A cycle of sexual activity that usually has a period of a year. It is largely under influence of a biological clock that controls the release of hormones, but exogenous factors to do with the weather may also play a part. Many animals have an annual breeding season, within which there may be a shorter oestrous cycle of female receptivity. Ovulation occurs during the cycle and the female becomes receptive to mating. Fertilization can take place only during this period. The oestrous cycle of the female rat (Rattus norvegicus) lasts 4–5 days, and may be repeated if fertilization does not take place. Domestic dogs (Caninae) have two periods of heat (oestrous cycle) per year, one in spring and one in autumn. In some species, copulation is necessary for ovulation. These include the rabbit (Oryctolagus cuniculus), cats (Felis catus), and the short-tailed shrew (Blarina brevicauda). Some primates, including humans, have a different system, called the menstrual cycle.
Subjects: Zoology and Animal Sciences.