Animal coloration is the general appearance of an animal resulting from the reflection or emission of light from its surfaces. The mechanisms for colour production in animals include pigments, chromatophores, structural coloration, and bioluminescence.
There are several separate reasons why animal coloration may evolve, including camouflage, enabling an animal to remain hidden from view, signalling to other animals, of the same or different species, diversion, physical protection, such as having pigments to protect against sunburn, and incidentally, such as having red blood because, as it happens, haem (needed to carry oxygen) is red. All of these can create striking natural patterns.
Animals use colour for signalling in several ways, including advertising, signalling services such as cleaning to animals of other species; in sexual selection, signalling sexual status to other members of the same species; warning, signalling to other animals not to attack; and mimicry, taking advantage of another species' warning coloration.
Animals use colour to divert attacks by startle, surprising a predator e.g. with eyespots or other flashes of colour, and by dazzle, confusing a predator's attack by moving a bold pattern (such as zebra stripes) rapidly.
History
In his 1665 book Micrographia, Robert Hooke describes the "fantastical" (structural, not pigment) colours of the Peacock's feathers:[1]
- "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."
- "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."
[edit] Camouflage
Main article: Camouflage
Protective resemblance is used by prey to avoid predation. It include 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.[5]
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.[5]
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.[5]
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). See also Category:Animals that can change color.[5]
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.[5]
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,[6] 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.[7]
Signalling
Further information: Cleaning symbiosis and Sexual selection
Further information: signalling theory
[edit] Advertising
Advertising coloration signals an animal's capability 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.[8][9][10][edit] Sexual selection
Main article: sexual selection
Darwin observed that the males of some species, such as Birds of Paradise (see illustration), were very different from the females.Darwin suggested an explanation of these differences in his theory of sexual selection (The Descent of Man, London, 1874): once the females begin to select males according to any particular characteristic, such as a long tail or a coloured crest, that characteristic will progressively be emphasized in the males. Eventually all the males will have the characteristics that the females are sexually selecting for strongly emphasized, as any male that does not will not reproduce. Note that this mechanism is so powerful that it is able to create features that are strongly disadvantageous to the males in other ways: for example, some male Birds of Paradise have wing or tail streamers that are so long that they may impede flight, while their brilliant colours may make the males more vulnerable to predators. In the extreme, it may be that sexual selection has driven species to extinction, as has been argued for the enormous horns of the male Irish Elk.[11]
Different forms of sexual selection are possible, including rivalry among males, and selection of females by males.
[edit] Warning
Main article: Aposematism
Further information: signalling theory
Warning coloration (aposematism) is effectively the "opposite" of camouflage. 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."[12] Warning colours work by being associated by potential predators with something that makes the warning-coloured animal unpleasant or dangerous. This can be achieved in several ways:- distasteful, for example a Cinnabar moth caterpillar has bitter-tasting chemicals in its blood
- foul-smelling, for example the skunk can eject a liquid with a long-lasting and powerful odour
- poisonous, for example a wasp can deliver a painful sting, while a viper can deliver a fatal bite
[edit] Mimicry
Main article: Mimicry
The existence of warning coloration (aposematism) makes it possible for mimicry to evolve, because it enables natural selection to drive slight, chance, resemblance to progressively more perfect mimicry. There are numerous possible mechanisms, of which by far the best known are:- Batesian mimicry, the resemblance of edible to distasteful animals, most commonly insects such as butterflies; a familiar example is the resemblance of harmless hoverflies (which have no sting) to bees
- Müllerian mimicry, the mutual resemblances among distasteful animals, most commonly insects such as wasps and bees (hymenoptera)
Müllerian mimicry was first described by 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.[14]
[edit] Distraction
[edit] Startle
Further information: Eyespot (mimicry)
Many insects, including the Peacock butterfly (Inachis io) use a combination of coloration strategies for survival. The underside, presented when the insect is resting in vegetation with wings closed, is cryptic, being a leaf mimic. But if disturbed by a predator, the butterfly flashes its wings, displaying the conspicuous eyespots, and startling the predator to hesitate, increasing the butterfly's chances of escape.[15] Since the eyespots do not resemble any particular animal, the startle coloration and behaviour are not exactly mimicry.
Butterflies with eyespots often survive predator attack for another reason also: birds typically attack the eyespots, not the body (see illustration).[16]
Many Noctuid moths, such as the Large Red Underwing, Catocala nupta which are highly cryptic when at rest, display a startlingly bright flash of colours – combinations of red, yellow, orange, pink, black, and white – when disturbed. Similarly, some Orthopterans such as grasshoppers are cryptic at rest, but flash bright wing colours including blue if disturbed. The moths then rapidly fly off; the grasshoppers jump, fly and glide, landing among cover and almost instantly 'disappear' as they fold their wings.[17]
[edit] Dazzle
Further information: Dazzle camouflage
Some prey animals such as Zebra are marked with high-contrast patterns which help to confuse their predators, such as Lions, during a chase. The bold stripes of a herd of running Zebra 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.[18] 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.[18] The zebra's stripes may also provide some protection from flies and biting insects.[19][edit] 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 (damage to living tissues caused by ultraviolet light).[edit] Incidental coloration
Further information: Biological pigment
Some animals are coloured purely incidentally because substances that they produce for other purposes happen to be pigments. For example, amphibians that live in caves may be largely colourless as colour has no function in that environment, but they may have red blood and show some red in their skin because the haem in their blood cells, needed to carry oxygen, happens to be red.[edit] Mechanisms of colour production in animals
[edit] Coloration by pigments
Main article: Biological pigment
Pigments are coloured chemicals (such as melanin) deposited into the animal tissues.[20] For example, the Arctic fox has a white coat in winter (containing little pigment), and a brown coat in summer (containing more pigment).[edit] Melanins and carotenoids
Many animals, including mammals, birds, and amphibians, are unable to synthesize most of the pigments that colour their fur or feathers, other than the brown or black melanins that give many mammals their earth tones.[21]For example, the bright yellow of an American Goldfinch, the startling orange of a juvenile Red-spotted Newt, the deep red of a Cardinal bird 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.[21]
[edit] Variable coloration by chromatophores
Main article: Chromatophores
Further information: Category:Animals that can change color
When Cephalopod molluscs like squid and cuttlefish find themselves against a light background, they contract many of their chromatophores, concentrating the pigment into a smaller area, resulting in a pattern of tiny, dense, but widely-spaced dots, appearing light. When they enter a darker environment, they allow their chromatophores to expand, creating a pattern of larger dark spots, and making their bodies appear dark.[23]"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."
Amphibians such as frogs have three kinds of star-shaped chromatophore cells in separate layers of their skin. The top layer contains 'xanthophores' with orange, red, or yellow pigments; the middle layer contains 'iridophores' with a silvery light-reflecting pigment; while the bottom layer contains 'melanophores' with dark melanin.[21]
[edit] 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.[21]Structural coloration means the production of colour by microscopically-structured surfaces fine enough to interfere with visible light, sometimes in combination with pigments: for example, peacock tail feathers are pigmented brown, but their structure makes them appear blue, turquoise and green.
Structural coloration can produce the most brilliant colours, often iridescent.[20] For example, the blue/green gloss on the plumage of birds such as ducks, and the purple/blue/green/red colours of many beetles and butterflies are created by structural coloration.[24]
Animals use several methods to produce structural colour, as described in the table.[24]
| Mechanism | Structure | Example |
|---|---|---|
| Diffraction grating | layers of chitin and air | Iridescent colours of Butterfly wing scales, Peacock feathers[24] |
| Diffraction grating | tree-shaped arrays of chitin | Morpho butterfly wing scales[24] |
| Selective mirrors | micron-sized dimples lined with chitin layers | Papilio palinurus, Emerald Swallowtail butterfly wing scales[24] |
| Photonic crystals | arrays of nano-sized holes | Cattleheart butterfly wing scales[24] |
| Crystal fibres | hexagonal arrays of hollow nanofibres | Aphrodita, Sea Mouse spines[24] |
| Deformed matrices | random nanochannels in spongelike keratin | Diffuse non-iridescent blue of Ara ararauna, Blue-and-yellow Macaw[24] |
| Reversible proteins | reflectin proteins controlled by electric charge | Iridophore cells in Loligo pealeii squid skin[24] |
[edit] Bioluminescence
Main article: Bioluminescence
Bioluminescence is the production of light, such as by the photophores of marine animals,[25] and the tails of glow-worms and fireflies.Bioluminescence, like other forms of metabolism, releases energy derived from the chemical energy of food. A pigment, luciferin is catalysed by the enzyme luciferase to react with oxygen, releasing light.[26]
Comb jellies such as Euplokamis are bioluminescent, creating blue and green light, possibly to attract prey; when disturbed, they secrete an ink which luminesces in the same colours, perhaps to distract predators.[27]
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.[28]
Some Angler fish 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.[29]
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