There is a division between animals on the basis of the rhythm of the sexual cycle on the mono - and polycyclic. The first group includes animals with a 1 the sexual cycle of the year. Have polycyclic animals occurs in several cycles, which in the course of a season or the whole year are repeated with a certain frequency depending on the animal species. Estrus lasts about 9 days. During this period, the vulva becomes soft. The first sign of the onset phase is the willingness of individuals to copulate. In this case, the discharge becomes thinner and less abundant. This is true only for those females, which is in the phase of proestrus there was intense selection. As already mentioned, depending on the breed highlight to dark colors can be a minor and continue throughout the first three periods of the sexual cycle. Ovulation occurs either at the beginning of estrus, or 2-4 day.