by James Morris

Animals sport a rich palette of colors. Think of a bright red cardinal, orange monarch butterfly, yellow warbler, green grasshopper, brown snake, or black-and-white striped zebra. Blue is less common. In fact, blue is one of the rarest colors in the animal world. Of course, there are blue eyes, peacock feathers, and butterflies. But these blues are not made in the same way that other colors in animals are commonly produced.

Most of the colors we see in nature are made from pigments. Pigments are colored chemicals. They absorb certain wavelengths of light, but not others. The color we see is the color that is not absorbed. For example, red pigment absorbs blue, green, and yellow, but not red, so appears red to us.

A blue feather, however, doesn’t contain blue pigment. How, then, does it appear blue? We need our first clue.

Take a feather and shine a light behind it. You’ll notice that the blue color disappears and it turns a grayish-brown color instead. This tells us that the blue color depends on the way that light reflects off the feather.

In fact, the blue in feathers results from the structure of the feather. Structural colors aren’t produced by pigments. Instead, they are made by the geometry of the object. In feathers, for example, there are microscopic structures that reflect blue light back to the observer, so they look blue. Beneath this structure is a pigment layer that absorbs all of the wavelengths of light, so the only color we see is blue.

This is true of all bird feathers. The blue of a blue jay, bluebird, and indigo bunting are all created by shape, not pigment. In fact, it’s true of the blues we see in nearly all birds, reptiles, mammals, and other vertebrates.

Structural colors are produced in different ways. In some feathers, for example, tiny air bubbles reflect blue light, as in blue jays. In other feathers, thin films bend and reflect light in such a way that we see blue, as in peacock feathers. This is like what happens when you see a film of oil on water. The film produces a rainbow of colors even though the water and oil are both colorless. Different types of structures produce different qualities of colors. The blue of a blue jay feather is bright blue, while the blue of a peacock feather appears iridescent.

One way to think about the difference between pigment color and structural color is by grinding the object into a powder and seeing what you get. If the color is the result of pigment, the powder retains the original color. If it is the result of structure, the powder loses its original color, as the color depends on the integrity of the structure.

You can demonstrate structural color yourself. Just put a few drops of milk into water, go into a dark room (don’t spill the milk!), and shine a light on it. Blue light scatters from the milk molecules, so it looks blue. You can get the same effect by dissolving a little flour in water, as shown here.

This is not unlike why the sky appears blue, especially on a clear summer day. Small particles in the air scatter light. Short wavelengths of light, like blue, are scattered more than long wavelengths, like red, so the sky looks blue to us. The scattering of light by small particles, called the Tyndall effect, is also responsible for producing blue eyes.

It turns out that the white color of feathers is also created by structure, just like the blue color of feathers. In this case, all of the wavelengths of light are reflected back to us, which produces a white color. This is the same effect we get with snow. Individual snow crystals are clear, like water. But all of the crystals in a snowflake or snow bank reflect light in different ways, creating white.

Then what about green? How do animals make green? We need our second clue.

Green in insects is often made by pigments, but green in birds is typically structural. In feathers, green is produced by the same structure that reflects blue light, but then this light passes through a yellow pigment layer, producing green. If you mix blue and yellow, you get green.

Plants are a different story altogether. Greens are made by chlorophyll, a pigment involved in photosynthesis. Blue in plants, like blue in animals, is relatively unusual. There are some examples, such as bluebells, delphiniums, and morning glories. The blue in plants comes from a pigment – called anthocyanin – that usually produces a red color, but is modified to produce blue.

Of course, all of this begs the question of why blue is so rare in animals, and why animals tend to use structure rather than pigments to create blue. We need one more clue.

Colors are used for all kinds of purposes – camouflage, warning, courtship, and communication, to name a few. And pigments do more than produce colors. In our skin, we have a pigment called melanin that protects us from ultraviolet radiation. Melanin also helps to keep bird feathers strong. In our eyes, we have a pigment called rhodopsin that absorbs light as the first step in the pathway for vision.

Why then is blue pigment so rare? It could be that it just never evolved in animals. So, although it might be useful, it’s hard to come by. Without blue pigment, animals came up with a different solution to make blue – structure. Then, to make green, which is all around us in our leafy world and therefore handy for hiding, many animals simply did some color mixing – blue structure and yellow pigment.

So, there is some truth to the statement that, for animals anyway, it is not easy being green. And that’s not anything to feel blue about.

© James Morris and Science Whys, 2020