Reproductive cycles are the cycles that occur in mammals which is crucial for fertilization to occur. There are two types of reproductive cycles – menstrual cycle which occurs in primates and the oestrus cycle which occurs in lower mammals. In the menstrual cycle, the cycle occurs monthly and occurs when the fertilization process does not occur. This is because the ovum is not fertilized which in turn leads to the shedding of the uterine lining. This is called menstruation. On the other hand, the oestrous cycle occurs either seasonally or during a year. Here, the females undergo various physiological changes induced by the reproductive hormones. This cycle occurs in animals like dogs, cows etc.