Why can't we have blue hair?

On January 6 this year, China Post issued stamps for the Year of the Rabbit, and one of them caused great controversy. Many friends said they couldn't appreciate it. Later, I took a look at it and felt that there was something wrong. Later, my son said, "Dad, are there really blue rabbits? Why haven't I seen them?"

China Post issued the Chinese Zodiac stamp for the year of Guimao. Image source: CCTV News

If we put aside the imagination in artistic creation and only look at the reality, then we can say that there is no blue-haired rabbit ("Rainbow Cat and Blue Rabbit" is a cartoon!). Not to mention rabbits, even if we put them in the big family of mammals, it seems that there is no species that is blue. So why is this?

01

A blue mammal?

It is rare indeed.

In fact, even if we don’t mention those colorful arthropods, there is no shortage of blue species among vertebrates. Just thinking about it can list a lot of them, such as Dory in "Finding Nemo": yellow-tailed surgeonfish, and Alan in "Rio": small blue macaw. In addition, there are cobalt blue poison dart frogs, white-lipped tree lizards, etc.....

Cobalt blue poison dart frog, copyrighted image, unauthorized reproduction

But it is really hard to think of any mammals with blue fur, except for the blue rabbit on the stamp, the kitten in "3000 Questions of the Blue Cat", the little mouse in "Blue Mouse and Big Face Cat", etc. Unfortunately, those three are fictional creatures and do not exist in reality.

Some friends may ask: "Aren't there blue whales?" Yes, the blue whale does have the word "blue" in its name, but to everyone's disappointment, the "blue" of the blue whale is not in line with its name. The blue whale's skin is actually lead gray or steel gray , but due to the refraction of light by seawater, our eyes see blue whales in the water from certain angles and find that they appear steel blue or other blue. There is a kind of pet cat called "blue cat". If you look closely , its fur is also gray, and the "blue" in its name comes from its origin .

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02

Blue hair

Why is it so rare?

Mammals, or beasts, except for some sea mammals, are mostly covered with fur. The color of these animals is mainly the color of their hair, and their fur color and texture are not monotonous. For example, cats such as cheetahs, leopards, and tigers have very complex patterns, and canines such as foxes, jackals, and African wild dogs have also evolved a variety of fur colors, but even so, they have not seen any blue species. Why is this?

To understand this question, we first need to understand how the color of mammalian fur is produced.

The color of mammalian fur that we see is expressed by the pigments present in the fur or medullary cells . Mammals have only two pigments, eumelanin and pheomelanin. Eumelanin can appear black and brown , pheomelanin can appear red and yellow, and can appear white under the influence of the albino gene, which makes the fur color of mammals mainly composed of black, brown and white.

This is why most mammals, although they have complex spots and stripes, and fur colors such as orange, yellow, gray, brown, and yellow, do not have very bright and vivid body colors like reptiles and birds.

03

These mammals

There is really blue on the body

Some primate species have blue or green colors that are rarely seen in mammals. For example, male green monkeys have two bright blue-green "testicles", and male mandrills also have bright blue cheeks and buttocks.

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How do they appear blue in the absence of corresponding pigments? In the skin of these animals, collagen fibers have neat and orderly arrangement differences, which can produce structural colors . The tiny unevenness on the surface causes the light shining on it to reflect or scatter, thus producing a special color effect. This color is called "structural color". The feathers of many birds are bright blue for the same reason.

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The hair of mammals is cylindrical and its arrangement is far less regular than that of bird feathers . The scales on the hair also grow randomly. As a result, light cannot be reflected and scattered regularly, making it difficult to produce blue structural colors like bird feathers.

So what is the function of the blue color on green monkeys and mandrills? It turns out that these primates generally use bright colors as a display of male ability and physical fitness . The brighter the color, the better the body, the stronger the male ability, the more favored by females, and the higher the status in the group.

In short, there are indeed very few blue mammals in real life, but if it is an artistic creation, there is naturally no restriction, you can paint it any color you want.

Author: Erzhu

Reviewer: Huang Chengming, Institute of Zoology, Chinese Academy of Sciences

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