Gray Cats Are Pulling Off An Optical Illusion

You might have heard that the sky – which is actually transparent – looks blue because of an optical illusion.

Some air molecules scatter parts of sunlight all across the atmosphere. When the visible violet and blue light waves carom off enough oxygen and nitrogen to reach our eyes, it looks to us like a solid blue color coming from every part of the sky.

Believe it or not, the blue-gray color in cat fur comes from a similar process.


In clear air, sunlight is white. Here are the other visible color waves it contains (most of the blues and violets are off somewhere, bouncing around the sky). Source

Of course, there are some major differences. In addition to the most obvious one – a cat is not the sky – light scattering on cat fur is done by clumps of a pigment called melanin, not gas molecules.

And since fur is a translucent glassy material, not a planetary atmosphere, there’s also a little other stuff going on

But we’re just talking about colors..

Melanin is one of the reasons why mammals and birds have different colored hair, skin, and eyes. Our bodies manufacture it in two forms:

  • Eumelanin: Browns and blacks
  • Pheomelanin: Yellows and reddish colors

Most hair and skin colors are complex combinations of these two polymers.

In people, eumelanin and pheomelanin mix in over ten different ways to give us every natural hair shade from black to light blonde.

Which seems a little strange, since hair is made of keratin–the same material as your fingernails. Like them, it is clear and a little shiny, like glass.

Nevertheless, it’s the optical qualities of keratin (refractive index, mostly), plus the amount of melanin present, and the geometry of the hair’s surface that give your hair color.

Here’s an oversimplified version of how it works:

  1. When light hits your hair, the outer surface reflects some of it back without any change in color. If it’s daylight, that color is white.
  2. The rest of the incoming light penetrates into the hair shaft, where the melanin particles are. Those particles scatter the light, causing a diffuse reflection of eumelanin and pheomelanin pigment in the keratin.

The surface of the hair shaft is scaly, sort of like a spruce cone, so both of the above reflections come out in slightly different directions. Thanks to this dispersion, reflection #1 is the same color as whatever light went in. Reflection #2 is from the pigments, giving us whatever hair color is appropriate to that combination of eumelanin and pheomelanin.

Since people and cats are both mammals, we have the same basic type of hair, but of course each group has followed its own evolutionary path. Human whiskers, for example, are very different from a cat’s.

African wildcats are the domestic cat’s ancestor, and they are tabby cats– a nice camouflage pattern.


An African wildcat in a zoo. Sonelle, CC BY-SA 3.0.

The light background is called agouti; these hairs have a bluish base, a dark tip, and a yellow band in between (all of these colors thanks to melanin, of course, and put into place by the agouti signalling protein gene).

Tabby stripes are hairs without the yellow band. The exact pattern of striples, spots, or blotches is controlled by different genes.

All domestic cats used to be tabbies, but around 2,500 years ago, perhaps in the eastern Mediterranean region, a non-agouti mutation happened. The yellow band disappeared and each hair was now a solid color.

Over time, other mutations happened, including, not necessarily in this order, black pigment, maximum pigmentation, and dense pigmentation.


The result of those mutations. Source

At some point the dense pigmentation gene (D) mutated into a dilution gene (d). Now, instead of lots of melanin granules distributed equally throughout the hair, there can be big clumps of granules here and there in each strand.

Of course, this changes the hair’s optical properties and the overall look.

Kitty still has the melanin levels of a black cat, but there are gaps in between these clumps of dark pigment. Reflection #2 contains some white light now, just like Reflection #1. This dilutes the overall color that we see to gray.

This happens with other solid colors, too. The dilution gene makes orange fur look cream-colored. It also lightens solid brown fur into fawn or a warm soft gray that cat breeders call either lilac or lavender.


A lilac British Shorthair and a white/sort-of-fawn Easter Bunny. Source

Cat fanciers have a special reason to be grateful for the dilution gene. Science has shown that black cat fur actually comes in a variety of shades, but we can’t see these very well. Thanks to dilution, a cat breeder can develop these shades (now blue-gray) and stabilize each one into its own feline breed.

Featured image: Gonzalo Díaz Fornaro. CC BY-ND 2.0


Ito, S., and Wakamatsu, K. 2003. Quantitative analysis of eumelanin and pheomelanin in humans, mice, and other animals: a comparative review. Pigment Cell and Melanoma Research. 16(5), 523-531.

Lyons, L. 2015. DNA mutations of the cat: The good, the bad, and the ugly. Journal of Feline Medicine and Surgery. 17(3):203-219.

Marschner, S. R.; Jensen, H. W.; Cammarano, M.; Worley, S.; and Hanrahan, P. 2003. Light scattering from human hair fibers, in ACM Transactions on Graphics (TOG). 22(3): 780-791). ACM.

Prieur, D. J., and Collier, L. L. 1984. Maltese dilution of domestic cats: a generalized cutaneous albinism lacking ocular involvement (abstract only). Journal of Heredity. 75(1), 41-44.

— 1981. Morphologic basis of inherited coat-color dilutions of cats (abstract only). Journal of Heredity. 72(3), 178-182.

Saini, R. 2011. Optical detection of hairs. International Journal of Trichology. 3(2):128-129.

Todd, N. B. 1977. Cats and Commerce. Scientific American. 237:100-107.

Wright, M., and Walters, S. 1980. The Book of the Cat New York: Summit Books.


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