Animal coloration is the general appearance of an animal resulting from the reflection or emission of light from its surfaces. Some animals are brightly coloured, while others are hard to see. In some species, such as the peafowl, the male has strong patterns, conspicuous colours and is iridescent, while the female is far less visible.

There are several separate reasons why animals have evolved colours. Camouflage enables an animal to remain hidden from view. Animals use colour to advertise services such as cleaning to animals of other species; to signal their sexual status to other members of the same species; and in mimicry, taking advantage of the warning coloration of another species. Some animals use flashes of colour to divert attacks by startling predators. Zebras may possibly use motion dazzle, confusing a predator's attack by moving a bold pattern rapidly. Some animals are coloured for physical protection, with pigments in the skin to protect against sunburn, while some frogs can lighten or darken their skin for temperature regulation. Finally, animals can be coloured incidentally. For example, blood is red because the haem pigment needed to carry oxygen is red. Animals coloured in these ways can have striking natural patterns.

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. 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

Classical antiquity

Animal coloration has been a topic of interest and research in biology for centuries. In the classical era, Aristotle recorded that the octopus was able to change its coloration to match its background, and when it was alarmed.Aristotle (c. 350 BC). Historia Animalium. IX, 622a: 2–10. Cited in Borrelli, Luciana; Gherardi, Francesca; Fiorito, Graziano (2006). A catalogue of body patterning in Cephalopoda. Firenze University Press. . Abstract

Early modern period

In his 1665 book Micrographia, Robert Hooke describes the "fantastical" (structural, not pigment) colours of the Peacock's feathers:Hooke, Robert (1665) Micrographia. Ch. 36 ('Observ. XXXVI. Of Peacoks, Ducks, and Other Feathers of Changeable Colours.'). J. Martyn and J. Allestry, London. Full text .

Darwinian biology

According to Charles Darwin's 1859 theory of natural selection, features such as coloration evolved by providing individual animals with a reproductive advantage. For example, individuals with slightly better camouflage than others of the same species would, on average, leave more offspring. In his Origin of Species, Darwin wrote:Darwin, Charles (1859). On the Origin of Species, Ch. 4. John Murray, London. Reprinted 1985, Penguin Classics, Harmondsworth.

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.Bates, Henry Walter (1863). The Naturalist on the River Amazons. John Murray, London.

Edward Bagnall Poulton's strongly Darwinian 1890 book The Colours of Animals, their meaning and use, especially considered in the case of insects argued the case for three aspects of animal coloration that are broadly accepted today but were controversial or wholly new at the time. It strongly supported Darwin's theory of sexual selection, arguing that the obvious differences between male and female birds such as the argus pheasant were selected by the females, pointing out that bright male plumage was found only in species "which court by day". The book introduced the concept of frequency-dependent selection, as when edible mimics are less frequent than the distasteful models whose colours and patterns they copy. In the book, Poulton also coined the term aposematism for warning coloration, which he identified in widely differing animal groups including mammals (such as the skunk), bees and wasps, beetles, and butterflies.

Frank Evers Beddard's 1892 book, Animal Coloration, acknowledged that natural selection existed but examined its application to camouflage, mimicry and sexual selection very critically.Beddard, Frank Evers (1892). Animal Coloration, An Account of the Principal Facts and Theories Relating to the Colours and Markings of Animals. Swan Sonnenschein, London. The book was in turn roundly criticised by Poulton.

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."Thayer, Abbott Handerson and Thayer, Gerald H. (1909). Concealing-Coloration in the Animal Kingdom. New York.

Hugh Bamford Cott's 500-page book Adaptive Coloration in Animals, published in wartime 1940, systematically described the principles of camouflage and mimicry. The book contains hundreds of examples, over a hundred photographs and Cott's own accurate and artistic drawings, and 27 pages of references. Cott focussed especially on "maximum disruptive contrast", the kind of patterning used in military camouflage such as disruptive pattern material. Indeed, Cott describes such applications:Cott, 1940.

Animal coloration provided important early evidence for evolution by natural selection, at a time when little direct evidence was available.

Evolutionary reasons for animal coloration

Camouflage

One of the pioneers of research into animal coloration, Edward Bagnall PoultonPoulton, Edward Bagnall (1890). The Colours of Animals, their meaning and use, especially considered in the case of insects. Kegan Paul, Trench, Trübner. London. pp. 331–334 classified the forms of protective coloration, in a way which is still helpful. He described: protective resemblance; aggressive resemblance; adventitious protection; and variable protective resemblance.Forbes, 2009. pp. 50–51 These are covered in turn below.

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.

Aggressive resemblance is used by predators or parasites. In special aggressive resemblance, the animal looks like something else, luring the prey or host to approach, for example when a flower mantis resembles a particular kind of flower, such as an orchid. In general aggressive resemblance, the predator or parasite blends in with the background, for example when a leopard is hard to see in long grass.

For adventitious protection, an animal uses materials such as twigs, sand, or pieces of shell to conceal its outline, for example when a caddis fly larva builds a decorated case, or when a decorator crab decorates its back with seaweed, sponges and stones.

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).

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.

Countershading was first described by the American artist Abbott Handerson Thayer, a pioneer in the theory of animal coloration. Thayer observed that whereas a painter takes a flat canvas and uses coloured paint to create the illusion of solidity by painting in shadows, animals such as deer are often darkest on their backs, becoming lighter towards the belly, creating (as zoologist Hugh Cott observed) the illusion of flatness,Cott, H. B. 1940 and against a matching background, of invisibility. Thayer's observation "Animals are painted by Nature, darkest on those parts which tend to be most lighted by the sky's light, and vice versa" is called Thayer's Law.Forbes, 2009. pp. 72–73

Signalling

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.

Sexual selection

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