How Whitetail Deer view their world!


The research was conducted from August 24 to 29, 1993, at the University of Georgia D. B. Warnell School of Forest Resources, in Athens, Georgia. Present were Dr. R. Larry Marchinton and Dr. Karl V. Miller of the University of Georgia with a staff of graduate students headed by Brian Murphy as research coordinator. The electroretinograph was administered by Dr. Jerry Jacobs and his assistant Jess Deegan of the University of California at Santa Barbara, assisting was Dr. Jay Neitz of the Medical College of Wisconsin. The Electroretinograph equipment, provided by Dr. Jacob's lab at the University of California, is the culmination of 12 years of refinements. Computer controlled light presentation and signal processing now enable scientists to accurately define the range of vision in animals.

The details of this work were publicized for the first time at the annual meeting of the Southeast Deer Study Group. This meeting was hosted by the Mississippi Department of Wildlife, Fisheries, and Parks and was held February 21 through 24, 1993, at the Holiday Inn North in Jackson, Mississippi. This study on the vision of the white-tail deer was presented by Dr. Karl Miller of the University of Georgia.

Following is the abstract of what was presented.


PHOTOPIGMENTS OF WHITE-TAILED DEER

Brian P. Murphy, Karl Miller, and R. Larry Marchinton, University of Georgia; Jess Deegan II, University of California; Jay Neitz, Medical College of Wisconsin; Gerald H. Jacobs, University of California.

All aspects of vision depend ultimately on the absorption of light by photopigments. The retinas of white-tailed deer (Odocoileus virginianus), like those of other ungulates, contain a mixture of rod and cone photoreceptors. We have used a noninvasive electrophysiological technique to measure the spectral absorption properties of the photopigments contained in these receptors. In this procedure, electroretinogram (ERG) flicker photometry, light-evoked potentials were sensed by a contact-lens electrode positioned on the eye of an anesthetized deer. The eye was stimulated with a rapidly-pulsed, monochromatic light; variations in pulse rate, stimulus wavelength and adaptation state of the eye allowed preferential access to signals from different classes of photoreceptor. Recordings were obtained from nine white-tailed deer. Three classes of photopigment were detected. One of these is the photopigment contained in rods; it has a peak sensitivity of about 496 nm, a value greatly similar to that found for rod photopigments of other mammals. These measurements also reveal the presence of two classes of cone. One contains a photopigment maximally sensitive in the middle wavelengths (peak value of c. 537 nm); The other cone class has a sensitivity peak in the short wavelengths, at about 455 nm. In light of what is known about the relationships between photopigments and vision in other species, these results suggest two likely characteristics of cone-based (i.e., daylight) vision in deer: (1) deer should be relatively less sensitive to long-wavelength lights than many other mammals (e.g., humans), and (2) white-tailed deer would be expected to have dichromatic color vision.


PHOTORECEPTORS AND DAYLIGHT VISION OF THE DEER

Vision is initiated when light is absorbed by photoreceptors of the retina, the light absorbing tissue that covers the back of the eye. The limits of vision depend on several factors which include:

1) The optical properties of the eye, i.e., the size of the eye, the size of the pupil, the refractive power of the eye's optical elements.

(2) The properties of light absorbing filters through which light must pass before reaching the photoreceptors. In humans these include filters in the lens and in the central region of the retina that absorb strongly in the short wavelengths, blue, violet and ultraviolet.

(3) The light absorbing properties of the photoreceptors themselves, the number of different classes of photoreceptors and their distribution in the retina.

(4) The reflectivity of tissues that lie behind the photoreceptors. For example many animals that are active in dim light have a reflective layer at the back of the eye that enhances sensitivity.

Some of these properties have been recently investigated for the eyes of white-tailed deer. A non-invasive procedure (harmless to the deer) was used on anesthetized deer to measure the sensitivity of the deer's eyes to wavelengths of light across the spectrum.

Deer like all other mammals have two types of photoreceptor, rods and cones. The rods are responsible for vision in dim light and the cones are responsible for vision in daylight. The light absorbing properties of the rods in deer were found to be similar to those found in other mammals, including humans. Two classes of cone photoreceptor were detected in the deer. One most sensitive to short-wavelength light (blue-violet); the other most sensitive to middle-wavelength light (green-yellow).

The lens of the human eye contains a yellow pigment that absorbs ultraviolet light almost completely; it absorbs strongly in the violet and into the blue spectral regions. In contrast, the transmission of short wavelength light is very high for the lens of many mammals that are active at dusk, dawn and at night. The recent experiments indicate that this is true for the deer. The relative sensitivity of deer eyes to short wavelengths (blue and violet) is high compared to that of humans, as expected because deer lack yellow pigment in their lens.

In humans, the very central region of the retina (the fovea) is specialized for high acuity vision. Among mammals, this specialization is found only in humans and other primates. Also unique to primates is an additional yellow pigment, the macular pigment, that covers and thus screens the central region of the retina. Humans use the central region of the retina whenever we look directly at an object; it is this region that we depend on most heavily for vision. Thus, when comparing the daylight vision of deer to that of humans it makes sense to consider human foveal vision.

The recent experiments suggest important differences between the daylight vision of deer compared to that of humans:

1) Humans have three classes of cone photoreceptors which are the basis of trichromatic (literally three-color) vision. In humans this three-receptor system confers excellent color vision. Humans can distinguish small differences in wavelength across the spectrum. In contrast, only two classes of cone photoreceptors were detected in deer. Deer can have no better than dichromatic (two-color) vision. Thus, the color vision capacities of deer are, at best, limited compared to humans. 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.

(2) Since humans have yellow pigments that screen out short-wavelength light, the relative sensitivity of deer to short wavelength light is much higher that the sensitivity of humans. This same difference would apply to low light conditions under which only rod photoreceptors operate.

(3) The three classes of cone photoreceptors in humans are each sensitive to a different region of the visible spectrum. Together these confer sensitivity to a wide band of wavelengths. The three classes of human cone photoreceptors can be termed red, green and blue cones. One of the two cone photoreceptors detected in deer is similar to the human blue cones; the other is similar to human green cones. Thus, compared to humans, deer effectively lack red cone photoreceptors. This suggests that deer should be relatively less sensitive to long-wavelength light (orange and especially red) than humans.

Human sensitivity is highest in the green-yellow region of the spectrum and, for equal intensities, these wavelengths are perceived as brightest. Humans are relatively insensitive throughout the short-wavelengths (blue and violet). Sensitivity also drops off rapidly in the very long wavelengths, e.g., we are relatively insensitive to deep reds. Humans can distinguish four basic colors; blue, green, yellow and red. We also distinguish dozens of intermediate colors, e.g., violet, blue-green, yellow-green, orange etc. Humans can make subtle color discriminations across the visible spectrum.

The region of highest sensitivity for the deer is at a shorter wavelength than that of humans. The relative sensitivity of deer to short-wavelength light is dramatically higher than human sensitivity to those wavelengths. For equal intensities, deer are expected to see short and middle-wavelengths as brightest. Because of the absence of red cones, the drop off in sensitivity at the long-wavelength end of the spectrum occurs at shorter wavelengths for deer. They are less sensitive in the spectral region that appears orange to humans and are virtually insensitive to deep reds. With only two classes of cone photoreceptors, deer can distinguish no more than two basic colors, one for the short wavelength end of the spectrum and another for the middle-to-long wavelength end of the spectrum. Animals with dichromatic color vision do not see an intermediate color in the spectral region between the two colors. That is, they do not see a color that appears bluish-yellow. Instead they see the intermediate spectral region as colorless (gray).

The issue of how deer see blaze orange is of considerable interest to hunters and those interested in hunter safety. Recent results lend insight into how deer may perceive blaze orange. Blaze orange is highly visible to humans because, for us, it is both intensely bright and intensely colored. The worst news for hunters would be if blaze orange was seen by deer as intensely colored and intensely bright as it is for humans. At the other extreme, perhaps the best news would be if blaze orange was not seen at all by the deer. Given what is known about deer vision neither of those extremes is likely to be true. The recommended specification of blaze orange requires a dominant wavelength between 595 and 605 nanometers. Deer are expected to see this band of wavelength. However, the deer's relative sensitivity to 605 nanometers is less than half the relative human sensitivity. Although 605 nanometers is expected to be seen by deer as colored, that color would not be different from long-wavelength lights (the ones we see as red, yellow and yellowish-green).

Wavelengths that deer are likely to be able to distinguish from 605 nanometers are the ones we see as violet, blue, blue-green, and pure green. A garment that emitted only an intense band of light at 605 nanometers would be less colored and less bright to deer than it is to humans. However, it is important to understand that such a garment would be far different from an ideal camouflage. It would still stand out as colored and/or bright against dark backgrounds, against bluish-greens, pure greens, browns, tans, and grays.

Finally, the issue of how deer see short-wavelength light has received considerable attention. Recent results also lend insight into this issue. "The difference between daylight human foveal vision and daylight deer vision is expected to be even more dramatic for short-wavelength light than it is for long-wavelength light. Humans are very insensitive to wavelengths below 450 nanometers. For example, relative to other wavelengths, deer are about eight times more sensitive than humans to lights of wavelengths near 430-440 nm (such as those emitted by U-V brighteners). Garments can reflect (or emit) considerable light in this spectral band. Because of the deer's high relative sensitivity to short wavelength light, the presence of blue, violet and U-V components would make a garment stand out as both bright and colored against natural backgrounds. Those same components could be barely noticeable to humans." Dr. Jay Neitz


We shall now examine the significance of these findings to the Hunter.

Much has been written lately about how U-V brighteners effect a deer's perception of camouflage , blaze orange, and other garments. In order to apply what has been learned about the visual systems of the deer we must define how brighteners effect the garment being seen. This is further complicated by the spectral composition of the ambient light in which the garment is viewed.

We can simplify the effects of variations in ambient light by simply assuming that, for the sake of a discussion about the effect of U-V brighteners, we are talking about a time and place where U-V is a high percentage of available light. In direct sun at high noon the longer wavelengths overwhelm our visual system completely and we see no effect from U-V brighteners. As we move to dusk, dawn, deep overcast, or shade the absolute amount of U-V and short blue light decreases, but the fraction of total light contributed by U-V increases greatly. We therefore confine discussion of brighteners to times and places where their effect is significant.

The garment's color and other optical characteristics are also significant. Ignoring most variations again allows us to focus on the effects of brighteners. It will be noted, that for humans (very insensitive to U-V and short blue wavelengths) the effects could only be observed (if at all) on white or light colored garments. The deer, however, should see these effects on almost any color. The background is also significant. If a hunter is forced to silhouette against a sky that is rich in U-V he may want to match this intensity just as he would try to match the lack of U-V from a tree trunk or grass.

Now lets consider what a U-V brightener is. There are about 200 compounds in a handful of families that absorb light energy in the U-V portion of the spectrum. Also called fluorescent whitening agents, brighteners undergo a temporary change (using up energy) and then release the remaining energy at a longer wavelength. The compounds that protect paint or plastic from U-V damage and the whitening agents in cloth and laundry detergents generally release the energy they gathered through the U-V spectrum in a small band of short blue wavelengths at about 440 nm.

Here we can apply what has been learned about the deer's ability to see short wavelengths. "Deer are much more sensitive than humans to the shorter wavelengths of light." They have been found to have a blue cone with peak sensitivity at 455 nm, just 15 nm from the 440 nm peak of spectral power caused by the brighteners. This is earth shaking news to a 2 legged predator that can't imagine the brightness of light he barely sees. This 440 nm light is seen as bright blue in the dichromatic eye of the deer. It occurs on garments of any color from camo to blaze orange if brighteners are present. In very low light the deer, like a human, switches to rod (black, white, and gray) vision and the 440 nm light caused by the brighteners is seen by the deer as a much brighter gray.

The research also verified that "Deer are much less sensitive to longer wavelengths than humans". This means that if a blaze orange vest had no brightener dyes and was purely 605 nm blaze orange, the deer would not see it nearly as well as we do. They lack our red cone completely. Their green cone peaks at 537 nm, almost 70 nm away and pigment sensitivity curves drop steeply on the long side. Dramatic as this difference in sensitivity is it is only part of the story. Remember the third finding of this study.

"White-tailed deer would be expected to have dichromatic color vision". Human dichromats called protanopes also lack the red cone function. A human with one dichromatic eye (blue/green cones) and one trichromatic eye (blue/green/red cones) can tell us the difference in color perception. They see blue as blue and the rest of the spectrum from green to red as the color yellow, with their dichromatic eye. Therefore, if blaze orange or most green/brown camouflage is without brightener effect, it is all yellow. It will all blend in well in a world of green leaves, yellow grass, and brown trees, because they too are all yellow.

Now consider what effect U-V brighteners would have on these garments that appear yellow in a yellow world. Blue flags? Yes, but only on blue, white, light shades of gray, and other colors that have some blue content. Other colors will simply appear brighter and whiter much as intended for humans. In low light the problem is even greater. Many subtle differences in physiology make the deer far more sensitive to dim light-especially shorter wavelengths. They switch to black and white rod vision as humans do but can detect light 1000X below our threshold in the blue and U-V wavelengths


COLOR VISION

The ability to see color is an important aspect of human vision. Color differences often allow us to easily identify objects from their backgrounds that would otherwise be invisible. For example, at a distance, ripe red tomatoes on the vine are much more easily seen among the leaves than unripe green ones. Humans are able to see color because of three different types of cone photoreceptor cells in the retinas of their eyes. One cone type is most sensitive to short wavelength (blue) lights a second is most sensitive to middle wavelengths (green) and a third is most sensitive to long wavelength (red) lights. The three different cone types are the basis for what has been termed trichromatic (literally three-color) vision in humans. It should be noted as an aside that the majority of the cone photoreceptors in the human retina are the long-wavelength sensitive type, the middle wavelength sensitive type are the next most common, and the short wavelength sensitive are rare-only about 10% of the cones. The blue sensitive cones are important for color vision, but because of their small number they provide little or no over-all sensitivity to short wavelength light. Scientists have studied color vision capacities in a number of animals. Among mammals, only primates (monkeys and apes) have been found to have trichromatic color vision like that of humans. However, a number of other mammals have color vision that is based on only two different cone types; this is dichromatic (two-color) vision. This simplified type of color vision seems to be common among mammals and has been observed in carnivores (e.g. dogs and cats) and ungulates (hoofed mammals). Although vision is predominantly based on rods in these animals (more than 90 percent of the total photoreceptors in their eyes are rods giving them excellent night vision), they have enough cones to provide color vision. Obviously color vision based on only two different cone types is not going to be as good as human color vision that is based on three types. The deficiency in dichromatic color vision is in the ability to discriminate among the colors of objects that reflect light in the middle to long wavelengths, i.e. green, yellow, brown, orange, and red. The ungulates and carnivores with color vision based on only short wavelength sensitive cones and long wavelength sensitive cones, would find these colors difficult or impossible to distinguish. However, for these animals, blue, violet and near ultraviolet (which is invisible to us because it is blocked by the lens) stand out from the other colors. The colors of earthly objects are mostly browns, tans, greens and yellows. To an animal with dichromatic color vision, a sportsman wearing garments that strongly reflect short wavelength light would stand out against these backgrounds like a ripe red tomato on a green vine.

Sincerely Yours, Jay Neitz, pH. D . Vision Scientist

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