According to a 2015 study, how do chameleons actually shift from calm greenish tones to vivid yellows and oranges when excited?
- By actively adjusting the spacing of a lattice of nanocrystals in a skin cell layer, changing which light wavelengths reflect
- By stretching pigment-filled cells called chromatophores to spread more colored pigment across the skin
- By rerouting blood flow to flush the skin with more red pigment
- By secreting colored oils onto the surface of their scales
Why A? And why not the others?
Correct answer: A. By actively adjusting the spacing of a lattice of nanocrystals in a skin cell layer, changing which light wavelengths reflect
A 2015 study using electron microscopy found that chameleon skin contains a layer of iridophore cells holding a lattice of guanine nanocrystals, and that the chameleon can actively relax or tighten the spacing of this lattice; that spacing change alters which wavelengths of light constructively reflect, shifting the visible color even though no new pigment is produced. The idea of stretching pigment-filled chromatophores is the popular assumption, borrowed from how octopuses and squid change color, but chameleon research shows pigment cells alone cannot account for the dramatic swings observed between resting and excited states. Flushing the skin with more blood-borne red pigment describes blushing in a few reptiles under specific conditions, not a chameleon's broad, mood-driven color shift across its whole body. No lizard is known to secrete colored oils onto its scales as a coloration mechanism.
Source: Teyssier et al., 'Photonic crystals cause active colour change in chameleons,' Nature Communications 6:6368, 2015.