The cycle of reproductive activity shown by most sexually mature nonpregnant female mammals except most primates (compare menstrual cycle). There are four phases:

(1) pro-oestrus (follicular phase)—Graafian follicles develop in the ovary and secrete oestrogens;

(2) oestrus (heat)—ovulation normally occurs, the female is ready to mate and becomes sexually attractive to the male;

(3) metoestrus (luteal phase)—corpus luteum develops from ruptured follicle;

(4) dioestrus—progesterone secreted by corpus luteum prepares uterus for implantation.

The length of the cycle depends on the species: larger mammals typically have a single annual cycle with a well-defined breeding season (they are described as monoestrous). The males have a similar cycle of sexual activity. Other species may have many cycles per year (i.e. they are polyoestrous) and the male may be sexually active all the time.