthe vagina or cervix hat nvade he uterusby hitchhiking ride on sperm.

She contends that the vaginal bleedingtypical of human menstruation flushesout myriad dangerous microorganismsthat could otherwise cause infertility, ll-ness, or even death.

This protection, says Profet, morethan offsets the nontrivial loss of ironand other nutrients that results frommenstrual bleeding.

Based on this supposition, Profet alsoargues that all mammals probably expe-rience menstrual bleeding, although the

blood loss may be less obvious than it isin women. Her assertion counters theconventional wisdom that only humans,higher apes, and a few other mammalsmenstruate.

Profet's theory, described in the Sep-tember 1993 QUARTERLYEviEW FBIOLOGY,quickly drew a barrage of criticism. Sci-entists, for example, noted that bloodactually serves as an excellent growthmedium for microorganisms and thatmany reproductive ract infections, suchas chlamydia and gonorrhea, occur morefrequently fter a women menstruates.

Profet retorts that evolutionary adap-tations are rarely perfect and that somemicroorganisms themselves may haveevolved to beat any menstrual protec-tion. We don't always win, says Profet,pointing out that the prevalence of AIDS,tuberculosis, and cholera isn't proof thatthe immune ystem didn't evolve to fightpathogens.

everly I.Strassmann, n anthropol-ogist at the University of Michiganin Ann Arbor who has studied

menstruation mong he Dogon or manyyears, has subjected Profet's antipatho-gen proposal to a critique by reviewing

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the scientific literature. Last summer, inthe June 1996 QUARTERLYEVIEW FBIOLO-GY, Strassmann published her analysisof Profet's hypothesis.

There are just so many logical argu-ments against what she said. To theextent you can generate predictions[from her theory], none were support-ed, says Strassmann.

Profet's hypothesis, for example, pre-dicts that the promiscuity of a speciescorrelates with its degree of menstrualbleeding. Strassmann contends that thepublished research does not back upthat notion. Several pri-mate species that are notpromiscuous have devel-oped copious menstrualbleeding.

Strassmann further notesthat women menstruateevery few weeks at most,making it a seemingly ineffi-cient means of infection con-trol. Indeed, most womenthroughout evolution spentmuch of their adult livespregnant or breast-feeding,which means they mighthave menstruated a mere100 or so times.

Assuming that menstruation was a rareevent in ancestral populations, then it isdoubtful that it evolved as a defenseagainst pathogens, Strassmann writes inan upcoming issue of EVOLUFIoNARYNTHRO-POLOGY.

How does Profet respond to Strass-mann's critique? Profet notes that she hasread only an early draft of Strassman'sQUAmRERLY EmEW OF BIOLOGY article and

therefore believes it is inappropriate toexpress publicly her specific criticisms ofStrassmann's work.

Still, Profet does say she finds Strass-mann's arguments flawed and unpersua-sive. Moreover, based upon her reading ofthe draft article, Profet says she has noplans to read the final published versionof the paper or to respond to it. Iconsid-er it a waste of time, says Profet, whorecently left the field of evolutionary biol-ogy to study physics and astronomy.

While Profet remains resolute in herbeliefs, Strassmann's critique confirmedthe opinion of many skeptics and evenproved persuasive to at least one earlychampion of Profet's theory. George C.Williams, an editor of the journal thatpublished both theories, was a strongsupporter of Profet's hypothesis but hassince changed his mind. I hink Beverlydid a pretty conclusive job of demolish-ing the main idea of Margie's paper,says Williams.

S trassmann has also offered herown evolutionary explanation formenstruation. The anthropologist

contends that Profet, and many otherswho discuss menstruation, incorrectly

consider the phenomenon solely interms of vaginal bleeding.

Strassmann uggests that the definingelement of menstruation s the cyclicalgrowth and regression of the endometri-um. Any bleeding is simply a side effectfor a small number of animals, ncludinghumans. It's important to distinguishbetween arguments that explain endo-metrial regression and those that explainmenstrual bleeding, ays Strassmann.

Strassmann poses the evolutionaryissue of menstruation n this form: Whydo women regularly row and shed their

endometrium rather thanconstantly keeping the tis-sue ready for implantationby an embryo?

Strassmann's answer isthat maintaining he endo-metrium would be a wasteof energy. It osts more todo that than to simplybuild it up when it's need-ed, she says. Tissues re-quire constant nutrientsand a support system.There are lots of examplesof other tissues that re-gress to save energy and

get built up in response to a particularneed.

In fact, from lizards to humans, repro-ductive tissues regress when not needed,says Strassmann.

To what extent might endometrialregression save energy? That's a difficultquestion. While a woman's menstrualcycle advances toward ovulation, theendometrial ining ncreases significantlyin mass as the uterine tissue

develops the vasculature andsecretory glands needed tosustain an implanted mbryo.

Around ovulation andimplantation, t consumes alot more energy than in theregressed state, says Strass-mann, citing one study whichfound hat the endometrium'soxygen consumption, a mea-sure of its energy expendi-ture, increases seven-foldfrom he beginning o the end of the men-strual cycle.

Indeed, the rising energy demands ofthe endometrium parallel he metabolicchanges that occur in the body as awoman progresses through her repro-ductive cycle toward ovulation. Severalstudies estimate that a woman's metabol-ic rate can increase by at least 7 percent.

It's not just the endometrium hat'sinvolved, bserves Strassmann, who hascalculated that if a woman could elimi-nate this extra energy demand for a year,she would save the energy equivalent ofhalf a month's food. The part of thatenergy savings that would result fromendometrial regression alone is any-body's guess, though t's likely o be verysmall, admits Strassmann.

lthough Strassmann's xplanationof menstruation has received amore positive reaction than Pro-

fet's, some scientists stress that it is farfrom he final word on the subject.

My irst reaction is that it's hard tobelieve the energy saving is significant.And I think that's lots of people's firstreaction, ays Hill. Yet Hill s unwilling odismiss Strassmann's proposal, notingthat even a small energy saving mighthave helped women during prehistorictimes, when they had to scramble des-perately or enough food to stay alive.

Peter T. Ellison, an anthropologist atHarvard University, admires Strass-mann's critique of Profet's heory, but hedoesn't believe the Michigan anthropol-ogist has offered a compelling counter-proposal.

Ellison contends that a cascade ofrequirements makes menstruation lmostinevitable for women. He argues that inhumans and other higher primates,embryos implant nto the endometriumin an unusually nvasive manner n orderto meet the oxygen and glucosedemands of their energy-hungry brains.In turn, the endometrium is forced toprepare itself for this implantation by

terminally ifferentiating ts cells.This, says Ellison, means that the

endometrial ells are committed to theirfates before an embryo arrives and thatthey have a limited ife span. That's whythere's only a day or two during whichembryos can successfully implant, hesays. The endometrium sn't good afterthat. You can't save it for later, ontendsEllison. Consequently, women shed the

outdated lining via menstrua-

tion each time they do notbecome pregnant.Colin A. Finn, a researcher at

the University of Liverpool Vet-erinary Field Station in Neston,England, independently offers asimilar line of reasoning in anarticle on menstruation's evolu-tion that appears in the Decem-ber 1996 EuRoPEAN OURNAL FOBs1TE-1Tcs NDGYNECOLOGYNDREPRODUCTIVEIOLOGY.

Strassmann responds that the argu-ments of Ellison and Finn seem flawed.She says that anatomists have estab-lished that the cycle of endometrialgrowth and regression is found in allmarnmals, ven those that lack invasiveimplantation.

The debate over menstruation s cer-tainly far from over. Perhaps the keypoint about this academic dispute, how-ever, s that it is occurring at all.

Hill says, I hink the most importantcontribution Margie Profet made, whichwill stand regardless of the utility of heridea, is bringing to light really clearlythat we have to start thinking about thefunctional ignificance of things. We can'tjust take something ike menstruation asa given. v

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