CHAPTER 25 Aggression in Horses
Because of their large size, aggression in horses can be very dangerous, whether it is directed to humans, other horses (conspecifics), or other animals. Aggression does not have a single cause. Current internal and external conditions, as well as prior learning, all can contribute to expression of aggressive behavior. Horses exhibit a wide spectrum of aggressive behaviors. Whereas biting, striking with one or both forelimbs, and kicking with one or both hind limbs are all likely to cause bodily harm to whomever or whatever the aggression is directed against, horses also exhibit a variety of behaviors, called threats or intention movements, that do not cause harm but signal the probability that the horse will proceed to harmful aggression if certain changes in their internal or external environment do not occur (Box 25-1). General reasons for the demonstration of aggressive threats or injury-inducing aggression include pain, fear, attempts to control important resources, defense of young, and lack of appropriate socialization or habituation. Use of the word aggression in this chapter refers to both threat behaviors and injury-inducing aggression.
Box 25-1 Intention Movements that Signal the Likelihood of the Horse Carrying Out Injury-Causing Aggression∗
|Threat to bite||The horse moves the head or its whole body closer to the individual being threatened and rapidly opens and closes the mouth.|
|Smack||The horse rapidly opens its mouth, making a loud “smacking” sound as the tongue disengages from contact with the hard palate. This behavior has been observed only in mares with nursing foals and is hypothesized to be a modification of the threat to bite that adapts to the fact that the foal cannot see its mother’s head while nursing.|
|Ears back or head threat||This is a mild form of the threat to bite in which the ears are laid back against the neck while the horse looks at or moves its head or its whole body closer to the individual threatened, without opening the mouth.|
|Threat to strike||The horse raises one or both (partial or full rear) forelimbs and moves them toward the target without making contact.|
|Threat to kick|
|Squeal||The horse emits loud, high-pitched vocalization.|
|Supplant||This may occur without laying back of the ears. A horse moves directly toward another horse, typically at a brisk walk, resulting in the horse being approached moving away, after which the aggressor occupies the space and uses the resource previously occupied by the supplanted horse.|
|Chase||One horse pursues another at a trot or canter with the ears laid back and sometimes a concurrent threat to bite.|
|Tail lashing||The horse vigorously lashes its tail.|
Most human-directed aggression is caused by pain, fear, or both. In some instances, aggression is an immediate response to the human engaging in a behavior that causes pain, such as lashing a horse with a crop or whip, hitting it with a fist or blunt instrument such as a piece of wood, jerking hard on the reins, or applying spurs along the horse’s side. In all these situations, the horse’s aggression is simply an attempt to defend itself from a creature that is causing pain. If the human causing the pain is on the ground, they are likely to be within reach of the horse’s defensive behaviors. If the human is on the horse’s back, the horse may buck or rear in an attempt to remove the human. The horse may also kick, bite, or strike ineffectively at the air or at a nearby human who is not causing it pain. This kind of pain-induced aggression can easily be prevented by avoiding use of aversive training techniques when raising and handling horses.
When horses are handled by humans who use harsh, aversive techniques, new owners must deal with the classically conditioned fear the horse has developed. In classic conditioning, a neutral stimulus that does not normally cause a particular response is paired with an unconditioned stimulus that does induce a particular response. Eventually the neutral stimulus is changed to a conditioned stimulus that leads to a conditioned response. For example, if a horse is raised by humans who pet it only in a gentle fashion and becomes habituated to them, a hand reaching toward the horse’s face should not cause a fear response. However, if someone punches the horse on the nose one or more times, it may become afraid of hands reaching toward its face and react with defensive aggression when a hand is extended toward its face to pet or halter the horse.
Veterinarians are often the object of horse aggression for several reasons. The horse may be in pain and the veterinarian is nearby; the veterinarian directly causes pain that, although necessary for the horse’s welfare, is still painful (for example, administering injections); and the horse has become classically conditioned to fear various stimuli associated with veterinarians, such as the sound of the veterinarian’s truck or a given veterinarian’s voice, the sight of someone in coveralls, or the smell of alcohol. For the first two categories, care of a sick or injured horse makes it essential that the veterinarian be in proximity and perhaps perform procedures that are painful to the horse. In this instance, adequate restraint, including sedation if needed, should be combined with avoidance of unnecessary pain. If a horse is being uncooperative and aggressive while being treated, hitting and yelling only increase its fear and are likely to make the situation worse. Although application of a twitch may temporarily inhibit movement because pain increases if the horse moves when twitched, learning associated with twitching can have long-term detrimental consequences. The horse may become afraid of sticks, of humans holding stick-shaped objects, of having its nose touched, or even of humans in general.