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Horse behavior is best understood from the view that horses are prey animals with a well-developed fight-or-flight response. Their first reaction to a threat is often to flee, although sometimes they 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 evolved from small mammals whose survival depended on their ability to flee from predators. This survival mechanism still exists in the modern domestic horse. Humans have removed many predators from the life of the domestic horse; however, its first instinct when frightened is to escape. If running is not possible, the horse resorts to biting, kicking, striking or rearing to protect itself. Many of the horse’s natural behavior patterns, such as herd-formation and social facilitation of activities, are directly related to their being a prey species.
When a horse reacts to a threat, it may initially ‘freeze’ in preparation to take flight. The fight-or-flight reaction begins in the amygdala which triggers a neural response in the hypothalamus. The initial reaction is followed by activation of the pituitary gland and secretion of the hormone ACTH. The adrenal gland is activated almost simultaneously and releases the neurotransmitters epinephrine (adrenaline) and norepinephrine (noradrenaline). The release of chemical messengers results in the production of the hormone cortisol, which increases blood pressure, blood sugar, and suppresses the immune system. Catecholamine hormones, such as epinephrine and norepinephrine, facilitate immediate physical reactions associated with a preparation for violent muscular action. The result is a rapid rise in blood pressure, resulting in an increased supply of oxygen and glucose for energy to the brain and skeletal muscles, the most vital organs the horse needs when fleeing from a perceived threat. However, the increased supply of oxygen and glucose to these areas is at the expense of ‘non-essential’ flight organs, such as the skin and abdominal organs.
Once the horse has removed itself from immediate danger, the body then returns to more 'normal' conditions by the parasympathetic nervous system. This is triggered by the release of endorphins into the brain, and it effectively reverses the effects of noradrenaline – metabolic rate, blood pressure and heart rate all decrease and the increased oxygen and glucose being supplied to the muscles and brain are returned to normal. This is also known as the ‘rest and digest’ state.
Horses are highly social herd animals that prefer to live in a group.
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."
Bands are harems, usually consisting of one adult male, several females, their foals and immature horses of both sexes, although it has been stated there may be one to five stallions. Each band is led by a dominant mare (sometimes called the "lead mare" or the "boss mare"). Bands are usually small, containing between 3 to 35 animals. The composition of bands changes as young animals are driven out of their natal band and join other bands, or as stallions challenge each other for dominance.
Horses have evolved to live in herds. As with many animals that live in large groups, establishment of a stable hierarchical system or "pecking order" is important to reduce aggression and increase group cohesion. This is often, but not always, a linear system. In non-linear hierarchies horse A may be dominant over horse B who is dominant over horse C, yet horse C may be dominant over horse A. Dominance can depend on a variety of factors, including an individual's need for a particular resource at a given time. It can therefore be variable throughout the lifetime of the herd or individual animal. Some horses may be dominant over all resources and others may be submissive for all resources.
Once a dominance hierarchy is established, horses more often than not will travel in rank order.
Aggressive interactions risk causing injury. Therefore, once the dominance hierarchy is established, aggressive behaviour is reduced between herd members. Higher-ranked animals often will assume a role of exercising control and moderating aggressive behavior in the herd.
The center of the herd offers the most protection from the elements and is further away from predators than the periphery. Because of this, "punishment" of misbehaving members is sometimes delivered in the form of temporary expulsion from the herd, or sometimes permanently.
Most young horses in the wild are allowed to stay with the herd until they reach full sexual maturity, usually in their first or second year. Studies of wild herds have shown that the herd stallion will usually drive out both colts and fillies; this may be an instinct that prevents inbreeding. The fillies usually join another band soon after and the colts driven out from several herds usually join together in small "bachelor" groups until those who are able establish dominance over an older stallion in another herd.
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 often the alpha or dominant mare. She is usually one of the more mature animals, familiar with the terrain and resources available. She tends to lead when the herd travels, although not always, and determines the route when the herd moves between locations. The dominant mare usually has priority of access to the best feed sources and to drink first from watering holes. Herd movements may be initiated by any individual, although higher-ranked members are followed more often by other herd members.
Stallions tend to stay on the periphery of the herd where they fight off both predators and other males. When the herd travels, the stallion is usually at the rear and apparently drives straggling herd members forward, keeping the herd together. Mares and lower-ranked males do not usually engage in this herding behavior. During the mating season, stallions tend to act more aggressively to keep the mares within the herd, however, most of the time, the stallion is relaxed and spends much of his time "guarding" the herd by scent-marking manure piles and urination spots to communicate his dominance as herd stallion.
Feral and wild bands may range from two to 25 individuals, mostly mares and their offspring, with one to five stallions.
Domesticated stallions, with human management, often mate with ("cover") more mares in a year than is possible in the wild. Traditionally, thoroughbred stud farms limited stallions to breeding with between 40 and 60 mares a year. By breeding mares only at the peak of their estrous cycle, a few thoroughbred stallions have mated with over 200 mares per year. With use of artificial insemination, one stallion could potentially sire thousands of offspring annually, though in practice, economic considerations usually limit the number of foals produced.
Some breeders keep horses in semi-natural conditions, with a single stallion amongst a group of mares. This is referred to as "pasture breeding." Young immature stallions are kept in a separate "bachelor herd." While this has advantages of less intensive labor for human caretakers, and full-time turnout[clarification needed] 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 foals. Therefore, keeping stallions in a natural herd is not common, especially on breeding farms mating multiple stallions to mares from other herds. Natural herds are more often kept on farms with closed herds, i.e. only one or a few stallions with a stable mare herd and few, if any, mares from other herds.
Mature, domesticated stallions are commonly kept by themselves in a stable or small paddock. When stallions are stabled in a manner that allows visual and tactile communication, they will often challenge each another and sometimes attempt to fight. Therefore, stallions are often kept isolated from each other to reduce the risk of injury and disruption to the rest of the stable. If stallions are provided with access to paddocks, there is often a corridor between the paddocks so the stallions cannot touch each other. In some cases, stallions are released for exercise at different times of the day to ensure they do not see or hear each another.
To avoid stable vices associated with isolation, some stallions are provided with a non-horse companion, such as a castrated donkey or a goat. (The Godolphin Arabian was 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 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 less dominant stallion.
Stallions and mares often compete together at horse shows and in horse races, however, stallions generally must be kept away from close contact with mares, both to avoid unintentional or unplanned matings, and away from other stallions to minimize fighting for dominance. When horses are lined up for award presentations at shows, handlers keep stallions at least one horse length from any other animal. Stallions can be taught to ignore mares or other stallions that are in close proximity while they are working.
Stallions live peacefully in bachelor herds in the wild and in natural management settings. For example, the stallions in the New Forest (U.K.) live in bachelor herds on their winter grazing pastures. When managed as domesticated animals, some farms assert that carefully managed social contact benefits stallions. Well-tempered stallions intended to be kept together for a long period may be stabled in closer proximity, though this method of stabling is generally used only by experienced stable managers. An example of this is the stallions of the Spanish Riding School, which travel, train and are stabled in close proximity. In these settings, more dominant animals are kept apart by stabling a young or less dominant stallion in the stall between them.
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.
Studies of domesticated horses indicate 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. 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 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 investigate 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.
Horses can sleep both standing up and lying down. They can 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 on the inside bottom end of the femur cups the patella and the medial patella ligament, preventing the leg from bending.
Horses do not need a solid unbroken period of sleep time. They obtain needed sleep by many short periods of rest. This is to be expected of a prey animal, 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 asleep; total sleep time in a day may range from several minutes to two 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, which horses may suffer from.
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.
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.
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.