Yea..well animals decide they’d rather have a baby than to go through that. So Everytime their cycle starts they get pregnant. Because that’s kinda the point. In zoos is pretty much the same. The exceptions are the ones that aren’t in a breeding program. Those are put on contraception.
Actually, it’s so much cooler than that: most animals don’t menstruate, and never would, regardless of their pregnancy status. Menstruation is a phenomenon limited to few mammals: elephant shrew, humans, primates, and a few bat species. What do these have in common? The presence of a hemochorial placenta (yep, science word—and it’s pretty metal—it means the embryo is bathed in blood! not that that really matters for the rest of it. Just thought you might need this for your next pub trivia contest).
So what’s the difference? Well, for starters, most mammals don’t build up a uterine lining (thickened endometrium, or what is actually shed during a woman’s period) unless they conceive. Humans and these other species, however, build it up every time they ovulate. That means that if there is no fertilization and no embryo, the fall in hormones (that would no occur if pregnant) sends a signal to get rid of all that excess tissue that is not needed right now. Bam! Periods (or menstruation, more correctly).
Okay, okay, but why? As for why the shedding of this tissue occurs, that’s to reduce the risk for infection and cancer. As you can imagine, having that rich, blood-filled tissue layer opens up all kinds of doors for infection. And, you never want cells that can replicate rapidly (as they do every month in response to ovulation to build up the endometrium) to hang around longer than necessary. After all, cancer cells are just regular cells that decided to divide too fast (for various regions)—therefore, cells that are normally meant to rapidly divide are just that much more predisposed to becoming cancer.
So why build up all this tissue in the first place just to shed it if she doesn’t become pregnant? It seems like a waste or resources, doesn’t it? Unfortunately, no one knows for sure. But current science has a damn good guess. If you try to look it up, you’ll see lots of references to “decidualization,” which is nonsense to you unless you, like me, have taken reproductive physiology and embryology classes. In plain English, decidualization refers to the extent to which the embryo invades into the mom’s tissue. There are lots of big, scary words to explain the different categories, but all you need to know is this: in animals that menstruate, the embryo is VERY invasive, and in animals that do not, the embryo is less invasive. Therefore, it seems like building up this tissue is defensive on the mother’s part. If the embryo is going to invade a certain amount, wouldn’t you want there to be a thicker barrier?
This all seems very convoluted, I’m sure. But remember: from the embryo’s perspective, it wants as many resources from the mother as possible without killing her (even if it makes her very sick), but from the mother’s perspective, she wants to live and not be sick! So the embryo wants to invade deeper, but the mom wants to push it away. So preemptively building up this extra tissue is defensive on the mother’s part!
But if you need even another reason a female would buildup this tissue before she knows she’ll need it, there is one! In case you didn’t know, reproduction is not perfect. Even if fertilization occurs, that definitely doesn’t mean there will be a baby. Aside from the many other reasons, sometimes the genetics of mom and dad don’t combine very well. That means the embryo is not viable (won’t live). The cool thing is, the endometrium that mom builds up before fertilization can sense a ton of these abnormalities, and therefore can reject an nonviable embryo before implantation can occur! This saves the female body from wasting resources on an embryo that won’t survive. Pretty cool, huh?
But back to zoos for a second. Those animals in zoo that would menstruate are generally bred or kept on contraception. But many zoos use similar contraceptive methods as are available for humans. Did you know that chimpanzees can take human birth control pills? Yep. Many zoos use those for their chimps, too. But, if you are a human with a uterus, you are probably aware that menstruation occurs on the pill too! So depending on the contraceptive, menstruation can still happen! However, there are lots of contraceptive methods used in zoos that would not result in menstruation even if the animals would otherwise menstruate.
So there you go, biological and evolutionary explanations for a period (or lack thereof!) Pretty cool, huh?
Questions? Hit me up.
(Also, I apologize for the cissexist language in this post, but I thought since it’s primarily referring to non-human animals it would be bulky to say “animals with uteri” instead of females or mom. However, humans can have uteri or menstruate without being women. And, humans can menstruate or have uteri if they are men.)
UCLA’s Steve Cole from The Social Life of Genes.
Your DNA is not a blueprint. Day by day, week by week, your genes are in a conversation with your surroundings. Your neighbors, your family, your feelings of loneliness: They don’t just get under your skin, they get into the control rooms of your cells.