How Can a Chameleon Change Its Colour?
Not With Pigment, but by Shifting a Crystal Pattern
When a chameleon gets excited, startled or wants to impress, its colour changes in moments. You might think it just shifts coloured grains around inside its skin. In fact, the main way it switches colour is not pigment but tiny crystals, arranged in regular layers in the skin.
Octopuses and squid can also change their colour and pattern quickly. Their method is thought to be this: cells holding grains of pigment spread out or shrink, changing how much colour you see.
Chameleons were long explained the same way, but research has shown that some of their colour changes can't be explained by pigment alone.
Besides pigment, a chameleon's skin is thought to contain layers of tiny crystals of a substance called guanine, set at regular spacings.
When the skin's tension changes and the crystals move further apart or closer together, the colour (wavelength) of the light that is reflected and strengthened changes too.
Let's look at these two ideas in turn: the regular crystal layers, and how changing their spacing switches the colour.
The crystal pattern strongly reflects only certain colours
When light hits regularly spaced crystal layers, it is reflected again and again between the crystals, and only light of a particular wavelength (colour) reinforces itself and comes back. Light of other colours gets out of step and cancels out. This is quite different from pigment, which absorbs certain colours and reflects the rest. It is called "structural colour". The basic physics is thought to be the same as the regular-structure colour explained in our article on why opal shimmers in rainbow colours.
Skin tension controls the crystal spacing
A chameleon is thought to change the spacing between the crystals themselves by relaxing or tightening its skin. When the skin is relaxed and calm, the crystals are packed closely. The spacing is narrow, so short-wavelength blue and green light is strongly reflected. When the animal is excited or displaying and the skin tightens, the crystals move apart, and long-wavelength light, yellow, orange and close to red, is strongly reflected.
Below the crystal layers that make the structural colour, there is also thought to be a layer of cells containing dark pigment (melanin). This layer is believed to darken the body and help regulate body temperature, so pigment control and crystal-based structural colour each play their own part, and both are at work.
It is a living crystal whose spacing can be changed.
What you can check yourself
- Hold the underside of a CD or DVD (the recorded side) up to the light
- Tilt it and watch the rows of fine grooves shine in rainbow colours
- Notice that as the viewing angle changes, the effective spacing the light meets changes, and so does the colour that is strengthened
It helps to picture the CD's groove spacing as something the chameleon actively changes through skin tension.
Summary
A chameleon can change its colour so much not only by moving pigment, but because it changes the spacing of tiny crystals, arranged in regular layers in its skin, through skin tension. Narrow spacing gives blue-green, wider spacing gives yellow or orange. The crystal pattern itself works as a dial for choosing the colour.
A chameleon is not "repainting" itself.
It is moving the scale of crystals that bounce light back.
The same idea, that a regular structure creates colour, is also explained in our opal article.
For those who want more: terms, numbers and links to textbooksFrom middle-school science to active research, each part is labelled with its level
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school basic physics
- High school+High-school physics, or advanced or sidebar material in textbooks
- UniversityUniversity-level content (biophysics) not taught in high school
- ResearchTopics researchers are still investigating, not yet settled even at university level
Middle schoolTerms: words used about chameleon colour change
- Structural colour: colour produced not by pigment but by light reflecting from a regular microscopic structure.
- Iridophore: a cell in chameleon skin that contains layers of crystals.
- Guanine: the name of the substance that makes up these crystals.
High schoolChecking with a formula: how does colour change as the crystal spacing grows?
Using the same relation as in the opal article, we can estimate how a change in crystal spacing changes the colour of the reflected light.
Reinforced wavelength ≒ 2 × crystal spacing × refractive index of the surrounding material
| Refractive index | About 1.4 for skin tissue (a rough guide value) |
| Crystal spacing (calm state) | About 130 nm (a rough guide value) |
| Crystal spacing (excited state) | About 180 nm (a rough guide value) |
| Wavelength, calm state | 2 × 130 × 1.4 ＝ 364 |
| Wavelength, excited state | 2 × 180 × 1.4 ＝ 504 |
| Result | The wavelength changes from about 364 nm to 504 nm |
As a rule of thumb, wavelengths of about 450 to 495 nm correspond to blue, 495 to 570 nm to green, and 570 to 590 nm to yellow. The calculation says that widening the crystal spacing by only about 50 nm moves the reflected colour from blue-green towards yellow. A tiny change, measured in nanometres, produces a difference in colour that the eye can clearly see.
High school+How it differs from octopus and squid colour change
Many octopuses and squid are thought to change colour by using muscles to stretch or shrink chromatophores, sac-shaped cells that contain pigment. This changes how the pigment itself looks. A chameleon's crystal-based colour change differs in principle, because it changes the wavelength of the reflected light itself, not the pigment.
UniversitySkin structural colour in biophysics
In biophysics, researchers study the mechanical means by which the crystal lattice inside an iridophore changes its spacing. One explanation, supported by observations such as electron microscopy, is that stretching the skin changes the symmetry and arrangement of the lattice itself, and so changes the reflected wavelength.
ResearchWhat is still unclear
- The detailed nerve- and muscle-level control of the skin stretching and shrinking that changes crystal spacing is still being studied.
- Comparative studies are also under way on how crystal arrangement and the range of colour change differ between chameleon species.
- Materials scientists are also working on artificial materials whose colour can be switched electrically (such as structural-colour displays), applying this mechanism.
Even a single piece of chameleon skin holds a rich, still-active research topic where biophysics and optics meet.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: properties of light | Structural colour, basic terms such as iridophore |
| High school | Basic physics: wave interference (advanced) | Calculating wavelength from crystal spacing |
| High school+ | Biology: animal behaviour (advanced) | Comparison with octopus and squid chromatophores |
| University | Biophysics | How the crystal lattice changes mechanically |
| Research | Biophysics and materials science (ongoing research) | Working out the control mechanism, comparing species, structural-colour materials |
- Research on structural colour in chameleon skin by Teyssier, J. et al., published in a scientific journal.
- Explanations of the link between iridophores and structural colour in biophysics materials.
- General descriptions of interference, diffraction and the Bragg condition in optics textbooks.
※ The crystal spacings and wavelengths are rough guide values to help explain the mechanism. Actual values are said to vary with species, individual and state.
※ This article is a general-audience science explainer. The figures given are rough estimates to help explain the mechanism. Actual values are said to vary with species, individual and state.