Aggression in Horses


   Aggression is a natural and necessary trait for horses.  It has enabled them over many generations to protect themselves from predators and to defend their offspring (mares) or harems (stallions).  While horses are not generally a territorial species, protection of food and other valued items does occur in domestic horses and resource guarding can be a common cause of aggression between captive herd members.   Man also selected for aggression when breeding warhorses, and certain lines of horses to this day are far more aggressive than others.   For an animal like the horse that relies on its speed and endurance to escape prey species, fighting is a potentially life-threatening proposition even for the winner.  Injuries sustained fighting can debilitate an animal to the point that it becomes easy prey, prevents it finding adequate food and water or from keeping up with its herd.  For this reason the horse, like most other species has developed a panoply of behaviors and expressions to convey its intentions to other horses so that actual fights can be avoided.  Many of the problems humans encounter in dealing with horses or other species could be avoided if we just learned to recognize what they were “saying” to us.  We will examine signs of aggression, dominance and submission in the horse, as well as the types of aggression seen in horses that stem from natural behaviors.  Finally, we will look at physiological problems that can cause horses to act aggressively.



What is he saying?


   The horse is super bly equipped for visual communication, which is probably the primary mode of expressing his feelings.  The first clue to his level of excitement comes from his posture.  In most situations either both his head and tail will be up or they will be down, rarely do the two extremes of his anatomy move in opposite directions.  With increasing excitement both head and tail tend to become higher.  The submissive horse has a lowered head and tail, and he will seem to be trying to slink along as low to the ground as he can.  The more aggressive and dominant he feels the higher the tail and head will be carried and the more confident he will appear.  A somewhat elevated head with a tucked or partially raised tail may accompany a defensive threat.  However, a sleepy horse will drop his head and tail, while a horse that is excited, eager to check out a new environment or one that is moving at a faster pace than a walk will also have an elevated head and tail carriage, so context is also important in judging posture.


   The next set of clues to a horse’s mood come from his facial expression, particularly the movements of his ears and his mouth and nose.  There are thirteen pairs of muscles adjusting the position of each ear, and ten pairs moving the nostrils, mouth and lips.  The aggressive horse’s ears are laid back against its head pushed hard against the skull.  Eyes will be wide open and generally focused on the object of aggression, the body of the horse lining up to follow the forward facing eyes.  The nostrils will also be dilated drawing in air in case the horse is called upon to take further action, they are drawn back so there will be wrinkles in the upper edge at the back.  The mouth may be open and if he intends to bite or threatens to do so the incisor (front) teeth will be seen.  If a bite or bite threat is being made the head will drop and be extended – giving the neck a snake like appearance.  The submissive horse’s ears tend to spread out to the side or they are held backwards but not pinned against the head.   In young horses in particular a behavior that is usually described as jaw snapping or Unterlegenheitsgebärde signals submission.  The head is extended with the mouth open and the lips drawn back.  The jaws are then opened and closed usually without the lips or teeth making contact.  There may be a slight sucking sound as the tongue hits the roof of the mouth.  This appeasing behavior has been variously described as either a ritualized grooming or eating display, both of which could be construed as calming, non-aggressive gestures.


   The tail can also tell us a lot about a horse’s intentions.  The dock of the tail is tensed in the aggressive horse so that the tip of the tail flows further out behind as the tail itself is raised.  If the horse is slashing its tail forcefully from side to side, or even more dramatically up and down too, there is a good chance he will kick or lash out.  This contrasts to the more leisurely tail swishing associated with brushing off flies.

Other signals that can help us judge a horse’s intentions would include the way he orients his body.  If he turns his quarters towards the object of his attention, or pushes at it with his shoulder this would indicate either a dominant display or mild aggression.  A submissive horse will try and scoot its lowered quarters away from the other animal or human.  The tail will tuck under and the hindquarters will drop, while the head will deflect to the side so he is not looking directly at the other animal.  If possible the horse will probably try to back away slowly.  As aggressive displays increase the horse will mime biting – a head swing with the neck extended and a slightly opened mouth or nipping towards the adversary, usually aimed at the forelegs, head, shoulder or chest; striking – one or both forelegs lift off the ground or the feet are stamped hard on the ground; or kicking – vigorous tail switching accompanied by lifting one or both hind legs and extending them out behind.  At this level it is clear from the lack of effort behind the motion that the movements are threats rather than intentional attacks.  As antagonism increases these movements will become more forceful.  In general, auditory behaviors tell us more about the level or arousal/excitement of the horse and less about his aggressive tendencies.  Certainly high pitched squeals may accompany aggressive displays or attacks, while throaty low pitched nickers would be more appropriate for the horse that wishes to appease others.



Aggression directed at other horses


   Horses are herd living animals, and allowing them opportunities to interact with other horses tends to promote good health and behavior.  They will be less likely to perform obsessive or stereotypical behaviors such as cribbing or stall walking if they can spend at least part of their day turned out either with other horses or where they can observe them over the fence.  In general, horses in a group have a fairly loose dominance hierarchy.  The more dominant horses are usually deferred to by the less dominant, but the arrangement is not strictly linear, and may, on closer observation, appear more triangular.  In general, with any hierarchy an animal may defer over some issues and stand up to a more dominant animal to protect more valuable resources.  For a mare this might mean defending her foal from bullying or injury.  If the horses get their grain ration at turnout a more submissive animal might defend this but not hay.  Some horses can definitely be bullies.  They may move other horses around constantly so that they cannot settle and graze, they may defend water or food piles from others, or they may show truly aggressive, antagonistic behavior to others.  Most of the fighting we see in an established group however tends to be play fighting.  Domestic horses are more likely to engage in such shenanigans than their wild counterparts, where the behavior is usually restricted to young colts.  While bites and kicks are inhibited they may still result in injury, whether intentional or accidental.  Horses should be watched at turnout and separated if the play gets too rough.  If one horse is consistently picking fights or acting the bully he should be removed from the group, although it may be possible to integrate him into a group of more self-assured animals.  Likewise if one horse is always the scapegoat he too should be removed and placed with more congenial companions, or allowed to watch the group from the safety of a different paddock.  Horses that have been raised in isolation from others of their species have a particularly hard time in groups, as do blind animals.  They have not learned, or cannot see, the signals of mood and intent being given by their peers and are liable to act in a socially inappropriate fashion.

A situation that is more likely to result in aggressive behavior occurs when it is time to introduce a new horse, or reintroduce a former companion after an absence, to the group.  It is preferable if horses can become used to the new horse over the fence at first.  It may also be helpful to introduce the newcomer to a few of the animals in the middle of the current hierarchy first – neither the most dominant nor the most submissive – in order to smooth his passage into the herd.

If possible it usually works best if domestic herds are composed either of all geldings or all mares.  Many geldings retain enough sexual memory that they can become pests when mares are in season, and they may not only annoy the mare but get into fights with each other.  It is extremely unusual for stallions to be turned out together, although some may tolerate a gelding as a companion.  Stallions may also be turned out with mares, although there is a risk that an unwilling or feisty mare might injure too ardent a suitor.  Mares with foals are likely to be more defensive with either geldings or a stallion present.



Aggression of horses towards people


   As was stated earlier, many of the behavioral problems between species occur because the two species signal their mood or intent in different ways.  Horses are wonderful at picking up subtle nuances in our body language because that is how they communicate with each other.  If we appear to be behaving submissively they are liable to be more confident or in some cases aggressive.  If we appear too dominant, a self-assured horse may decide to display his position to us.  If our posture doesn’t change this has the potential for leading to an aggressive standoff or escalation.  Given that even the smallest pony far outweighs us, getting into a battle of brawn with a horse is never a good idea.  


   Much of what we perceive as aggression may just reflect a lack of education.  Horses need to be trained from their earliest interactions with humans to respect our space and accept our leadership.  This is particularly important with orphaned foals.  While their bumping and jostling us, mouthing or even nipping may seem cute when they are tiny, they will not be appropriate when the horse is older, and they will be much harder, or in some cases impossible, to eliminate.  Kind but firm handling of foals is appropriate.  Some people advocate imprint training as described by Robert Miller DVM in his book Imprint Training of the Newborn Foal.  Certainly getting a young foal used to being handled all over, accustomed to having his body manipulated and to strange sights and sounds at an early age will make these experiences easier for him when he is older.  He should remain relaxed and cooperative for these exercises.  Any resistance on his part should not be bullied into submission, but shaped gradually.  Whether it is essential that these exercises are done immediately at birth, or whether they can wait for the succeeding days has been much debated.  Provided they are completed in a timely fashion and the mare is not disturbed by human intervention, performing this program immediately after birth appears neither to prevent the foal receiving sufficient colostrum, nor the formation of an appropriate maternal bond.

With older horses certain interactions with humans tend to be more likely to produce aggressive responses.  Often these reflect lack of preparation or warning to the horse so that it is taken by surprise.  Things which often result in handler directed aggression include grooming – particularly sensitive spots with harsh implements; girth tightening – this may be because it is rushed all at once, the horse has been punched in the belly, or skin and hair caught and pulled in the past, in general fearing that their saddle might slip most riders tend to err on the side of over rather than under tightening girths; entering the stall – the owner may be pinned against the wall, or the horse turns its hind quarters to face the owner as she enters; striking or yelling at the horse, or acting in a manner the horse perceives as aggressive/dominant; handling feet and legs; clipping; syringe use and other potentially painful procedures; approaching the horse at pasture; cornering a horse; trying to coerce a horse to go somewhere – for example onto a trailer – it fears; leading a horse that has not learned to properly respect the handler, so it becomes distracted, shies, bolts or otherwise invades the handler’s space – horses that are fine at home may become liabilities when taken to more exciting venues such as shows.  Horses may aggressively demand treats and bite and nip if they are not delivered promptly.  Hand feeding is not the problem, the problem arises because the horse has been permitted to get away with this behavior, and even rewarded for it in so far as it results in the owner swiftly coughing up the treat to ward off attack.  Other horses nip owners for attention.  They cannot distinguish between biting on thick winter clothing and on bare summer arms, so this behavior should never be tolerated and allowed to become established.  In general, when designing a behavior modification program for human directed aggression in horses it is important to establish whether the behavior is motivated by fear or dominance.



Aggression towards other species


   Horses generally get on well with other grazing animals.  They may get into mock fights with more feisty companions, but frequently these animals will be acceptable buddies if it is not feasible to keep another horse.  Usually horses will also accept dogs and cats around the barn.  Dogs that chase or dart at horses may well be treated as predators and are liable to be kicked.  Cats invading the horse’s space, jumping on their backs etc may also not be tolerated by some horses. 



Maternal aggression


   As has been mentioned mares usually protect their foals and may resort to aggression in their defense, be it against other animals or humans.   They should be given adequate time to bond away from other horses and people, and should not be reintroduced into groups that include individuals liable to be excessively nosy, pushy or aggressive.


   Some mares, particularly young first time mothers, may reject and even attack their foals when they try to approach them or nurse.  This behavior is most frequently seen in Arabians.  For some of these mares, normal maternal instincts will kick in if the foal can successfully nurse.  The mare should be restrained with a trusted handler at her head; the handler should be calm but matter-of-fact.  Animals often misconstrue reassurance to mean that the handler is also fearful and the situation becomes proportionally more frightening accordingly.  Hobbles, to prevent the mare kicking, may be advisable.  A second handler should manoeuver the foal to the dam, keeping it as close as possible to her flank so that if she does kick or bump it she won’t have much force behind the movement.  The foal should be guided to the mare’s teat; hand milking her first may help induce her to letdown milk.  If the mare continues to reject the foal, this method is usually too labor intensive for the long term.  The foal may be successfully placed with a foster mother, or the mare’s colostrum can be milked out and fed to the foal, which can subsequently be hand raised.  If possible the hand-reared foal should be placed with an older horse (a mare with no foal of her own or a calm gelding) so that it is properly socialized.  It is not advisable to continue to breed mares that have rejected a foal, unless maternal behavior did kick in after the initial rejection, as the behavior will almost certainly be repeated with subsequent foals and in subsequent generations.



Physiological causes of aggression


   Because aggression is a natural behavior certain parts of the brain – the hypothalamus, amygdala and frontal lobe of the cerebral cortex – are involved in the regulation of aggression.  Any disease or condition that affects these areas has the potential to either increase or decrease aggression – although the latter is rarely of concern.  Viral (rabies, eastern, western or Venezuelan encephalomyelitis), bacterial (Strep equi), protozoal or fungal diseases, tumors, cysts or abscesses can potentially, if rarely, cause increased aggression in horses.  Thiamine deficiency, leukoencephalomalacia, lead or organophosphate pesticide poisoning might also be causes.

Probably the most common “condition” we tend to relate to aggression is testosterone.  While stallions generally need more careful management than most mares or geldings, they can generally be safely handled.  Aggression is most frequently seen if two stallions come into close contact, particularly if mares in season are nearby.  Even these situations can be dealt with with careful training.  Horses with one or both testicles retained in the abdomen may be passed off to unsuspecting buyers as geldings.  These rigs however usually display all the normal stallion behaviors, and will have serum testosterone levels similar to those of stallions with both testicles descended into the scrotal sack.  Stallion-like conformation and behavior together with elevated testosterone is usually diagnostic of the condition, and the retained testicles should be removed.  Because of the elevated temperature inside the abdomen these horses are normally sterile.  If they have one descended testicle they will be fertile, but shouldn’t be bred as the condition is inherited.  Retained testicles are also far more likely to develop cancerous tumors than descended ones.  Another condition in which elevated testosterone can lead to increased aggression is sexy gelding syndrome.  This is sometimes seen in older geldings and it is thought to be caused by a tumor of the pituitary gland.  This releases a hormone that stimulates production of steroid hormones, including testosterone, by the adrenal glands.  Others signs of hyperadrenocorticism (Cushing’s disease) would be expected to accompany the behavioral changes, such as brittle coat, trouble shedding, a pot belly and sway back as well as increased eating, drinking and urination.  The condition is treated with cyproheptadine.  In mares, ovarian granulose cell tumors also produce excessive testosterone and they can become aggressive too.  Some mares will become aggressive during their seasons.  If the mare is not to be bred, spaying her may be curative.  For others progesterone or synthetic steroid hormones or some herbal remedies may help reduce the aggression and other unwanted behaviors.

Another hormonal cause of aggression in horses is hypothyroidism. The aggression can be directed at other horses, people or both.  Thyroid hormone levels affect the levels of both dopamine and serotonin.  These are neurotransmitters – substances that transmit nerve signals from nerve to nerve – that are particularly important in controlling behavior.  It also affects the level of other hormones that normally are released in response to stress.  Hypothyroid animals seem to live in a perpetual state of stress, and may respond to this with aggression.  Replacing the missing hormone can completely control abnormal aggression in these horses.


   Aggression may be inherited.  Certain lines of horses are renowned for their aggression.  The other merits of such horses should be carefully weighed in any breeding decision.  We can create enough problems with aggressive horses without deliberately breeding for the trait.