Rats and mice are polyestrous with short estrous cycles of approximately 4–6 days. The mouse estrous cycle is ~4–5 days; the rat cycle is ~4–5 days. The cycle has four stages: proestrus, estrus (ovulation), metestrus (diestrus 1), and diestrus (diestrus 2). This short cycle makes rodents valuable research models for reproductive biology. Humans have a ~28-day menstrual cycle; cattle have a ~21-day cycle.