Why Do Women Menstruate?

Science News, April 12, 1997 by John Travis

Scientists seek a reason for this feminine phenomenon

Myths, taboos, and jokes concerning menstruation--the periodic shedding of the uterine lining and consequent vaginal bleeding--are legion and go back centuries. At least one female comedian, for example, has offered the whimsical thought that this phenomenon is compelling evidence that God is indeed a man.

Ancient philosophers thought menstrual blood was the source of new life: Aristotle speculated that it harbors a substance, the materia prima, that a man's sperm shapes into an embryo. This theory persisted for almost 2,000 years, but it certainly hasn't been the only false belief about menstruation. Hippocrates argued that men cleanse their blood by sweating but that women menstruate to remove impurities. Another theory held that women generate more blood than they can handle and that menstruation allows them to expel the excess.

Menstruation has also often been used to cast women in a threatening light. Parts of the Bible contend that menstruating women are polluted and dangerous to men. The Roman historian Pliny wrote that menstruating women cause wine to sour, vines to wither, grass to die, and fruit to fall. As recently as 1974, LANCET published a letter speculating why flowers will if held by menstruating women.

Menstrual taboos, usually based on the notion that menstruating women are unclean, persist in many societies. Prohibition of sex during menstruation is common, and in some cultures menstruating wives aren't allowed to cook meals for their husbands. At one time, the Catholic Church didn't allow menstruating women to receive communion. Among the Dogon people of West Africa, menstruating women must sleep in special huts.

Even in the United States today, menstruation essentially remains a taboo topic. "It's a conversation-ender," says Harry Finley, who several years ago founded the Museum of Menstruation in New Carrolton, Md., in part to demystify the phenomenon.

In truth, there are few mysteries left about menstruation. The details of how it happens are well understood. Perhaps the one puzzle that remains is menstruation's biological significance. Just why do women menstruate? In the last few years, a debate has erupted over that fundamental question.

To many biologists and physicians, a sufficient explanation for menstruation is that it marks a women's failure to become pregnant during her reproductive cycle. They would argue that if an ovulated egg is not fertilized and implanted in the uterine lining, or endometrium, a women simply sheds the complex tissue that has readied itself to nourish an embryo and starts building it anew.

Yet some scientists press the issue further and seek an evolutionary explanation for menstruation. To these biologists, bodily features or phenomena necessarily serve a functional role. According to the Darwinian idea of natural selection, they wouldn't have persisted if they didn't offer an advantage.

Fever, for example, has been dismissed as a mere side effect of an infection. Yet evolutionary biologists have suggested that fevers may instead represent an integral part of the body's attempt to eliminate the infection.

Can menstruation be subjected to a similar analysis? "My assumption is that if menstruation wasn't advantageous, there are ways over long periods of time, in large populations, for natural selection to abolish it. And since it's still there, our job as good evolutionary biologists is to figure out what are the advantages maintaining it," says Kim Hill of the University of New Mexico in Albuquerque.

Of the serious attempts to explain menstruation from this perspective, the first that gained widespread scientific and public recognition was made several years ago by Margie Profet, a self-taught evolutionary biologist who has a history of arguing controversial theories.

Profet, who lacks a Ph.D. but won a so-called genius grant from the MacArthur Foundation in 1993, reverses the historical perception that menstruating women are unclean by proposing that the phenomenon defends women from pathogens in the vagina or cervix that invade the uterus by hitchhiking a ride on sperm.

She contends that the vaginal bleeding typical of human menstruation flushes out myriad dangerous microorganisms that could otherwise cause infertility, illness, or even death.

This protection, says Profet, more than offsets the nontrivial loss of iron and other nutrients that results from menstrual bleeding.

Based on this supposition, Profet also argues that all mammals probably experience menstrual bleeding, although the blood loss may be less obvious than it is in women. Her assertion counters the conventional wisdom that only humans, higher apes, and a few other mammals menstruate.

Profet's theory, described in the September 1993 QUARTERLY REVIEW OF BIOLOGY, quickly drew a barrage of criticism. Scientists, for example, noted that blood actually serves as an excellent growth medium for microorganisms and that many reproductive tract infections, such as chlamydia and gonorrhea, occur more frequently after a women menstruates.