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Ultra awesome: Ultraviolet eyesight in animals

Posted by: Kirsten Pisto, Communications

Roses are red, violets are blue…unless you’re a tetrachromat, it’s true. Apologies for the obvious segue to a post about color, but I couldn’t resist!

A blue bellied roller and a brightly colored orchid show us examples of some of the beautiful colors here at Woodland Park Zoo. Photos by Ryan Hawk and Kirsten Pisto/WPZ.
Throughout the animal kingdom, there is enormous diversity in the structure and faculty of eyesight. Oftentimes, we relate our own human eyesight to the visual capabilities of animals, but most evidence points to the contrary: in fact, humans might be an underdog in visual perception.

A rainbow sits over the North Meadow. Red has the longest wavelength and blue has the shortest. Photo by Dennis Conner/WPZ. 

The human eye sees a wide range of what we call visual color, that is, measurable wavelengths in the range of about 390–700 nanometers. Our eyeballs have a ton of tiny little cones and rods in the retina, which are super sensitive photoreceptors. Cones determine which colors are perceived, rods determine light perception. Most people are trichromats, have three types of cone receptors; red, blue and green, and can see in what is referred to as 3-dimensional color.

So many colors can be found here at Woodland Park Zoo! All photos WPZ.
But guess what? Even humans can’t actually see all of the colors we perceive! Take magenta for example. Because magenta, or hot pink, is a mixture of multiple wavelengths, our eyes cannot actually distinguish it as a color; instead, our brain fills in the gaps with what we perceive to be the color magenta. Weird, huh? That’s a pretty cool trick, but a lot of animals have an even more extreme adaptation of their visual senses, much beyond the limits of human perception.

This color spectrum shows us visible light, that is, colors most humans can perceive. Ultraviolet light has shorter wavelengths. Via Science Blogs.
If you were to walk past violet on the color spectrum, you would get to ultraviolet rays in the range of 10 – 400 nm. As trichromats, (with red, blue and green cones) our eyes aren’t built to see this level of color, but animals with an ultraviolet cone receptor can!
So, which animals actually have UV vision? 

  • Monochromats (many undersea animals and nocturnal animals), have only one type of color receptor cone. In fact, many bats, nocturnal snakes and lizards have no cones at all, sacrificing the ability to distinguish color for increased absolute sensitivity. Some monochromats might see in ultraviolet, but it’s sort of an unknown. 
  • Some dichromats (animals that have only two types of color receptors), such as scorpions, can see UV color. Their cones are built to perceive ultraviolet and green/yellow colors, but they cannot see blue, green or red. Most mammals, however, are dichromats, with only red and green receptor cones.
  • As far as trichromats (humans and some other primates, marsupials, and honeybees) go, bees have the upper hand in UV vision. They have three receptor types, although unlike humans they are sensitive to ultraviolet light, with loss of sensitivity at the red end of the spectrum. Because color perception is a mixture of receptor types, this means that bees do not simply see additional UV colors, but will perceive even human-visible spectra in different hues to those which humans experience.

Tetrachromacy is suspected among some arachnids, fish, reptiles and amphibians. Photos by Ryan Hawk/WPZ.
  • Tetrachromats (animals with four types of cone receptors such as some birds, turtles and fish), can see UV wavelengths perfectly well because they have 4-dimensional color vision and the ability to see in ultraviolet. That means they can see all the colors we see (red, blue, green) plus an additional color, which of course is ultraviolet.
  • Pentachromats (butterflies and some birds), actually have five different color receptors and maybe more! Just imagine the colors we are missing out on!
It is well known that many bees and birds follow UV-reflecting nectar guides on flowers which lead them to the most nectar-rich part of the plant. These animals depend on UV colors to guide them to their food source, yet research done within the last few years has revealed that many animals use UV colors for much more than just finding the sweetest spot on a flower.

On the left is a daisy under a UV light lens. You can see the nectar pattern on the petals, although we still can’t actually see the correct UV color because, well, we are only human. On the right is what the flowers look like to us in visual color—no pattern! UV photo by Leonard Less, butterfly photo by Ryan Hawk/WPZ
For example, it is thought that desert iguanas might mark their paths with UV-absorbing urine, leaving behind territorial signals against the sand. Scorpions glow or appear yellow and green under UV illumination, keeping camouflaged to mammalian eyes, but standing out to each other.

Woodland Park Zoo hosts a wide variety of colored feathers, but just imagine if we could see these birds with ultraviolet vision! Photos by Dennis Dow/WPZ.
Many birds and butterflies have patterns in their plumage and wings that are invisible to human color vision but observable in ultraviolet. Recent research suggests that birds and butterflies might see about 10 billion colors, whereas humans can only see 10 million! This assists them in finding the correct mating species. The next time you look at a little brown bird such as the common sparrow and think, how drab, think again—to the bird world that little guy might have some majorly flamboyant feathers!

To us, this peacock resembles the painted pipes behind him, but would he stand out if we had ultraviolet vision? Photo by Ryan Hawk/ WPZ.
In May 2010, researchers at the University College London tested the electrical response of the retina of anaesthetized reindeer to UV light. What they found is that a reindeer’s retina responded to near UV, about 320 – 420 nm. That’s not up to par with a hummingbird or honeybee’s UV sight, but its pretty impressive for a mammal!

Could our arctic residents have an ultraviolet secret? Photo by Ryan Hawk/WPZ.
The research suggests that reindeer, an arctic animal that often experience white-out conditions, might use their UV eyesight to forage for food or even detect predators. UV rays are actually absorbed up by things like lichen, a plant which reindeer munch on, and the fur of a wolf’s coat, which reindeer would like very much to avoid. The UV rays appear darker to the reindeer, very helpful against a snowy landscape.

Researchers predict that other arctic mammals may also share this ultraviolet vision. One clue, arctic foxes, polar bears and seals are not known to suffer from snow blindness, pointing to their ability to see in extreme white out conditions. Of course, their sense of smell could be another reason these creatures outmaneuver people when it comes to blizzards. (Polar bears can smell their prey up to 20 miles away!). So, until more research is done on these individual species, it’s sort of an arctic mystery!

Imagine being able to see an entirely new color! What would our world look like if we had the ability to see in ultraviolet? Photo by Mat Hayward/WPZ.
Ultraviolet vision is still sort of a new science. Advances in access to tools such as the fiber-optic spectrophotometer allow researchers to measure ultraviolet color in different environments leading to a better understanding of how animals might use their UV vision. When you come to the zoo, you must use your imagination, because even though we can measure UV light, we still have no idea what it really looks like!



Anonymous said…
My Mother has the human version of Tetrachromic vision.

She carries genes for "normal" (received from her mother ) and "color-blind" (received from her father) versions of the red sensitive cones. During embryonic development of the eye, a cone cell randomly selects to express the red, green, or blue sensitive pigment, and then for that gene, it randomly selects which chromosome to use: the one from the mother or the one from the father. One third of the cones in my Mother's eyes are sensitive to red, but of those, half use the "normal" version of the gene from her mother and half use the "color-blind" version from her father.

This means she sees in four colors, which is evident with the color purple. She sees differences in shades of purple that "normal" people cannot see.

Tetrachromic vision is not possible in men because the color pigment genes are carried on the X chromosome of the XY pair that make a male. Men therefore only have the color pigment genes received from their mothers, which for me is the "color-blind" version my Mother had received from her father.
Electrician IN said…
A study released in the summer of 2011 reported that birds' eyes can see ultraviolet light in addition to the band of red-through-blue that we humans perceive.
How do birds use their avian ultraviolet eyesight?
Anonymous said…
I don't think it's the white-out conditions that are problematic for reindeer vis-a-vis UV vision. For removing glare I think you'd want polarization vision. Also, for much of the winter in the Arctic circle, the sun barely rises above the horizon. The bigger problem then is a lack of perceptible light - at least if you can't see in the UV. Being able to do so, you'd be fine though, as the highly scattered light that does fall onto the earth is disproportionately blue and UV. Snow strongly reflects UV, but not reindeer food items, and not reindeer predators, due to their fur.

The big problem with seeing red through UV light is that it's not always feasible to keep all of the light in focus at once. Because light tends to move through non-vacuum media at different speeds (think prism), it is bent and thus focused differentially by a lens. Since not all of the light can be brought to an acceptable focus at once, some wavelengths of light are not good for acute vision in a typical single chambered eye. It is for this reason that blue photoreceptors are not present in the human fovea. Having a mutlifocal lens is one way around this, so that light of differnt colors is focused at different lateral points along the retina, but unfocused light causes a kind of light pollution that has to be dealt with. Another solution is to have a tiered retina, where light of different wavelengths is focused at different distances away from the lens. This works well for a large enough eye (like we have), but the capacity to form a tiered retina has to be available to whatever lineage. Another solution is to have less acute but more color-rich eyesight. This is basically what butterflies have.

So on balance, human vision - in daylight, at any rate - isn't much of an underdog.