Transient hues

under my skin The skin of the male chameleon contains two layers of a mesh of minute crystals whose orientation and separation affects the shades of l

Colour plays a major role in the way the natural world gets about. In plants, it is the chemicals found in leaves or petals, that give rise to colours; but in animals, especially birds and insects, variation of shade and pattern (more than what is possible with chemicals) is achieved by optical tricks. And the chameleon is a creature that has muscular control over the mechanism and can change colour at will.

It was initially thought that the  chameleon does it by managing the movement of light reflecting cells, of different colours, in the animal’s skin. But Jérémie Teyssier, Suzanne V Saenko, Dirk van der Marel and Michel C Milinkovitch of the University of Geneva report in the journal Nature Communications, that they have discovered a different mechanism.

They find that the skin of the male chameleon contains two layers of a mesh of minute crystals whose orientation and separation affects the shades of light that the skin reflects. Stress or excitation brings about changes in the distances between the nanocrystals, which changes the wavelengths of light that are strongly reflected, thus changing the colours of the animal’s skin.

Biological pigments
The main function of plant surfaces is collection of light for photosynthesis. The prominent colour of plants is hence the colour of chlorophyll, which absorbs the yellow and blue wavelengths. Other colours, apart from the red or yellow of carrots, are the anthocyanins (meaning flower blue) which take shades from red to blue, and reflect light that has passed through the leaf or petal to parts that contain chlorophyll, for optimising photosynthesis. Another group of pigments is the red or yellow betalains, found where there are no anthocyanins, like in beetroot or bougainvillea.

Unlike pigments in plants, structural colouration, found in birds and insects is not because of absorption of wavelengths, but from suppression of reflected components by interference of waves, from the step-like surface of wings of birds and insects. When there is a change in the angle at which light falls on these surfaces, or the angle from which they are seen would affect which wavelengths get suppressed, there can be rapid change of colours or patterns as the animal moves.

Colours in animals,which are mainly for camouflage or communication, generally arise from cells called chromatophores. The pigment expressed can be controlled and many animals, particularly marine animals, are able to make an extent of changes in colours.

But the colouration in animals can also come from structures known as photonic crystals, which are patterns of microscopic crystals separated by distances of about a quarter of the wavelength of light. The distribution of theses crystals affects the passage of particular wavelengths and leads to a variety of colours. Changes in the geometry of the reflecting surfaces have been suggested as a way of changing colours too.

The chameleon
While manipulating chromatophores can lead to changes of shade, there are kinds of chameleons which display changes of basic colour, not just variation of shade. The panther chameleon, native of Madagascar, possesses two types of dark pigment, with which both sexes and all ages can control to modulate the brightness of the skin.

But adult males have various combinations of white, red, green and blue skin and the ability to rapidly change colour. In the presence of a male competitor or a potentially receptive female, a mature male panther chameleon can shift the background colour of its skin from green to yellow or orange, while blue patches turn whitish and red becomes brighter, all within a couple of minutes – the change being fully reversible.

The researchers at Geneva used high resolution videos and analysed the mix of wavelengths involved in the slide show. The colour changes, from blue to green to red, were so marked, that they were clearly not only due to pigment change within
chromatophores and had to involve structural effects, like multilayer interference.

Sensitive microscopic analysis then showed that the chameleon skin contained two layers of reflective cells, called iridophores, which contained a matrix of transparent crystals of different shapes and sizes. In the adult males, the upper layer was stocked with small, close packed crystals, arranged in a triangular lattice.

The lattice of transparent crystals created a pattern of alternating path length for light and the effect was like that of a photonic crystal, which is an optical device that can block out a range of wavelengths, through interference effects. Further, study of chameleon skin just when it was blue or green (relaxed) and yellow or white (excited) showed that nanocrystals were of the same size, but the distances separating the crystals were about 30 per cent greater when the animal was excited. As the slightest change in geometry of photonic crystals leads to large changes in the wavelengths suppressed, variation of crystal separation appeared to be the way the chameleon managed wide its colour changes.

To confirm this mechanism, samples of chameleon skin in the excited (white) state were subjected to chemical media of different salinities, to create pressures that would shrink the lattice to the relaxed state. “This treatment indeed results in a blue shift..” says the paper.

The group has followed up with computing the colour-changing effects that a model optical crystal, with the same properties and dimensions of the chameleon skin lattice would show, and they find that this tallies with what is observed. The finding is that the wide colour changes of the chameleon come from physical changes in the layout of photonic crystal elements in the skin, rather than changes in pigments or even orientation of reflecting surfaces.

Another result of the study is that the second layer in the chameleon skin also contains crystals, which reflect in the infra-red or radiation that carries heat. As chameleons have evolved in environments of bright sunshine, the function of this layer appears to be regulation of the temperature of the animal in conditions of intense heat.

“The organisation of iridophores into two superposed layers constitutes an evolutionary novelty for chameleons that allows some species to combine efficient camouflage with spectacular display. Additional analyses are warranted to identify whether the deep layer of iridophores in chameleons further provide them with improved resistance to variable sunlight exposure,” the paper says.