Just 1.5% of mammals menstruate and 99.9% of those are primates. That's why scientists are amazed by the spiny mouse—the first rodent shown to menstruate with a cycle remarkably similar to humans, according to a study that still needs to be peer-reviewed. Researchers at Monash University in Australia who were studying a lab colony of 14 female spiny mice found they bled for three days out of an average menstrual cycle of nine days. That means they shed the uterine lining for 20% to 40% of their cycle, similar to the 15% to 35% of time women bleed during a typical 28-day cycle, per Nature. "When you do science you're not surprised at anything—but wow, this was a really interesting finding," says a reproductive biologist not involved in the study. Scientists hope the mice can now be used to study conditions related to menstruation.
The current animal model for menstruation-related conditions is the baboon, which menstruates, but research on primates is expensive. The study suggests spiny mice could eventually replace the baboon, "opening up a new, cheaper way to study disorders like endometriosis and pre-menstrual syndrome," reports Popular Science. Researchers—who flushed the mice with a saline solution for 18 days, and dissected the uteri of four mice at various stages of the menstrual cycle—are still looking at genes to discover the source of cells that regrow the uterine lining after it is shed. As for why it took them so long to realize a rodent was menstruating, "the answer, as with many discoveries in science, is that no one really looked," says a study author. "Everyone knew that rodents didn't menstruate." (This company has a period policy.)