What causes the colors we see in animals?

Animals are showing a wonderful range of colors which make them look very beautiful, and also helps the animals in ways of survival, and in attracting mates. A lot of insects, reptiles and especially birds tend to show many different and wonderful color combinations, but how do they get or create this wide range of colors?

Colors in animals can be created by two different mechanisms, one is by producing pigments, the other on is by producing specific structures that interact with light in a special way that results in different colors.

Pigments

Pigments are chemical substances that individual animals usually can produce themselves. They have genes that code for different precursors and proteins, that are combined in manners that create very specific substances that can absorb different wavelengths. Humans and close relatives see light in the specter from 400 nm to 750 nm, and this is called the visible specter, but other animals can sometimes see shorter and/or longer wavelengths. When light hits a pigment, the pigment will absorb a portion of the light, and reflect the rest. Depending on which portions of the visible light that are reflected from the pigment , we will perceive a certain color. If all light in the visible specter is reflected, we will perceive an object as white, opposite of this is black, when all wavelengths in the specter are absorbed an nothing reflected. Some chemical substances will as an example absorb light in the green and blue part of the specter and reflect most of the red and orange specter, which might make an object look reddish. Different chemical substances put together might absorb different wavelengths, and therefor will the composition of different substances altogether dictate which color we might perceive, depending on the reflection. Different compositions might give different colors. In the animal kingdom melanin is the pigment that is most abundant, but you might find other pigments as well, all though less common. Melanin give rise to colors like black, brown, and reddish brown . Lots of animals show colors that are a result of pigments from their diet, like carotenoids that give animals a yellow, orange and red color, and flavonoids is another example. Birds are a good example of animals that gain red and pink from their diet, this colors are not produced, so to get their beautiful plumage, they need to eat certain types of food that is rich in this pigments. Pigments are usually found in skin and underlying tissue, but can also color gut content, fur, and feathers. Pigments are usually causing the warm colors you find in animals, like red, orange, and yellow, but it is possible to find green and blue pigments as well.

Illustration 1: This lion looks brownish due to melanin in the fur, were different amounts and different types of melanin can cause small changes in the wavelengths that reach the observer, and hence give slightly different colors.

Illustration 2: This finch has a beautiful red color due to pigments from its diet. It consumes berries with carotenoids that give it the strong color.

Structural colors:

These colors are based on interactions between white light and arrays on or in materials. Here is the architecture of the material more important than the chemical makeup of material. Structural colors are due to reflection, refraction, diffraction and scattering of light, but never absorption. Here it is often important that the material are structurally stable and stiff, and are often based on non living materials. Bird's feathers that is showing green and blue colors are often due to specific structures and architectures in the feathers that interact with light in a special way. Structural colors are usually cool colors like, blue, green, violet and ultraviolet. Some times a combinations of pigments and structural colors are used to create new color effects.

Some examples off effects that can cause structural colors: (there are many more effects than explained here, I just talk about two possible structural effects that create different colors)

Scattering effects: Here white light will encounter a cloud or array of molecules, particles or other structures. What happens is that wavelengths will be spread in different directions, including in the direction of the observer. Depending on the wavelength that goes in the observers direction, the observer will see a specific color. Normally, if the structure that scatters the light is bigger than 700 nm, the color perceived will be matte white. A smaller structure around 400 nm will scatter more of the short wavelengths, and less of the long wavelengths, which will only pass through.

Illustration 3: This picture show how light of short wavelengths are scattered while longer wavelengths are just passing through. The scattered light here can as an example be wavelengths in the blue part of the scale, and can then be perceived as Tyndal blue that you find in feathers and in blue eyes.

Interference: Here white light is separated when it reaches a structure, and is then brought back together. When the light does so, some rays will have travelled a longer distance than other rays, and as a result some wavelengths will be in phase and reinforced, while others will be out of phase and chancel out, and this is what gives the brilliant iridescent.

Illustration 4: These hummingbirds are showing structural iridescent colors, where the light is refracted, and depending on the angle of the bird and the observer, the color might change a bit, due to different wavelengths being reflected back after refraction.

References:

Resh, V &Carde, R(eds) 2009,Encyclopedia of insects, second edn, Elsevier Science & Technology, Chicago.

Illustration 1: Viewed 20 May 2014<http://www.webexhibits.org/causesofcolor/7I.html >

Illustration 2: Viewed 20 May 2014<http://www.webexhibits.org/causesofcolor/7I.html >

Illustration 3: From Resh & Carde 2009

Illustration 4: Viewed 20 May 2014<http://www.birds.cornell.edu/AllAboutBirds/studying/feathers/color/document_view >