How animals change colour

By The Economist

CHAMELEONS' colour-changing ways are well-known, but precisely how they do it has remained a mystery until this week. They are far from the only animals with the skill: often deployed for camouflage, in reaction to stress, to warn predators, woo partners, or even to regulate temperature. The golden tortoise beetle does its colour-changing while mating. How do animals change colour?

In many cases the process starts in the eye, where the colours of the environment, a rival or a mate are acquired. That triggers a signal from the brain to other cells, called chromatophores, which initiate the change in hue. Some contain sacs of various pigments. In animals such as the octopus, squid or cuttlefish, colour changes when a complex network of muscles adjusts the sizes and positions of different sacs within the skin.

Other creatures use optical trickery in related cells called iridophores. In damselfish, light waves bounce between thin plates within these cells. The colour reflected depends on the spacing of the plates. The fish can precisely shift that separation across millions of cells by just billionths of a metre, all in under a second. Chameleons, scientists have just worked out, use a bit of nanotechnology. They have two layers of iridophores, each containing regularly spaced crystals of the chemical guanine. When combative males strut their colourful stuff, they are changing the spacing within each crystal lattice, creating what are called bandgaps that permit or preclude the passage of certain colours.

Humans experience a rather simpler colour change. Blushing is just a rush of blood to the face, parts of which have larger blood vessels closer to the skin's surface. Although it's easy to induce the effect, the biological purpose of blushing remains a matter of debate.