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Horse behavior is best understood from the view that horses are prey animals with a well-developed fight-or-flight instinct. Their first response to a threat is to flee, although they are known to stand their ground and defend themselves or their offspring in cases where flight is untenable, such as when a foal would be threatened.
Nonetheless, because their physiology is also suited to a number of work- and entertainment-related tasks, humans domesticated horses thousands of years ago, and they have served humans ever since. Through selective breeding, some breeds of horses have been bred to be quite docile, particularly certain large draft horses. On the other hand, most light horse riding breeds were developed for speed, agility, alertness and endurance; building on natural qualities that extended from their wild ancestors.
The instincts of horses can be used to human advantage to create a bond between human and horse. These techniques vary, but are part of the art of horse training.
Horses as herd animals 
Horses are highly social herd animals that prefer to live in a group. Like all creatures, equine social behavior developed to help the species survive.
There also is a linear dominance hierarchy in any herd. They will establish a "pecking order" for the purpose of determining which herd member directs the behavior of others, eats and drinks first, and so on. This behavior pattern also applies to their interrelationship with humans. A horse that respects the human as a "herd member" who is higher in the social order will behave in a more appropriate manner towards all humans than a horse that has been allowed to engage in dominant behavior over humans.
Horses are able to form companionship attachments not only to their own species, but with other animals, including humans. In fact, many domesticated horses will become anxious, flighty and hard to manage if they are isolated. Horses kept in near-complete isolation, particularly in a closed stable where they cannot see other animals may require a stable companion such as a cat, goat or even a small pony or donkey to provide company and reduce stress.
When anxiety over separation occurs while a horse is being handled by a human, the horse is described as "herd-bound". However, through proper training, horses learn to be comfortable away from other horses, often because they learn to trust a human handler, essentially ranking humans as a dominant member of a "herd."
Herd behavior in the wild 
Feral and wild horse herds are usually made up of several separate small bands who share a given territory. Bands are organized on a "harem model" in that they usually consist of one adult male and a group of females. Each band is led by a mare who is dominant in the hierarchy, called the "dominant mare," the "lead mare" or the "boss mare." The band contains additional mares, their foals, and immature horses of both sexes. There is usually a single "herd" or "lead" stallion, though occasionally a few less-dominant males may remain on the fringes of the group.
Bands are usually on the small side, as few as 3 to 35 animals. The makeup of bands shifts over time as young animals are driven out of the band they were born into and join other bands, or as young stallions challenge older males for dominance. However, in a given closed ecosystem such as the isolated refuges in which most wild horses live today, to maintain genetic diversity the minimum size for a sustainable wild horse or burro population is 150-200 animals.
Hierarchical structure 
Survival dictates that the herd members ultimately cooperate and stick together. As with many animals that live in large groups, establishment of a stable hierarchical system or "pecking order" is important to smooth group functioning. This is often, but not always, a completely linear system. Situations arise where horse A may be dominant over horse B who is dominant over horse C, yet in certain circumstances horse C could be dominant over horse A in competition for a particular resource. Dominance depends on a variety of factors, including an individual's need for a particular resource at a given time. It is therefore variable throughout the lifetime of the herd or individual animal. Some horses may be dominant over all resources and some may indeed be submissive over all resources.
Contention for dominance can be risky since one well-placed kick to a leg could cripple another horse to such an extent that it would be defenseless, exposed, and possibly unable to get to food or water. Therefore, once this system of dominance and submission is generally established aggressive behaviour is minimal between herd members, though higher-ranked animals often will assume a role of exercising control and moderating aggressive behavior in the herd.
In times of stress, whether from predators or extreme weather, the center of the herd is the safest because it offers the most protection from the elements and is further away from predators than any other part. Because of this, "punishment" of misbehaving members is sometimes delivered in the form of expulsion from the herd—temporarily or sometimes permanently.
Most young horses in the wild are allowed to stay with the herd until sometime in their yearling or 2-year old year, when they reach full sexual maturity. Studies of wild herds have shown that the herd stallion will usually drive out both young colts and fillies. Experts who study wild horses theorize that this may be an instinct that prevents inbreeding, so that the herd stallion does not mate with his own female offspring. The fillies usually join another band in fairly short order, and the colts driven out from various herds usually join together for safety in small "bachelor" groups until those who are able establish dominance over an older stallion in another herd.
Role of the lead mare 
Contrary to traditional portrayals of the herd stallion as the "ruler" of a "harem" of females, the actual leader of a wild or feral herd is the alpha or dominant mare, commonly known as the "boss mare" or "lead mare." She is usually one of the more mature animals, responsible for the overall safety of the herd, familiar with the terrain and resources available. She takes the lead when the herd travels, determines the best route, when to move from one place to another, and claims the right to drink first from watering holes and stake out the best location for grazing.
Role of the stallion 
The edge of the herd is the domain of the herd stallion, who must fight off both predators and other males. When the herd travels, the stallion brings up the rear, watching for predators and driving straggling herd members on, keeping the group together. During mating season, stallions tend to act more aggressively, in order to keep the mares from straying off. However, most of the time, the stallion is fairly relaxed, spending much of his time "guarding" the herd not by herding the mares around, but by scent-marking manure piles and urination spots in order to make clear his position as herd stallion.
By living on the periphery of the herd, exposed to weather, predators, and challenges from other stallions, the herd stallion endures a somewhat vulnerable existence. He is exposed to more risks than any other herd member and can be replaced by a stronger successor at any time. Interestingly, a herd stallion will occasionally tolerate one young stallion to live at the edge of the herd, possibly as a sort of designated successor, even though the young horse will eventually gain mastery over the older stallion and claim the herd.
Ratio of stallions and mares 
Biologically, and depending on the physical environment available to a herd in the wild, there is only a need for one stallion for every 10 to 20 mares, though most bands are smaller than this. Domesticated stallions, with careful human management, often "cover" more mares in a year than is possible in the wild. Traditionally, Thoroughbred stud farms limited stallions to breeding between 40 and 60 mares a year. Today, by carefully breeding mares only when at the peak of their Estrous cycle, a few thoroughbred stallions have "covered" over 200 mares per year. With use of technologies such as artificial insemination one stallion could sire thousands of offspring annually, though in actual practice, economic considerations usually limit the number of foals produced.
Domesticated stallion behavior 
Some horse breeders keep horses in semi-natural conditions, allowing a single stallion to run with a group of mares. This is referred to as "pasture breeding." Young stallions who are not of breeding age live in a separate "bachelor herd." While this has advantages of less intensive labor for human caretakers, and full-time turnout may be psychologically healthy for the horses, pasture breeding presents a risk of injury to valuable breeding stock, both stallions and mares, particularly when unfamiliar animals are added to the herd. It also raises questions of when or if a mare is bred, and may also raise questions as to parentage of ensuing foals. Thus, keeping stallions in a natural herd is not particularly common, especially on breeding farms standing multiple stallions to outside mares. It is more often seen on farms with closed herds; only one or a few stallions with a stable mare herd and few, if any, outside mares.
More often, mature domesticated stallions are commonly kept by themselves in a stable or small paddock with a strong, high fence that prevents escape. When stallions are stabled in such a way that they can both see each other and touch nose to nose, such as through bars or a grill, they will often challenge one another and sometimes attempt to fight. For this reason, stallions are often kept from sight and touch of other stallions, due to risk of injury and disruption to the rest of the stable, even if several are kept in the same barn. If they have access to paddocks, there often is a corridor between the paddocks so the stallions cannot touch one another, and in some cases, stallions are let out for exercise at different times of day so they do not see or hear one another.
To avoid stable vices associated with isolation, some stallions, though not all, do well when stabled with a non-horse companion, such as a gelded donkey or a goat. (The Godolphin Arabian was said to be particularly fond of a barn cat). While many domesticated stallions become too aggressive to tolerate the close presence of any other male horse without fighting, some tolerate a gelding as a companion, particularly one that has a very calm, unflappable temperament. One example of this was the racehorse Seabiscuit, who lived with a gelding companion named "Pumpkin." Other stallions may tolerate the close presence of an immature and thus less dominant stallion.
While stallions and mares often compete together at horse shows and in horse races, stallions generally must be kept away from close contact with mares, both to avoid unintentional or unplanned matings, but also to minimize their instincts to fight one another for dominance when in the presence of mares. When horses are lined up for placing at shows, handlers keep stallions at least a horse's length from any other animal and do not allow their horses to touch noses with any other animals. Stallions can be taught to ignore any mares or other stallions that are in close proximity while they are working.
However, stallions do live peacefully in bachelor herds in the wild and in natural management settings. For example, the stallions in the New Forest region of the United Kingdom live in stallion-only bachelor herds on their winter grazing. Stallions are still herd animals, and when managed as domesticated animals, some farms assert that carefully managed social contact benefits stallions. Well-tempered stallions who will be kept together for a long period of time may be stabled in closer proximity, though this method of stabling is generally used only by experienced stable managers. An example of this are the stallions of the Spanish Riding School, who travel, train and are stabled close together. In these settings, more dominant animals are kept apart by stabling a young or less dominant stallion in the stall between them. However, sometimes a stallion raised in isolation from other horses cannot be stabled in this fashion at all due to aggressive behavior.
Dominance in domesticated herds 
Because domestication of the horse usually requires stallions to be isolated from other horses, either mares or geldings may become dominant in a domestic herd. Usually dominance in these cases is a matter of age and, to some extent, temperament. It is common for older animals to be dominant, though old and weak animals may lose their rank in the herd. There are also studies suggesting that a foal will "inherit" or perhaps imprint dominance behavior from its dam, and at maturity seek to obtain the same rank in a later herd that its mother held when the horse was young.
In recent studies of domesticated horses, new evidence indicates that horses appear to benefit from a strong female presence in the herd. Groupings of all geldings, or herds where a gelding is dominant over the rest of the herd (for example, if the mares in the herd are quite young or of low status), may be more anxious as a group and less relaxed than those where a mare is dominant.
Horses communicate in various ways, including vocalizations such as nickering, squealing or whinnying; touch, through mutual grooming or nuzzling; smell; and body language. Body language is probably the predominant means of communication. Horses use a combination of ear position, neck and head height, movement, and foot stomping or tail swishing to communicate. Discipline is maintained in a horse herd first through body language and gestures, then, if needed, through physical contact such as biting, kicking, nudging, or other means of forcing a misbehaving herd member to move. In most cases, the animal that successfully causes another to move is dominant, whether it uses only body language or adds physical reinforcement.
Horses can interpret the body language of other creatures, including humans, whom they view as predators. If socialized to human contact, horses usually respond to humans as a non-threatening predator. Humans do not always understand this, however, and may behave in a way, particularly if using aggressive discipline, that resembles an attacking predator and triggers the horse's fight-or-flight response. On the other hand, some humans exhibit fear of a horse, and a horse may interpret this behavior as human submission to the authority of the horse, placing the human in a subordinate role in the horse's mind. This may lead the horse to behave in a more dominant and aggressive fashion. Human handlers are more successful if they learn to properly interpret a horse's body language and temper their own responses accordingly. Some methods of horse training explicitly instruct horse handlers to behave in ways that the horse will interpret as the behavior of a trusted leader in a herd and thus more willingly comply with commands from a human handler. Other methods encourage operant conditioning to teach the horse to respond in a desired way to human body language, but also teach handlers to recognize the meaning of horse body language.
Horses are not particularly vocal, but do have four basic vocalizations: the neigh or whinny, the nicker, the squeal and the snort. They may also make sighing, grunting or groaning noises at times.
Ear position is often one of the most obvious behaviors that humans notice when interpreting horse body language. In general, a horse will direct the pinna of an ear toward the source of input it is also looking at. Horses have a narrow range of binocular vision, and thus a horse with both ears forward is generally concentrating on something in front of it. Similarly, when a horse turns both ears forward, the degree of tension in the horse's pinna suggests if the animal is calmly attentive to its surroundings or tensely observing a potential danger. However, because horses have strong monocular vision, it is possible for a horse to position one ear forward and one ear back, indicative of similar divided visual attention. This behavior is often observed in horses while working with humans, where they need to simultaneously focus attention on both their handler and their surroundings. A horse may turn the pinna back when also seeing something coming up behind it.
Due to the nature of a horse's vision, head position may indicate where the animal is focusing attention. To focus on a distant object, a horse will raise its head. To focus on an object close by, and especially on the ground, the horse will lower its nose and carry its head in a near-vertical position. Eyes rolled to the point that the white of the eye is visible often indicates fear or anger.
Ear position, head height, and body language may change to reflect emotional status as well. For example, the clearest signal a horse sends is when both ears are flattened tightly back against the head, sometimes with eyes rolled so that the white of the eye shows, often indicative of pain or anger, frequently foreshadowing aggressive behavior that will soon follow. Sometimes ears laid back, especially when accompanied by a strongly swishing tail or stomping or pawing with the feet are signals used by the horse to express discomfort, irritation, impatience, or anxiety. However, horses with ears slightly turned back but in a loose position, may be drowsing, bored, fatigued, or simply relaxed. When a horse raises its head and neck, the animal is alert and often tense. A lowered head and neck may be a sign of relaxation, but depending on other behaviors may also indicate fatigue or illness.
Tail motion may also be a form of communication. Slight tail swishing is often a tool to dislodge biting insects or other skin irritants. However, aggressive tail-swishing may indicate either irritation, pain or anger. The tail tucked tightly against the body may indicate discomfort due to cold or, in some cases, pain. The horse may demonstrate tension or excitement by raising its tail, but also by flaring its nostrils, snorting, and intently focusing its eyes and ears on the source of concern.
The horse does not use its mouth to communicate to the degree that it uses its ears and tail, but a few mouth gestures have meaning beyond that of eating, grooming, or biting at an irritation. Bared teeth, as noted above, are an expression of anger and an imminent attempt to bite. Horses, particularly foals, sometimes indicate appeasement of a more aggressive herd member by extending their necks and clacking their teeth. Horses making a chewing motion with no food in the mouth do so as a soothing mechanism, possibly linked to a release of tension, though some horse trainers view it as an expression of submission. Horses will sometimes extend their upper lip when scratched in a particularly good spot, and if their mouth touches something at the time, their lip and teeth may move in a mutual grooming gesture. A very relaxed or sleeping horse may have a loose lower lip and chin that may extend further out than the upper lip. The curled lip flehmen response, noted above, most often is seen in stallions, but is usually a response to the smell of another horse's urine, and may be exhibited by horses of any sex. Horses also have assorted mouth motions that are a response to a bit or the rider's hands, some indicating relaxation and acceptance, others indicating tension or resistance.
Horses and humans 
Horses are creatures of habit and have excellent long-term memory, which makes consistent training extremely important to the horse. Untrained young horses, even with top bloodlines, can be bought for relatively little money compared to those with training. Once a horse is started under saddle and demonstrates that it is trainable, ridable and has some athletic talent for its work, the price easily triples.
Humans are usually viewed by wild horses as potential predators. However, horses are also innately curious and may investigate any creature that is interesting but not threatening.
Any domesticated horse with some experience of humans usually views people as generally harmless objects of curiosity worth at least minor notice, especially if they know that humans may bring food or treats. Rarely will any domestic horse become truly vicious unless it has been spoiled or abused by humans, though many stallions have a great deal of naturally aggressive, dominant behavior that requires that they be managed only by knowledgeable handlers. However, any horse is a large animal that retains some wild instincts, so can react unpredictably by running, biting, striking, or kicking. Thus humans must always be alert around horses because they can accidentally harm people.
The ability of humans to work in cooperation with the horse is based on both the natural curiosity of the horse and the strong social bonds that horses have with each other. Horses do not like to be separated from their herd, because to be alone is to be exposed to predators on all sides. Also, in a herd, less dominant horses tend to gravitate toward the most mature and confident members. Therefore, many horse training principles are based upon having the horse accept a human as the dominant herd member. Ideally this is not done by force, but by the horse developing trust in the ability of the human and confidence that the human will be a responsible "herd leader."
Horses are also adapted to covering large amounts of territory and must have a certain boldness to do so. A horse that is afraid more than necessary will expend energy needlessly and then may not be able to escape when a threat is real. Thus, horses have an ability to check out the unusual and not immediately flee from something that is merely different.
This willingness to consider new things can also be used by a human trainer to adapt the horse's behavior to an extraordinary range of activities that are well outside the range of instinctive horse behavior, including acts considered naturally dangerous by the average horse such as bullfighting, jumping off cliffs, diving into water, jumping through a ring of fire, or walking into a modern television studio, complete with enclosed space, bright lights, and tremendous noise.
People who train horses first have to educate them that some normal herd behavior is inappropriate around humans. For example, biting and "shadow boxing" (rearing, striking) that is common play among young horses, colts in particular, could be injurious or fatal to people. Other instinctive traits, such as running away when frightened, bucking off anything that lands on a horse's back (like a mountain lion or other predator), or never entering a small enclosed area, also have to be overcome before the horse is useful to humans.
Even when trained, most horses will still test boundaries, at least mildly, and some horses with dominant personalities will openly challenge a weak or inexperienced handler. For example, if handled with incompetence or abuse, a horse may ignore its training and attempt to nip, bite, kick, refuse to be led, or try other ways to challenge human dominance. Without consistent handling, some horses, especially young ones, will revert to their untrained ways. However, due to their good memory, horses with solid training from trustworthy handlers often retain what they have learned, even after a gap of many years.
Sleep patterns 
Horses are able to sleep both standing up and lying down. They are able to doze and enter light sleep while standing, an adaptation from life as a prey animal in the wild. Lying down makes an animal more vulnerable to predators. Horses are able to sleep standing up because a "stay apparatus" in their legs allows them to relax their muscles and doze without collapsing. In the front legs, their equine forelimb anatomy automatically engages the stay apparatus when their muscles relax. The horse engages the stay apparatus in the hind legs by shifting its hip position to lock the patella in place. At the stifle joint, a "hook" structure situated on the inside bottom end of the femur cups the patella and the medial patella ligament, preventing the leg from bending.
Unlike humans, horses do not need a solid, unbroken period of sleep time. They obtain needed sleep by means of many short periods of rest. This is to be expected of a prey animal, one that needs to be ready on a moment's notice to flee from predators. Horses may spend anywhere from four to fifteen hours a day in standing rest, and from a few minutes to several hours lying down. However, not all this time is the horse actually asleep; total sleep time in a day may range from several minutes to a couple of hours. Horses require approximately two and a half hours of sleep, on average, in a 24-hour period. Most of this sleep occurs in many short intervals of about 15 minutes each.
Horses must lie down to reach REM sleep. They only have to lie down for an hour or two every few days to meet their minimum REM sleep requirements. However, if a horse is never allowed to lie down, after several days it will become sleep-deprived, and in rare cases may suddenly collapse as it involuntarily slips into REM sleep while still standing. This condition differs from narcolepsy, though horses may also suffer from that disorder.
Horses sleep better when in groups because some animals will sleep while others stand guard to watch for predators. A horse kept entirely alone may not sleep well because its instincts are to keep a constant eye out for danger.
Eating patterns 
Horses have a strong grazing instinct, preferring to spend most hours of the day eating forage. Horses and other Equids evolved as grazing animals, adapted to eating small amounts of the same kind of food all day long. In the wild, the horse adapted to eating prairie grasses in semi-arid regions and traveling significant distances each day in order to obtain adequate nutrition. Thus, they are "trickle eaters," meaning they have to have an almost constant supply of food to keep their digestive system working properly. Horses can become anxious or stressed if there are long periods of time between meals. When stabled, they do best when they are fed on a regular schedule; they are creatures of habit and easily upset by changes in routine. When horses are in a herd, their behavior is hierarchical; the higher-ranked animals in the herd eat and drink first. Low-status animals, who eat last, may not get enough food, and if there is little available feed, higher-ranking horses may keep lower-ranking ones from eating at all.
Psychological disorders 
When confined with insufficient companionship, exercise or stimulation, horses may develop stable vices, an assortment of compulsive stereotypies considered bad habits, mostly psychological in origin, that include wood chewing, wall kicking, "weaving" (rocking back and forth) and other problems. These have been linked to a number of possible causal factors, including a lack of environmental stimulation and early weaning practices. Research is ongoing to investigate the neurobiological changes involved in the performance of these behaviors.
See also 
- Domestication of the horse
- Equus (genus)
- Glossary of equestrian terms
- Horse breeding
- Horse care
- Horse training
- Sacking out
- Stable vices
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