Why Is Blue So Rare In Nature?


Originally published at: https://boingboing.net/2018/01/16/why-is-blue-so-rare-in-nature.html




I always thought it was because animals don’t like to be eaten, but I guess there can be other reasons too.


Blue is super common in the animal world. All the other species can see it. Just not humans.

Very anthropocentric post.


I have hated this pop science anecdote since I was a boy. First, it isn’t a claim about blue coloration, which is quite common, it’s a claim about blue pigment, which is just one of many ways life forms make color. Second, it isn’t even true. Anthocyanin, the most common plant pigment after chlorophyll, is literally “flower blue”.



Ultraviolet is not blue. It is sort of like blue but if you just say close enough you can say green is blue as well. It’s kind of silly article but that is not why. Also “all the other species”? Hardly. I bet most species of macroscopic animals don’t see colors at all, much less most species. Humans actually have pretty good color vision.


Well, yeah, but it is more mis-titled then anything. It is a rare pigment in animals, right? Most animal ( and plant, I think) coloration is pigment, right? So it is a fair question to ask why? The video, of course does not answer that question, just notes it. I think the answer to HOW animals get blue is pretty interesting, though I would also like to know why it is such a rare pigment.



Yeah, this is another of those “everybody knows it’s true” claims that really isn’t that true. No blue in Nature? Sky is blue, that’s half your field of vision on a nice day, and water’s blue, so that the other half on two-thirds of the planet’s surface. Oh, you didn’t mean Nature, you meant plants and animals? OK. Flowers and berries and beetles and butterflies and birds are blue (and I wish “flowers” started with a b), so that’s all over the place, actually. Oh, you didn’t mean animals and plants you meant pigments in animals and plants but not structural colorations? Then say what you mean, and still: anthocyanins are all over the place.

But not exactly that all over the place.

The etymology is fine, but “anthocyanin” isn’t a single chemical compound. The term refers to a wide range of chemical structures with slightly different colours, and the colours vary with acidity: they are as likely to appear as red or purple as they are to appear blue. And they are not the most common pigment after chlorophyll. If they were, leaves would turn purple in the fall. Do a quick paper chromatography experiment on most ground up plants and you’ll see greens (chlorophylls), oranges and reds (carotenes), and yellows (xanthophylls) all in far greater abundance than anthocyanins. Unless you choose a red cabbage or something.


False. Plants and bacteria cannot see anything. Most animals are monochromatic and cannot see blue at all. Many species are dichromats and see blue no better than humans. Some species (birds, mostly) are tetrachromatic and can see past blue and violet and into the ultraviolet.


Short answer: physics and molecular orbital theory.

Long answer:
Pigment based colours work like this. All molecules have electrons, those electrons lie in their lowest-energy states, but they can be excited to higher energy levels by photons. If a photon bumps an electron up to a higher level, that photon of exactly that energy gets absorbed, but all other photons of all other energies pass through. You observe what’s left. If a pigment absorbs violet photons, you see the colour complementary to violet, which is yellow. If a pigment absorbs both violet and red photons, you’ll see the colours in between: OYGCB, which you’ll perceive as green.

For you to observe blue, the pigment needs to absorb orange, but no other colours in the higher-energy range of the visible spectrum. No yellow or green or cyan or violet absorption, or else the perceived colour is skewed. So it has to absorb one specific type of photon, and excite electrons by about 2 eV, but there cannot be any other energy levels above it: no other places to excite the electron at 2.2 or 2.4 or 2.5 or 3.0 eV, just the one excited state at 2eV and nothing else.

That’s very hard to do. Most molecular structures will give you lots of excited states that are relatively close together. It requires some unusually restrictive structural features in the molecule to limit the excited states like that. And it’s difficult for Nature to synthesize those molecules.

That’s not a problem for other colours. You want yellow? Fine. Absorb violet. And absorb everything above it, too, but that’s UV and humans can’t see it. You want green? Fine? Absorb red and also absorb violet, leave the green. Multiple excited states at 2.0 and 3.0 eV, no problem. But just absorb orange and nothing else? Tricky.



The cool thing about blue is not how (supposedly) rare is is as a naturally-derived pigment (1.3 billion pairs of blue jeans each year would like a word with you) but how many different ways there are to make it.

  • the daytime sky
  • water
  • indigo and anthocyanins and other organic pigments
  • sapphires
  • Prussian blue
  • lapis lazuli
  • cobalt blue
  • blue beetles and morpho butterflies
  • dinoflagellate marine plankton after mechanical disturbance

Each of those blues is due to a different physical process in the object to shoot mostly blue photons at your eye.



Awsome, thanks. That would have been cool to see in the video with nice info-graphics and all. I suppose animals that can see in to UV have trouble with yellow.