Tuesday, January 7, 2014

Color Vision

Even a wash of golden afternoon sunlight couldn’t seem to warm the icy air. I put my head down and skied harder, hoping to send warmth to my fingertips. Despite being out in full sun—a treat after several weeks of skiing only at dusk-- the woods seemed stark and colorless. Soon a steep hill came to my rescue, and as I churned up it determinedly, I could feel my extremities turning warm and pink under their protective layers.

Now I could relax a bit, and look around. With greater awareness, it seemed that color had seeped into the woods just like it had seeped into my fingers. Blue sky shone through the intricate pattern of twigs, although it paled gradually toward the horizon. The trees themselves revealed complex mosaics of browns and grays. Thin stripes of orange, even, lay in the bottom of the deep furrows low on big tooth aspen trunks. Clinging to the bark was a whole other palate of pale green, gray green, sage green, and sea green lichens. A few unique lichen species glowed with summery shades of tangerine and goldenrod.

The peeling bark of young birch trees exposed innocent, peach-colored skin. A granite boulder, not quite buried in the snow, added a splash of coral pink. Through the understory, the buds of red maple trees lived up to their name, and dogwood twigs matched their scarlet hue. High in those brown twigs, silhouetted against the blue sky, the bright red crest of a hairy woodpecker livened up his otherwise black and white feathers.

When we look close enough, color is everywhere -- even in the winter woods. But what if we didn’t have to look so hard? What if the world glimmered with twice as many – or a million times as many -- hues?

It’s hard to imagine, but not every creature is limited to the same basic seven colors we see. We think of “visible light” as those wavelengths between 380 nanometers to 760 nanometers. Those are the electromagnetic waves that the cone cells in our eyes perceive as violet, indigo, blue, green, yellow, orange, and red. Humans have three types of cone cells – red, green, and blue – that each absorb light in overlapping sections of the spectrum. We perceive the intermediate colors when light is absorbed by two types of cone cells at once. For instance, we see yellow when the red cones are stimulated slightly more than the green cones. In this way, we can see a few hundred hues.

Birds, as well as some species of fish, amphibians, reptiles and insects, however, have a fourth type of cone cell that allows them to see wavelengths of light down to 300 nanometers, which includes the ultraviolet section of the spectrum. This means that the animals see wavelengths beyond those of a typical human being's eyesight, and may be able to distinguish colors that appear identical to humans.

What does ultraviolet look like? We don’t know!

We do know that things like flower petals, feathers, caterpillars, beetles, and moths reflect light in the ultraviolet spectrum, and seeing UV light gives critters an advantage in finding food and attracting mates.

While UV radiation is invisible to the human eye, illuminating certain materials with UV light causes the objects to fluoresce, and re-emit light at a slightly longer wavelength. The UV light is altered enough that it becomes visible light, and we see things glow. I think that this is how a black light works when it makes your white t-shirt glow.

In addition to seeing beyond the spectrum of human vision, birds can see more colors within our normal range as well. They accomplish this first by having a higher density of photo receptor cells than humans, and then by having a drop of colored oil on each cone cell. The oil droplets act as light filters, and reduce the range of wavelengths that can trigger each cone cell. This increases the precision of their color vision, and allows them to distinguish between more subtlety different hues.

With their four types of color receptors, birds are considered “tetrachromats” as opposed to humans, who are “trichromats” with three color receptors. Incredibly, pigeons and some butterflies are “pentachromats” with five color receptors, and, the ability (theoretically at least) to distinguish up to 10 billion colors. Talk about a different way to see the world!

Now, before you get too jealous, you should know that such stellar color vision has its drawbacks. Birds who focus on color vision have poorer night vision and must roost after dark. That’s why the woods are so silent on our starlit hikes. Humans, on the other hand, have the versatility to see both in bright sunlight and nearly total darkness. We also can move our eyes within our sockets, and use binocular vision to aid in depth perception, something that many birds cannot do. In addition, birds devote a greater proportion of their brain volume to visual processing, at the expense of their senses of smell and taste.

It might be exciting to see a million colors, but it might also be a little overwhelming. I don’t think I’m willing to trade the scent of wood smoke or the taste of chocolate to try it out. I’ll leave the visual bonanza to the flock of chickadees chattering overhead. Instead, I’ll appreciate the subtle beauty of blue-gray tree shadows laying brush strokes across the trail, and enjoy the fact that I can still see well enough to ski as the dusk falls on another winter day.

For over 45 years, the Cable Natural History Museum has served to connect you to the Northwoods. Come visit us in Cable, WI, at 13470 County Highway M. The current exhibit, “Deer Camp: A Natural and Cultural History of White-tailed Deer,” opened in May 2013 and will remain open until April 2014.

Find us on the web at www.cablemuseum.org to learn more about our exhibits and programs. Discover us on Facebook, or at our blogspot, http://cablemuseumnaturalconnections.blogspot.com/.

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