The rarity of blue in nature is highlighted because no vertebrate, bird, mammal, or reptile has been found to make a blue pigment on its body. While animals come in pretty much every color, blue is the rarest, making blue animals look more astounding when we find them. In contrast to other colors, blue doesn’t come from pigments but rather from the shape of the microscopic structures on the surface of the animal’s skin, fur, or feathers. How these structures reflect light, specifically blue light, creates the blue color we see.
Butterflies are one of the best examples of animals that use color to communicate. The colors on butterfly wings deliver messages, such as “I’m toxic” or “I’m a male, and this is my territory.” The colors come from tiny scales containing pigments, organic molecules that absorb every color except the one we see. Other colors, such as oranges, reds, yellows, and browns, also come from pigments derived from the animal’s diet.
However, blue is different. Blue Morpho butterflies, for example, are not blue; their blue color comes from the shape of the wing scale itself. If we zoom in on a blue wing scale, we can see tiny ridges shaped like Christmas trees. These ridges reflect blue light, absorbing other colors to create the iridescent blue we see. Other animals, such as blue jays, peacocks, and even humans with blue eyes, also use microscopic structures to make their blue color.
While blue as a pigment in nature is scarce, one known butterfly has cracked the code for making an actual blue pigment. However, outside of this exception and the ocean, almost exclusively, the bluest living things make their colors with microscopic structures, and each one is a bit different. Overall, understanding the rarity and mechanism of blue coloration in animals requires exploring topics such as evolution, chemistry, and physics, and it highlights the beauty and complexity of nature.