Aggression in Horses

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Section: Aggression towards other species

Horses, generally, get along 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 horses' space, jumping on their backs, etc., may also not be tolerated by some horses.

Maternal Aggression

Mares usually protect their foals and may resort to aggression in their defense, 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. Hobbles, to prevent the mare kicking, may be advisable. A second handler should maneuver the foal to the dam, keeping him as close as possible to her flank so that if she does kick or bump him 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 let down 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 he 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.

Medical 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.

Probably the most common "culprit" we tend to associate with aggression is testosterone. While stallions generally need more careful management than most mares or geldings, they can generally be handled safely. 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 by means of 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 in 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 usually treated with cyproheptadine.

In mares, ovarian granulosa cell tumors also produce excessive testosterone and affected mares may become aggressive. 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. Hypothyroidism 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.