Why do chameleons change color?

For a long time people assumed chameleons change color purely to blend in, but research shows communication and temperature control are the primary drivers. Their skin contains special cells called iridophores filled with tiny crystals; by actively spacing these crystals closer or farther apart, the chameleon changes which wavelengths of light are reflected, producing different colours. A calm chameleon may be green, while an excited or aggressive one can flush bright yellow, orange, or red to warn rivals. Colour also helps with heat: a cold chameleon may darken to absorb more sunlight, then lighten as it warms. Camouflage still plays a role, but many colour changes are really social signals.