The Biology of Indigo

This post isn’t so much about an animal as about its color: indigo. You see, the other day, a little rainbow came through a prism at my window and landed on my hand. I studied the little rainbow, and it reminded me of a fact of nature that I had forgotten: that indigo is its own color.

To be crudely scientific, indigo is any color on the electromagnetic spectrum between 420-450 nanometers in wavelength. It was first designated a spectral color by Sir Isaac Newton, but it has clearly been there between “blue” and “violet” in a rainbow ever since light first diffracted in water. However, we do not discern it as being different from blue. Physics does. But we do not. We lump it in with the nearly endless variations of blue that we have categorized on our paint wheels: phthalo, cerulean, baby, robin’s-egg, midnight, navy, cyan. It turns out that the human eye is relatively insensitive to changes in hue between blue and violet, which is why we may not be able to “see” indigo, even when we’re looking straight at it. Which makes me wonder: If physics calls indigo its own color, does biology? How do other animals perceive it?

There were two species that came immediately to mind. One is the Indigo Snake, the longest snake in the United States, at over 9 feet. It lives in the Deep South, where it preys on anything large enough to swallow, including rattlesnakes, to whose venom it is immune. Its color is iridescent, turning red-purple in bright light.

The other species is the innocuous Indigo Bunting, a widespread North American bird in the Cardinal family. Typically brown, the males turn this lovely hue during the breeding season to attract mates.

Now, I know that technically, we could still look at the snake and the bird and think “blue.” But what color do they see themselves? Another commonality between the two species is that their colors are structural – in other words, they’re produced not by pigmentation but by light diffraction in special criss-crossed proteins. In other words, they make indigo much the way rainbows make indigo, which accounts for their iridescent glow. They want to stand out. Snakes can’t see color, as far as we know, but such a stark and unusual spectral color as indigo clearly tells potential predators that this thing is bad-ass, and best left alone.

The bunting has better color vision than ours. Given that its indigo is used for sexual display, it also seems to be using indigo specifically to advertise. Indigo, the thinnest strip of color on the spectrum, the bastard daughter of the rainbow, is better than blue for saying Look Up and Watch Out. All kinds of things around us are blue. It’s a blue sky. It’s a blue sea. It’s a blue planet. But indigo, that’s a whole different story. To our primate eyes, developed in darkness, it may not be any different from a deep cyan or light violet. But to things evolved for light, indigo isn’t a mere hue. It’s a whole different frequency of sex, danger, and intrigue.

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About quantumbiologist

Christian Drake, AKA The Quantum Biologist, is a naturalist and poet formerly of Albuquerque, NM and currently living deep in the backwoods of the Connecticut Berkshires. He has worked in aquariums and planetariums, national parks and urban forests. When not birding or turning over rocks to find weird bugs, he enjoys rockabilly music, gourmet cooking, playing harmonica and writing dirty haiku. View all posts by quantumbiologist

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