By Bob Humphrey
When it comes to the latest and greatest products, theories or technology in the hunting world, most hunters fall into one of two categories. First, there are the bandwagon jumpers. "If a pro hunter says it works, that's good enough for me." Then, there are the skeptics. "Sounds fishy to me. And there's no way you can prove it works."
One of the classic examples of the latter group are those who dispute claims that deer don't see blaze orange. They all have a story that usually goes something like, "I was wearing orange once, and a deer saw me." Well, I guess you convinced me, Bubba. Not!
And if you really want to wind them up, mention a product called UV Killer. It blocks the blue/white fluorescence of clothing that has been treated with fabric brighteners, which are intended to make "whiter whites." It was developed because those brighteners also allegedly enhance the reflection of light in that portion of the visible spectrum that a whitetail's eyes are most sensitive to. It sounds great in theory, but our skeptics quickly counter with, "How can them scientist fellas possibly know just what a deer sees without being able to read their minds?"
Well, Bubba, it turns out they actually do read the deer's mind — sort of.
Advertisement
The most intensive and extensive work on deer vision has been conducted by Dr. Karl Miller and the staff and students at the University of Georgia's D.B. Warnell School of Forest Resources. They've used everything from dissection and physical inspection to literally implanting electrodes in a deer's brain to better understand the intricacies of how whitetails perceive their world. And much of what they've learned has valuable application to hunters. The easiest way for most people to understand deer vision is by comparing it to ours.
Deer and humans both have round eyes and a pupil that dilates and constricts to alter the amount of light reaching the retina (rear of the eye). That's where things start to diverge.
Advertisement
While the human pupil is round, the deer's is more oval — almost rectangular. Miller suggests the selective advantage of this: minimizing excessive light energy from above that could be distracting, while maximizing light from the horizon where danger lies. That, and the physical arrangement of light-sensing rods and cones (more on that in a bit) are why deer don't see elevated hunters as well as those at "eye" level. They still pick up movement, and can shift their attention with a simple tilt of the head, but they're generally less concerned with what's above them.
Deer's eyes and their pupils are also bigger than ours. And Miller says these larger openings increase light gathering ability to about nine times that of a human. This alone allows them to see better than us in low light, but it's not the only thing.
On the back of both deer and human eyes is a retina, and on it are light receptors called rods and cones. But a whitetail's retina also has a reflective layer called the tapetum lucidum (what we see as the eye-shine when a deer is illuminated at night). When light strikes the retina, some is absorbed by the rods and cones. The rest strikes the tapetum lucidem and then gets bounced back over rods and cones a second time. According to Miller, this gives deer about 18 times greater ability to see at night than humans. So just about the time your vision is fading, theirs is just getting revved up.
The two types of light receptors — rods and cones — perform different functions. Rods are very sensitive to light, and what we use to see in low light, which we perceive as a black and white image. Rods are much bigger and are bundled three to each nerve ending. They're not as good at picking up detail or movement and provide a very coarse image. Still, deer have more rods than humans, further enhancing their ability to see at night.
But deer don't get all the advantages. Each cone has its own nerve fiber so it is more fine-tuned for picking up detail and movement. Humans have more cones so we see more detail in good light. Humans are also trichromats. Our cones have three types of photo pigments that peak at different parts of the visible spectrum: blue, green and red. Every other color is some combination of those, so we tend to see colors better than deer in general.
We also have another advantage. Human eyes have a "yellow" filter that partially blocks out UV (blue) light to aid in visual acuity. We see very well in the red and green (and yellow) part of the spectrum, but not so good in the blue and UV (the region enhanced by fabric brighteners). And because deer lack that filter, they see into the blue part of the spectrum 20 times better than humans.
Also unlike humans, deer dichromats, having two photo pigments. One is for detecting blue and the other for about halfway between what we see as red and green.
According to Miller, "Light reflected in other parts of the spectrum, in the neutral zone, probably looks gray to them. Anything that's dark red looks black to a deer, and they don't see blaze orange as well as we do."
That last part bears repeating before we move on. Deer do not see blaze orange as well as we do. They see it, and to their eyes it probably falls somewhere between gray and pale yellow — shades and colors that are quite common in the woods.
And for the skeptics out there, it's this part and the next that Miller's team determined using a Deer Training Apparatus and an operant conditioning experiment. By attaching sensors to the deer's brain, they monitored sensory reaction and behavioral reactions to various wavelengths of light. So, in a sense, they're reading the deer's mind.
They also learned another difference between human and deer vision. Humans have something called the fovea centralis, a small pit of densely packed cones on the retina that allow us to focus on an individual point. Deer don't. Instead of having to constantly move their eyes to focus on a point, deer use the blue cones are scattered across their retina (and about double what humans have) to see the "big picture."
Why is this important? At twilight, light gets diffused and there is about two to three times the ambient blue light compared to red and green. Visual acuity becomes less important, but the ability to see blue, and in low light offers a dual asset. Add a large pupil, the tapetum and more rods and you have an animal that sees very, very well in low light, which explains why they're more active then.
Know If You Glow Science has proven that deer do indeed see very well in the blue-UV portion of the spectrum, and they see it best during low-light periods of dawn and dusk when they are most active. We don't. So how can you tell if your clothes glows? Use a blacklight. Popular back in the psychedelic 60s, blacklights or UV lights will literally make whites and colored fabric treated with brighteners glow in the dark. If yours do, you might want to consider treating them with UV Killer, particularly if you're a bowhunter.
