The menstrual cycle occurs only in fertile female humans
and other female rodentia, such as rats. Instead of 28 days as
the average length in human, the length of rats menstrual cycle
is about 4e5 days.1 Based on vaginal smears, there are four
phases of the menstrual cycle for rats: proestrus (12e14 h),
estrus (25e27 h), metestrus (6e8 h), and anestrus (55e57 h)