How Do Chameleons Change Colour? Nanocrystals, Not Pigments

Ask almost anyone how a chameleon changes colour and they will say it is to blend in. It is a lovely idea, and completely backwards. Chameleons mostly change colour to be seen, not to hide. And the way they do it is not by mixing paint, but by tuning tiny crystals in their skin like a living optical device.

Two kinds of colour

Animals make colour in two main ways.

• Pigments absorb some wavelengths of light and reflect others. The melanin that makes our hair dark and the carotenoids that make flamingos pink are pigments.
• Structural colours come from tiny structures that reflect certain wavelengths because of their size and spacing. The shimmering blue of a morpho butterfly or a peacock feather is structural.

Chameleons use both. Their skin contains pigment cells with yellow, red and dark brown pigments, but the dramatic changes come from a structural trick.

The secret: a lattice of nanocrystals

In 2015 a Swiss team led by Jérémie Teyssier and Michel Milinkovitch studied panther chameleons from Madagascar. Under the outer skin they found a thick layer of cells called iridophores. Each iridophore is packed with tiny crystals of guanine, a substance also found in DNA, arranged in a regular three-dimensional lattice.

The spacing between the crystals decides which colour is reflected, much as the spacing of grooves on a CD decides the rainbow you see.

• When the chameleon is calm, the crystals are close together and reflect mainly blue light. Blue combined with yellow pigment above it makes the skin look green.
• When the chameleon is excited, the cells take up water and swell, pushing the crystals apart. The lattice spacing increases and longer wavelengths are reflected: yellow, orange and red.

The researchers confirmed this by bathing pieces of skin in salt solutions that made the cells swell or shrink. The colour changed just as it does in a living animal.

Beneath this layer lies a second layer of larger, less organised crystals. It reflects much of the near-infrared light from the sun, which may help protect the chameleon from overheating.

Why change colour at all?

If not for camouflage, then for what? Research points to several reasons.

Talking with colour

In 2008 Devi Stuart-Fox and Adnan Moussalli compared many species of South African dwarf chameleons. They found that the species with the greatest ability to change colour were those that used bold social displays, not those living in the most varied backgrounds. Colour change seems to have evolved mainly as a way to communicate.

A male confronting a rival flushes bright, contrasting colours. The winner stays bright; the loser quickly turns dark and drab, signalling surrender. Females can show whether they are receptive or not.

Mood and stress

Colour also reflects a chameleon’s state. Frightened, stressed or sick chameleons often turn dark brown or grey. Pet owners learn to read this as a warning sign.

Controlling temperature

Chameleons are cold-blooded and depend on their surroundings for heat. On cool mornings they can darken to absorb more sunlight and warm up faster, then lighten again in the heat of the day.

And camouflage?

Camouflage does play a part, but mostly through a chameleon’s resting colours, which already match leaves and bark. Some species also darken or pale to match the brightness of their surroundings, and they can adjust their colour depending on whether a predator is a bird or a snake. But the spectacular changes are about signalling.

Other remarkable features

Colour is not the only trick. A chameleon’s eyes sit in cone-shaped turrets that move independently, giving almost all-round vision. When it spots an insect, both eyes focus on it to judge distance. Then the tongue fires out at extreme acceleration, sticky tip first, and in some species reaches more than twice the length of the body.

There are more than 200 species of chameleon, most of them in Africa and Madagascar. The common chameleon also lives around the Mediterranean, including parts of southern Turkey.

In short

• Chameleons change colour mainly with structural colour, by tuning a lattice of guanine nanocrystals in skin cells called iridophores.
• Calm: crystals packed tightly reflect blue, which looks green with yellow pigment. Excited: crystals spread out and reflect yellow, orange and red.
• The main purposes are communication and temperature control, not hiding.
• Dark, dull colours often signal stress, cold or defeat.
• A deeper crystal layer reflects infrared light and may protect against overheating.