Profet reasons that monthly bleeding is so costly in terms of iron depletion and lost fertility time, that it would have failed as an evolutionary trait unless it also performed some adaptive function. That function, she says, is preventing sperm-borne microbes like streptococcus from taking hold in the uterus. Profet thinks menstrual blood is specifically designed for this, with its rich supply of disease-fighting immune cells. She notes that women with infections of the reproductive tract, or the chronic inflammation an IUD normally causes, respond by bleeding heavily. One implication of her theory, she says, is that physicians should routinely consider infections as the cause of unexplained vaginal bleeding.
George C. Williams, co-editor of The Quarterly Review of Biology, is convinced by Profet’s hypothesis on menstrual bleeding–though he says it may have additional functions: for example, signaling that a woman is fertile. Other experts think she has not yet proved her case. For one thing, “Over most of human evolutionary history, women weren’t menstruating very often.” observes University of Michigan anthropologist Beverly Strassmann, because they were usually either pregnant or breastfeeding (which suppresses menstruation). Furthermore, says epidemiologist Dr. John S. Moran of the Centers for Disease Control and Prevention, sperm are not major culprits in spreading diseases. A sore on a man’s penis, or infected secretions from his urethra, can easily transmit an infection even without ejaculation.
Another problem with Profet’s theory is that sexually transmitted diseases are common in menstruating women-and some STDs, notably gonorrhea, actually cause more complications during menstruation. “We’re in a co-evolutionary race with many other organisms,” Profet responds. “Sometimes they win, but without menstruation there would be much more infection.”
Profet also says many kinds of mammals menstruate, at least in small amounts. That’s not true, argues Kurt Benirschke, professor of pathology and reproductive medicine at the University of California, San Diego, and former research director at the San Diego Zoo. “Lots of researchers have looked at this issue,” he says. “There are huge volumes on it.”
Love it or hate it, the new theory is already stirring up vigorous discussion about why menstruation works the way it does and that’s a healthy development.
