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.
Animal coloration has been a topic of interest and research in biology for well over a century. According to Charles Darwin's 1859 theory of natural selection,[1] features such as coloration evolved by providing individual animals with a reproductive advantage. For example, an individual with slightly better camouflage than others of the same species would, on average, leave more offspring.
There are several separate reasons why animal coloration may evolve:
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One of the pioneers of research into animal coloration, Edward Bagnall Poulton[2] classified the forms of protective coloration including camouflage in a way which is still helpful[3]:
The main mechanisms to create the resemblances described by Poulton – whether in nature or in military applications – are:
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[4], 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.[5]
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."[6] 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:
Warning coloration can succeed either through inborn ("instinctual") behaviour on the part of potential predators, or through a learned avoidance. Either can lead to various forms of mimicry.
Colour is often used in startling 'deimatic' displays that have evolved to scare off predators. These combine warning coloration with behaviour.
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.[7] 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).[8]
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.[9]
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 was first described by pioneering naturalist Henry W. Bates. When an edible prey animal comes to resemble, even slightly, a distasteful animal (not necessarily closely related to it), natural selection favours those individuals that even very slightly better resemble the distasteful target. 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).[10]
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.[11]
Müller's argument runs basically (using a simple example rather than equations) as follows:
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. [12] 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. [12]
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.[13]
Different forms of sexual selection are possible, including rivalry among males, and selection of females by males.
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).
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.
Animal coloration may be the result of any combination of Pigments, Chromatophores, Structural coloration and Bioluminescence. [14]
Pigments are coloured chemicals (such as melanin) deposited into the animal tissues.[14] For example, the Arctic fox has a white coat in winter (containing little pigment), and a brown coat in summer (containing more pigment).
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.[15]
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.[15]
Chromatophores are special pigment-containing cells that can change their size, so varying the colour and pattern of the animal.[14] For example, cuttlefish and chameleons can rapidly change their appearance, both for camouflage and for signalling, as first noted by Aristotle over 2000 years ago.[16]
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.[17]
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.[15]
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. [15]
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.[14] 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.
Bioluminescence is the production of light, such as by the photophores of marine animals[18], 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.[19]
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.[20]
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.[21]
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