From its birth up to one year of age, a young horse is known as a foal. It can either be a colt (male) or a filly (female). After reaching one year old, the foal is called a yearling. The mare's gestation lasts 11 months, and her young will usually stay with her up until a year after birth. The mother's milk constitutes the initial food source of the foal that usually drinks it for at least four months. After these first months, the foal grows more able to eat grass, fodder, and other solid foods. The young horses can stand, walk, and trot as soon as two hours after birth.