When your horse jumps out of his skin because of the door in the arena that he sees every day, what is going on? Is he just being silly? Does he know better? If so, you’re justified in the frustration this situation produces. But frustration never trained a horse. Is there a more helpful way to look at this behaviour?
Behaviourally speaking there is a better explanation. Every horse has a ‘window of tolerance’. This window isn’t static, it can be expanded through training and it can also be narrowed by stress. Inside this window the horse can operate well. Each stimulus the horse is exposed to can fill up the window a bit, and as he responds to it or relaxes his level of arousal within the window goes up or down.
When this window is already almost full due to various small (or large) stressors like being alone, pain, a windy day, a sudden noise, or a new training concept, the arena door can become the tiny thing that pushes him out of his window of tolerance. This is called stimulus stacking.
Stimulus stacking means that by the time the arena door became the last straw, the horse had already done a lot of self-regulating of the previous stressors that were narrowing his window. It can seem like out-of-window behaviour comes out of the blue, but there are actually many signs of increasing stress that you may notice beforehand.
Signs that the horse is trying to self-regulate very mild stress include yawning, blinking, and shaking the head or body. More stress produces displacement behaviours like licking objects, pawing, rubbing the head and neck, and sniffing the ground but not eating. Finally stress signals appear, such as characteristics of the equine pain face, startling, and frequent pooping. Pushing through all of these signs takes you to the threshold of the window of tolerance and beyond.
What is beyond the window of tolerance? Fight, flight, or freeze. This is where the horse’s parasympathetic nervous system is engaged. He is now beyond learning from the situation or responding to your cues.
This is why staying within the window of tolerance gets the most training work done. It is also desirable to grow the window of tolerance so the horse becomes more resilient to daily stressors and able to easily handle larger ones. Reading the horse’s body language and responding accordingly will help the horse to regulate his responses to the world around him, resulting in more relaxing and safer interactions between him and you!
In training my clients’ horses, this is what I aim to do—remain within the window of tolerance and grow it to produce a calm and relaxed horse that can cope well with the many stressors that come with a relationship with people.
Draaisma, Rachaël (2018) Communication Ladder: Fight or Flight. In: Language Signs and Calming Signals of Horses, Taylor & Francis Group.
Thompson, Kirrily. (2020) The Window of Tolerance. Horses and People, March-April.