The ovaries are the source of mature female gametes (ova) and hormones necessary for reproduction.

Oogenesis is formation of ova. Estrus (heat) is the period of sexual receptivity in the female. An estrous cycle is the interval from the beginning of one estrus to the begin­ning of the next.

Animals that have only one estrous cycle per year are called monestrous animals, while those that have several estrous cycles per year are polyestrous. Many animals have successive periods of estrus if a pregnancy is not estab­lished and maintained. The timing of these periods is often determined by seasonal changes in length of day, so these animals are seasonally polyestrous. The relatively long period of inac­tivity in seasonally polyestrous animals is termed anestrus and is not properly a part of the sexual cycle.