estrus

Estrus, commonly known as heat, is the period in many female mammals during which sexual receptivity and fertility coincide. It is part of the estrous cycle and is distinct from the human menstrual cycle, in which there is no single, universal phase of receptivity.

Physiologically, estrus is driven by rising estrogen from developing ovarian follicles. The timing of ovulation relative to estrus varies by species: in many animals, ovulation occurs toward the end of estrus or shortly after; in others, ovulation follows estrus. Some species are induced ovulators, releasing oocytes in response to mating.

Most estrous cycles are described as four stages: proestrus, estrus, metestrus, and diestrus. Proestrus features follicular growth and increasing estrogen; estrus is the phase of sexual receptivity and peak fertility; metestrus and diestrus follow with luteal activity and rising progesterone. Anestrus may occur in non-breeding seasons for some species.

Duration and signs of estrus vary widely by species. Small mammals may have an estrous period lasting from several hours to a few days; dogs commonly experience a 5–14 day estrus with visible signs such as sexual receptivity and vulvar changes; cows often exhibit estrus for about 12–24 hours. Behavioral cues include changes in mounting behavior, tail deviation, vocalization, and mucus discharge.

Human reproduction is governed by the menstrual cycle rather than a true estrus; fertility follows ovulation, but there is no universal estrous-phase receptivity.