Why Women Bleed: U-M Study Suggests New Explanation For One Of Reproductive Biology's Most Persistent Puzzles.

November 07, 1996

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 daysí 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."
-end-


University of Michigan

Related Bacteria Articles from Brightsurf:

Siblings can also differ from one another in bacteria
A research team from the University of Tübingen and the German Center for Infection Research (DZIF) is investigating how pathogens influence the immune response of their host with genetic variation.

How bacteria fertilize soya
Soya and clover have their very own fertiliser factories in their roots, where bacteria manufacture ammonium, which is crucial for plant growth.

Bacteria might help other bacteria to tolerate antibiotics better
A new paper by the Dynamical Systems Biology lab at UPF shows that the response by bacteria to antibiotics may depend on other species of bacteria they live with, in such a way that some bacteria may make others more tolerant to antibiotics.

Two-faced bacteria
The gut microbiome, which is a collection of numerous beneficial bacteria species, is key to our overall well-being and good health.

Microcensus in bacteria
Bacillus subtilis can determine proportions of different groups within a mixed population.

Right beneath the skin we all have the same bacteria
In the dermis skin layer, the same bacteria are found across age and gender.

Bacteria must be 'stressed out' to divide
Bacterial cell division is controlled by both enzymatic activity and mechanical forces, which work together to control its timing and location, a new study from EPFL finds.

How bees live with bacteria
More than 90 percent of all bee species are not organized in colonies, but fight their way through life alone.

The bacteria building your baby
Australian researchers have laid to rest a longstanding controversy: is the womb sterile?

Hopping bacteria
Scientists have long known that key models of bacterial movement in real-world conditions are flawed.

Read More: Bacteria News and Bacteria Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.