Why is blue rare among the animals?

There are no blue tigers. No blue bats, no blue squirrels, or blue dogs. Even blue whales aren’t that blue. Animals come in pretty much every color, but blue seems to be the rarest. What’s cool, though, is when we do find a blue animal, they’re awesome looking. Nature doesn’t do halfway with blue. To understand why this is, we’re going to journey through evolution, chemistry, and some very cool physics. But first we’re going to need to understand why animals are any color at all, and to do that, we need to go look at some butterflies because butterflies are awesome.

Butterflies are awesome. Make no mistake about it. They’re a group of moths that evolved to be active during the day, and if you’re active during the day, you have an advantage: You can use light to communicate. You probably realize this, but out of all insects, butterflies display the brightest and most detailed patterns. And there’s a good reason for that: The colors in butterfly wings deliver messages, like “I’m toxic”, or “I’m a male and this is my territory”, but not all butterfly colors are created equal. If we zoom way in on a butterfly wing, we see the colors come from tiny scales. It’s actually how moths and butterflies get their scientific name. Oranges, reds, yellows browns…those scales all contain pigments, organic molecules that absorb every color except what we see. Black scales absorb all colors. Animals, from butterflies to birds to you and me, don’t make these pigments from scratch, they’re made from ingredients in our diet.

You might know this thanks to flamingos: They’re born gray, but turn pink thanks to pigments called carotenoids in crustaceans they eat. So, when it comes to these colors: You are what you eat. But not so for blue. Blue is *different* If we zoom way in on a blue wing scale, we see little ridges. If we slice across the scale, and look closer, we see the ridges are shaped like tiny Christmas trees. The arrangement of the branches is what gives Morpho wings their blue color. When light comes in, some bounces off the top surface. But some light passes into the layer and reflects off the bottom surface. For most colors of light, waves reflecting from the top and bottom will be out of phase, they’ll be canceled out, and that light is removed. But blue light has just the right wavelength: the reflected light waves are in sync, and that color makes it to our eye.

There’s even a pigment at the base that absorbs stray red and green light to make the blue even more pure. That’s how we get the awesome iridescent blue. The microscopic structure of the wing itself. All of this happens because of the way light bends when it moves from air into another material. So, if we fill all those tiny gaps with something other than air, like alcohol, the blue disappears.

Technically, this “changes the index of refraction”, but in plain English that means blue light is no longer bent the right way. The microscopic light filter is broken. Until the alcohol evaporates. And the color returns. But these butterflies live in the rainforest. You think they’d lose their color any time they got wet, right? Unlike the highly-ordered structures we find in butterfly wings, these feather structures are messier, like a foam, so instead of changing as we move, the color’s more even from every direction. Even that color is made by the adding and subtracting of light waves thanks to structures in the skin… not pigment. And yes, even your blue eyes, are colored by structures, not pigments.

Outside of the ocean, almost exclusively, the bluest living things make their colors with microscopic structures, and each one’s a little different. No vertebrate, not a single bird or mammal or reptile that we know of, makes a blue pigment on its body. In fact, there’s only one known butterfly that has cracked the code for making a true blue pigment. Blue as a pigment in nature is incredibly rare. But there’s one exception so far that we know about, and these are over here called the Olive wings. They have evolved a blue pigment. They’re not very common and we don’t know much about them. That’s a really special butterfly. Why is almost all of nature’s blue made from structures and not pigments like everything else?

At some point way back in time, birds and butterflies evolved the ability to see blue light. But they hadn’t yet evolved a way to paint their bodies that color. But if they could, it’d be like going from early Beatles to Sgt. Pepper’s Beatles. It meant new opportunities for communicating and survival. Creating some blue pigment–out of the blue–would have required inventing new chemistry, and there was no way to just add that recipe to their genes. It was much easier for evolution to change the shape of their bodies, ever so slightly, at the most microscopic level, and create blue using physics instead. They solved a biology problem with engineering. These colors have fascinated curious people for hundreds of years. After looking at peacock feathers through one of the first microscopes back in the 1600’s Robert Hooke wrote: “these colors are only fantastical ones” Even Isaac Newton noticed there was something unusual about these blues, and scientists have been studying it ever since. Not only because the science is interesting, but because it’s beautiful.