Common sense tells us that chameleons have green skin. But science tells us that chameleons have no green pigments in their skins at all. So, why is common sense so wrong?
To answer that we need to look at what a chameleon does have in its skin and that’s more complicated than you would think.
Chameleon skin has many layers
There are several layers of cells in the skin. The outermost layer of skin is transparent, so that whatever goes on underneath it can be seen through the top layer.
Below the transparent layer are cells that contain yellow and red pigment. These are known as ‘chromatophores’. These pigments are what colour areas of the chameleon’s skin yellow and red. Cape Dwarf Chameleons have mostly yellow chromatophores. Chameleons from Madagascar have more red chromatophores.
But chameleons are not predominantly yellow (or red), so we need to go further into the skin.
Below the chromatophore layer is a second layer of cells called ‘iridophores’. These cells contain tiny crystals of a chemical called ‘guanine’. These crystals are so small that they are referred to as ‘nanocrystals’ (‘nano’ meaning that the size is 10-9 or one billionth of a metre).
The role of these crystals was discovered by a team of scientists headed by Prof M Milinkovitch from the University of Geneva.
They hit upon the importance of the crystals when they used an electron microscope to look inside the iridophores. From whichever angle they looked, the crystals formed an incredibly neat, regular pattern – just the sort of arrangement that creates colours by reflecting different wavelengths of light. This is similar to raindrops reflecting the different wavelengths of white light to form a rainbow.
So Prof Milinkovitch and his colleagues correctly proved that these crystals explain the different colours that might colour a chameleon’s skin. But the crystals also explain how chameleons change colour.
The researchers found that the distance between the crystals was critical. Most light will go right through them, except for very specific wavelengths. If the distance between the layers of crystals is small, it reflects short wavelengths, like blue; if the distance is large it reflects longer wavelengths, for example, red.
This gives us a clue to chameleon colour. Chameleons have yellow pigment in the chromatophore layer and a layer of iridophores with crystals that can reflect blue light. We all know yellow and blue make green and hence the usual green colour of the chameleon.
The colour pattern on a chameleon is dependent on the arrangement of the different chromatophores into these patterns.
How does a chameleon change colour?
It was found that the mental state of chameleons changes the distance between the iridophore crystals. When a chameleon is relaxed, the crystals in the iridophores are tightly packed and they reflect blue light. The blue light shining through the yellow chromatophores produces green light – the colour we associate with chameleons.
When a chameleon is agitated, for example by the presence of a predator or an unwelcome male, the crystals are excited, spread apart and reflect longer wavelengths towards the red spectrum – and hey presto, the chameleon changes colour.
Another crystal layer
Not only do chameleons change colour, they can also adapt to temperature.
Below the layer of crystal-packed iridophores is another layer of larger cells that contain crystals, but these crystals are much more randomly packed. Because that higgledy-piggledy structure reflects near-infrared light particularly well, it is believed that it might serve to reflect the sun’s warming rays and keep the chameleons cool.
A fourth layer – melanophore
The story doesn’t end there. There is another fourth layer of cells called ‘melanophores’, which contain black melanin pigment. The melanophores play a crucial role in colour change. They are large, star-like cells with long ‘arms’ that extend towards the skin’s surface. A chameleon can change from dark to light through the movement of ‘packets’ of melanin pigment within the melanophores. When melanin pigment is aggregated within the centre of the cell, the skin appears very pale, whereas when it is dispersed through the arms of the melanophores towards the skin’s surface, the animal appears dark.
Chameleons will not only become dark in response to stress, but also to regulate body temperature. They are cold-blooded reptiles, so they go dark when they want to absorb heat from the sun, and become lighter in colour to reflect heat.
It is a myth that chameleons change colour to match their background. Blind chameleons change colour even though they can’t see their own background. Colour changes are much more dependent on the mood of the animal. It is now accepted that chameleons change colour to stand out (e.g. when confronting a rival) and not to blend in.
So next time you see a chameleon, ponder a moment on how the chameleon got its green – and other colours too.
About the Author
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