If a whitetail deer were a human, he’d need glasses. Think you know what deer see? Think again.
Years ago when I first became a deer hunter, there was a lot of talk about deer vision. The general consensus at that time was that whitetails had super-vision—they could pick out the slightest movement at great distances and their eyes could bore a hole right through you. But they were color-blind. That’s right, they lived in a world of black and white and shades of gray.
These beliefs have been proven wrong. Research conducted at the University of Washington and the University of Georgia has revealed some remarkable things about deer vision. They dissected the eyes of deer and looked at them through high-powered microscopes. This revealed some very interesting findings about the differences between the eyes of predators and prey. Before I get into that, here’s a quick primer on the eye itself.
The eye and how it works
Eyes of all mammals are similar in structure, but there are significant differences in how they use them and each mammal has various subtleties that help them survive in their environment. The eye of a deer is quite a bit larger in comparison to their body size than many animals. Their position does not allow for much movement, which is one characteristic that differs from ours.
The eye is made up of five basic parts, contained within the ciliary body: the cornea, lens, retina, pupil, and optic nerve head. The cornea is the protective layer over the lens and it is perfectly clear in deer, while in humans it has a UV filter. The lens is right behind the cornea and it serves to collect the light and direct it onto the retina. The pupil opens and closes to change the amount of light that passes into the eye. The retina is the back of the eye and the light that hits it is sent to the brain through the optic nerve. The retina in a deer is different in several ways, some of which give them significant advantages over predators. We will get deeper into that in a minute.
Here is how the eye works in a nutshell: the retina is like a movie screen with light being cast upon it. Light comes through the lens, and is monitored by the opening and closing of the pupil. It is focused by the lens.
Predator versus prey
Here is where things get really interesting. The eyes of predators, like you and me, are optimized in a different way than those of prey species, like the deer. If you pay attention, you will notice that your eyes are moving along the lines of type as you read this. Your eyes have a very small point of focus, and everything within your field of view around that point of focus is peripheral vision, but it is out of focus. Look at something in the distance, and bounce your eyes around to look at different things around you. Your eyes move and settle on objects and then your eyes focus on that object. It’s actually a small point that is in focus. That’s because there is a small area on your retina (that screen in the back of the eye) that interprets the light coming through the lens.
The eyes of a deer are very different. They actually have a wide band of area on their retina that can interpret light. Scientists call this a “visual streak.” It’s not very tall from top to bottom, but it is quite wide. Think of it this way: When a deer is looking straight ahead, almost the entire horizon is in focus at once! They do not have to bounce their eyes around like you and I. They do not even have to move their eyes at all. In fact, the deer’s eyes are almost fixed in place and can move very little.
The eyes of a predator are positioned more towards the front of the head, while prey species have them more to the side. It takes two eyes at once (binocular vision) to correctly read depth perception. The brain calculates the difference in distances between the object and each eye, and provides an ability to see in three dimensions.
With the head stationary, a deer can see a 300-degree band around him. A slight turn of the head either way reveals the other 60 degrees! Compare that to the small point of focus you have in your 120-degree field of view.
But there are some trade-offs. The visual acuity of the whitetail is surprisingly poor. In fact, they have about 20/40 vision. Plus, deer only have depth perception for that 60-degree area where their vision from both eyes overlaps in the front of them. While they may be terrific at picking up your movement anywhere around them, they can’t really focus on you unless their nose is pointed at you so they can see you with both eyes.
Have you ever had a deer pick you off from across a field? They stand there and stare you down. Sometimes they take a few steps towards you, sometimes they stomp a foot to get you to move. They may get nervous and leave, or they may go back to feeding if you don’t move a muscle for a few minutes. Remember they have poor visual acuity and they simply can’t figure out what they are seeing. Anything across a field is too far away to see clearly.
Rods and cones
How can you get away with wearing a bright “hunter orange” coat? Are deer able to see that color? There has been a lot of debate through the years about the color vision of whitetails. Because of the latest scientific research mentioned above, that debate can now be settled once and for all. Deer actually see some colors better than we do, and some colors they can barely detect. Without getting too technical, we need a basic understanding of the cellular structure of the retina. The two primary photoreceptors on the retina are rods and cones. Generally, cones are responsible for interpreting color, and rods are responsible for collecting light, which helps with low-light and night vision.
It’s no surprise that deer have a much higher density of rods than we do. It’s one of several reasons they can see at night so much better than we can. Rods also help detect motion.
In layman’s terms, certain kinds of cones collect certain colors of light from the spectrum. Deer do not see the longer wavelengths, so they do not see much red. Orange is near the red end of the spectrum, so they do not see it well. Yet they see the short wavelength colors very well. They see blues better than we do. The yellows are somewhere in the middle. Additionally, human eyes have UV filters, and deer eyes do not. Imagine a hunter wearing a blaze orange jacket and blue jeans. The blue jeans are going to jump right out at the deer and the orange is going to be a subdued color—just the opposite of what a human would see.
Night time is the right time
One of the most fascinating aspects of learning more about deer vision is in understanding why they see so well in low-light conditions. The reason deer see so well with so little light is found in three parts. I mentioned that they have a high density of rods in their eyes. Another component is that their pupil is a horizontal slit, which allows it to open very wide and cast a lot of light onto the lens. Compare that to your eyes, which have a round pupil that can open and close a small amount. This fact is responsible for the proverbial deer-in-the-headlights look. Because there is so much movement in the pupil, it takes them a long time to close it down when they see a bright light. This factor has caused a lot of skid marks on the highways.
But the third component to this is even more interesting. On the back of the deer’s eye, across the retina, is a reflective substance called tapetum lucidum. When you see a deer’s eye reflecting back at you from a light source, you are actually seeing the reflection of this shiny substance on the back of the eye.
Here’s how it works: when light enters the eye, it passes over the lens and hits the retina. Because of the reflective tapetum, the light then bounces back to the front of the eye and reflects back to the retina again. This effectively doubles the amount of light that the eye can send to the brain for interpretation. All told, the combination of a wide pupil, the density of rods, and the tapetum lucidum makes for some very good night vision. One of the scientists who did some of the research, Dr. Jay Neitz, estimated that a whitetail deer’s eye can take in about 50 times as much light as a human’s eye!
Knowing how and what deer see can help you be a better hunter. Use this newfound knowledge to your advantage next time you are hunting deer.
Follow Bernie’s bowhunting adventures on his blog, bowhuntingroad.com.