Periods are a strange phenomenon. We don’t know why humans have them, or, to look at it another way, why most other animals don’t. Scientists say only 1.5 percent of mammal species have periods, and most of those are primates like us. The ranks of the menstrually afflicted grew a little bit recently, as researchers learned that female spiny mice have periods, too. They shared their findings on the bioRxiv preprint server.

The spiny mouse (Acomys cahirinus, also known as the Egyptian spiny mouse) is a perculiar little animal. It has spines like a hedgehog and will indiscriminately devour just about anything in its path, including mummified bodies. When in danger, a spiny mouse can shed (and later regrow) large chunks of its fragile skin. Needless to say, the spiny mouse has got scientists’ attention.

Curious about the spiny mouse’s reproductive system, researchers decided to get up close and personal with their nether regions. They brought 14 young, never-pregnant female mice into the lab and, once a day for 18 days, administered tiny saline douches into the rodents’ vaginas, watching to see what, if anything, would be washed out.

They found blood. Once every nine days, for about three days, the mice shed the lining of their endometrium and expelled it, along with some blood, just like humans do. The mice spent between 20 to 40 percent of their nine-day cycle menstruating, which is approximately the same proportion seen in human women. These findings make the spiny mouse the first known rodent with a human-like menstrual period.

Reproductive biologist Francesco DeMayo of the U.S. National Institute of Environmental Health Sciences was not involved with the study, but was impressed by the results. “When you do science you’re not surprised at anything—but wow, this was a really interesting finding,” he told Scientific American.

The discovery has implications not only for the rest of the animal kingdom (how many other menstruating species have scientists overlooked?) but for us. At the moment, the most popular animals used in research—mice, rats, birds, and fish—are period-free, which means they can’t be used to study the menstrual cycle. Learning about mouse menstruation could one day help us understand our own.

Why did it take scientists so long to notice that these curious creatures were part of the period posse? “The answer, as with many discoveries in science, is that no one really looked,” said Hayley Dickinson, a reproductive physiologist and long-time spiny rat researcher at the University of Monash. “Everyone knew that rodents didn't menstruate.”

Know of something you think we should cover? Email us at tips@mentalfloss.com.