Animal coloration

Animal colouration is the general appearance of an animal resulting from the reflection or emission of

There are several separate reasons why animals have evolved colours.

Animals produce colour in both direct and indirect ways. Direct production occurs through the presence of visible coloured cells known as pigment which are particles of coloured material such as freckles. Indirect production occurs by virtue of cells known as chromatophores which are pigment-containing cells such as hair follicles. The distribution of the pigment particles in the chromatophores can change under hormonal or neuronal control. For fishes it has been demonstrated that chromatophores may respond directly to environmental stimuli like visible light, UV-radiation, temperature, pH, chemicals, etc.^[1] colour change helps individuals in becoming more or less visible and is important in agonistic displays and in camouflage. Some animals, including many butterflies and birds, have microscopic structures in scales, bristles or feathers which give them brilliant iridescent colours. Other animals including squid and some deep-sea fish can produce light, sometimes of different colours. Animals often use two or more of these mechanisms together to produce the colours and effects they need.

History

Animal coloration has been a topic of interest and

In his 1665 book Micrographia, Robert Hooke describes the "fantastical" (structural, not pigment) colours of the Peacock's feathers:^[3]

The parts of the Feathers of this glorious Bird appear, through the Microscope, no less gaudy then do the whole Feathers; for, as to the naked eye 'tis evident that the stem or quill of each Feather in the tail sends out multitudes of Lateral branches, ... so each of those threads in the Microscope appears a large long body, consisting of a multitude of bright reflecting parts.
... their upper sides seem to me to consist of a multitude of thin plated bodies, which are exceeding thin, and lie very close together, and thereby, like mother of Pearl shells, do not onely reflect a very brisk light, but tinge that light in a most curious manner; and by means of various positions, in respect of the light, they reflect back now one colour, and then another, and those most vividly. Now, that these colours are onely fantastical ones, that is, such as arise immediately from the refractions of the light, I found by this, that water wetting these colour'd parts, destroy'd their colours, which seem'd to proceed from the alteration of the reflection and refraction.

— Robert Hooke^[3]

According to

When we see leaf-eating insects green, and bark-feeders mottled-grey; the alpine ptarmigan white in winter, the red-grouse the colour of heather, and the black-grouse that of peaty earth, we must believe that these tints are of service to these birds and insects in preserving them from danger. Grouse, if not destroyed at some period of their lives, would increase in countless numbers; they are known to suffer largely from birds of prey; and hawks are guided by eyesight to their prey, so much so, that on parts of the Continent persons are warned not to keep white pigeons, as being the most liable to destruction. Hence I can see no reason to doubt that natural selection might be most effective in giving the proper colour to each kind of grouse, and in keeping that colour, when once acquired, true and constant.

— Charles Darwin^[4]

Henry Walter Bates's 1863 book The Naturalist on the River Amazons describes his extensive studies of the insects in the Amazon basin, and especially the butterflies. He discovered that apparently similar butterflies often belonged to different families, with a harmless species mimicking a poisonous or bitter-tasting species to reduce its chance of being attacked by a predator, in the process now called after him, Batesian mimicry.^[5]

Abbott Handerson Thayer's 1909 book Concealing-Coloration in the Animal Kingdom, completed by his son Gerald H. Thayer, argued correctly for the widespread use of crypsis among animals, and in particular described and explained countershading for the first time. However, the Thayers spoilt their case by arguing that camouflage was the sole purpose of animal coloration, which led them to claim that even the brilliant pink plumage of the flamingo or the roseate spoonbill was cryptic—against the momentarily pink sky at dawn or dusk. As a result, the book was mocked by critics including Theodore Roosevelt as having "pushed [the "doctrine" of concealing coloration] to such a fantastic extreme and to include such wild absurdities as to call for the application of common sense thereto."^[12][13]

the effect of a disruptive pattern is to break up what is really a continuous surface into what appears to be a number of discontinuous surfaces... which contradict the shape of the body on which they are superimposed.

— Hugh Cott[15]

Animal coloration provided important early evidence for evolution by natural selection, at a time when little direct evidence was available.^{[16][17][18][19]}

Evolutionary reasons for animal coloration

Camouflage

One of the pioneers of research into animal coloration,

Protective resemblance is used by prey to avoid predation. It includes special protective resemblance, now called mimesis, where the whole animal looks like some other object, for example when a caterpillar resembles a twig or a bird dropping. In general protective resemblance, now called crypsis, the animal's texture blends with the background, for example when a moth's colour and pattern blend in with tree bark.^[20]

In variable protective resemblance, an animal such as a chameleon, flatfish, squid or octopus changes its skin pattern and colour using special chromatophore cells to resemble whatever background it is currently resting on (as well as for signalling).^[20]

The main mechanisms to create the resemblances described by Poulton – whether in nature or in military applications – are crypsis, blending into the background so as to become hard to see (this covers both special and general resemblance); disruptive patterning, using colour and pattern to break up the animal's outline, which relates mainly to general resemblance; mimesis, resembling other objects of no special interest to the observer, which relates to special resemblance; countershading, using graded colour to create the illusion of flatness, which relates mainly to general resemblance; and counterillumination, producing light to match the background, notably in some species of squid.^[20]

Countershading was first described by the American artist

Colour is widely used for signalling in animals as diverse as birds and shrimps. Signalling encompasses at least three purposes:

• advertising, to signal a capability or service to other animals, whether within a species or not
• sexual selection, where members of one sex choose to mate with suitably coloured members of the other sex, thus driving the development of such colours
• warning, to signal that an animal is harmful, for example can sting, is poisonous or is bitter-tasting. Warning signals may be mimicked truthfully or untruthfully.

Advertising services

Advertising coloration can signal the services an animal offers to other animals. These may be of the same species, as in sexual selection, or of different species, as in cleaning symbiosis. Signals, which often combine colour and movement, may be understood by many different species; for example, the cleaning stations of the banded coral shrimp Stenopus hispidus are visited by different species of fish, and even by reptiles such as hawksbill sea turtles.^[23][24][25]

Sexual selection

Darwin observed that the males of some species, such as birds-of-paradise, were very different from the females.

Darwin explained such male-female differences in his theory of sexual selection in his book

Different forms of sexual selection are possible, including rivalry among males, and selection of females by males.

Warning

Main article: Aposematism

Warning coloration (aposematism) is effectively the "opposite" of camouflage, and a special case of advertising. Its function is to make the animal, for example a wasp or a coral snake, highly conspicuous to potential predators, so that it is noticed, remembered, and then avoided. As Peter Forbes observes, "Human warning signs employ the same colours – red, yellow, black, and white – that nature uses to advertise dangerous creatures."^[28] Warning colours work by being associated by potential predators with something that makes the warning coloured animal unpleasant or dangerous.^[29] This can be achieved in several ways, by being any combination of:

• distasteful, for example caterpillars, pupae and adults of the
  starlings vomit.^[31]
• foul-smelling, for example the skunk can eject a liquid with a long-lasting and powerful odour^[32]
• aggressive and able to defend itself, for example honey badgers.^[33]
• venomous, for example a
  viper or coral snake can deliver a fatal bite.^[28]

Mimicry means that one species of animal resembles another species closely enough to deceive predators. To evolve, the mimicked species must have warning coloration, because appearing to be bitter-tasting or dangerous gives natural selection something to work on. Once a species has a slight, chance, resemblance to a warning coloured species, natural selection can drive its colours and patterns towards more perfect mimicry. There are numerous possible mechanisms, of which the best known are:

• bees
  .
• wasps and bees (hymenoptera
  ).

Batesian mimicry was first described by the pioneering naturalist

Henry W. Bates. When an edible prey animal comes to resemble, even slightly, a distasteful animal, natural selection favours those individuals that even very slightly better resemble the distasteful species. This is because even a small degree of protection reduces predation and increases the chance that an individual mimic will survive and reproduce. For example, many species of hoverfly are coloured black and yellow like bees, and are in consequence avoided by birds (and people).^[5]

Müllerian mimicry was first described by the pioneering naturalist Fritz Müller. When a distasteful animal comes to resemble a more common distasteful animal, natural selection favours individuals that even very slightly better resemble the target. For example, many species of stinging wasp and bee are similarly coloured black and yellow. Müller's explanation of the mechanism for this was one of the first uses of mathematics in biology. He argued that a predator, such as a young bird, must attack at least one insect, say a wasp, to learn that the black and yellow colours mean a stinging insect. If bees were differently coloured, the young bird would have to attack one of them also. But when bees and wasps resemble each other, the young bird need only attack one from the whole group to learn to avoid all of them. So, fewer bees are attacked if they mimic wasps; the same applies to wasps that mimic bees. The result is mutual resemblance for mutual protection.^[39]

Distraction

praying mantis in deimatic

or threat pose displays conspicuous patches of colour to startle potential predators. This is not warning coloration as the insect is palatable.

Startle

Main article: Deimatic behaviour

Some animals such as many

honest signal.^[40][41]

Motion dazzle

Some prey animals such as zebra are marked with high-contrast patterns which possibly help to confuse their predators, such as lions, during a chase. The bold stripes of a herd of running zebra have been claimed make it difficult for predators to estimate the prey's speed and direction accurately, or to identify individual animals, giving the prey an improved chance of escape.^[42] Since dazzle patterns (such as the zebra's stripes) make animals harder to catch when moving, but easier to detect when stationary, there is an evolutionary trade-off between dazzle and camouflage.^[42] There is evidence that the zebra's stripes could provide some protection from flies and biting insects.^[43]

Physical protection

Further information: Biological pigment

Many animals have dark pigments such as melanin in their skin, eyes and fur to protect themselves against sunburn^[44] (damage to living tissues caused by ultraviolet light).^[45][46] Another example of photoprotective pigments are the GFP-like proteins in some corals.^[47] In some jellyfish, rhizostomins have also been hypothesized to protect against ultraviolet damage.^[48]

Temperature regulation

Further information:

Temperature regulation

Some frogs such as Bokermannohyla alvarengai, which basks in sunlight, lighten their skin colour when hot (and darkens when cold), making their skin reflect more heat and so avoid overheating.^[49]

Incidental coloration

Further information: Biological pigment

Some animals are coloured purely incidentally because their blood contains pigments. For example, amphibians like the

albinos and people with fair skin have a similar colour for the same reason.^[51]

Mechanisms of colour production in animals

chromatophores

(dark spots) respond to 24 hours in dark (above) or light (below).

Animal coloration may be the result of any combination of

structural coloration and bioluminescence.^[52]

Coloration by pigments

Main articles:

Seasonal camouflage

Pigments are coloured chemicals (such as

red-spotted newt, the deep red of a cardinal and the pink of a flamingo are all produced by carotenoid pigments synthesized by plants. In the case of the flamingo, the bird eats pink shrimps, which are themselves unable to synthesize carotenoids. The shrimps derive their body colour from microscopic red algae, which like most plants are able to create their own pigments, including both carotenoids and (green) chlorophyll. Animals that eat green plants do not become green, however, as chlorophyll does not survive digestion.^[53]

Variable coloration by chromatophores

Main article:

Chromatophores

Further information:

Animals that can change color

Chromatophores are special pigment-containing cells that may change their size, but more often retain their original size but allow the pigment within them to become redistributed, thus varying the colour and pattern of the animal. Chromatophores may respond to hormonal and/or neurobal control mechanisms, but direst responses to stimulation by visible light, UV-radiation, temperature, pH-changes, chemicals, etc. have also been documented.^[1] The voluntary control of chromatophores is known as metachrosis.^[52] For example, cuttlefish and chameleons can rapidly change their appearance, both for camouflage and for signalling, as Aristotle first noted over 2000 years ago:^[2]

The octopus ... seeks its prey by so changing its colour as to render it like the colour of the stones adjacent to it; it does so also when alarmed.

— Aristotle

When

melanophores' with dark melanin.^[53]

Structural coloration

Main article: Structural coloration

While many animals are unable to synthesize carotenoid pigments to create red and yellow surfaces, the green and blue colours of bird feathers and insect carapaces are usually not produced by pigments at all, but by structural coloration.

butterflies are created by structural coloration.^[55] Animals use several methods to produce structural colour, as described in the table.^[55]

Mechanisms of structural colour production in animals

Mechanism	Structure	Example
Diffraction grating	layers of chitin and air	Iridescent colours of butterfly wing scales, peacock feathers[55]
Diffraction grating	tree-shaped arrays of chitin	Morpho butterfly wing scales[55]
Selective mirrors	micron-sized dimples lined with chitin layers	Papilio palinurus, emerald swallowtail butterfly wing scales[55]
Photonic crystals	arrays of nano-sized holes	Cattleheart butterfly wing scales[55]
Crystal fibres	hexagonal arrays of hollow nanofibres	Aphrodita, sea mouse spines[55]
Deformed matrices	random nanochannels in spongelike keratin	Diffuse non-iridescent blue of Ara ararauna, blue-and-yellow macaw[55]
Reversible proteins	reflectin proteins controlled by electric charge	Iridophore cells in Doryteuthis pealeii squid skin[55]

Bioluminescence

comb jelly is bioluminescent

.

Main article: Bioluminescence

Bioluminescence is the production of

Comb jellies such as Euplokamis are bioluminescent, creating blue and green light, especially when stressed; when disturbed, they secrete an ink which luminesces in the same colours. Since comb jellies are not very sensitive to light, their bioluminescence is unlikely to be used to signal to other members of the same species (e.g. to attract mates or repel rivals); more likely, the light helps to distract predators or parasites.^[58] Some species of squid have light-producing organs (photophores) scattered all over their undersides that create a sparkling glow. This provides counter-illumination camouflage, preventing the animal from appearing as a dark shape when seen from below.^[59]

Some anglerfish of the deep sea, where it is too dark to hunt by sight, contain symbiotic bacteria in the 'bait' on their 'fishing rods'. These emit light to attract prey.^[60]

See also

• Albinism in biology
• Chromatophore
• Dog coat colours and patterns
• Cat coat genetics
• Deception in animals
• Equine coat colour
• Equine coat colour genetics
• Roan (colour)
• Fish coloration

References

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Sources

Wikimedia Commons has media related to Animals by color.

Wikimedia Commons has media related to Camouflage.

• Cott, Hugh Bamford (1940). Adaptive Coloration in Animals. Methuen, London.
• Forbes, Peter (2009).
  ISBN 0300178964

External links

• Theme issue 'Animal coloration: production, perception, function and application' (Royal Society)
• NatureWorks: Coloration (for children and teachers)
• HowStuffWorks: How Animal Camouflage Works
• University of British Columbia: Sexual Selection (a lecture for Zoology students)
• Nature's Palette: How animals, including humans, produce colours