Note: This is one article in a series about horse vision. I’ve compiled all of them, including more information and references, in this blogpost: Horse Vision.
Horses are animals who evolved in open grasslands. They’re designed to be awake and graze most of the day and night. Because of this they have excellent vision in scotopic (low-light) conditions. It’s hard for us to imagine because we have opposite needs – to see with acuity and color in the daytime. To have good scotopic vision, the anatomy of a horse’s eye is different than ours. This affects their behavior in basic ways.
Sometimes your horse’s pupil looks bluish-grey. What you’re seeing is the tapetum lucidum, a structure that reflects light back through the photoreceptor layer of the eye so that the horse has a high sensitivity to light, especially light reflecting off of the ground. For an animal that needs to graze at night, and also to see predators, and then move quickly away from them, over uneven terrain, without stumbling, this is a useful adaptation. But there’s a trade-off. We humans don’t have a tapetum lucidum and our night vision is poor. However, we have the ability to adjust our eyes quickly from bright light to dark. Horses do not. It takes horses much longer to see in scotopic conditions, but once they do, they see far more than us.
Walk from sunlight to the interior of a barn.
In five minutes, your eyes have adjusted and this is what you see.
This is what your horse sees. Yes. That dark.
No wonder there are behavior “issues” going into enclosed spaces, or why your horse trips over a lead rope on the aisle’s floor and panics. And why you should always put those saddle racks down when not in use, and why door latches should be smoothly tucked out of the way. The horse can’t see them!
However, in twenty minutes, this is what your horse sees – it’s brighter and in more detail than you can ever perceive. Which is why you won’t notice a sudden movement at the far end of the barn, but your horse will.
Horses need to flee from danger, and so have an innate fear of small, enclosed, dark spaces. Here’s one.
The above photo is how you see the trailer. It doesn’t look so bad. But the photo below is how the horse sees it – the interior is much more of a mystery. Remember that they see in panorama, so there are wide open spaces on both sides. (This photo is also adjusted for their visual acuity and color blindness.) I can understand why the horse, of all of their options in this scene, wouldn’t want to go into that black hole!
This doesn’t mean that loading your horse into the trailer has to be fraught with conflict and pressure. The first thing to do make it brighter inside. If your trailer has them, open the front doors. Here’s the same trailer, viewed with a horse’s vision, with that one easy change. Now the horse can see what they’re getting into.
Many trailers have interior lights. Make use of those, too.
Here’s a horse who is an experienced show horse and usually loads easily. However, she’s never seen this trailer. Note how her body language is tense, she’s stopped at the ramp, and is lowering her head to try to get as much light as possible reflecting off of her tapetum lucidum, and also to get the floor of the trailer into focus using her line of acuity.
We walked her away, opened the doors at the front of the trailer, and had her approach again. She walked right on.
Of course, thoughtful, reward-based training should be part of the picture, too. (I’ve blogged about that. Check the trailering category on the right-hand side of this page.)
When going from shade to sun, a human’s pupil constricts in diameter to let less light in. A horse evolved in wide open spaces, where brightness remains rather constant over stretches of time. They didn’t need to adjust quickly from light to dark, so the horse’s pupil can constrict only a tad. Instead they have corpora nigra – that’s the raggedy part of the iris that hangs into the pupil and is particularly effective at shading out sunlight from above.
This design is perfect for a grazing animal whose head is usually on the ground and whose eyes are scanning low across the horizon. The corpora nigra acts as an awning! When a horse’s head is up and in very bright light, the pupil constricts and the corpora nigra partially covers the iris to protect it from being harmed by the sun. It’s like when we try to see through squinted eyes – you see only enough to fumble your way around.
The takeaway from this is that you might want to try a moonlit ride on your horse. You won’t be able to see where you’re going, but your horse will. On the other end of the (literal) spectrum, when riding out on a wonderfully sunny day, if you head into the shade of the woods, slow down and take your time. Your horse’s eyes can’t be rushed.
NOTE: The science of sight is complex. There are several ways for an eye to adjust to different lighting conditions, including the one that we are most aware of – the dilating pupil. There’s another one – chemical – the photoreceptors go through a chemical change to alter their sensitivity to light. When in bright light, the photoreceptors go through bleaching, and it takes more light to trigger them. When in darkness, they chemically increase their sensitivity to light to enable them to register detail in dark conditions. This chemical change is the major way that the horse adjusts from light to dark, and this is what takes so long. For more information, see this text book Equine Ophthalmology, 3rd Edition, Brian C. Gilger (Editor) especially Chapter 12 “Equine Vision” by Paul E. Miller and Christopher J. Murphy. You can read excerpts here on Google Books. An earlier version of the “Equine Vision” chapter is available here.