estrous

Estrous refers to the recurring reproductive cycle observed in most mammalian species other than some primates. The cycle describes the physiological and behavioral changes that prepare the female for ovulation and potential mating. The period of sexual receptivity is called estrus or standing heat. The broader cycle typically includes several stages—proestrus, estrus, metestrus, and diestrus—with some species also exhibiting a period of anestrus when reproductive activity is paused.

During proestrus, the ovaries recruit follicular growth and estrogen levels rise. In estrus, the female is typically receptive to males and ovulation occurs, often triggered by a surge of luteinizing hormone. Metestrus follows ovulation as the corpus luteum forms and begins to secrete progesterone. In diestrus, progesterone production generally dominates while the reproductive tract prepares for potential pregnancy. The length of each stage and the overall cycle length vary widely among species, ranging from several days to several weeks.

Hormonal control centers on the hypothalamus-pituitary axis. Gonadotropin-releasing hormone stimulates FSH and LH, leading to follicle development, ovulation, and luteal function. Estrogen and progesterone regulate the tissues of the reproductive tract and the behavioral signs of receptivity. If fertilization does not occur, the corpus luteum regresses, cycle conditions reset, and the animal returns to anestrus until the next cycle.

Variation among species is common: some are polyestrous with ongoing cycles, others are seasonal breeders, and a few may be monoestrous. Behavioral and chemical signals often indicate estrus, aiding mating. Estrous contrasts with the human menstrual cycle, in which the uterine lining is shed rather than reabsorbed and ovulation is not tied to a single conspicuous receptive phase.