A lot of research on human health involves mice and rats, partly because their physiology is well-understood, and partly because their biology and behavior is resemblant to that of humans. But because typical mice and rats don’t menstruate, they aren’t good models for studying female reproductive health. The first human-like menstrual cycle in a rodent was identified this year — in a species of spiny mouse (a type of rodent, different than a common mouse). The spiny mouse’s cycle isn’t exactly the same as the human cycle — it lasts 9 days, about 3 of which are spent bleeding — but the ratio of cycle length to bleeding time is similar. It’s still unknown whether the spiny mouse will prove to be a good model for research, but if it is, it could help us in reaching a better understanding of common conditions like endometriosis and in developing effective treatments.