According to the BBC, the only other animals that have menstrual cycles—apart from humans and our close primate relatives—are the elephant shrews and certain bats, which certainly seems unfair.
As you probably learned in health class years ago, a menstrual cycle is the shedding of the uterine lining, which happens once a month or so. Every month, the uterus prepares a "fluffy and plush lining of blood vessels and stuff for the [egg and sperm] to implant into," Dr. Suzanne Gilberg-Lenz, an OB GYN, explains. "When this fails to happen and a female releases the egg but done not conceive, the lining of the uterus is shed—this is the period."
Lovely symptoms of this monthly process include blood pouring out of your vagina, cramps, PMS, and pimples. While humans bleed a lot for their body size, other animals, such as dogs, may have a small or moderate amount of discharge, and other animals, such a mice and mares, do not bleed at all.
There's no scientific consensus regarding why human menstruation is so damn bloody, but one idea suggests that the bleeding may be to prevent complications. "The tissue that is lost during menstruation is a mom's way of keeping the fetus in check—it's like an evolutionary balance," explains Dr. Elizabeth Rowe, an anthropologist from Perdue University who researches menstruation, the uterus, and genetics.
"In animals that bleed during pregnancy, the fetus digs deeply into a mom's uterus so it can have access to her blood supply," says Rowe.
While this allows nutrients for the growing little creatures, it can be bad news for mom. "The problem with that is, if you're a mother mammal—if you let a fetus just dig into your tissues willy-nilly, that could ultimately kill you," Rowe explains. A period acts as a preemptive strike of sorts to ensure this doesn't happen.
"The tissue that is lost during menstruation forms something of a shield between fetus and mom. I call it pre-gaming for pregnancy," says Rowe, who tested this hypothesis using data from primates. This notion explains why some mammals shed blood and others don't. "It turns out you don't see menstrual bleeding in species where they don't have an aggressive fetus," says Rowe.
To further her research, Rowe wants to compare brain size to blood loss. "What I want to look at is differences in brain size relative to body size, because one of the things that's really important about our species: We have really big brains compared to our body sizes. If you have big a brain, you have a big-brained fetus, and that big brain is greedy for nutrients and oxygen," Rowe says. "That's why they dig so deeply into mom's tissues."
With a solid notion in place as to why we're doomed to bleed, what about other less-than-luxurious period symptoms, like PMS and cramps? Do other animals experience them? In the 80s, biologists did report observing PMS symptoms in baboons; unfortunately, scientists still aren't sure to what extend most animals experience them. Today, there's even a debate over the legitimacy of PMS in humans. The uterus works in mysterious ways.
However, we do know that in some animals, like cats, are even known to actually act friendlier while bleeding. "Cats will be extremely friendly, where they will rub against objects, knead their paws on things, vocalize randomly, and even posture with their hind end in the air," when they're on the cat-rag, says Dr. Ashley, "the Southern Pet Vet."
Personally speaking, I'm more likely to spend my period with ice cream and cannabis suppositories, but now I can feel less aggrieved knowing my blood is just my body trying to protect me from the threat of an aggressive, greedy, large-brained fetus.