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!
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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.
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A rainbow sits over the North Meadow. Red has the
longest wavelength and blue has the shortest. Photo by Dennis Conner/WPZ.
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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.
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| 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.
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| 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.
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| Tetrachromacy is suspected among some arachnids,
fish, reptiles and amphibians. Photos by Ryan Hawk/WPZ. |
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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.
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| 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.
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| 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!
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| 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!
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| 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!
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| 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!
Resources:
http://www.nwf.org/News-and-Magazines/National-Wildlife/Animals/Archives/2002/Seeing-Colors-in-a-New-Light.aspx
http://en.wikipedia.org/wiki/Ultraviolet_vision
http://www.ucl.ac.uk/news/news-articles/1105/11052502
http://www.sciencedaily.com/releases/2011/06/110622224455.htm
http://www.dailymail.co.uk/sciencetech/article-473897/A-bees-eye-view-How-insects-flowers-differently-us.html
