What separates us from the animals, even more so than our opposable thumbs or larger brains? That would be our massive amounts of period blood.In fact, only a few species of animals (other than primates) have a menstrual cycle at all. Certain animals, including cats and dogs, will have some discharge and bleed when they're in heat , but they just can't compare to the four to 12 teaspoons of blood and fluid we human ladies shed during our Time Of The Month. So, Broadly decided to investigate this discrepancy; we are all mammals, after all.Turns out, we might bleed as much as we do to avoid an evolutionary mishap. "If you're a mother mammal — if you let a fetus just dig into your tissues willy-nilly, that could ultimately kill you... The tissue that is lost during menstruation forms something of a shield between fetus and mom," Dr. Elizabeth Rowe told Broadly. "If you have big a brain, you have a big-brained fetus, and that big brain is greedy for nutrients and oxygen," she added. "That's why they dig so deeply into mom's tissues."In other words, menstrual bleeding is only a thing among species that breed hungry, aggressive fetuses. Our bodies are just trying to make sure our own young don't siphon off all of our nutrients and kill us. (Side note: Aggressive Fetuses should be the name of someone's punk band .)