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Estrous cycle variability differs among species, but cycles are typically more frequent in smaller animals. Even within species significant variability can be observed, thus cats may undergo an estrous cycle of 3 to 7 weeks. [15] Domestication can affect estrous cycles due to changes in the environment.
The cycle is controlled by several hormones which regulate the estrous cycle, the mare's behavior, and the reproductive system of the mare. The cycle begins when the increased day length causes the pineal gland to reduce the levels of melatonin, thereby allowing the hypothalamus to secrete GnRH.
The estrous cycle, also known as "season" or "heat" of a mare occurs roughly every 19–22 days and occurs from early spring into autumn. As the days shorten, most mares enter an anestrus period during the winter and thus do not cycle in this period.
Horses are a precocial species, and foals are capable of standing and running within a short time following birth. [53] Foals are usually born in the spring. The estrous cycle of a mare occurs roughly every 19–22 days and occurs from early spring into autumn.
The second breeding phase of the rut takes place three to four weeks after the first breeding phase. This is due to younger cows coming into estrus, as well as older cows that were not bred on their first estrus cycle coming back into estrus. Herd bulls are less aggressive towards satellite bulls at this phase in the rut due to exhaustion. [14]
Many zoologists regard this as different from a "true" menstrual cycle. Female domestic animals used for breeding—for example dogs, pigs, cattle, or horses—are monitored for physical signs of an estrous cycle period, which indicates that the animal is ready for insemination.
Jeju horses belong to long day breeders, whose mating occurs from March to August in Jeju. The estrous cycle is 20 to 24 days (e.g. 22 days on average), and the estrus usually lasts 4 to 11 days (e.g. 7 days on average). Within 7 to 10 days (e.g. 8 days on average) after delivery, the mare undergoes another estrus, which reoccurs every 22 days.
Female seasonal breeders will have one or more estrus cycles only when she is "in season" or fertile and receptive to mating. At other times of the year, they will be anestrus, or have a dearth of their sexual cycle. Unlike reproductive cyclicity, seasonality is described in both males and females.