Horses are social animals that under feral conditions (or on pasture) live in bands (harems) that consist of several mares, their offspring up to 2–3 yr of age, and at least 1 and as many as 6 adult males. The core of the group is the mares, which stay together even if the stallion leaves or dies. The group size ranges from 2 to 21 horses; multiple-male bands are larger than single-male bands. Groups are not limited to a specific geographic area and will travel in search of resources. Colts and fillies leave the group usually before 2 yr of age (when they become sexually mature), stay alone for a few months, and then join a different group or establish a new one. Some colts may form a “bachelor band” with up to 16 males, and later join other groups in which the stallion has died or been chased away.
Hierarchy in horses appears to be linear (with occasional triangular relationships) and not necessarily based on age, weight, height, gender, or time in the group. These are important factors when considering problems in stabled horses, and attentive management is required before introducing horses to each other. Offspring of high-ranking mares appear to be high-ranking later in life, which might indicate both genetic and experience components. Hierarchical rank in females is determined by observing group behavior (eg, seeking out resources such as water holes). Females make the decision about whether to leave or to stay within a band based on factors such as assessment of food resources or number of stallions in the group. High-ranking females can successfully interfere with the nursing of foals by lower-ranking females. Mares have preferred associates, are preferentially groomed, and will groom certain individuals. As in many other species, hierarchy in horses is based on deference by lower-ranking animals, not fighting.
The highest-ranking stallion in a band does most of the breeding, because it is the first to secure access to a receptive female and the first to displace a female from another band. In the absence of conception, horses cycle every 21 days during the spring and summer. There are three phases of sexual behavior in horses: courtship, mating, and postmating behavior. During courtship the stallion will approach the mare, prance, sniff her, nuzzle her, and groom her. The mare may squeal, kick, or move away to show the stallion she is not ready, or she may stand still, deviate her tail, and urinate, leading the stallion to mount her. Pasture breeding can achieve 100% success rates, versus 50%–60% for “in hand” or controlled breeding. This is probably because of familiarity between the horses, higher fertility of the mare, and less aggression between horses.
Ovulation usually occurs 36 hr before estrous behavior ends. Gestation lasts 315–365 days, with an average of 340 days. Factors that affect gestation length include nutrition, time of year (shorter time for late summer breeding), and gender (slightly longer for colts). Mares usually deliver at night, even when provided with artificial light. Bonding between mare and foal occurs in the first 24 hr. Most nursing behavior is initiated by the foal and terminated by the mare, especially in the first month.
During the first month of life, foals show maximal dependence on their dams and have minimal contact with other horses. They spend most of the time resting near the dam. The socialization period is 2–3 mo of age; the foal starts playing with other foals and explores its environment. It is important to provide gentle handling during the first 42 days of life. At this time, snapping (teeth clapping) peaks. This is a behavior shown by foals toward adult horses, presumably to reduce aggression. It may also be a displacement nursing behavior, such as air nursing. Snapping is not the same behavior as smacking, which is an aggressive behavior or threat. Allogrooming also peaks at this time.
From 4 mo of age, the foal starts developing independent relationships and spends more time in adult maintenance behaviors such as grazing and resting while standing. There are sex differences in play; colts mount more and fight more than fillies, who focus on grooming and running. Colts groom only fillies, while fillies groom both sexes.