The Oestrous Cycle
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The oestrous cycle is the reproductive cycle found in most mammalian placental females whereby there are recurring periods when the female is fertile and sexually receptive (oestrus) interrupted by periods in which the female is not fertile and sexually receptive (anoestrus). Animals that have oestrous cycles reabsorb the endometrium (inner membrane of the mammalian uterus) if conception does not occur during that cycle. The oestrous cycle is contrasted with the menstrual cycle in which the endometrium is shed through menstruation when pregnancy does not occur and in which the female may be sexually receptive at any time during the cycle.
There are many variations among animals in terms of their oestrous cycles. Some may undergo oestrous only one time a year during a particular season (white-tailed deer, foxes), while others may undergo a succession of cycles during a certain time of the year if they do not become pregnant (horses, sheep), and others may undergo cycles throughout the whole year (mice, cows, pigs).
Below is an overview of the hormonal changes during the oestrous cycle: (Download article to see this)