Domestic horses are social animals. In the wild, they live in a harem group or band with one to several stallions, multiple mares, and the mares' offspring. One stallion (the highest ranking or dominant animal) does most of the breeding. In many horses, rank is associated with age or the ability to survive and thrive in challenging environments. 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. Within a harem group, the highest-ranking individual is usually, but not always, a stallion. This high-ranking stallion will force colts to leave the group once they are 2 years old, as they begin to become sexually and socially mature. 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. Social maturity is not attained until 5 years of age.
Most fillies and all colts leave the herd they were born in by about 5 years of age. Fillies that remain in their original group may have fewer offspring. 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. 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.
Rank within groups also depends on the ages and sexes of the group members. The more members of the herd, and the more within each age and sex group, the less likely it is for a dominance hierarchy to exist. 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.
Many behavior problems in horses are associated with confinement. Under free-ranging circumstances, horses will wander and spend 60% of their day foraging. The reminder of their time is spent standing, lying down, or engaging in another activity. This same pattern is the preferred one under barn conditions—even with free choice of grain, horses will choose to eat many small meals a day.
Free-ranging movement is also part of play development in horses. Until 3 months of age, most play is solitary. Interactive play peaks at 3 to 4 months of age. There are also sex differences in play. 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 while colts tend to groom only fillies, which has been interpreted as practice for later courtship behavior. The social experience of play is important for normal social interaction in adult life.
Last full review/revision July 2011 by Karen L. Overall, MA, VMD, PhD, DACVB, ABS Certified Applied Animal Behaviorist