Do You See What I See? - Equine Vision (Part 1)

Think owls are the only animals that can see nearly 360 degrees around their bodies? Think again. The horse can do the same thing without moving its head! Monocular vision (focusing with each eye) and binocular vision (using both eyes together) combined with high, wide eye placement allows equines to have a very large field of vision.

How a Horse Sees

It's common knowledge that good human visual acuity is 20/20. The average horse's vision, on the other hand, is 20/33. Horses can see better when using the "visual streak" - an area in the retina with higher acuity - that is a concentration of ganglion cells. If something looks fuzzy to a horse, the animal will move its head so that the object comes into focus in the visual streak. This is like having built-in bifocals.

Horses use a combination of monocular and binocular vision. Monocular vision is used to see what's happening on each side of the head. Typically, it's helpful for spotting predators. Monocular vision allows the horse about 350 degrees of viewing around its body. Binocular vision is best used for movement and looking ahead. The range of this vision works best when the horse looks directly at something – just like when you use a pair of binoculars.

Color and Night Vision

Friends often ask me if horses can see in color. Although it was thought originally that horses could not perceive colors, a "recent study by researchers at the Medical College of Wisconsin and the University of Wisconsin-Madison's Schools of Medicine and Veterinary Medicine has shown that horses do possess color vision, albeit a reduced form compared to most people". It has been determined that horses can distinguish yellow, blue, and green - but probably not red.

Let me tell you a story from my own experience. My horses had two identical grain buckets: one red and one blue. We'd place them at different places in the paddock each day. Yes, they smelled different, of course, but the horses would identify them from a great distance away – probably too far to smell.

Horses have great night vision. When the sky is even partially lit by the moon, they can see as well as during the day. This is due to a high 20:1 ratio of rods to cones. Horses also have tapetum lucidum – that's the stuff that reflects light back through the retina, or makes an animal's eyes look "possessed" when you try to take its picture at night.

Blind Spots

While horses can see a lot without even turning their heads, there are two areas they can't see – especially when their heads are restrained. One of those spots is directly behind them. If you've ever been told not to walk or sneak up behind a horse, this is why. Because the horse can't see you, its first assumption may be that you're a predator - and kick. The farther away you are, the harder the impact will be – so the more it'll hurt! (Although I'm experienced with horses, I was kicked last fall. I'll write a blog entry about the biomechanics of a horse kick sometime soon.)

Can you guess where the other blind spot of the horse is? Check back next week for Part 2. I'll explain what it is and why it's so important for one of the equestrian sports.

Resources:

http://www.extension.org/pages/Horse_Vision

http://www.thehorse.com/ViewArticle.aspx?ID=3326

http://www.equisearch.com/horses_care/health/anatomy/nightvision_091003/

http://en.wikipedia.org/wiki/Equine_vision