How and why do chameleons change colour?
I've always thought, like many of my students, that chameleons change colour in response to their environment. However, after doing some research it seems they change colour based on temperature, light intensity, and mood. If this is true how does the colour change always seem to match that of their environment? Could you explain how this process works at a molecular level and how this produces the change in colour?
It's nature's example of Joseph and his Technicolour Dream Coat, the chameleon. They're just phenomenal. There is this myth that chameleons change colour to blend in with their surroundings, but this is actually not true. Most of the reason chameleons change colour is as a signal, a visual signal of mood and aggression, territory and mating behaviour.
The way that chameleons actually do this is really molecular - they're molecular masterminds, really. If you look at the skin of a chameleon, you find that they have several layers of specialised cells called chromatophores and these are cells that can change colour. On the outer surface of the chameleon, the skin is transparent and just below that is the first layer of these cells, and they contain various pigments. These are xanthophores, containing particular specialised pigments that have a yellow colour. Beneath that are pigment cells which are called erythrophores which have a red colour in them. Beneath that, another layer of cells called iridiphores have a blue coloured pigment called guanine, which is actually also used in making DNA. And underneath that is another layer of cells called melanophores which have a brown pigment - melanin - in them. Now, how does the chameleon change colour? Well those chromatophores are wired up to the nervous system. They are also sensitive to chemicals that are washing around in the blood stream of the chameleon. What happens is that the colours are locked away in tiny vesicles, little sacs inside the cells that keep them in one place, so the cells don't look coloured. But when a signal comes in from the nervous system or from the blood stream, the granules or vesicles can discharge, allowing the colour to spread out across the cell, and this alters the colour of the cell. It's rather like giving the cell a coat of paint. By varying the relative amount of activity of the different chromatophores in different layers of the skin, it's like mixing different paints together. So if you mix red and yellow, you get orange for example, and this is how chameleons do this. They mix different contributions of these chromatophores. It's a bit like on your television screen. When you mix different colours together on the screen to get the colour that the eye ultimately perceives and so, that's how the chameleon changes colour, and usually does so to convey mood.
So a calm chameleon is a pale greeny colour. When it gets angry, it might go bright yellow, and when it wants to mate, it basically turns on every possible colour it can which shows that it's in the mood. This is not unique to chameleons. Other animals also have these chromatophores. Cuttlefish are another very elegant example of how this works. So it's not so much to do with camouflage. It's more to do with communication.