Domestic horses are social animals. In the wild, they live in a harem group or band of 2 to 21 horses, with one to several stallions, multiple mares, and the mares’ offspring. One stallion (the highest ranking or dominant animal) does most of the breeding. High-ranking stallions are the first to gain access to mares in heat and the first to displace a mare from another band. Unless they become pregnant, mares cycle over 21 days during the spring and summer months.
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. When she is receptive to breeding, she may stand still, deviate her tail, and urinate, leading the stallion to mount her. Pasture breeding is usually more successful than "in hand" or controlled breeding.
Ovulation (the release of an egg from an ovary) usually occurs while a mare is sexually receptive to a stallion. Pregnancy lasts 315–365 days, with an average of 340 days. Factors that affect the length of pregnancy include nutrition, time of year (shorter time for late summer breeding), and sex of the fetus (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 hours. 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 the most dependence on their mothers and have minimal contact with other horses. They spend most of the time resting near their mother. It is important that caregivers provide gentle handling during the first 42 days of life. At approximately 2 to 3 months of age, foals become more social and start playing with other foals and exploring their environment. Snapping (tooth clapping or champing) is a facial expression given by young horses to adults, particularly stallions. It peaks in frequency at 2 months of age, after which it decreases. It may function to decrease aggression from adults, but is also compatible with displaced nursing behavior. This is not the same behavior as smacking, which is an aggressive threat in which the ears are laid back and the mouth is open with smacking lips, but the lips are not retracted.
At approximately 4 months of age, foals become more independent from their mothers and start developing relationships with other horses. They also start spending more time performing adult behaviors, such as grazing and resting while standing. Colts play more than fillies and play different games than fillies do. Colt games focus more on fighting and mounting, while filly games focus more on running and mutual grooming. Fillies will groom both colts and fillies whereas colts tend to groom only fillies, which has been interpreted as practice for later courtship behavior. The social experience of play learned at these young ages is important for normal social interaction in adult life.
Most fillies and all colts leave the herd they were born in before 2 years of age, when they become sexually mature. They stay alone for a few months and then join a different group or establish a new one. Young stallions form bachelor herds, and the highest-ranking stallion within this group is usually the next one to acquire a mate. Fillies can join a bachelor herd but are often incorporated into other bands. Stallions are rarely solitary; when this occurs, they are usually old and infirm.
Rank in horses is not necessarily associated with age, weight, height, sex, or time in the group. Offspring of high-ranking mares appear to be high-ranking later in life, which might indicate both genetic and experience factors. While rank in males is based primarily on access to females, rank in females is determined by which mares lead group activities (for example, seeking out resources such as water holes). Horse groups are largely structured by females, and females make the decision about whether to leave or to stay within a harem. Such decisions are usually based not on specific stallions or their characteristics, but on a female’s assessment of food resources. High-ranking females can successfully interfere with the nursing of foals by lower‑ranking females. Mares form friendships and are more likely to groom each other. This pattern is typical of many animals—rank is determined mainly by the way that lower-ranking animals defer to higher-ranking ones, not by the results of outright combat.
Relationships within most horse bands are complex and depend on multiple factors and their interactions (age or length of residence in the group, sex, size, and rank of the mother). These factors are important to consider when addressing problems that may arise in stabled horses.
Rank effects also exist between herds. Multi-stallion herds are dominant over single-stallion bands, possibly because lower-ranking stallions within a herd conduct most of the fighting that occurs between groups. Herds that are currently occupying an area or using a resource (for example, a water hole) tend to retain it. Groups, as well as individuals within them, follow specific patterns of fecal marking.
Also see professional content regarding normal social behavior in horses.