Why Do Women Menstruate?

Science News, April 12, 1997 by John Travis

Strassmann poses the evolutionary issue of menstruation in this form: Why do women regularly grow and shed their endometrium rather than constantly keeping the tissue ready for implantation by an embryo?

Strassmann's answer is that maintaining the endometrium would be a waste of energy. "It costs more to do that than to simply build it up when it's needed," she says. "Tissues require constant nutrients and a support system. There are lots of examples of other tissues that regress to save energy and get built up in response to a particular need."

In fact, from lizards to humans, reproductive tissues regress when not needed, says Strassmann.

To what extent might endometrial regression save energy? That's a difficult question. While a woman's menstrual cycle advances toward ovulation, the endometrial lining increases significantly in mass as the uterine tissue develops the vasculature and secretory glands needed to sustain an implanted embryo.

"Around ovulation and implantation, it consumes a lot more energy than in the regressed state," says Strassmann, citing one study which found that the endometrium's oxygen consumption, a measure of its energy expenditure, increases seven-fold from the beginning to the end of the menstrual cycle.

Indeed, the rising energy demands of the endometrium parallel the metabolic changes that occur in the body as a woman progresses through her reproductive cycle toward ovulation. Several studies estimate that a woman's metabolic rate can increase by at least 7 percent.

"It's not just the endometrium that's involved," observes Strassmann, who has calculated that if a woman could eliminate this extra energy demand for a year, she would save the energy equivalent of half a month's food. The part of that energy savings that would result from endometrial regression alone is anybody's guess, though it's likely to be very small, admits Strassmann.

Although Strassmann's explanation of menstruation has received a more positive reaction than Profet's, some scientists stress that it is far from the final word on the subject.

"My first reaction is that it's hard to believe the energy saving is significant. And I think that's lots of people's first reaction," says Hill. Yet Hill is unwilling to dismiss Strassmann's proposal, noting that even a small energy saving might have helped women during prehistoric times, when they had to scramble desperately for enough food to stay alive.

Peter T. Ellison, an anthropologist at Harvard University, admires Strassmann's critique of Profet's theory, but he doesn't believe the Michigan anthropologist has offered a compelling counterproposal.

Ellison contends that a cascade of requirements makes menstruation almost inevitable for women. He argues that in humans and other higher primates, embryos implant into the endometrium in an unusually invasive manner in order to meet the oxygen and glucose demands of their energy-hungry brains. In turn, the endometrium is forced to prepare itself for this implantation by "terminally differentiating" its cells.