Monday, August 2

How do chameleons change color?



For a long time it was defended that the ability of chameleons to exhibit color changes in a rapid and complex way was exclusively due to a battery of pigments that they contain in specialized cells called chromatóforos.

However, for some time, it has been possible to verify that the process is much more complex and that structures known as nanocrystals also play a fundamental role in it.

The outer surface of the skin of these reptiles is transparent, below it is a first layer of xanthophoric cells with yellow pigments, a little further down there is another layer of erythrophorous cells, which contain the red color, which is followed by a third of iridophorous cells with bluish pigment and, finally, a layer of melanophorous cells with brown pigment.

Chromatophore pigments are not free in the cell cytoplasm, on the contrary, they are contained in small vesicles, which explains why these cells do not adopt any type of coloration.

These pigments are connected to the nervous system and are especially sensitive to certain chemicals that arrive through the bloodstream. When the agreed signal arrives, the vesicles discharge the pigments and allow the color to spread through the cells, altering the color of the cell. It’s kind of like coloring them with a coat of paint.

If the activity affects different chromatophores, different shades are achieved, for example, if red and yellow are activated, the resulting color is orange.

All that glitters is not chemistry

In addition to this chemical process, these reptiles are capable of exhibiting the so-called ‘structural colors’ and they do so from an intracellular and self-organizing optical system, in which optical inference is the main actor.

Cells are made up of many transparent guanine nanocrystals, of different shapes, sizes and organizations, which act like photonic crystals, which have a variable refractive index both in space and time.

Nanocrystals have the property of reflecting different wavelengths, for example, when the animal is relaxed the nanocrystals are close together and reflect short-wave light (blue), while when the chameleon is excited the crystals separate and the light that is reflected is long wave (yellow, orange or red).

The end result of skin coloring is the combination of light reflected from pigmented surface cells and light reflected from nanocrystals from deeper cells.

Thus, for example, when we see that the chameleon adopts a greenish coloration, it is because the blue light of the nanocrystals has been added to the yellowish light of the pigments.

Chromatic changes occur in relation to mood, environment and temperature, in such a way that it has been shown that in relaxation situations the reptile exhibits a pale greenish color, but when it is in a stressful situation or if pretends to mate the dress the skin turns bright yellowish.

M. Jara
M. Jara

Pedro Gargantilla is an internist at the Hospital de El Escorial (Madrid) and the author of several popular books.

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