How much do we really know about how deer experience the world around them? Not much, according to Wildlife Biologist Dr. Bradley Cohen.
Cohen and his colleagues at the Warnell School of Forestry and Natural Resources at the University of Georgia would know. For the past few years they have been studying deer vision like never before.
"If our knowledge of [deer vision] was a puzzle, I'd say it would be about 10 percent complete," Cohen said. "The whitetail is the most studied animal on the planet, but there have been few extensive behavioral studies on how deer perceive their world."
That all changed when, led by renowned biologist Dr. Karl V. Miller, Cohen and a team of researchers set out to examine a deer's ability to see different wavelengths of light. The group trained seven does (some wild, some semi-wild, and some tame) to associate light with a food reward, like modern day Pavlov's Dogs.
Scientific efforts in the past had primarily been focused on the anatomical dissection of a deer's eye. In fact, it wasn't until the 1950s that rod and cone structures were recognized in deer retina. In the following decades, researchers were able to identify pigments that were proven to respond to colors of light and break down the optical properties in comparison to the human eye.
The most significant findings originated from a 1994 study by Dr. Jerry Jacobs and Dr. Jay Neitz, also completed at the Warnell School in Georgia with Dr. Miller. Jacobs and Neitz used a non-invasive procedure on anesthetized deer to measure the sensitivity of the deer's eyes to wavelengths of light across the spectrum.
The study found that while humans have three different classes of photoreceptors (the basis of our exceptional tri-color vision) and can distinguish small differences in wavelength across the spectrum, deer have only two classes of receptors (two-color vision). What does this mean?
"The color vision capacities of deer are, at best, limited compared to humans," Neitz and Jacobs concluded. "The two classes of cones in deer allow for the ability to see color differences between short and long-wave lights, e.g., blue and yellow, however, they lack the photoreceptor basis for seeing differences in the color of objects that reflect middle-to-long wavelength light, e.g., yellow-green, green, yellow, orange, and red."
This is where things get rather scientific, maybe too in-depth, for this purpose. But it's important to remember that, according to this study, blue, violet, and near ultraviolet light stand out from the other colors for deer.
While early biologists like Jacobs and Neitz were able to make these educated guesses about deer vision based on the optical makeup of the eyeball, the real-world applications were largely intangible. Enter Cohen's group.
Using classical (appetitive) conditioning, his team allowed conditioned stimuli to help deer "tell us what they could see."
"This study has so many implications in the deer hunting community," Cohen said. "Our results will provide a more complete picture for hunters on the effectiveness of concealment products on the market, and help us all understand how best to avoid detection. After spending years on this study, all the hunters on the research team have changed their habits in the woods."
The right reward
Before you understand the function of Cohen's study, it's important to get an idea not only of the wavelengths of light deer see, but also of all facets of their vision.
"In truth, a deer's vision is based mostly on movement," he said. "Most of their perception is based on determining a stationary object versus a moving object. They also see everything in equal focus, and it's across 310 degrees of their head."
According to Cohen, there's an elementary exercise that will give us all a vague idea of the reality. Squint your eyes until they are blurry and hold it there. That's what a deer sees. Everything is blurry and in the same focus.
It's with all this in mind that the team at the Warnell School went to work. They determined the parameters of how they would train the deer, which deer would make the best subjects, and how they could use the conditioned stimuli to get the desired result.
It was decided that the subjects must be all does, mostly because bucks get very agitated during the rut and simply could not be left alone with the expensive machinery.
As with most animals, food and water are the biggest motivating factors besides sex, so the team created an automated training device inside a pen with infrared sensors featuring two food troughs, each had an LED light on or near the trough. During the test, both troughs opened when approached, but a deer could choose to eat from only one. If they chose the trough with the light, they could eat. If not, the trough closed. Pretty simple.
"We wanted them to respond positively to the light," Cohen said. "By giving them a reward, we were able to train all seven deer in about two weeks."
Now that the deer were trained, and even having fun with the study, the team began the next step.
"We took the very specific wavelength lights and turned them to highest intensity, then we dropped the intensity, halving it every day. Once they could not make the association with the right trough, we knew the intensity of light they could not see. From there we were able to paint the picture of how deer perceive light and the environment."
Results and implications
All of this scientific jargon and exhaustive studying means nothing if it doesn't make us better hunters. Cohen has an answer for that, too.
"We find that deer see blue really, really well," he said. "They see blue almost 20 times better than us, into the ultra-violet spectrum. It turns out that if you measure the light available at sunrise and sunset, it's blue and UVs that are most prevalent."
The same amount of light is available to us that is available to deer at dawn and dusk, but they perceive the blues and UVs better, so they can see better then we can. Of course, deer move primarily at these times, so having better eyesight to avoid predators seems natural.
What about camo? What can deer see and what can't they? This is where it gets interesting.
"Because of the way deer see, depth in camo patterns really doesn't matter," Cohen said. "Deer cannot distinguish minute detail, everything looks like a blob.
"Some parts of the camo you wear, especially the grey parts creating the depth, are reflecting blue light or UV light. This doesn't mean you're glowing, it just means there's some blue tint to your silhouette. Camo is made to be attractive to us. To do that it often needs greys and subtleties, but the deer sees those as blue and can see them better. Certain camo patterns reflect this spectrum differently."
On the topic of UV brighteners, the story gets even more compelling. Here's the official conclusion of the group: "Camouflage clothing worn by hunters should not be washed with laundry detergents containing brightening agents. These agents absorb light in the ultraviolet region of the spectrum and re-emit light in the blue region, thus increasing the intensity of short-wavelength light, which may be perceived by deer against an otherwise darker background."
Cohen wants hunters to know that while the hunting industry and its products have historically been produced before science has identified their effectiveness, that might change in the future.
"We've held onto this for years, we wanted rigourous peer review," he said. "We wanted hunters to have solid, verified information and be able to make buying decisions based on this knowledge. I'm a hunter and this made me put on different camo."
And this isn't the end. Cohen and company are already working on more in-depth research. Stay tuned.
The video evidence
All scientists and biologist record their work. This study was no different. Bradley Cohen and the team at the Warnell School were nice enough to provide us with video from the study, showing the deer responding to the stimuli and feeding from the troughs. See the astounding footage for yourself: