White-tailed deer are a prey species. Their sight, hearing and sense of smell are especially adapted to evade predators, human hunters included.
While the whitetail’s ability to hear and smell potential danger is probably most critical for survival purposes, I’ve been impressed by their ability to detect even the slightest bit of potentially dangerous movement.
As Leonard Lee Rue III notes: “A deer’s eye is geared to detect motion, the slightest motion, and to move while the subject is in the deer’s range of vision is to ensure detection.”
Although the whitetail’s ability to differentiate colors has been hotly debated, even among scientists, recently reported study findings may have finally answered this intriguing question: Can whitetails detect colors?
Vision Basics Like most prey species, a deer’s eyes are set on the side of the head, providing a wide field of view. Their slightly bulging eyes also increase their field of vision. Hence, a deer’s eye structure permits it to distinguish distant objects across 310 degrees field of view without moving its head.
Day or night, whitetails have excellent vision. Like other animals adapted to being active at night, deer have a membrane in the back of the eye referred to as the tapetum lucidum, which allows light to pass through the receptor layer a second time, enhancing vision during dim light and producing eyeshine when exposed to bright light at night.
Unlike other mammals that are strictly crepuscular and nocturnal, deer can also see well in bright light. This ability is probably because of a ring of pigment surrounding the cornea in the eye that acts as an antiglare device.
Given the deer’s ability to see in bright light, noted scientist Dietland Muller-Schwarze suggests the following: “Under strong light, the pupils of the eye close into a slit, focusing light onto a horizontal band across the eye’s retina. In exactly this streak are clustered the nerve cells that function as signal conductors, carrying messages from the photo receptors to the brain. The arrangement and density of the nerve cells, called ganglion cells, in the visual streak account for the deer’s ability to detect danger from afar.”
Other researchers theorize that the visual streak corresponds to the horizon that dominates an animal’s view in open-country. If so, then deer are especially adapted to detect danger in flat country.
“As another aid in their vision,” notes Rue, “deer often use triangulation.” This generally involves a deer bobbing its head up or down or moving it from side to side, giving the animal two vantage points of vision and helping in depth perception.
Debating Color Vision Early reports concerning deer color vision were conflicting. Some suggested that whitetails had no ability to detect colors. However, other scientists reported the presence of cones (color receptors) on the deer’s retina and their ability to distinguish different colors.
According to Muller-Schwarze, “Deer appear to have the anatomical apparatus for color vision, though again, not all studies agree. In vertebrate eyes there are two kinds of photo receptors: rods, which respond to single photons and enable vision in dim light, and cones, which account for color and daylight vision. The mix of rods and cones – or the absence of one form altogether – determines what kind of vision an animal has.”
Until recently, speculation regarding deer color perception revolved mostly around an understanding of physiological and anatomical characteristics of the deer eye. One study measured the electrical activity of the tails. These investigators concluded that deer are less sensitive to light of long wavelengths (orange and red) and actually rely upon their perception of only two colors – yellow and blue.
At least one study found that deer had difficulty in picking out green. Considering that deer typically forage in a myriad of grassy and green vegetation this seems puzzling, leading the investigators to speculate that deer might not need to distinguish colors in their natural environments, even though they can do so in the laboratory.
Although the available data suggest that deer can differentiate colors, until recently there have been relatively few studies based on the behavioral response of deer to color.
Forced Choice Test One of the most recent published studies concerning white-tailed deer color vision was conducted by Bradley Cohen and others at the University of Georgia. They used seven semi-tame whitetail does in a so-called “forced choice test,” using a form of food reward when deer made the right choice.
Initially, deer were trained to associate a lighted trough with a food reward. That is, during a given trial deer were offered two similar troughs, but only the lighted trough held accessible food. Hence, deer were trained to feed from the lighted trough.
After training, deer were tested to determine their sensitivity to light of various wavelengths, ranging from 360nm to 650nm (bright blue to orange-red). Then, the intensity of a given light was varied until deer no longer responded.
Of the wavelengths tested, deer were most sensitive to 430nm and least sensitive to 650nm. Deer were successfully trained to detect ultraviolet and near-ultraviolet light (360nm and 380nm), but had difficulty detecting long-wavelength (590nm and 650nm) colors such as blaze orange.
These findings, based on behavioral study, generally agree with previous speculation concerning the whitetail’s ability to perceive color and that this ability influences their behavior.
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Implications The whitetail’s crepuscular behavior is well-suited to its improved sensitivity to short-wavelength light, because when deer are most active the available light is in the short-wavelength part of the visible light spectrum.
According to the Georgia-based researchers, “The [whitetail’s] sensitivity to blue wavelengths across a wide visual field likely facilitates movement through their environment during times humans would typically perceive as low-light conditions, and also aids in predator detection along a horizon.”
In addition, these researchers point-out the following: “Some predator pelts exhibit low near-UV reflectance; also, the twilight sky is rich in the wavelengths at this end of the spectrum. Thus, increased sensitivity to short-wavelength light might enhance the ability of white-tailed deer to detect predators during crepuscular periods of activity. The visual perception of the white-tailed deer is well-adapted for a crepuscular prey species with an array of both coursing and stalking predators, being specialized for sensitivity during the times they are most active and enabling detection of predators along their horizon.”
Of practical importance to the deer hunter, Rue emphasizes the following: “While talking about wearing camouflage, it is important that you do not wash such clothing in any of the supermarket ‘super’ detergents, because they all contain ultraviolet (UV) brighteners that not only make your whites whiter than white, but makes your camo clothing brighter than bright. It has been discovered that deer can see the ultraviolet light emanating from clothing so washed.”
Conclusions Despite earlier disagreement among scientists, the most recent studies clearly show that white-tailed deer not only have the anatomical structures necessary to distinguish colors, but also demonstrate behavioral responses to prove so. However, deer perceive their world in colors of blue to yellow-green – which is considerably different from humans.
When it comes to employing vision-based mechanisms to reduce deer-human conflicts, such as deer-vehicle collisions, Cohen and his group recommend increased consideration of the differences in deer versus human vision sensitivity. They suggest that vision-based deterrents focus on the blue-wavelength spectrum to which deer are more sensitive. For example, car headlights expressing longer wavelengths would prevent deer from becoming blinded by headlight glare.
As noted above, the hunter should also be aware of the whitetails’ sensitivity to shorter wavelengths than humans and avoid laundering their hunting apparel in detergents containing brightening agents. Also, be alert of the whitetails’ keen ability to detect movement across a wide field of view.
— John Ozoga is a retired deer research biologist from Michigan. He has been D&DH’s research editor since 1994.
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