Horses breed by sexual reproduction, usually during the longer days of the year. When a mare comes into estrus, or "heat," she signals her status by raising her tail, squatting, urinating and winking her vulva. Stallions respond by engaging in courtship behaviors, concluding in mating.
During the breeding season, mares usually go through estrus cycles every 19 to 26 days. Each cycle consists of two to 10 days of estrus followed by an average of 21 days until the next period of estrus begins. These cycles continue until the mare mates and becomes pregnant or until the days begin growing shorter, signaling the end of the breeding season. A few mares continue cycling into the fall or even the winter.
Under natural conditions, a mare in estrus seeks out a stallion and assumes a rigid, wide-based breeding stance. As the stallion begins to nip and nuzzle her, working from her nose toward her hindquarters, she may urinate or wink her vulva. While courtship behavior progresses, the mare's vaginal secretions increase and the stallion's penis becomes erect. At the conclusion of courtship, the stallion often attempts to mount the mare several times before actually coupling with her. Following mating, the stallion and mare usually stay near each other and may mate repeatedly until the mare's estrus passes.