Why do women menstruate?

Menstruation is a peculiar phenomenon that women go through on a roughly monthly cycle, and it’s not immediately obvious from an evolutionary standpoint why they do it. It’s wasteful — they are throwing away a substantial amount of blood and tissue. It seems hazardous; ancestrally, in a world full of predators and disease, leaving a blood trail or filling a delicate orifice with dying tissue seems like a bad idea. And as many women can tell you, it’s uncomfortable, awkward, and sometimes debilitating. So why, evolution, why?

One assumption some people might make is that that is just the way mammalian reproduction works. This isn’t true! Most mammals do not menstruate — they do not cycle their uterine linings, but instead only build up a thickened endometrium if fertilization occurs, which looks much more efficient. Of the mammals, only most primates, a few bats, and elephant shrews are among the lucky animals that menstruate, and as you can see from the phylogeny, the scattered diversity of menstruating mammals implies that the trait was not present ancestrally — we primates acquired it relatively late.

i-58b845535df701c0ab264735f8049106-menstrual_phylogeny-thumb-500x494-71491.jpeg

Phylogeny showing the distribution of menstruation in placental mammals and the inferred states of ancestral lineages. Menstruating species/lineages are colored in pink, non- menstruating species/lineages in black. Species in which the character state is not known are not colored, and lineages of equivocal state are represented with black lines. Monodelphis represents the outgroup. Inference of ancestral states was performed in MacClade 4 by the parsimony method. Note that there is strong evidence for three independent originations of menstruation among placental mammals.

I suppose we could blame The Curse on The Fall, but then this phylogeny would suggest that Adam and Eve were part of a population of squirrel-like proto-primates living in the early Paleocene. That’s rather unbiblical, though, and what did the bats and elephant shrews do to deserve this?

There are many explanations floating around. One is that it’s a way to flush out nasty pathogens injected into the reproductive tract by ejaculating males — but that phenomenon is ubiquitous, so you have to wonder why only a few species bother. Another explanation is that it’s more efficient to get rid of the endometrium when not using it, than to maintain it indefinitely; but this is a false distinction, because other mammals don’t maintain the endometrium, they just build it up in response to fertilization. And finally, another reason is that humans have rather agressive embryos that implant deeply and intimately with the mother’s tissues, and menstruation “preconditions” the uterine lining to cope with the stress. There is, unfortunately, no evidence that menstruation provides any boost to the ‘toughness’ of the uterus at all.

A new paper by Emera, Romero, and Wagner suggests an interesting new idea. They turn the question around: menstruation isn’t the phenomenon to be explained, decidualization, the production of a thickened endometrial lining, is the key process.

All mammals prepare a specialized membrane for embryo implantation, the difference is that most mammals exhibit triggered decidualization, where the fertilized embryo itself instigates the thickening, while most primates have spontaneous decidualization (SD), which occurs even in the absence of a fertilized embryo. You can, for instance, induce menstruation in mice. By scratching the mouse endometrium, they will go through a pseudopregnancy and build up a thickened endometrial lining that will be shed when progesterone levels drop. So the reason mice don’t menstruate isn’t that they lack a mechanism for shedding the endometrial lining…it’s that they don’t build it up in the first place unless they’re actually going to use it.

So the question is, why do humans have spontaneous decidualization?

The answer that Emera suggests is entirely evolutionary, and involves maternal-fetal conflict. The mother and fetus have an adversarial relationship: mom’s best interest is to survive pregnancy to bear children again, and so her body tries to conserve resources for the long haul. The fetus, on the other hand, benefits from wresting as much from mom as it can, sometimes to the mother’s detriment. The fetus, for instance, manipulates the mother’s hormones to weaken the insulin response, so less sugar is taken up by mom’s cells, making more available for the fetus.

Within the mammals, there is variation in how deeply the fetus sinks its placental teeth into the uterus. Some species are epithelochorial; the connection is entirely superficial. Others are endotheliochorial, in which the placenta pierces the uterine epithelium. And others, the most invasive, are hemochorial, and actually breach maternal blood vessels. Humans are hemochorial. All of the mammalian species that menstruate are also hemochorial.

That’s a hint. Menstruation is a consequence of self-defense. Females build up that thickened uterine lining to protect and insulate themselves from the greedy embryo and its selfish placenta. In species with especially invasive embryos, it’s too late to wait for the moment of implantation — instead, they build up the wall pre-emptively, before and in case of fertilization. Then, if fertilization doesn’t occur, the universal process of responding to declining progesterone levels by sloughing off the lining occurs.

Bonus! Another process that goes on is that the lining of the uterus is also a sensor for fetal quality, detecting chromosomal abnormalities and allowing them to be spontaneously aborted early. There is some evidence for this: women vary in their degree of decidualization, and women with reduced decidualization have been found to become pregnant more often, but also exhibit pregnancy failure more often. So having a prepared uterus not only helps to fend off overly-aggressive fetuses, it allows mom a greater ability to be selective in which fetuses she carries to term.

The authors also have a proposed mechanism for how menstruation could have evolved, and it involves genetic assimilation. Genetic assimilation is a process which begins with an environmentally induced phenotype (in this case, decidualization in response to implantation), which is then strengthened by genetic mutations that stabilize the phenotype — phenotype first, followed by selection for the mutations that reinforce the phenotype. They make predictions from this hypothesis. In species that don’t undergo SD, embryo implantation triggers an elevation of cyclic AMP in the endometrium that causes growth of the lining. If genetic assimilation occurred, they predict that what happened in species with SD was the novel coupling of hormonal signaling to the extant activation process.

If either of these models were correct, we would expect an upregulation of cAMP- stimulating agents in response to pro- gesterone in menstruating species like humans, but not in non-menstruating species such as the mouse.

Results from experiments like those described above will elucidate the evolutionary pathway from induced to spontaneous decidualization, allowing us to answer long-unanswered questions about the evolutionary significance of menstruation. In addition, they will provide mechanistic insights that might be useful in the treatment of common reproductive disorders such as endometriosis, endometrial cancer, preeclampsia, and recurrent pregnancy loss. These disorders involve dysfunctional endometrial responses during the menstrual cycle and pregnancy. Thus, clarifying mechanisms of the normal endometrial response to maternal hormones, i.e. SD, will facilitate identification of genes with abnormal function in women with these disorders. An analysis of how SD came about in evolution can aid in identifying these critical molecular mechanisms.

Evolution, genetic assimilation, a prediction from an evolutionary hypothesis, and significant biomedical applications … that all sounds powerful to me.


Emera D, Romero R, Wagner G (2011) The evolution of menstruation: A new model for genetic assimilation: Explaining molecular origins of maternal responses to fetal invasiveness. Bioessays 34(1):26-35.

(Also on FtB)

Catholic compassion

I guess the Catholic church is concerned about the rights of breast cancer tumors: they probably identify too much with a deadly metastatic disease, and don’t want to seem them hindered. At least, that’s the only way I can interpret the decision of the Toledo, Ohio diocese to ban all support for the Susan G. Komen Foundation. Or maybe it’s just because breast cancer primarily affects women, so letting them die is of minimal consequence to them.

Their published reasoning is almost as ludicrous as that. Church officials say they won’t support Komen because 1) some of the money might go to Planned Parenthood, and we all know how much sanctioned church doctrine hates rational family planning, and 2) some of the money might someday support stem cell research. The Foundation doesn’t have any specific programs funneling money to stem cell research right now, but they might, someday, so let’s cut ’em off for something they don’t do!

I’m also rather peeved about another thing. The head of the Ohio Komen affiliate said, to persuade the church to reverse its declaration, that the local affiliate had not used any of its money to support Planned Parenthood. Screw that; maybe you haven’t, but you ought to be willing to do so if appropriate, and don’t throw an ally under the bus like that.

Something has stirred up the anti-circumcision brigade

It’s been a rough weekend. It was a rough night, with little sleep. This morning I had the double whammy of a doctor’s appointment and a dentist’s appointment. So I’m feeling a little cranky, and my teeth are all coated with this gummy gritty cherry-flavored fluoride goop. You don’t want to cross me today.

And then some idiot going by the name ‘eident9’ charges into a year-old thread and first demands an audience with me, and then demands an apology. WTF? This is a blog. Just post your comment and be done with it. And what bug has crawled up his butt suddenly? He says, “Recent matters have reminded me of this board.” Which had me wondering. What matters?

I’ve found out. It seems Russell Crowe had a twitter tantrum over circumcision. He’s against it. So am I. But apparently, I am not sufficiently angry about circumcision, and I belittled people who make it an obsession. I’m not going to apologize for that; I consider circumcision to be needless cosmetic surgery and a barbaric practice, and I’m not going to condone it, but neither am I going to go off the deep end like that guy and consider my life ruined by it. A majority of American males have been circumcised, and we’re a randy, raunchy, sexed-up lot who don’t seem to be suffering from an epidemic of sexual inadequacy. Well, maybe eident9 is, but one frustrated guy does not an epidemic make.

Now Salon has followed up with an article that suggests that circumcision may actually have some health benefits. I am not impressed. They cite a couple of incomplete epidemiological studies in African populations for HIV infection, and they come up with some astounding figures: a 50-60% reduction in infection rates. Wow, with that kind of advantage…sign me up.

However, these are deeply flawed studies. None of them were completed: they all abandoned the protocol and stopped the research as soon as preliminary results gave them positive values. This is like shooting craps and announcing that all your dice throws were practice…until you get a good roll, and then, yeah, that was the real deal. That one counts.

They all overstate their results. That 50-60% reduction was in relative rate, in comparison across the two groups. The actual calculated protection in absolute terms conferred by circumcision was a 2% reduction in the likelihood of infection. That doesn’t dazzle me, either, and given that the studies were terminated when they got their best results, I’m not persuaded.

And finally, give me a plausible mechanism for how circumcision would achieve these remarkable gains. Tell me how it is supposed to work. If it’s something to do with hygiene, it seems to me that better sex and health education should have the same or better effect than lopping off bits of skin.

So to my crackpot complainer, I say no apologies. I also say as I always have that I oppose circumcision, think it is a pointless and petty bit of suffering to put children through and ought to be discouraged, but I also don’t think it’s as hideously damaging as the obsessive nuts want to claim. Also, in the context of the original post, I consider it a prime example of selfish privilege to invade discussions of female genital mutilation, which does cause serious sexual and medical problems, with demands that we pay more attention to the lesser concerns of males getting lightly scarred penises.

Abortion needs to be taught in our medical schools

Read this horror story of a failed pregnancy.

I was taking an afternoon nap when the hemorrhaging started while my toddler napped in his room when I woke up to find blood gushing upward from my body. Though I didn’t know it at the time, I was experiencing a placental abruption, a complication my doctor had told me was a possibility. My husband was at work, so I had to do my best to take care of me and my toddler on my own. I managed to get to the phone and make arrangements for both of my children before going to a Chicago hospital.

Everyone knew the pregnancy wasn’t viable, that it couldn’t be viable given the amount of blood I was losing, but it still took hours for anyone at the hospital to do anything. The doctor on call didn’t do abortions. At all. Ever. In fact, no one on call that night did. Meanwhile, an ignorant batch of medical students had gathered to study me — one actually showed me the ultrasound of our dying child while asking me if it was a planned pregnancy. Several wanted to examine me while I lay there bleeding and in pain. No one gave me anything for the pain or even respected my request to close the door even though I was on the labor and delivery floor listening to other women have healthy babies as the baby I had been trying to save died in my womb.

Fortunately, a nurse called in a competent doctor to abort the fetus and stop the bleeding — or this woman would have been dead.

My two kids at home almost lost their mother because someone decided that my life was worth less than that of a fetus that was going to die anyway. My husband had told them exactly what my regular doctor said, and the ER doctor had already warned us what would have to happen. Yet none of this mattered when confronted by the idea that no one needs an abortion. You shouldn’t need to know the details of why a woman aborts to trust her to make the best decision for herself. I don’t regret my abortion, but I would also never use my situation to suggest that the only time another woman should have the procedure is when her life is at stake. After my family found out I’d had an abortion, I got a phone call from a cousin who felt the need to tell me I was wrong to have interfered with God’s plan. And in that moment I understood exactly what kind of people judge a woman’s reproductive choices.

The story also highlights the subversive strategy the right wing has followed: there is now a serious dearth of doctors trained to do abortions, so when a necessary abortion case shows up in an emergency, you’ve got a muddle of the self-righteous and the ignorant, all incompetent to do anything, milling about with their thumbs up their asses. She might as well have stumbled bleeding into a church and asked for help…which is exactly what the Coathanger Coalition wants them to do.

Imagine if someone showed up in an emergency room having a heart attack, and for religious reasons, no one had any training in using a defibrillator, and the only one available was in an underfunded clinic across town. That’s the direction we’re going, only we’re suppressing information and skills that would help just women’s lives. Which makes it OK, I guess. No men will die of a placental abruption, so it’s a low priority.

The power of faith

It’s amazing what religion can do. In this case, it motivated some dim old fart who ought to have been loafing about watching Glenn Beck and drowning his anger with a six-pack of Bud to go out and try to murder gynecologists. He didn’t actually succeed, fortunately: he was playing with his gun in his cheap room at the Motel 6 when it went off and sent a bullet flying into the room next door…so bad-ass that he is, he called up the front desk to mention that he was worried he might have hit someone else.

Then the police came and found out what he was really up to. He didn’t want to accidentally shoot someone, but he definitely intended to march into Planned Parenthood and murder as many people as he could.

Ralph Lang, 63, told a Madison police officer at the Motel 6, 1754 Thierer Road, that he had a gun “to lay out abortionists because they are killing babies,” according to a criminal complaint filed Thursday in U.S. District Court.

Lang said he planned on shooting the clinic’s doctor “right in the head,” according to the complaint. Asked if he planned to shoot just the doctor or nurses, too, Lang replied he wished he “could line them up all in a row, get a machine gun, and mow them all down,” the complaint said.

And he’s proudly confessing all this to the police! These religious excuses do attract the dumb ones, that’s for sure. And yes, he had a vague plan to go on a nationwide shooting spree, and he was driven by his religion.

Sgt. Bernie Gonzalez looked around Lang’s motel room and saw a box that contained several documents, including a map of the U.S. with dots in each state and the handwritten words “some abortion centers.”

Also written on the map was “Blessed Virgin Mary says Hell awaits any woman having an abortion.”

I think someone needs to lock up Ol’ Grandpa Gunman in a nice institution somewhere with a chapel and an absence of firearms and a multituded of locks on the doors, for the safety of society.

Support Planned Parenthood on Good Friday

We have a weekend of reason and lunacy coming up. On the reason side, atheists will be gathering in Iowa to discuss science and a sensible interpretation of the universe, taking advantage of a holiday none of us find particularly interesting. On the other side, that holiday celebrating the magical ‘death’ and imaginary resurrection of a weird Jewish mystic is regarded as also a good time to come out and oppress people. As is traditional, mobs of fanatics will turn out on Good Friday to surround local women’s clinics and harass and threaten the patients. Good Friday is not a good day to get a pap smear.

Planned Parenthood is well aware of this nasty tradition, and they are asking people of good will to come out and stand in solidarity with women looking for health care and reproductive freedom. There will be a rally at the Highland Park Clinic in St Paul tomorrow — they are asking people to register for two hour shifts. They are unfortunately marred by a couple of multifaith services, but stand with them anyway.

Your body isn’t yours, it belongs to the conservative Christians

Brace yourselves for a new onslaught of ridiculous Republican anti-woman bills. Alabama is working on bills to declare fetuses ‘persons’ by fiat — they’ll just legally redefine humanity to be a fertilized ovum and all derivatives thereof. They’ll probably get it passed, too.

I suppose if we could get enough dumbass legislators together to declare that vegetables are persons, we’d have to start treating carrots as if they were real people, like laborers and single mothers and college students and Mexicans and all those others the Republicans hold sacred. It won’t make them so, though, not that it makes much difference.

And just to show another tack they’re willing to take, Idaho wants to make all abortions illegal, no exemptions for rape or incest. It doesn’t matter how you got knocked up, lady, you’re having that baby…well, unless you’re the daughter of one of those Republican legislators, in which case you’ll get a ride in daddy’s SUV to that liberal hotbed of sin and perversion, Seattle, where you can get taken care of in a nice clean clinic with caring professionals.

The logic behind this decision is predictable.

“Is not the child of that rape or incest also a victim?” asked Rep. Shannon McMillan, R-Silverton. “It didn’t ask to be here. It was here under violent circumstances perhaps, but that was through no fault of its own.”[…]

The Idaho bill’s House sponsor, state Rep. Brent Crane, R-Nampa, told legislators that the “hand of the Almighty” was at work. “His ways are higher than our ways,” Crane said. “He has the ability to take difficult, tragic, horrific circumstances and then turn them into wonderful examples.”

God is so powerful, he also has the ability to take simple, manageable situations and turn them into tragic, horrific, oppressive circumstances in which the devil’s whores, i.e. all women, can suffer and feel guilt.

By the way, Brent Crane is no more of an authority on this god’s ways than I am, and what he’s really saying is that he can’t think of a rational justification for his evil law, so he’ll just take a shortcut straight to his god the psychopathic joker.

So pregnancy means a total surrender of autonomy?

This is how to handle a depressed person:

On December 23, 2010, Shuai, a 34-year-old pregnant woman who was suffering from a major depressive disorder, attempted to take her own life. Friends found her in time and persuaded her to get help. Six days later, Shuai underwent cesarean surgery and delivered a premature newborn girl who, tragically, died four days later.


On March 14, 2011, Shuai was arrested, jailed, and charged with murder and attempted feticide.

Don’t get pregnant, ladies! It means that every risk you might take becomes a criminal offense! I always thought it was a joke to criminalize suicide, too, but here’s a case where they’re actually prosecuting a woman for the crime.

Next up: every time you kick a man in the balls, you are a mass-murderer.

Who believes Governor Jan Brewer actually wants to help non-whites?

I don’t think I do — her history of policies in the state just screams racist. But she has suddenly become a concerned egalitarian, signing a law to protect minorities and women. She says. The law is actually designed to increase control of the reproductive rights of women and minorities — it criminalizes abortion if it is done for certain reasons.

Under the new Arizona statute, doctors and other medical professionals would face felony charges if they could be shown to have performed abortions for the purposes of helping parents select their offspring on the basis of gender or race.

First, is this a real problem? Are there very many white women demanding abortions because, to their great surprise, the father might have been black or brown? Are there many Hispanic women clamoring to get that pale-skinned mutant out of their uteruses?

Second, it’s a peculiar approach: abortion is legal. It is invasive and irrelevant to intrude on a woman’s personal decision in these matters, yet they plan to arrest people for their reasoning? Weird.

Third, look who’s being punished: the doctors. So a woman may be referred to a specialist ob-gyn who, with an appropriate evaluation of the patient’s health and emotional stability, carries out a legal medical procedure…and then later the doctor can be arrested if the patient makes a racist or sexist remark about the aborted fetus? That makes no sense.

This isn’t a bill to defend minorities. It’s a bill to make the position of abortion doctors even more precarious and difficult, and to give the state a pretext to lock them up in jail. It’s nothing but intimidation.

Jesse Bering responds

I was not kind in my assessment of Jesse Bering’s story about the evolutionary psychology of homophobia. He was quite irate with me and several other people who pointed out the tattered fabric of his evidence. Now he has gone scurrying back to the author of the study he described, Gordon Gallup, and gotten his take in a rather tendentious interview. Between the two of them, unfortunately, they still can’t manage to address anyou of our criticisms. It’s a very weird conversation: here’s how he handles me.

One common complaint lodged against evolutionary psychology is that its methods, which typically do not track the claimed fitness benefit, are inadequate for testing its hypotheses. PZ Myers, in surveying your homophobia studies, writes:

They know nothing about heritability, they’ve shown nothing about differential survival or fecundity … Is this to be the fate of evolutionary psychology, that it shrivels away into irrelevancy as its proponents overhype (sic) feeble, pathetic data sets?

Myers is, of course, notorious for such over-the-top statements—like the Jim Bakker of New Atheists, a caricature of sweat, histrionics and stage glitter, he sees religious conspiracies as often as evangelicals see the Devil.

Oh, man, glitter! I just knew I’d been forgetting something for my stage show.

Despite that useful bit of information, however, it’s dodging the issue with irrelevancies. I did not accuse Gallup or Bering of committing some religious conspiracy, nor did I even mention religion in my complaint. I said there was no evidence to back up their claims that homophobia conferred a fitness benefit, or even that it was a heritable trait. I know, expecting evidence of an evolutionary psychologist may be a sign of hysteria, and certainly is over-the-top, but I would expect that a reply to shoot me down would be the presentation of evidence to show I was wrong.

They don’t do that.

Instead, Gallup mentions a series of papers he’s published that have nothing to do with homophobia. For instance, he claims that they’ve “shown that a person’s voice is also related to fitness.” But they haven’t! I looked at the paper: it’s another self-reporting exercise in which they determined that people perceive women’s voices as more or less attractive in different stages of their menstrual cycle. Again, there is no attempt to examine inheritance, or whether this perception actually affects survival or fecundity…so we’re right back at my original complaint.

I’m not even going to touch Bering’s unwarranted moral indignation at the idea that Gallup might be a homophobe, which is not only irrelevant but wasn’t even suggested by any of his critics, or his silly conclusion that he’ll do anything to understand why gay men and women are bullied and murdered around the world…as if all of his critics are somehow just fine with the oppression of homosexuals.

Again, Jeremy Yoder has another solid response to the nonsense.