Menstruation: Another Way Humans Are Unique In The Animal Kingdom

The following is an excerpt from Vagina Obscura: An Anatomical Voyage by Rachel E. Gross.

Disclaimer: When you purchase products through the Bookshop.org link on this page, Science Friday may earn a small commission which helps support our journalism.

Get The Book

Vagina Obscura: An Anatomical Voyage by Rachel E. Gross

Buy

The art of growing and then sloughing off the entire uterine lining is a rare quality throughout the animal kingdom, limited to a handful of primates, four species of bats, and a couple shrews. Even fewer experience menstrual disorders like endometriosis. Altogether just 84 species—1.6 percent of all placental mammals—are known to menstruate. If you look at a phylogenetic tree, they’re all over the place, suggesting that uterine bleeding has evolved at least three times independently. This underscores a fundamental mystery: Menstruation, with all its requirements and regularity, is why the human uterus is so dynamic. But it is also costly, requiring an animal to shed an entire organ and regrow it every month. So why would any animal bleed? 

One of the most common explanations is the idea that the female body needs to get rid of something dirty or harmful. In the 1920s, a doctor named Béla Schick theorized that women had special toxins in their menstrual blood, which he called “menotoxins.” His questionable experiments suggested that menstruating women sweated toxins from their skin that made flowers wilt and die. Although none of his findings were replicable, others latched onto the idea, arguing that menstruating women did indeed wither plants and spoil beer, wine, and pickles. Even today, many theories about menstruation come from the idea of the vagina as dirty and in need of cleansing: In 1993, a physician and mathematician named Margie Profet made waves when she suggested that menstruation’s function is to “defend against pathogens transported to the uterus by sperm.”

The real explanation may be not about getting rid of something harmful but defending against harm in the first place, says Dr. Günter P. Wagner, a Yale researcher in the ecology and evolutionary biology department who studies the evolution of menstruation.

Consider that motherhood isn’t all warm and fuzzy. It’s a fight for resources, often waged brutally between mother and offspring. Since an offspring holds only half the genome of its mother, its evolutionary interests aren’t exactly the same as hers—​and sometimes, they directly conflict. Evolutionarily speaking, the goal of the fetus is to suck as many resources as possible from Mom, whom it basically sees as its personal Giving Tree. Mom’s goal, by contrast, is to survive her pregnancy and limit the aggressiveness of her offspring. The evolutionary tug-​of-​war that takes place between mother and child’s genomes is called “maternal-​fetal conflict.”

Animals that menstruate have particularly conflict-​riddled relationships with their offspring, Wagner says. They tend to have more invasive fetuses and placentas, which burrow deeply into the mother’s body to gain access to her nutrients and blood supply. This poses an existential threat: In blurring the boundaries between the mother’s body and her offspring’s, the fetus runs the risk of siphoning off too many resources and weakening or even killing its host.

Fortunately, Mom has some tricks up her sleeve. The key event in menstruation is not bleeding, but the differentiation of the uterine lining. Over a period of about three days, uterine cells called fibroblasts transform into what are known as “decidual cells”—​meaning that they eventually fall off, like the leaves of deciduous trees. While these cells are necessary for an embryo to implant, they simultaneously create a matrix that is more difficult to penetrate. They also help tamp down the inflammatory response that occurs when a fetus implants, an event akin to a wound. All of these developments make sure the embryo burrows deeply enough to stay viable, but not so deep that it harms the mother.

In most species, this crucial differentiation happens only when an embryo appears. But in menstruators, it happens about once every month, spontaneously. (Menstruating animals ovulate, or release eggs, spontaneously—​as compared to animals who ovulate in response to light and temperature, like frogs, or copulation, like dogs.) “You don’t want to be defenseless when this pesky embryo is coming along,” says Wagner. You want to be ready for it—“sort of like a standing army.” These animals get a head start by erecting their defenses with every ovulatory cycle—​no fetus required.

Once that lining has differentiated, and once the body realizes there is no embryo, it has nowhere to go but down. The progesterone drop causes the blood vessels to violently die, killing the surrounding tissue and causing the rest to disintegrate and exit the body through the vagina.

So what really links animals that menstruate? Evolutionarily, they’re Type-​A planners. They anticipate conflict, priming the uterus and shielding themselves should an unwanted visitor happen to implant, rather than waiting until it’s too late. Mom’s body does this independently, regardless of whether a male, or fetus, comes along. There may be another advantage to regular menstruation: The uterine lining could play a role in sensing the “quality” of the fetus and deciding whether a prospective embryo should live or die. By taking into account chromosomal errors, aging sperm and eggs, and other quality-​control issues, the mother might summarily eject an embryo that isn’t worth the investment. The uterus may even learn from its mistakes and adapt to new conditions.

Many researchers argue that the remarkable dynamism of the uterine lining is a double-​edged sword. In the past, Critchley points out, women menstruated only around forty times in their lifetime, and spent the rest of the time pregnant or nursing. Today, the average Western woman menstruates up to five hundred times—​meaning it’s statistically more probable that some step in the intricate process will go askew. Consider endometriosis: Taken out of its natural context of the womb, the dynamic nature of the uterine lining proves catastrophic, intent on executing its life cycle in places where it caused scarring, pain, and inflammation.

Others counter that this logic is just the modernized version of Hippocrates’s wandering womb: it presumes that women’s uteruses are set up for disease, and that pregnancy is protective. There’s no reason that the frequency of menstrual cycling should be inherently pathological, says Dr. Kate Clancy, a biological anthropologist who studies reproduction at the University of Illinois Urbana-​Champaign. There are other changes to modern women’s bodies that deserve deeper investigation, including external factors like toxins from the environment that have been linked to endometriosis. Perhaps the problem is not in women’s heads, or even their pelvises, but in the world they inhabit.

“I’m increasingly thinking that it’s not a system flaw,” Clancy says. “At a certain point we need to start to think about this with the same rigor we would if this was a cisgender male body that we were exploring.”

That starts with understanding the basic mechanisms of menstruation. Shedding light on processes like uterine differentiation will help reveal what makes the cells of endometriosis different from other uterine cells and, ultimately, help scientists disrupt the process. It isn’t a woman’s lot to suffer just because she isn’t pregnant. We just haven’t been asking the right questions about how the uterus truly works.

Want to keep reading? Thanks to our friends at W. W. Norton, SciFri fans can download the rest of Chapter 7 from Rachel E. Gross’s Vagina Obscura.

Excerpted from Vagina Obscura: An Anatomical Voyage by Rachel E. Gross. Copyright © 2022 by Rachel E. Gross. Used with permission of the publisher, W. W. Norton & Company, Inc. All rights reserved.