This document discusses reproductive cycles in female animals. It defines estrus, estrous, and related terminology. Female reproductive cycles can be polyestrus, meaning they occur repeatedly, or monoestrus, occurring once. The average cycle lengths and characteristics are provided for common farm animals like cows, ewes, and sows. Causes of anestrus, or lack of cycling, include pregnancy, season, stress, nutrition and the presence of offspring. The human menstrual cycle phases of follicular, ovulation and luteal are also outlined.