Why, you ask, don't most female animals get periods like humans? Well, turns out that humans aren't actually the rule to menstruation - we're the exception! (No, your female dog or cat going through heat is not actually the same as it getting a period - who knew!) Although other animals that are closely related to us, like chimpanzees, orangutans, and rhesus monkeys also get periods, the only other non-relative animal that menstruates like humans are, get this, bats! (Though there is some research that suggests that elephant shrews also get periods.)
So what do other animals do if ovulation does not end in pregnancy? Instead of shedding the uterine lining, most animals just reabsorb it back into their bodies. Humans, it's thought, are different because our uterine lining is thicker and thus can't be fully reabsorbed. But the reasoning behind human periods actually isn't fully understood - there are a bunch of different controversial theories and though many studies have posed varying hypotheses, none have been scientifically proven. Want to hear all the ideas for why you go through that cramping every month? Watch the video to learn more!