How to Tell If Your Horse Is Stressed Out

What side preferences can tell us about the emotional state of horses.

Posted Jan 13, 2021

Most people are right-handed, but 10.6% are left-handers (Papadatou-Pastou et al. 2020). While scientists initially thought that only humans show handedness and other forms of so-called hemispheric asymmetries (i.e., left right-differences in brain and behavior) we now know that they are quite common in the animal kingdom (see "Seven Surprising Facts About Left-Handedness in Animals").

For example, pets like cats and dogs (Ocklenburg et al. 2019), but also rats and mice (Mundorf et al. 2020) show so-called paw preferences — they prefer one paw over the other when interacting with food just like humans prefer one hand for writing. Besides handedness, hemispheric asymmetries are also reflected in many other forms of preferences in daily life e.g., footedness, a favorite eye, or a preferred side for chewing. (For more examples, see the blog post "Seven Surprising Ways Brain Asymmetries Affect Your Daily Life"). 

In animals, hemispheric asymmetries have not only been investigated in cats and dogs but there is also a considerable amount of research in horses. You might wonder why it is important to investigate left-right preferences in horses. Does it even matter what side a horse prefers? 

Turns out: Yes, it does.

One of the main reasons is that assessing left-right preferences can tell us about the emotional state of an animal and thus knowing about it can help improve their wellbeing (Leliveld et al. 2013). Many scientific studies have related the right half of the brain to negative emotions and the left half of the brain to positive emotions (Silberman & Weingartner, 1986). Since the right half of the brain controls the left side of the body it has been suggested that an increasing leftward preference in animals might be indicative of stress (Ocklenburg et al. 2016). Otherwise, a reduced left-side preference might be indicative of a relaxed or happy animal. 

This is demonstrated nicely when looking at tail wagging in domestic dogs as a response to seeing the dog's owner or a dominant unfamiliar dog (Quaranta et al. 2007). Seeing the owner led to more tail wagging to the right side (left brain activation) while seeing a dominant unfamiliar dog was associated with more tail wagging to the left side (right brain activation) (Quaranta et al. 2007). Thus, the study showed that left-brain activation (reflected in right-sided behavior) was associated with positive emotions whereas right-brain activation (reflected in left-sided behavior) was associated with negative emotions.

In contrast to humans, cats, and dogs, horses have laterally placed eyes. Other species with laterally placed eyes such as chicks use them in an asymmetrical way as in turning the left eye to scan the sky for predators while turning the right eye to the ground in search of food (Güntürkün & Ocklenburg, 2017). Thus, the question arises whether horses show similar asymmetries. 

Surprisingly, more horses show a left side preference than humans. For example, 90% of domestic horses favor one direction when under saddle in ridden work, and of these 52% favor the left side (Murphy & Arkins, 2008). But does this side preference come naturally, or is it trained from the side preference of the owner? 

To answer this question, researchers came up with different ideas on how to test asymmetries in horses in a way that asymmetrical behavior comes naturally. One way to investigate asymmetry in horses is to test if they have a preferred eye when looking at an unknown person or novel object. This was tested by Farmer and colleagues (2010) who investigated domestic riding horses in a neutral area. They found that 72% of horses showed a left-eye preference when looking at a novel object or unknown person (Farmer et al. 2010). 

A previous study found similar results when investigating if horses show an eye preference in a frightening situation. Therefore, a stranger holding an open umbrella (as an unknown object) was asked to approach the horses standing on the pasture. New and unexpected things frighten horses. Then, the researchers observed how the horses reacted depending on the side (left/right eye) the person was approaching the horse (Austin & Rogers 2007).

Interestingly, horses moved further away from the umbrella when it was coming from the left side as when the umbrella was approaching from the right side (Austin & Rogers, 2007). So, an approach in the visual field of the left eye activated the right hemisphere. Given that the right hemisphere is associated with negative emotions, a right brain activation (through the left visual field) triggers a greater fear response in horses.

Other scientists investigated if horses have a preferred foreleg. Therefore, they analyzed the position of the forelimbs to one another while the horses were standing on the pasture. Here, the focus was on the foreleg the horses moved first when switching from standing to moving. They found that 61% of horses favor one side and of these 80% showed a stronger left foreleg preference (McGreevy & Rogers, 2005). This way of testing a foreleg preference in horses is fairly similar to how footedness is tested in humans. 

Taken together, there seems to be an overall preference for the left side in horses, especially when naturally using their forelegs and when observing an unknown situation with the eye. Newer studies also reveal that besides novel or frightening situations social interactions are left-lateralized in horses as well (Farmer et al., 2018). 

How can we now use this knowledge of a left-side preference in aversive and social situations in horses to improve their wellbeing?

One study has tried to answer this question by investigating the asymmetry in horses in acute and long-lasting stress conditions. Stress was induced by switching the horses from group housing to single boxes and by starting a new training as acute stress (Marr et al. 2020). Asymmetry was then measured in the preferred forelimb and by observing the head-turning of the horses over time. Interestingly, overall head-turning preference shifted directly to the left upon single housing while the foreleg preference increased to the left after one week of single housing compared to measurements before the stress started (Marr et al. 2020). So, horses show a preference for the left side, and the strength of this preference can tell us about their emotional state. Observing the side preference of your horse in new situations might render helpful insights into how stressed your horse is e.g., a strong left-side preference might be indicative of a stressed horse. 

Written together with guest blogger Dr. Annakarina Mundorf.


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