Most aggressive behaviour in horses is our fault

Part 2 of 4: What causes aggression, what makes it worse, and why some horses are more aggressive than others.

In my last article I explained what aggression is and how we can learn to read horses’ warning signals. I also talked about the three purposes of aggression, which are:

1. Escaping from danger

2. Guarding a resource

3. Creating distance to another individual

This time I’ll dive into the causes of aggression. While the three purposes of aggression remain the same, there can be many different causes that trigger aggressive behaviour.

What causes aggression?

Let’s look at some of the main causes of aggressive behaviour in horses. I’ll go through them one by one and explain how they relate to each of the three purposes.

• Acute or chronic pain. Pain is a signal to the nervous system that the organism is in danger and needs to act immediately to survive. There is a strong link between pain and aggression in all species, and studies have found specific links between aggressive behaviour and painful conditions such as stomach ulcers and spinal deformities in horses¹².

  Purpose: escaping from danger

• Expectation of pain. Even horses that have been successfully treated and are no longer considered by veterinary professionals to be in pain can retain their aggressive behaviour. This is because they remember that a particular situation caused them pain and expect to experience it again.

  Purpose: escaping from danger

• Guarding a resource. Horses behave aggressively as a way of communicating to other horses that they want a particular resource for themselves. This happens in situations where the resource is in limited supply and therefore needs to be monopolized. A resource can be anything a particular individual finds important: food and water are the obvious ones, but another horse or a person can also be considered resources, as can access to shelter, gates and so on. The herd is also a resource and will be guarded from intruders, such as new horses that have just moved to the barn. Horses guard resources from each other but also from people. A horse may behave aggressively towards a person who approaches its food or takes their friend out of the pasture, for example.'

  Purpose: guarding a resource

• Being too close to another horse. Horses have a particular distance that they like to keep to other horses. This distance varies depending on the dynamics between the individuals. Pair-bonded friends will stand close to each other, engage in mutual grooming or graze nose to nose. Horses that don’t particularly like each other will keep a larger distance and avoid interaction. When they come close to each other they will use aggressive behaviours to communicate to the other horse that he or she needs to move away.

  Purpose: creating distance to another individual

• Not being able to escape from something frightening or dangerous. Horses are prey animals that will primarily respond with flight when they are frightened by something. However, if they are not able to escape – for example because they are confined or tied up, or because what frightens them chases them – they will respond with aggression as a last resort to protect themselves.

  Purpose: escaping from something dangerous

A perfect illustration of resource guarding - Julle, the horse in the middle, is letting the others know that he wants the water for himself. As you can see by the hose, the water is being filled which means they might be thirsty. That would make the water a very valuable resource.

What makes aggression worse?

There are some factors that make horses more likely to resort to aggression in the above cases, through a process known as trigger stacking. It means that negative experiences add up and cause a stronger response than they would on their own. For example, a horse might be able to cope just fine with tractors or with hacking out alone, but when meeting a tractor on a ride away from the safety of the barn he becomes frightened.

The same phenomenon applies to aggression. Here are some factors that make a horse more likely to respond with aggression in a situation:

• Hunger. Anyone who’s ever been hangry will relate to this: hunger increases aggression. Horses are particularly prone to feeling hungry and being stressed by it, because they have a biological and behavioural need to eat for up to 15 hours a day.

• Not having their behavioural needs met. Horses have three main behavioural needs that can be summed up with the three F’s: friends, forage, and freedom. It means that they need to be able to engage in physical contact with other horses that they like, that they need to spend most of their day grazing, and that they need to move over large areas at a walk (when given the option horses will move roughly 5-15 km/day under domestic conditions and up to 80 km/day under feral conditions). When these needs aren’t met, they will have heightened overall stress levels.

• Unstable social environment. Being part of a stable social group is the ultimate safety for horses and something they work hard to achieve when given the opportunity. Horses that live in fluctuating social conditions such as competition barns, where horses regularly come and go, will lack this baseline of safety and security. A change in the herd composition – such as a horse leaving or a new one arriving – will also heighten stress levels until the new group dynamic has stabilized, which can take anything from months up to a year.

• Other pre-existing physical and emotional discomfort. This can be anything from poorly fitted tack, an unbalanced rider, being forced to hack out alone, correction-based training and so on.'

• Hormones. Seasonal fluctuations in hormones, for example gestation status in mares, can affect behaviour and make some horses behave more aggressively.

• Temperament. While there are no innately aggressive horses, there are horses who see to be more prone to display aggressive behaviour than others. Studies of temperament and personality in horses are not conclusive, but there seems to be one or several genetic components that determine a horse’s predisposition to behave aggressively.

Real-life aggressive behaviour is often multi-factorial, with more than one cause and more than one factor affecting it.

We are the problem

After reviewing the causes of aggression and the factors that make aggression worse, it becomes clear that our way of keeping and handling horses has a huge impact on their behaviour. I will go as far as to say that most of the aggression we see in domestic horses is caused by us.

Many management practices that are widely accepted and often considered hallmarks of good horse keeping are in fact very detrimental to horses’ physical and emotional wellbeing.

Take for example the standard practice of housing horses in individual stalls. This denies them two of their three basic behavioural needs: engaging in physical contact with other horses and moving around. Add to this a near-constant flux of horses coming and going at the yard, which creates social instability in their home environment, and small unenriched fields that don’t provide nearly enough movement or foraging opportunities. All this creates high levels of stress that can trigger aggressive responses.

Even horses that are allowed the company of other horses often lack enough resources and space to avoid aggressive confrontations. Studies suggest that a field needs to provide at least 330 square meters of space per horse to keep aggression levels at a minimum³, which most stables cannot provide. In addition, we rarely think to provide enough resources such as food, water, shelter, gates, and friends to remove the need for resource guarding.

Feeding concentrated feeds and not providing ad lib forage such as hay or straw increases the risk for gastric ulcers – up to 90% of competition horses are believed to have some stage of ulceration!⁴ – and colic, and forces many horses to spend a large part of their day hungry.

On top of this, many of our interactions with horses cause both physical and emotional discomfort: overtightening girths, using sharp bits and closing their mouths with tight nosebands, for example. Not to mention the many correction-based training methods that rely heavily on using punishment instead of reinforcement.

The depressing truth is that we give horses a lot of reasons to be aggressive. But what can we do about it? The first principle of dealing with aggression is to not punish it. In my next two articles I’ll dive into the practicalities of dealing with aggressive horses, starting with the cardinal rule: don’t punish an aggressive horse!

1. Fureix et al., 2010. Partners with Bad Temper: Reject or Cure? A Study of Chronic Pain and Aggression in Horses

2. Millares-Ramires, E.M & Le Jeune, S.S., 2019. Girthiness: Retrospective Study of 37 Horses (2004-2016)

3. Flauger, B and Krueger, K., 2013. Aggression level and enclosure size in horses (Equus caballus)

4. Sykes, B.W. et al., 2015. European College of Equine Internal Medicine Consensus Statement: Equine Gastric Ulcer Syndrome in Adult Horses