Art in Nature: Structural Coloration Part I

Most of us are probably familiar with pigments. They are, after all, one of the most important components of visual art, and new pigments have been a highly sought-after resource by artists for thousands of years. In both art and nature, they are chemicals that cause skin, hair, eyes, and feathers to reflect certain colors. In humans, melanin is the primary force for skin color. It is also in our hair, eyes, and apparently even our inner ears and brain stem. For us, more melanin is naturally better in some environments, worse in others, and we have evolved as a result. More melanin can help prevent skin cancer, but it can also hinder vitamin D synthesis. Like a great artist, such a simple thing can do so much. In most hair coloration, there are two types of melanin and the ratios of the two types give us most of the amazing array of hair colors we have. A mutation in melanin production for red hair as well as ultraviolet radiation (think of sun bleaching in the summer) add even more variation. Obviously there is much more to this and to human evolution, but this blog post is not about that, it is about art in nature.

In most American birds, feathers are colored either by melanin or carotenoids. For carotenoids, think of carrots, since that is literally what they are named for, and the orange of a carrot is a carotenoid. Typically carotenoids provide the "pretty" colors you see on a bird, while melanin provides the dark feathers. Melanin actually adds strength, so it is more important on flight feathers, especially the tips. If you have ever cracked brown eggs, you probably noticed that they are firmer. This is due to melanin. The dark spots on wild bird eggs? Also melanin.

This need for carotenoids is one reason why nutrition for baby birds is so important, and one reason why even seed-eaters do not feed their young seeds, instead favoring caterpillars and spiders. Also, I'm sure we can all appreciate the concept of having something squishy jammed into our throats over hard seeds, if not the thought of what that squishy thing actually is.

So pigments are great and do a lot of amazing things from giving strength to color to protection. However, they are not the only ways in which wildlife develops color. One very significant and easily overlooked method is structural coloration. Structural colors are colors that come from the physical dimensions of objects that actually cause light waves to interfere with each other, changing the color we see. Everyone is familiar with structural coloration whether or not they realize it. Gasoline is clear; water is clear, but gasoline on water is colorful. This is a result of the structure of the two together. The same goes for soap bubbles you might blow to a child. Specifically this is type of structural coloration—and there are many types—is called "thin-film interference."

We often think of light reflecting off an object, but don't consider how, on a clear object, some of the light may reflect off the front of an object while some of the light will travel through the object and reflect off the back side of it. This is actually what happens in the thin film of oil on water, or the thin layer of a soap bubble. The films are very thin, but they still have depth. So some light reflects to your eyes from the front of the film, and some light reflects off the back of the same thin film of soap where it touches water. This is where it gets more complicated.

As we know, light has wavelengths and waves can add or subtract with each other, just like waves on a beach. So light reflecting off the back of the layer can interfere with the light reflecting off the front. As the light hits the film at odd angles--especially as the oil moves--the color changes. If the light waves line up perfectly, they will add together to make a stronger reflection at that wavelength. However, if they are exactly opposite, say where one wave is going "up" and the other is going "down," then they will cancel out and there will be no color at that wavelength. Thus some colors can be stronger while others can be absent, which in turn makes some colors stand out. Of course, rarely will the light be perfectly in-sync like this, especially as the angle of light changes (like as a bubble floats away), but even being close can have a big impact. Thus, we end up with iridescence.

In nature, you have seen thin-film interference in the colorful sheen on wings of flies and wasps, colorful butterfly wings without pigment, and even glossy flower petals, however perhaps the most popular iridescence in North America is found in hummingbirds. Much of hummingbird coloration is pigment, but much is also structural thin-film interference. Wait, thin-film interference on a bird or insect? Where is the oil? Although birds do produce oil (like we do) to clean their feathers (preen oil), the “thin-film” is created by the structure of their feathers.

So for starters, it is important to understand that melanin is produced by organelles in cells called melanosomes. Now, bird feathers are generally made of keratin, just like our hair and nails. In the tiny leafy parts of the feathers, there are melanosomes, and in a hummingbird—as well as some other birds—they are structured in a specific way to create iridescence. Specifically, these melanosomes will be relatively flat and wide with air bubbles inside. This is where exciting physics comes into play. Just like the thin-film interference of soap bubbles, melanosomes can create thin-film interference. So say red light hits a melanosome, and the melanosome reflects some of the red light back out. But some of the red light is allowed into the melanosome and hit the back end, then reflect back out. Because the melanosome is the perfect width for that wavelength, both these reflections of red light end up combining together into a super wave, much like you sometimes see in the ocean. So with the flat and wide melanosomes, if a bird moves slightly and the light reflects at a slightly different angle, then the two waves of red light can match up slightly differently, creating that shimmer as the bird moves.

Thus, one of the most amazing features of color in a very colorful genus of birds comes not from pigment, but the structure of their feathers. Just as the beauty of art is not simply the pigments, but how it’s structured, how it’s layered, and how it creates a story.

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