Biological Colour

How living things make colour and what the evidence says it is for: pigments, structural colour, fluorescence and bioluminescence, cells that change colour, and the research on camouflage, warning, mimicry and signalling.

A mechanism that has been measured is kept apart from a function that has been proposed: where a purpose rests on experiment it says so, and where it is inferred it is described as associated with, not for. Colour as other animals see it is not colour as a human or a screen shows it.

14 topics · 76 evidence-labelled claims · 103 sources

How the colour is made

Pigments, nanostructures, fluorescence and light made by chemistry.

How can you tell whether a colour in nature comes from pigment or from structure?A pigment colours by absorbing part of the light and returning the rest; a structural colour comes from transparent material arra…What pigments do living things use, and why can so few animals make a blue one?Animals rely mainly on melanins, which they make, and carotenoids, which they must eat, with pterins, porphyrins and the parrots'…How do stacks of colourless layers produce the metallic colours of beetles, hummingbirds and blue fruit?Light reflected from the top and bottom of a transparent layer a few hundred nanometres thick interferes, reinforcing some wavele…Which living things use diffraction gratings, and do the rainbows they make matter to anything?A diffraction grating is a surface of evenly spaced ridges that spreads white light into its spectrum by angle; some beetles, the…What is a photonic crystal, and how do butterflies, weevils, peacocks and chameleons come to have one?A photonic crystal is a material whose refractive index repeats in two or three dimensions at the scale of light's wavelength, so…Why are a kingfisher's back and a mandrill's face blue without shimmering like a beetle?Their blue comes from a sponge-like or fibrous nanostructure with only short-range order: regular enough for scattered light wave…What is the difference between an animal that glows and one that only fluoresces?Bioluminescence makes light: an enzyme oxidises a small molecule and the reaction releases a photon, so the animal shines in comp…

Colour that changes

Chromatophores, seasons and colour across a lifetime.

How do cuttlefish, chameleons and fish change colour, and are they doing the same thing?They are doing three different things.Why do some animals and leaves change colour with the season or with age?Hair, feathers and leaves cannot be recoloured once grown, so slow colour change means replacing the tissue or letting its chemis…

What the evidence says it does

Camouflage, warning, mimicry and signalling, with the strength of each case.

How much of what is said about what an animal's colour is 'for' has actually been tested?Less than the confident phrasing of most accounts suggests.Which kinds of camouflage have actually been tested, and what did the tests find?Background matching, disruptive coloration and countershading are distinct mechanisms, and each now has field experiments behind…Do warning colours and mimicry really protect animals, and how do we know?Field experiments say yes, within limits.Is bright colour an honest signal of quality, or is that still an open question?It is partly supported and still open.Why is the colour you see on an animal not the colour its own species sees?Colour is a response of a visual system, and other animals' visual systems differ from ours in the wavelengths they detect, the n…