Horse Aggression Towards Other Horses
Behavioral Causes of Horse Aggression
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.
Physiological Causes of Horse 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.
Hormones and Horse Aggression
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.
Aggression when introducing a new horse to the barn
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.
Aggression between genders
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.
By Linda Aronson, DVM, MA