How do horses happen to see our world?
Horses happen to have one of the largest eyes of any land-based mammal. Because it is a “prey” animal, horses have active vision throughout daylight and nighttime hours. With the position of their eyes, on each side of the head, a horse has about 350 degrees of total vision, including 65 degrees of dual-eye vision.
Every horse breed has dichromatic vision. That means they are not color blind, but have two-color vision. In comparison, humans have trichromic, or three-color vision. Horses can see green and blue colors within the spectrum, but are unable to distinguish red colors. That makes their vision be similar to what red-green color blindness is like in humans.
The reason why horses have two-color vision is because there are two types of cones in their eyes. They have a cone that is sensitive to short wavelengths that senses blue colors well, while the longer cone is sensitive to colors in the yellow-green spectrum. One explanation for this development is that horses tend to be active from dusk until dawn, when this type of color discernment is most important.
Just as horses struggle to distinguish red colors, they can struggle to distinguish between yellows and greens.
What Is Notable About the Vision of a Horse?
Horses may only have two-cone vision, but they have a very high proportion of rods within their vision structure. Horses have a 20-1 ratio for rods to cones, which works with a retroreflector called a “tapetum lucidum” that allows the eye to find definition and color in low-light conditions. That is how horses have such excellent night vision.
The size of the eye for horses allows them to detect movement with greater regularity as well. Changes can be sensed very well in conditions with low light levels, allowing the horse to sense danger and respond appropriately. In low light, horses can distinguish shapes. In virtual darkness, they can still see enough to navigate safely in virtually any situation. Humans, on the other hand, would struggle to adapt to the conditions and stumble into obstacles that the horse could avoid.
At the same time, the ciliary muscles for horses is relatively weak when compared to other land-based mammals. That means the eyesight of the horse has a poor accommodation when attempting to focus on specific objects. Horses usually track items of interest at a distance, so there is relatively little need to track items at close range, so this aspect of their vision is usually not a disadvantage to the horse.
Horses are very sensitive to motion. It is their first alert, in most circumstances, that a potential predator is approaching. Horses use their periphery vision to detect the initial motion and then move to track it with their visual acuity. Because they must track the movement after it has been detected, horses tend to tilt or raise their heads so a better understanding of what they say can be obtained.
That is why a horse can be “spooked” easily sometimes. If a potential threat is detected and the horse cannot track it, then their natural instinct is to run away from the danger.
What Is the Structure of a Horse’s Eye?
The eye of a horse is not shaped like a sphere, nor does it have a ramped retina. It is somewhat flattened in shape, moving from the anterior to the posterior of the eye. The wall of the eye for a horse is then made up of 3 specific layers.
The nervous tunic is the retina of the eye. It contains cells that are extensions of the brain, working with the optic nerve to generate an image. The receptors in this layer are receptive to light, allow the horse to see in dichromatic tones, and provide night vision. About 70% of the horse’s eye can receive light, so the receptor cells do not cover the entire interior of the eye.
The vascular tunic, which is also known as the uvea, contains the iris. Much of the pigment within the eye of a horse is contained in this layer, especially within the choroid. It helps to form the reflective layer that gives the horse a superior level of night vision, reflecting the light back to the nervous tunic layer. The iris is situated between the lens and the cornea, providing a specific eye color for the horse and helps to control the pupil as well.
Meet a Horse Monday: Allie! Allie started her life as a strong, sure footed trail mount and has become a lovely and reliable part of the herd at Hearts & Horses focusing her time as our primary vaulting mule. Allie has lovely long ears and kind eyes. #HopeHorsesHealing pic.twitter.com/skeicZStmn— Hearts & Horses (@HeartsnHorses) November 13, 2017
The fibrous tunic contains the cornea and the sclera of the eye. These protect the structures of the eye. The sclera, which is the white part of the eye, is comprised of collagen and elastin. In horses, the cornea is regularly coated in lacrimal fluid and a clear fluid called “aqueous humour” to provide it with nutrition. The fluids contain low levels of protein, in a consistency that is similar to blood plasma, and this helps to support the structure of the eye.
What Color of Eyes Do Horses Have?
Most horses have eyes that are dark brown. The iris of a horse can be a wide variety of colors, however, ranging from amber and green to blue and hazel. Blue eyes in horses are often associated with white markings on their coat. Pinto patterns are often linked to blue eyes. When the coat color is the direct link to eye color, a horse may have one blue eye and one iris of another color. Both eyes may be blue or both eyes may be part-blue as well.
The eyes of the horse are then protected by a unique eyelid that is composed of three tissue layers. The outer layer is a thin skin layer that is protected with hair. The middle layer is composed of muscle tissues, allowing the horse to blink. The inner layer, called the “palpebral conjunctiva,” lies against the eye. These layers work with a nictitating membrane that closes diagonally over the eye so the cornea can be protected.
Horses may not be color blind, but they do not see colors in the same way humans do. Because they struggle to distinguish between greens and yellows and do not discern red colors well, an understanding of how a horse sees can make it easier to work with and interact with the animal on a regular basis.