How the chameleon glows in technicolour

Chameleons don’t change colour to blend in – they do it to show off. Their dazzling displays are intended to face down rivals or catch the eye of a potential mate.

According to the textbooks, chameleons achieve this colour change by shuffling skin pigments around. But new research shows they are exploiting a masterful trick of nanotechnology. A team of biologists and physicists from the University of Geneva, reporting in Nature Communications in March, has discovered chameleons change colour by adjusting the spacing between tiny crystals within their skin, a trick modern physicists are at a loss to reproduce.

“Chameleons have evolved to have the most efficient [light-reflecting] structure that can exist with the organic materials they have available,” says Jérémie Teyssier, lead author of the study. “It’s really amazing that nature can be so smart.”

From the dyes in your clothes to the paint on your walls, most of the colours around you arise from chemistry – pigment molecules that absorb some light and reflect the rest, which is the colour we see. But the shimmering slick of oil on water, and the readable side of a CD, are coloured by physics. These “structural” colours arise from patterns on the object’s surface, not its chemical properties.

When the team looked deeper into the chameleon’s skin, they uncovered a trick never seen in any animal before

This dazzling form of colour emission was first noted by English scientist Robert Hooke in 1665, when he realised there was something “fantastical” about the peacock’s tail. The hues seemed to change depending on the angle he looked at them, and if the feathers got wet, the colours disappeared. He was the first to guess that some colours in nature could come from structure.

We now know the colour in the peacock’s tail comes from its surface, which acts like a nanoscale sieve, allowing most light to pass through, but reflecting specific wavelengths to produce pure colour. Butterflies, beetles, seashells, lizards, fish and squid all use the same trick. A rare few of them can change their colours at will.

And the male panther chameleons of Madagascar are capable of perhaps the most dramatic changes of hue. “They switch on these fabulous, high-contrast, gorgeous display colours when they’re courting a female or fighting a male,” says Devi Stuart-Fox, a zoologist at the University of Melbourne who studies colour-changing animals.

When a mature male panther chameleon sees a rival or a potentially receptive female, he can shift the background colour of his skin from green to yellow or orange, and also switch the colour of his stripes.

Until now, we thought chameleons changed colour by moving packets of melanin – the same pigment that gives our skin, hair and eyes their colour – around on their skin. Melanin is like a stage curtain that the chameleon can pull across to conceal brightly coloured skin cells filled with red, blue or yellow pigments. Changing which coloured cells are “on stage” determines the chameleon’s colour.

But to Michel Milinkovitch, an evolutionary biologist at the University of Geneva, this explanation “did not make sense”. Other animals that achieve similarly swift and dazzling colour changes – including some squid – use a combination of pigment and structural colour. Such structural colour changes are generally faster and more intense than pigment movements alone.

So Milinkovitch and his team delved into the inner structure of chameleon skin by taking thin slices of individual “iridophore” colour-changing skin cells and peering inside them using high-powered electron microscopes. The images revealed neat stacks of nanoscale guanine crystals within the cell cytoplasm.

But by looking at sections of cells in relaxed and excited states, the Geneva team discovered the chameleons’ colour-changing trick. By controlling the spacing between the guanine crystals – like changing the size of the holes in a nanoscale light sieve – they could rapidly alter which wavelengths of light were allowed through, and change the reflected colour.

So while chameleons do use melanin pigment “stage curtains” to lighten or darken their skin, the team found the colour itself comes from the shape-shifting guanine crystals. That’s a similar method of colour control to the one that evolved independently in squid. But when the team looked deeper into the chameleon’s skin, they uncovered a trick never seen in any animal before.

What the team uncovered was a deeper layer of iridophores containing bigger guanine crystals with a messier structure. After modelling the interaction of these crystals with light, the team realised they reflected infrared light – in other words, heat. This layer could be important for regulating body temperature, an important consideration for a cold-blooded animal. “We simply don’t know whether any other lizards or reptiles might have this second ‘deep’ layer of iridophores,” says Stuart-Fox.

According to Philipp Reineck, a physicist at the RMIT node of the Australian Research Council Centre of Excellence for Nanoscale BioPhotonics, engineers have much to learn from the chameleon. We understand the physics behind it, but being able to reproduce these tiny shape-shifting structures in our own gadgets and devices is a different story, he says: “The goal really should be to mimic the fabrication process. That’s the most amazing feature.”

Such structures could lead to clothing or paints that change colour, dazzling LED displays or improved fibre optic cables.

Milinkovitch is now orchestrating his team of cell biologists and biochemists to work out how the chameleon changes the spacing between crystals at will to change its colour.

“It’s probably the beginning of a very long story,” says Teyssier.

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