Geometry, physics, and the chameleon’s change of colour

For many years, scientists thought that the chameleon’s ability to change colors was due to its capacity to transfer pigments around inside its cells; however, it appears that this is not the case.

Instead of pigments, the little animals rely heavily on geometry and physics. To understand this, you must first understand how light and our sense of color work.

Pigments, such as those found in our own skin, usually work by absorbing all but one hue of visible light. The color we perceive is caused by the one wavelength that is not absorbed. To put it simply, a purple pigment absorbs the majority of visible light wavelengths and allows just the purple wavelengths to bounce off its surface.

Many vertebrates can rapidly change color for camouflage, communication, and thermoregulation, but these so-called physiological color changes are generally mediated by changes in skin brightness (that is, diffuse and/or specular reflectivity) caused by pigment-containing organelles, particularly melanosomes, dispersion/aggregation within dermal chromatophores.

Rapid active tuning of skin hue, on the other hand, has been documented in just a few species and typically includes structural, rather than pigmentary, components, such as multilayer nano-reflectors with alternating high and low refractive indices that create light wave interference.

Chameleons use specialized cells called iridophores, in order to present different appearances.

Combining histology, electron microscopy and photometric videography techniques with numerical band-gap modelling, here we show that chameleons have evolved two superimposed populations of iridophores.[..] This combination of two functionally different layers of iridophores constitutes an evolutionary novelty that allows some species of chameleons to combine efficient camouflage and dramatic display, while potentially moderating the thermal consequences of intense solar radiations.

Color change and iridophore types in panther chameleons. (a) Reversible colour change is shown for two males (m1 and m2): during excitation (white arrows), background skin shifts from the baseline state (green) to yellow/orange and both vertical bars and horizontal mid-body stripe shift from blue to whitish (m1). Some animals (m2 and Supplementary Movie 2) have their blue vertical bars covered by red pigment cells. (b) Red dots: time evolution in the CIE chromaticity chart of a third male with green skin in a high-resolution video (Supplementary Movie 3); dashed white line: optical response in numerical simulations using a face-centred cubic (FCC) lattice of guanine crystals with lattice parameter indicated with black arrows. (c) Haematoxylin and eosin staining of a cross-section of white skin showing the epidermis (ep) and the two thick layers of iridophores (see also Supplementary Fig. 1). (d) TEM images of guanine nanocrystals in S-iridophores in the excited state and three-dimensional model of an FCC lattice (shown in two orientations). (e) TEM image of guanine nanocrystals in D-iridophores. Scale bars, 20 μm (c); 200 nm (d,e).

Structural color is a system that manipulates geometry on the nanoscale by bending certain wavelengths of light. Scientists eventually discovered two layers of skin with iridophores (these are the cells that utilize structural color). These were packed with small guanine nanocrystals organized in a lattice pattern on the top layer. The formations on the chameleons contain exact spacings between each crystal, which permits the creatures to emit varied hues.

Blues are at the short end of the light spectrum, whereas reds are at the long end. As a result, when the gaps between the guanine crystals are small, the bluer wavelengths are reflected. The longer wavelengths are reflected when the crystals are placed further apart. Chameleons alter their appearance by combining these substances with pigment cells.

In-vivo skin colour change in chameleons is reproduced ex vivo. (a) TEM images of the lattice of guanine nanocrystals in S-iridophores from the same individual in a relaxed and excited state (two biopsies separated by a distance <1 cm, scale bar, 200 nm). This transformation and corresponding optical response is recapitulated ex vivo by manipulation of white skin osmolarity (from 236 to 1,416 mOsm): (b) reflectivity of a skin sample (for clarity, the 19 reflectivity curves are shifted by 0.02 units along the y axis) and (c) time evolution (in the CIE chromaticity chart) of the colour of a single cell (insets i–vi; Supplementary Movie 4); both exhibit a strong blue shift (red dotted arrow in b) as observed in vivo during behavioural colour change. Dashed white line: optical response in numerical simulations (cfFig. 1b) with lattice parameter indicated with dashed arrows. Note that increased osmotic pressure corresponds to behavioural relaxation; hence, the reverse order (white arrowhead in CIE colour chart) of red to green to blue time evolution in comparison with Fig. 1b. (d) Variation of simulated colour photonic response for each vertex of the irreducible first Brillouin zone (colour outside of the Brillouin zone indicates the average among all directions) shown for four lattice parameter values (from Supplementary Movie 5) of the modelled photonic crystal. L-U-K-W-X are standard symmetry points.

The organization of iridophores into two superposed layers constitutes an evolutionary novelty for chameleons that allows some species to combine efficient camouflage with spectacular display.

Certainly, some chameleon species live in open settings where they are exposed to a lot of sunshine. Panther chameleons and veiled chameleons (researched here) live in dry, hot settings (Northern Madagascar and Yemen, respectively) and are highly exposed to sunlight, thus the 45 percent reduction in solar absorption produced by D-iridophores is likely to be beneficial for survival.

Photonic crystals cause active colour change in chameleons, Jérémie Teyssier, Suzanne V. Saenko, Dirk van der Marel & Michel C. Milinkovitch

Published: March 2015

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