Horsing around – equine vision!

It was a sunny, slightly chilly Sunday morning at my parents’ place at Swanfels, outside of Warwick, when I decided to wander out to see what Dad was up to in his shed, and say hi to his two horses Kinsman and Pepper. Pepper (above left in the image) can be a tetchy horse, and after I fell off him on Christmas Day 2006 my horseriding days have concluded, but on Sunday he was lapping up the attention I was giving him, enjoying the head and neck rubs and not wanting them to end. I spent quite a bit of time looking at his fascinating dark eyes, and wondered what he could see. So I asked Dr Google (veterinarian version).

What can a horse see?

It turns out that there’s quite a few scientific articles on horse vision, including one review comparing horse and human vision published in the International Journal of Zoology a few years ago.(1) Generally, a horse doesn’t see as well as we do in terms of clarity of vision but sees better than a dog or a cat. Pepper can’t focus well up close to see detail, but as he doesn’t read very much, this doesn’t matter to him. Horses have dichromatic vision, which means Pepper isn’t colour blind but only sees shades of blue and green, and cannot detect red, similar to the experience of some people with colour vision deficiency. Above right is an image I found hilarious, from the website of the Equine Research foundation (www.equineresearch.org) of a horse undergoing colour vision testing. He’s doing it wrong! Hahaha!

A horse’s field of vision is designed to allow them to keep ‘half an eye’ on everything. If you could see around your whole head, you would have a field of vision of 360 degrees, and horses almost can. A human with normal vision will be able to see a field of vision of almost 180 degrees out to either side horizontally, with about 120 degrees of this being binocular, meaning we use two eyes and can better appreciate depth. By comparison, with its eyes set on the side of its head, a horse’s field of vision is about 350 degrees - most of this is monocular vision, meaning the right eye sees things on the horse’s right side and the same for the left eye. The horse only has about 65 degrees of binocular vision, which he will use to spot distant objects when his head is raised. When he holds his muzzle down and his head is vertical, he will have depth perception to see objects in front of him on the ground. Apparently pulling a horse’s head down as occurs in particular equestrian disciplines shortens his field of view even more to what’s right in front of him.

But despite this, the horse has hardly any blind spot, fitting its general characteristics as a ‘flight’ type of animal, being a herbivorous prey species in the wild.

How large is a horse's eye?

The equine eye is the largest of any land mammal, being around 40mm (2) compared to the human average of around 24mm. I researched the size of an elephant eye just to check this claim, but it turns out their eyes are around 34mm long.(3) On my interesting travels down the Google rabbit hole (I know you’ve been there too!) I even read an article equating eye size in land mammals to their speed. Apparently the fastest runners in the animal kingdom generally have the biggest eyes when compared to their body size, improving sensitivity of vision which is helpful when you’re moving quickly through the environment. Even after adjusting for body size, researchers found that nearly 90% of the variation in eye size amongst mammals related to maximum running speed.(4) There’s even evidence that male horses have exhibited superior visuospatial ability compared to females, again likely linked to speed, and possibly contributing to the relative success of male over female horses at high levels of equestrian competition.(1)

We take for granted that humans will have variation in our visual ability from person to person. This is also the case for horses, and could influence their performance, especially in equine sports where assessing vision could predict how well a particular horse may perform. The horse has a unique relationship with the human, where two very different visual systems have to work together to control what is essentially one pattern of movement.(1)

So it turns out that my friend Pepper can’t see clarity, colour or close objects as well as me, but has bigger eyes and an almost 360 degree field of view. He also tolerates the chilly weather much better than me, so after an extended pat session I left him to his Sunday morning and headed inside for a cup of tea.

REFERENCES

  1. Murphy J, Hall C, Arkins S. What horses and humans see: a comparative review. Int J of Zoology 2009, Article ID 721798.
  2. McMullen RJ, Gilger BC. Keratometry, biometry and prediction of intraocular lens power in the equine eye. Vet Ophthalmol 2006;9(5)357-60.
  3. Bapodra P, Bouts T, Mahoney P, Turner S, Silva-Fletcher A, Waters M. Ultrasonographic anatomy of the Asian elephant (Elephas maximus) eye. J Zoo Wildl Med 2010;41(3):409-17.
  4. Heard-Booth AN, Kirk EC. The influence of maxiumum running speed on eye size: a test of Leuckart’s Law in mammals. The Anat Record 2012;295:1053-62.
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About Kate

Dr Kate Gifford is a clinical optometrist, researcher, peer educator and professional leader, and a co-founder of Myopia Profile and the My Kids Vision websites. In her spare time she is also a slow-and-steady runner, aspiring triathlete and dark chocolate addict.