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Reproductive cycles, also known as estrous cycles or heat cycles, are recurring physiological processes that prepare female mammals for mating and potential pregnancy. These cycles are regulated by hormones, primarily estrogen and progesterone, which are produced by the ovaries and the pituitary gland. The cycle typically consists of several distinct phases, each characterized by specific hormonal changes and physical manifestations.

The follicular phase, driven by estrogen, leads to the development of ovarian follicles, which contain the eggs. As follicles mature, estrogen levels rise, causing the female to become receptive to mating. Ovulation, the release of a mature egg from the ovary, usually occurs at the peak of estrogen levels. Following ovulation, the luteal phase begins, dominated by progesterone, which is produced by the corpus luteum, a structure formed from the ruptured follicle. Progesterone prepares the uterus for implantation of a fertilized egg and maintains pregnancy if it occurs.

If fertilization does not happen, the corpus luteum degenerates, leading to a drop in progesterone and estrogen levels. This decline triggers the start of a new cycle, which may include menstruation in some species, or a period of anestrus (inactivity) in others. The length and specific characteristics of reproductive cycles vary significantly among different species, influenced by factors such as genetics, environment, and season. Understanding these cycles is crucial for successful breeding programs in both domestic and wild animal populations, as well as for reproductive health management.