Why Do Women Menstruate?

Science News, April 12, 1997 by John Travis

Profet retorts that evolutionary adaptations are rarely perfect and that some microorganisms themselves may have evolved to beat any menstrual protection. "We don't always win," says Profet, pointing out that the prevalence of AIDS, tuberculosis, and cholera isn't proof that the immune system didn't evolve to fight pathogens.

Beverly I. Strassmann, an anthropologist at the University of Michigan in Ann Arbor who has studied menstruation among the Dogon for many years, has subjected Profet's antipathogen proposal to a critique by reviewing the scientific literature. Last summer, in the June 1996 QUARTERLY REVIEW OF BIOLOGY, Strassmann published her analysis of Profet's hypothesis.

"There are just so many logical arguments against what she said. To the extent you can generate predictions [from her theory], none were supported," says Strassmann.

Profet's hypothesis, for example, predicts that the promiscuity of a species correlates with its degree of menstrual bleeding. Strassmann contends that the published research does not back up that notion. Several primate species that are not promiscuous have developed copious menstrual bleeding.

Strassmann further notes that women menstruate every few weeks at most, making it a seemingly inefficient means of infection control. Indeed, most women throughout evolution spent much of their adult lives pregnant or breast-feeding, which means they might have menstruated a mere 100 or so times.

"Assuming that menstruation was a rare event in ancestral populations, then it is doubtful that it evolved as a defense against pathogens," Strassmann writes in an upcoming issue of EVOLUTIONARY ANTHROPOLOGY.

How does Profet respond to Strassmann's critique? Profet notes that she has read only an early draft of Strassman's QUARTERLY REVIEW OF BIOLOGY article and therefore believes it is inappropriate to express publicly her specific criticisms of Strassmann's work.

Still, Profet does say she finds Strassmann's arguments flawed and unpersuasive. Moreover, based upon her reading of the draft article, Profet says she has no plans to read the final published version of the paper or to respond to it. "I consider it a waste of time," says Profet, who recently left the field of evolutionary biology to study physics and astronomy.

While Profet remains resolute in her beliefs, Strassmann's critique confirmed the opinion of many skeptics and even proved persuasive to at least one early champion of Profet's theory. George C. Williams, an editor of the journal that published both theories, was a strong supporter of Profet's hypothesis but has since changed his mind. "I think Beverly did a pretty conclusive job of demolishing the main idea of Margie's paper," says Williams.

Strassmann has also offered her own evolutionary explanation for menstruation. The anthropologist contends that Profet, and many others who discuss menstruation, incorrectly consider the phenomenon solely in terms of vaginal bleeding.

Strassmann suggests that the defining element of menstruation is the cyclical growth and regression of the endometrium. Any bleeding is simply a side effect for a small number of animals, including humans. "It's important to distinguish between arguments that explain endometrial regression and those that explain menstrual bleeding," says Strassmann.