PZ Myers has a really cool blog post up now about a new theory of menstruation put out in a paper by Emera, Romero, and Wagner, who appear to be actual biologists, instead of those psych profs and anthropology profs who get called "evolutionary biologists" every time they wank off in public with unevidenced theories about how we evolved to have 50s-era gender roles. It's one of those things I want to flag for feminists particularly, because I think really understanding the scientific discourse around human female biology can go a long way to chilling some of the uglier debates that go on about "nature" and things like reproduction, menstruation, etc. Basically, the problem for biologists regarding human menstruation is that it's surprisingly uncommon for mammals to have monthly menstruation. Not all mammals---as PZ says, other primates, bats, and elephant shrews menstruate---but by and large, most mammals only build up a uterine lining after an embryo implants and begins the pregnancy cycle. On paper, this seems like the smarter move, survival-wise. It uses fewer resources and avoid the health problems that can accompany menstruation if you're not lucky enough to live in a hygienic environment. (Or, as PZ puts it, "filling a delicate orifice with dying tissue seems like a bad idea.") I'll add that it's particularly confusing for it to happen in humans, who are social animals who tend to be private about our body functions. How much so changes across cultures, sure, but overall, we're private animals. Finding ways to conceal menstruation in order to participate in public life has been a hassle for women throughout history, and unfortunately for many, the answer today is still "avoid leaving the house until it's over". So, the question is why: why would we evolve a unique-ish trait that is a physical and social burden to an extent that it also impacts our ability to survive and optimally reproduce? There's a lot of theories, but this new one is pretty interesting:

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.

I don't know that I can make it even more clear that PZ, who is a gifted science educator. What I want to talk about is how critical theories like this, and understanding the science of human reproduction, are to really understanding why our social/political debates over female reproductive systems are completely bonkers. That's because those debates are often built on the debate over what's "natural". Obviously, the best answer to anyone who says that women can't or shouldn't do X (have an abortion, use contraception, use the pill for suppressing a period) because of nature should be dismissed out of hand as using the naturalistic fallacy. But when it comes to ladies and our leaky bodies (though, of course, men's are just as leaky---leaky is the sort of natural state of bodies), a lot of people insist stridently that the naturalistic fallacy should be put on hold. Here's the thing that understanding the science should really help you realize, however: Nature doesn't have a single, unchangeable "intent" for women and our bodies, nor are women's wills automatically in conflict with nature.  I bracketed out some contentious areas for some thoughts on what PZ explains means for various debates on women's reproductive capacities and what's "natural".

1) Abortion. This one is a no-brainer. Anti-choicers claim abortion is unnatural, but as PZ's writing explains, it's actually perfectly natural. Women's bodies go through a lot of unconscious processes to determine if now is a good time to have a baby. Biologically speaking, the idealized reproductive strategy for a woman is to have babies when she's in the best possible state to raise them. The unconscious body does some of this work, but what makes human beings awesome is that we have these large brains that can supplement our natural processes and make them more efficient. "I'm not ready to have a baby" is an equally valid message coming from the brain as from inside the uterus. Unless, of course, you believe that women are inherently inferior creatures who should be constrained from self-care and family care in order to satisfy the desires of mostly strangers who have psychological issues around sex. To which I say, you have an entirely different argument to prove then. 

2) Contraception. Anti-choicers like to portray menstruation as the product of some inherent female tendency to nurture at all costs. In fact, PZ caught David Barton before making facetious arguments about how all animals but humans will sacrifice the health and lives of mothers for the young, which is not only false but obviously false. (Death of mothers tends to equal death of young from lack of care.) The image that anti-choicers paint of the monthly uterine build-up is that it's like a baby nest that you're making, and efforts to keep "babies" from making their homes there are somehow unnatural. The reality is far more complex. It's not that uterine lining isn't about nurture, but it's about so much more. It seems it's also likely about protecting a woman's body from the parasitic (biologists' word, not mine!) qualities of the embryo and fetus. It's also about sorting the good from the bad. But most importantly, it's a system that's got wastefulness built into it. That women menstruate so much means that saying no to babies a lot more than saying yes is a built-in part of the system. Contraception, alongside abortion, is simply a logical extension of the pre-existing system. 

3) Contraception, part two. Unfortunately, many feminists run with the naturalistic fallacy to bash hormonal contraception, saying that you shouldn't take it because it's not "natural". Again, neither are cars or clothes or condoms, for that matter, but for some reason, this argument has its hooks in many. A lot of people find it weird to stop the process of ovulating and then having a real period (as opposed to the fake one the pill creates, or suppressing your period altogether with continuous pill use) every month, which they assume must have some value in and of itself. But if you actually look at the science, the notion that we "should" be having a monthly cycle even while not trying to conceive doesn't really compute. Whether or not this particular theory is the truth, the reality is that constant ovulation and menstruation serves no purpose outside of being the best that evolution could come up with to reproduce. Humans have come up with better ways of handling these functions, so why not use them? An honest look at evolution shows that nature doesn't always know best, and some times it creates biological processes that look like what you rigged up as a home repair to avoid having the money to do it right. If evolution could have created a situation where women simply will their uterine lining to start building as they get closer to wanting to conceive, and then and only then ovulated, that would be in women's best biological interests. That technology goes ahead and does that for us is a blessing, it really is. 

Now, if you can't take the pill or don't want to, that's great. Don't. Please. I'm serious. This is not a guilt trip. The point is that there's no reason to make broad arguments about how it's "unnatural" or that there's some great purpose to menstruation that we can't know and so shouldn't suppress it to be safe.*

So, in sum: PZ's post is about one of the many theories to explain why humans menstruate. It may or may not be the best theory, but what it shares in common with all other theories is a baseline understanding that the ovulation-menstruation cycle is, at best, inefficient and often dangerous. It's not necessarily bad, but it's certainly not good. And definitely not good enough to overrule women's express desires to abort any one pregnancy, prevent ovulation, or prevent menstruation.

*The other argument that I hear from feminists on why menstrual suppression is bad is that men benefit from women not bleeding on their dicks when they have sex. Okay, but I figure women also benefit. It's weird to cast men and women's interests as always opposed, when mostly they're in line. You know who really doesn't benefit from bleeding all over the place during sex? Sheets. If you have something against sheets, I suppose you can start from there, but be assured that most people are simply not going to get on the anti-sheets train. Especially women, who do most of the washing of sheets.