by Rachele Malavasi, ethology consultant for the Ethical Riding School
In the wide steppes of Asia, where the gaze extends as far as the eye can see and everything is tinged with ocher: here is where the horse has evolved its sight, hearing, smell, taste and touch. Its senses are ideal for probing the wide steppes, avoiding ingesting the frequent stones among the grasses of these arid pastures, and saving itself from attacks by predators. We on the other hand, are collectors and we have built around him a world made of details and close distances: how can we help the horse to extricate itself in this environment?
A specialized sight
Consistent with the breadth of the steppes, the horse has evolved a rectangular pupil through which it sees a rectangular world, just as we with our round pupil see reality in a frame of the same shape. The upper and lower part of the horse's field of vision are blurred, while just below the midline there is an area rich in cones, the receptors responsible for clear vision. This is why the horse, to see something far away, slightly raises his chin: it must frame it in this area so as to identify it well. And if this "something" is in motion, it is easy for the horse to turn its nose in its direction, to frame it in the binocular vision area. While the visual range of humans overlaps by 120° (binocular vision), allowing us to perceive the depth practically wherever our gaze spans, the horse has eyes placed laterally and with binocular vision in an arc of only 60°. So, think about how important it is for a horse in motion to be able to move his head freely to understand the distance of the objects around him. Often in jumping competitions the riders keep the reins tight preventing the horses from stretching their heads and therefore calculating the distance well from the obstacle. For similar reasons, when horses are held in the "hooding" or "rollkur" position they cannot help but rely on the rider. Given their sensory characteristics, with their heads so bent they do not know where they are going. In this case, it must also be considered that horses cannot see much above the forehead line, precisely because of their rectangular pupil.
But the perception of depth is at the expense of visual accuracy: if the horse wants to see details well, it must use only one eye. For this reason, you will see it turn its head slightly as you approach with something in your hand. In fact, do it a favor, stop about 2 meters away to let it observe, because below that distance it sees less well, another adaptation to life in the steppes where nearby capabilities are of little use. Furthermore, just under the nose the horse has a shaded area in which it does not see anything: we do not think about it because our nose is not very bulky (it depends ...) and, looking down, we see our chest. Not the horse: if you want to go on a treasure hunt, in which you take it to find treats hidden in the meadow, remember this and show it the places from a certain distance! And I recommend: if you do the treasure hunt at night, go without a torch! The horse has an excellent scotoscopic vision, due to a light reflecting carpet (tapetum lucidum) which amplifies even the slightest glare. It is an adaptation to the nightlife of many predators which, however, makes it difficult for them to adapt to bright light or sudden changes in lighting, for example by entering the stable after an evening activity.
Now close your eyes and imagine the expanses of Asia: what colors come to mind? The blue sky and a thousand shades of yellow and brown, certainly not the red of apples or buckets! It is therefore obvious that the horse has evolved a dichromatic vision, that is with only two cones and not three like us, in which the red is missing: it sees shades from blue to yellow, passing through green and brown, and does not perceive bright colors. This certainly does not prevent it from aiming at a nice red bucket on the lawn, although for him it is almost tone on tone!
Super-hearing at high frequencies
With the 16 muscles of the fin, compared to only the 3 of our auricle, the horse is practically able to speak with his ears. From their movements we know not only what it is paying attention to at that moment, but also what it wants to show us. But we will talk about it better when we tackle the issue of communication.
The horse's ear is specialized for hearing high frequencies: it perceives from 50Hz to 33KHz (compared to 20Hz - 20KHz for humans) and thanks to the shape of the auricle it amplifies the highest frequencies of 10-20Db. Therefore, the horse perceives with particular sensitivity sounds that we do not hear: be careful when using the radio in the stables, especially with younger horses. In fact, hearing acuity decreases with age (starting around 15-18 years), as indeed happens in humans.
It is not only the sight that has adapted to life in large spaces: the horse has a poor ability to locate the sound source, which places him in a radius of about 20-30°, better than that of the mouse (33°) and worse than the cat (5°), while we humans and elephants have an error range of a fraction of a degree. In fact, if a predator is approaching, a horse does not need to know exactly where it is, as much as the area it comes from to escape in the opposite direction. And it also needs to hear it from a distance: it is thought to be able to perceive sounds up to 1km, even 4km according to some, but obviously it depends on the intensity and giving a unique value is impossible.
The fact of not being able to immediately identify the sound source means that it needs to hear that sound at least a second time to identify its origin, waiting with his head held high: we must let it listen and immerse ourselves in his reality. Beyond this deficiency, between the broader perception of the sound spectrum, the amplification due to the fin and the ability to move it in any direction, the horse has a very detailed stereophonic picture of reality!
The other senses
Research on the other senses is less thorough. Regarding touch, which should be one of the most studied senses since it is the main means of communication from the saddle, we know that the horse perceives pressures on the body that we are not able to perceive, and that its brain is particularly developed in the areas assigned to the tactile area of ??the mouth and nose: how many reflections we should make on the use of the bit! The tongue is also equipped with fungiform papillae that allow it to discriminate foreign bodies and this explains why very few foreign objects are found in the stomach of horses living among the debris, as opposed to what happens for cows that lack these papillae.
The sense of smell is fundamental for individual recognition, both through the skin and the feces: when they smell each other's breath, horses perceive the hormonal status of the other and react accordingly; smelling the feces they recognize who produced them and, if it is the feces of several horses, they could also interpret their social bond. Like other mammals, the horse also has a vomero-nasal organ placed under the palate: during the flehmen,in which the horse stretches the neck and raises the upper lip, the entrance to the organ is opened, which allows it to quickly deliver some particularly relevant molecules to the brain, especially pheromones.
Bibliographic references:
Deshpande K, Furton KG, Mills DEK. The equine volatilome: volatile organic compounds as discriminatory markers. J Equine Vet Sci. (2018) 62:47–53. doi: 10.1016/j.jevs.2017.05.013
Guarneros, M., Sánchez-García, O., Martínez-Gómez, M., & Arteaga, L. (2020). The underexplored role of chemical communication in the domestic horse, Equus caballus. Journal of Veterinary Behavior.
Murphy, J., Hall, C., & Arkins, S. (2009). What horses and humans see: a comparative review. International Journal of Zoology, 2009.
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Rørvang, M. V., Nielsen, B. L., & McLean, A. N. (2020). Sensory abilities of horses and their importance for equitation science. Frontiers in Veterinary Science, 7, 633.