most marsupial and eutherian mammals have a reproductive cycle known as the oestrous cycle (u.s: estrous cycle). females are sexually active only during the oestrous stage, when they are 'on heat' for a few days each month. if an ovum is not fertilized, the endometrium (uterus lining) is resorbed. oestrus cycles may occur once or twice a year, or many times a year. each group of mammals has its own frequency.