Wild horses are socially complex mammals. In a free-roaming state, they display a variety of behavioral patterns that are predictable and directly linked to survival, reproduction, and communication in their natural environment. With over 57 million years of evolutionary history in North America, the horse has developed behaviors that were selected out by nature to promote viability and continuation of the species.. The social organization of wild horses closely parallels that reported for plains and mountain zebras and can be seen in remnant form in certain domesticated horse behaviors, like play fighting.
In behavioral research conducted between May 1 and November 1, 1970, a graduate student from the University of Michigan, James D. Feist, observed wild horse behavioral patterns within the Pryor Mountain Wild Horse Range, located on the Montana-Wyoming border. Feist kept careful notes related to behavior and reproduction; mare-foal relations; nursing; home range and spacing between groups (territoriality); reaction to threat; drinking; feeding; play and play fighting; rolling; rest and sleep; and communication and social behavior patterns, including perceptions; individual recognition; vocalizations; aggressive behavior; submissive behavior; mutual grooming; leadership and herding behavior; dominance hierarchy; stallion encounters and fights; and reemergence of wild behaviors in domesticated horses. He felt that an impressive aspect of the social behavior of horses was the conservatism of social organization. This means that old, established traits and patterns of behavior in horses are not easily lost with time and change or, even, with domestication.
Domesticated horses have a long history of human-controlled, artificial selection for characteristics having no bearing on survival in the wild. They are also managed in captivity in ways that do not allow the expression of normal social structure. Feist observed that, despite this period of manipulation by man, once horses manage to escape and live in a wild or semi-wild state, the typical wild social organization emerges. This social organization is typical of what is known of the well-studied plains and mountain zebras, and somewhat less well known in studies of wild horse herds. Re-emergence of wild behavior patterns, in once-domesticated horses, has been reported not only by Feist in Wyoming-Montana but by researchers in Japan, Nova Scotia, England, and Nevada.
Wild horses exist in stable harem groups, called "bands," comprised of two or more individuals, with 5-7 being average. Some bands may grow to as many as 20 horses. Bands normally consist of mares, foals, and yearlings protected by the harem stallion. The stability of the band group comes from strong dominance by dominant stallions and the fidelity of harem members. These family groups are so cohesive that mares will often stay together, even after the stallion dies or is replaced by a younger, stronger stallion. When young males within a band reach the age of about two, they are normally driven out by the harem stallion, after which the young male joins with other males, of the same age, to form a bachelor band. These all-male bands are less stable than harem bands, from which they came, as the bachelor band stays together only until each bachelor finds an unattached, wandering mare or wins over a band of mares through successful combat with its harem stallion. By the time a bachelor finds his own band, he is normally between 3-6 years old. Aggression between stallions is common, owing to attempts by bachelors to secure mares and by harem stallions struggling to keep their bands intact. When one band encounters another, fighting behavior is common amongst harem stallions and young males that still remain within a family group. Stallions sometimes create large piles of manure, called "stud piles" that serve to mark their territory for rival males.
A hierarchy exists amongst mares in a band, with one mare becoming dominant and acting as co-leader, along with the stallion. The stallion actually protects his harem from predators and other stallion-intruders, while the dominant mare leads the band in its quest for forage, water, and shelter. Members of a band communicate with each other through body language, using gestures, posturing, and vocalization. Movement of the head, ears, body, and feet, along with neighing, squealing, and scenting, communicates a vast array of information that, through field observation, wild horse behaviorists have slowly been able to translate.