One of the major reasons we know so little about menarche, and menstruation in general, is that humans belong to a very small group of animals that menstruate. Most researchers who study how human organs work use animal models such as mice and rats as proxies for humans, but these animals do not menstruate. Therefore, we need new systems to study the onset of menstruation. Recently, a rodent called the spiny mouse was found to menstruate. The spiny mouse gives us a unique opportunity to study menarche in a laboratory setting. Using this animal, we will be able to look at the uterus immediately before and after menstruation begins and to determine how its cells are altered by this milestone.