ANN ARBOR— The cyclical growth and retreat of the endometrium, the lining of the uterus, consumes less energy than keeping the blood-rich lining in a steady-state of readiness to receive an embryo.

That is why menstruation evolved, according to University of Michigan biological anthropologist Beverly I. Strassmann” not, as a rival theory would have it, to cleanse the uterus of sperm- borne bacteria.

In a report published earlier this year in the Quarterly Review of Biology, Strassmann analyzes data from a wide variety of studies, to show that whole-body metabolic rate changes over the course of the menstrual cycle, becoming about 7 percent lower after menstruation than around the time of implantation by an embryo.

This metabolic cycling saves an amount of energy equivalent to six days’ worth of food over four menstrual cycles, economizing on the energy costs of reproduction.

” The energy economy of menstruation may be of ancient origin,” predating the evolution of mammals, says Strassmann, assistant professor of anthropology at the U-M. ” The uterine endometrium in mammals is similar to the epithelium of the oviducts in reptiles,” she explains. ” Both are secretory linings that transfer nutrients from mother to embryo.

” During the mammalian menstrual cycle, the lining of the uterus grows until it is ready to support an embryo, but if pregnancy does not occur, the lining is reabsorbed or shed during menstruation.

” Just like the mammalian endometrium, the secretory activity of reptilian oviducts is restricted to the time when a fertilized egg or embryo is likely to be present. The epithelium of reptilian oviducts grows larger in the breeding season, when it is most biochemically active, and saves energy by regressing in the non-breeding season.”

Strassmann has studied menstruation in both the laboratory and the field. For 31 months, she worked among the Dogon, a farming people who live in Mali, West Africa. There, she studied the menses of Dogon women who are forced by taboos to use menstrual huts, displaying their reproductive status to the entire community.

Dogon women between the ages of 20 and 34 have only about four periods in two years, on average, because they are usually either pregnant or nursing, Strassmann found.

” Since menstruation is a rare event in societies that do not practice birth control and since sexual activity is often present during long stretches when menstruation is absent, it is doubtful that menstruation evolved as a defense against pathogens carried by sperm,” notes Strassmann.

Furthermore, she points out, the bacteria that cause gonorrhea are more prevalent in the female reproductive system during and after menstruation than before it. ” A likely reason,” she says, ” is that blood is an excellent culture medium for bacteria.”

Finally, Strassmann points out, if menstruation evolved to protect the uterus against bacteria delivered by sperm, then species in which females mate often, and with lots of different partners, should have heavier menstrual flows. But an analysis of primate species shows no relation between copious menstruation and female promiscuity.

” In short,” says Strassmann, ” a large body of evidence contradicts the pathogen defense hypothesis, suggesting that it is rash to use it as the springboard for medical recommendations.”