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During rifle season, hunters are required to wear blaze orange (some states also allow bright pink).

I have heard that deer and elk are color blind to blaze orange, is there scientific evidence to back this up?

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Humans have trichromatic vision. Deer, Elk (wapiti) and some other animals have dichromatic vision. It's not that deer and Elk can't "see" oranges and reds, they just perceive them differently.

Here is an explanation from Polk County Iowa

Deer Color Blindness

Are deer are color-blind? Is this why hunters can wear bright orange clothing and the deer don’t seem to notice?

Yes, deer have a form of color blindness. All mammals have a retina located in the back of the eye that consists of two types of light sensitive cells called rods and cones. Rods function in the absence, or near absence, of light and permit vision in darkness. Cones function in full light and permit daytime and color vision. Deer have more rods (nighttime cells) and fewer cones (daytime and color cells) than humans. Therefore, deer have better nighttime vision than humans but poorer daytime and color vision. Deer lack red cones so they can't distinguish between green, yellow, orange, red or brown. All of these colors appear as shades of yellow. So the blaze orange color hunters wear appears yellow, as does almost everything else to a deer.

Elk have similar vision as deer. According to this site,

When it comes to sight, elk aren’t endowed with precise vision. In fact, they see around the human equivalence of 20-60. Additionally, they don’t see the full color spectrum humans do. Elk vision is dichromatic, which means their world is seen in two colors, not trichromatic like our vision. There is a whole science to ungulate vision, but to keep it simple, just know this. Elk don’t have a red cone like humans, so the upper end of the color spectrum appears yellowish to elk. They aren’t color blind, but they don’t see color the way we do.

As a bonus, it's speculated that deer may have the ability to perceived ultraviolet light better than humans.

In August 1992, a group of leading deer researchers and vision scientists gathered at the University of Georgia to conduct this landmark study....

Our study confirmed that deer possess two (rather than three as in humans) types of cone photopigments allowing limited color vision. The cone photopigment deer lack is the “red” cone, or the one sensitive to long wavelength colors such as red and orange. These colors aren’t invisible to deer, but rather are perceived differently. Deer are essentially red-green colorblind like some humans. Their color vision is limited to the short (blue) and middle (green) wavelength colors. As a result, deer likely can distinguish blue from red, but not green from red, or orange from red. Therefore, it appears that hunters would be equally suited wearing green, red, or orange clothing but disadvantaged wearing blue.

The results regarding the UV capabilities of deer were equally fascinating. Our results confirmed that, unlike humans, deer lack a UV filter in their eye. In humans, this filter blocks about 99 percent of damaging UV light from entering the eye. It also functions much like a pair of yellow shooting glasses and allows us to focus more sharply on fine detail. The trade-off is a loss of sensitivity to short wavelength colors, especially in the UV spectrum. Because deer do not have a UV filter, they see much better in the UV spectrum but lack the ability to see fine detail. This helps explain why deer often move their head from side to side when they encounter a hunter.

Article Source

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    "Elk have similar vision as deer." Biologically speaking, elk are deer. – David Richerby Jun 7 '18 at 11:35
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    Also, your first two sources seem to think that "colour blind" means "see in black and white", which isn't what it means at all. Colour blind means exactly the inability to distinguish between colours that humans with normal vision can distinguish, and your third source hits the nail on the head: deer are red-green colour-blind so bright orange doesn't stand out to them. Anyway, your answer covers all the bases so +1. – David Richerby Jun 7 '18 at 11:39
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    Oh, and if you'll permit me to blather on for one more moment, it's interesting that the last quote says that deer's eyes are sensitive to UV. Fluorescent colours work by absorbing UV and re-emitting it in the visible (to humans) spectrum. So fluoro looks unusually bright to us because it transforms light we can't see into light we can. But, to a deer, it presumably wouldn't look anything special because it's just changing the colour of the light from one they can already see to a different one. OK, I'm done now! :-) – David Richerby Jun 7 '18 at 11:42
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    FWIW, almost all placental mammals have (at most) dichromatic color vision, i.e. they're all essentially red-green color blind. Deer are not at all unusual in this respect. The only exception to this rule is a subgroup of primates (including the great apes, and among them humans) that appear to have re-evolved a third color receptor through duplication and mutation of the red/green receptor pigment gene. – Ilmari Karonen Jun 7 '18 at 13:27
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    The UV bit is really quite important, because if you wash your outdoor gear with "colour brightening" detergents, these actually contain UV enhancers that produce a type of fluorescence that will make your entire outfit stand out, especially in lower light situations. – J... Jun 7 '18 at 13:36
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I can't take any credit for already knowing this, all my information comes from an article from Penn State University, entitled Deer-Forest Study: The eyes have it!

Some of the information in this article appears to contradict the claim at first glance, for example it begins with "Day or night, a deer’s vision is excellent. This is good if you’re a deer but bad if you are a hunter. So how do deer see the world?" As you read on however, there are some details that clear things up.

Relevant facts are that deer eyes have a higher cone-to-rod ratio than humans, and the rods are sensitive to different wavelengths than humans, in particular more sensitive in the blue-violet end of the spectrum, and even into the UV, and less sensitive in the red end. Quoting the article:

Deer are essentially red-green color blind (like some people). Deer can distinguish blue from red, but not green from red. Red, orange, or green all look the same to a deer. Because deer lack a UV filter in their eye, their sensitivity to short wavelength colors is enhanced in the UV spectrum.

If you remember your rainbow R-O-Y-G-B-I-V, to a deer it is more like X-B-I-V-UV where X is some color combination only a deer would understand.

Thus it is not accurate to say "deer can't see orange" (which is not what the question claims, but I have heard said) but rather that what is obviously orange to a human, is just dim-dull blob to a deer. As a hunter, you would hope this makes you look similar to the dim-dull fall vegetation around you.

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