I recently started reading a very captivating book by Ed Yong, An Immense World: How Animal Senses Reveal the Hidden Realms Around Us (in Russian translation – Необъятный мир: Как животные ощущают скрытую от нас реальность). I want to summarize each chapter on Facebook to get a better grasp of the subject. Also, as a side effect, to entertain people, although I am doing this more for myself 🙂
I’ve just got hooked on a topic, and went online to dig up details. The topic is – why are there so few blue creatures? There are many red, yellow, brown ones, but few blue ones. It turns out, it’s even more interesting—there are practically no animals with blue pigment. But it’s curious that it’s not “none,” but “practically none”. Interestingly, the same nearly holds true for green.
Let’s start with what a pigment is. Pigments are substances that physically absorb certain wavelengths of light and reflect others, which determines their color. For example, a pigment absorbing red and green light will appear blue. But most likely all the blue you see on animals is not the result of blue pigment. It’s the result of a special physical structure of the surface. These structures are microscopically small and manipulate light through diffraction, interference, and scattering. As a result, the color appears bright and sometimes changes depending on the viewing angle. For example, peacock feathers do not contain blue pigment, but thanks to microscopic structures, they reflect blue light. Pigments work through chemical composition, while structural colors through the physical structure of the surface.
So, everything that comes to mind as blue—butterflies, peacocks, birds like blue jays, blue spiders, blue sharks—none of them have blue pigment. But there are interesting exceptions. For example, the butterfly obrina olivewing has a natural chemical blue pigment in its wings.
You might ask, what about blue eyes? The trick is that blue eyes don’t really exist. Generally, humans have several important pigments that mix to give new colors. For instance, the main pigment, melanin, determines the color of the eyes (and skin, and hair). It comes in two main types: eumelanin (which can be black or brown) and pheomelanin (which gives reddish-yellow hues). So, blue eyes are those without any melanin. The blue color of the eyes is formed by a combination of structural color and the absence of melanin pigment in the front part of the iris. Here, a phenomenon known as Rayleigh scattering also plays a role, which also accounts for the blue color of the sky. The light scatters in the tissues of the iris, with shorter waves of light (blue and green) scattering more than longer wavelengths (red and green). By the way, it participates in the green color of the eyes too. Humans and animals do not have green pigment. The green color in animals is often the result of a combination of yellow pigment and blue structural coloring. In other words, almost all natural life primarily consists of a combination of black/brown/reddish-yellow (melanin), red (hemoglobin), yellow-orange (carotenoids, bilirubin). That’s why earthy paints—sienna, umber, ochre—are often used in portraits. Speaking of carotenoids – flamingos are actually born gray, but because they eat small red crustaceans containing carotenoids, they acquire a pink color.
Speaking of beautiful butterflies. It turned out that butterflies actually barely see each other as we see them. I mean, they almost don’t see everything clearly. Unlike the human three-color retina (blue, green, and red cones; plus rods) and honeybees (ultraviolet, blue and green photoreceptors), the retina of butterflies usually has six or more classes of photoreceptors with different spectral sensitivities. I attached a picture from the book—how a flower with a butterfly on it map butterfly sees a reed bunting and a Eurasian blackbird. There are convincing reasons to believe that butterfly vision is almost non-existent. But what they do have is very, very specific and probably just one component of the olfactory picture of the world. So all this beauty on their wings—it’s not even for each other. It’s sort of a side effect. That is, evolution selected for large spots, but all sorts of small patterns just went along for the ride.
Oh, also about the perception of colors. I attached a picture of how the Himba people of African ethnicity perceive colors. They don’t immediately notice the blue square on the right side, because their language uses the same word for blue and green. But they have no trouble distinguishing the square encircled, because from childhood they are trained to discern this difference and they have two words designating them in their language.
Anyway, if you’re interested, I’ll keep writing stuff like this. It’s quite simple for me to do, and generally interesting to Google more about what I’ve read than the book goes into.
Hi, I'm Rauf Aliev. 