Blue Butterflies, Green Frogs, Red Fish - Colors in Animals

Colour in animals arise from pigments and/or structure. Pigments are coloured chemical substances produced in animals tissues that can be chemically separated.

The colour blue is rarely from pigment, but arises from structure.

The most widespread pigment in animals is melanin. Melanin is found in all forms of animal life, and has even been found in fossils over 1 million years old. Melanin is usually brown or black and is the pigment that colours skin, fur and hair. Albinos totally lack melanin, including in the eyes, which makes eyes appear pink from blood pigment.

Haemoglobin is the red respiratory pigment in blood that contains iron. Not all animals have red blood; crustaceans, cuttlefish, spiders and many others have blue blood. Haemocyanin is a blue respiratory pigment that contains copper instead of iron.

Bilins are pigments from broken down haemoglobin. They are responsible for the colour of bile and bruises and the patterns on birds' eggs.

Carotenoids are yellow/orange/red pigments obtained from diet. They are plant pigments that dissolve in fat and result in colour in frogs, fish, egg yolks, sponges and lobsters.

Cytochromes are pigments that are present in very small amounts in all animals and many yeasts, bacteria and plants. Cytochromes are involved in the release of energy from carbohydrates.

Structural colours arise from microscopic features in an animal's surface. The surface proteins have a regular arrangement of fine grooves or barb which reflect rays of light. Structural colours are a feature of the animal's structure and cannot be extracted like pigments.

Structural colours may disappear when an animal dies (as proteins shrink) or temporarily when wet (as water fills air spaces).

Tyndall blue is the blue colour observed when the microscopic surface of an animal scatters blue light. A dark melanin backing layer absorbs all other wavelengths of light. The animal appears blue, yet there is no blue pigment. Tyndall blue can be seen in many birds, fish, amphibians, reptiles and jellyfish, as well as blue eyes.

If there is pigment and Tyndall blue together, an intermediate colour results. Green snakes, frogs, birds and eyes contain yellow pigment and also have Tyndall blue, which results in green. Violet/purple result when red pigment occurs with Tyndall blue, for example in birds.

Iridescence is when shimmery colours appear to change in the light. Iridescence is caused by inteference of light waves reflecting from the different layers such as oil in a water puddle. Iridescence is common in fish, birds and insects. Thin layers result in blue and green iridescence (most common). Thicker layers result in red, gold and orange colours. The colour in peacock feathers are from iridescence. The colours are more intense because the feathers have a dark melanin backing layer.

Lustre and shine can result from reflective and iridescent cells.

The shimmery, metallic appearance of silvery fish and sea-slugs are from cells containing waste products.

Many amphibians, fish and reptiles have iridescent cells. Rainbow trout have iridescent cells in their skin.

White (and albino) results from the absence of pigment in fur, skin, teeth, ivory, bone and whites of eye. There is no pigment to absorb specific wavelengths of light, so all wavelengths are reflected, which recombine to form white light. Black is when pigment absorbs all wavelengths of light.

Grey is usually an optical mix of black and white hair or fur.

A lacquered, glistening effect results when the cuticles of hair or fur is flattened and smooth. Some birds have a powdery coating which makes their feathers look pale and powdery blue, greys and pinks.

Some animals have bioluminescence - they produce light. Marine bioluminescence is usually blue-green.

Some creatures are transparent, often with pigmented organs eg juvenile crustaceans, shrimps and fish.

Many animals change colour in certain situations - response to temperature, humidity, season, light/dark, sexual cycles, age and mood.

Some animals contain chromatophores which are responsible for rapid colour changes according to emotional state/mood. Creatures that contain chromatophores include cuttlefish, octopus and chameleons.

Fast colour changes are connected to the nervous system. The chromatophores of octopus and cuttlefish expand when they are aggressive, resulting in a colour change, usually to red.

Slow-colour changes are from hormones. Chameleons have transparent surface layers containing melanin which contract revealing colours underneath.

Fish are paler at night and when frightened; darker during day or when aggressive. Lizards are darker in the morning and become paler as they soak up sun.

Exposed skin (eg wattles of birds) allows blood vessels to show through. Exposed skin becomes paler with fear or shock and redder with aggression.

Colours can change during moulting of hair, feathers or reptile skin. Colours are often obtained from diet. Flamingos are white/grey and only turn pink when they moult if they have obtained pigment from diet.

Some animals change colour with humidity eg hercules beetles are black when conditions are humid; yellow when conditions are dry.

Usually the male in a group is the most brightly coloured. Males that incubate eggs (like the emu) are typically dull in colour. Anemonefish (clownfish) live in groups with one brighter-coloured dominant male. If the male dies/disappears, one of the females undergoes a sex change and takes on male colouration.

Young animals are usually paler and develop pigmentation as they mature. They are typically most colourful at sexual maturity. The final colour change is at death, when hormone systems that control colour and pigments break down.