Animal coloration

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

There are several separate reasons why animals have evolved colors. Camouflage enables an animal to remain hidden from view. Animals use color 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 color 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 colored 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 colored incidentally. For example, blood is red because the heme pigment needed to carry oxygen is red. Animals colored in these ways can have striking natural patterns.

Animals produce color in both direct and indirect ways. Direct production occurs through the presence of visible colored cells known as pigment which are particles of colored 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] Color 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 colors. Other animals including squid and some deep-sea fish can produce light, sometimes of different colors. Animals often use two or more of these mechanisms together to produce the colors and effects they need.

Orientalischer Süßlippfisch
A brilliantly-colored oriental sweetlips fish (Plectorhinchus vittatus) waits while two boldly-patterned cleaner wrasse (Labroides dimidiatus) pick parasites from its skin. The spotted tail and fin pattern of the Sweetlips signals sexual maturity; the behaviour and pattern of the cleaner fish signal their availability for cleaning service, rather than as prey
Elephant-ear-sponge
Bright coloration of orange elephant ear sponge, Agelas clathrodes signals its bitter taste to predators

History

Micrographia title page
Robert Hooke's Micrographia

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.[2]

In his 1665 book Micrographia, Robert Hooke describes the "fantastical" (structural, not pigment) colors 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 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:[4]

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]

Brazilian Skunk from Edward Bagnall Poulton Colours of Animals 1890
Warning coloration of the skunk in Edward Bagnall Poulton's The Colours of Animals, 1890

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.[6][7] 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".[8] The book introduced the concept of frequency-dependent selection, as when edible mimics are less frequent than the distasteful models whose colors 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.[8]

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.[9][10] The book was in turn roundly criticised by Poulton.[11]

Roseate Spoonbills 1905-1909 Abbott H Thayer
In Roseate Spoonbills 1905–1909, Abbott Handerson Thayer tried to show that even the bright pink of these conspicuous birds had a cryptic function.

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]

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:[14]

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, Edward Bagnall Poulton[8] 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.[20] These are covered in turn below.

Orange oak leaf bottom
A camouflaged orange oak leaf butterfly, Kallima inachus (centre) displays protective resemblance

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 color and pattern blend in with tree bark.[20]

Insect camouflage PP08338
A flower mantis, Hymenopus coronatus, displays special aggressive resemblance

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.[20]

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.[20]

In variable protective resemblance, an animal such as a chameleon, flatfish, squid or octopus changes its skin pattern and color 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 color 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 color 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 Abbott Handerson Thayer, a pioneer in the theory of animal coloration. Thayer observed that whereas a painter takes a flat canvas and uses colored 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,[21] 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.[22]

Signalling

Color 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 colored members of the other sex, thus driving the development of such colors
  • 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 color 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

Paradesia decora Keulemans
Male Goldie's bird of paradise displays to a female

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 The Descent of Man.[26] Once the females begin to select males according to any particular characteristic, such as a long tail or a colored crest, that characteristic is emphasized more and more in the males. Eventually all the males will have the characteristics that the females are sexually selecting for, as only those males can reproduce. This mechanism is powerful enough 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 impede flight, while their brilliant colors may make the males more vulnerable to predators. In the extreme, sexual selection may drive species to extinction, as has been argued for the enormous horns of the male Irish elk, which may have made it difficult for mature males to move and feed.[27]

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

Warning

Micrurus tener
A venomous coral snake uses bright colors to warn off potential predators.

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 colors work by being associated by potential predators with something that makes the warning colored animal unpleasant or dangerous.[29] This can be achieved in several ways, by being any combination of:

Tyria jacobaeae caterpillar
The black and yellow warning colors of the cinnabar moth caterpillar, Tyria jacobaeae, are instinctively avoided by some birds.

Warning coloration can succeed either through inborn behaviour (instinct) on the part of potential predators,[34] or through a learned avoidance. Either can lead to various forms of mimicry. Experiments show that avoidance is learned in birds,[35] mammals,[36] lizards,[37] and amphibians,[38] but that some birds such as great tits have inborn avoidance of certain colors and patterns such as black and yellow stripes.[34]

Mimicry

FinnBirdMimic
Hawk-cuckoo resembles predatory shikra, giving cuckoo time to lay egg in songbird's nest unnoticed

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 colored species, natural selection can drive its colors and patterns towards more perfect mimicry. There are numerous possible mechanisms, of which by far the best known are:

  • Batesian mimicry, where an edible species resembles a distasteful or dangerous species. This is most common in insects such as butterflies. A familiar example is the resemblance of harmless hoverflies (which have no sting) to bees.
  • Müllerian mimicry, where two or more distasteful or dangerous animal species resemble each other. This is most common among insects such as wasps and bees (hymenoptera).

Batesian mimicry was first described by 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 colored black and yellow like bees, and are in consequence avoided by birds (and people).[5]

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 colored 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 colors mean a stinging insect. If bees were differently colored, 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

Gottesanbeterin Abwehr
A praying mantis in deimatic or threat pose displays conspicuous patches of color to startle potential predators. This is not warning coloration as the insect is palatable.

Startle

Some animals such as many moths, mantises and grasshoppers, have a repertory of threatening or startling behaviour, such as suddenly displaying conspicuous eyespots or patches of bright and contrasting colors, so as to scare off or momentarily distract a predator. This gives the prey animal an opportunity to escape. The behaviour is deimatic (startling) rather than aposematic as these insects are palatable to predators, so the warning colors are a bluff, not an 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] Another theory is that the zebra's stripes could provide some protection from flies and biting insects.[43]

Physical protection

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]

Temperature regulation

Bokermannohyla alvarengai01
This frog changes its skin color to control its temperature.

Some frogs such as Bokermannohyla alvarengai, which basks in sunlight, lighten their skin color when hot (and darkens when cold), making their skin reflect more heat and so avoid overheating.[47]

Incidental coloration

Proteus anguinus Postojnska Jama Slovenija
The olm's blood makes it appear pink.

Some animals are colored purely incidentally because their blood contains pigments. For example, amphibians like the olm that live in caves may be largely colorless as color has no function in that environment, but they show some red because of the haem pigment in their red blood cells, needed to carry oxygen. They also have a little orange colored riboflavin in their skin.[48] Human albinos and people with fair skin have a similar color for the same reason.[49]

Mechanisms of color production in animals

Zfishchroma
Side of zebrafish shows how chromatophores (dark spots) respond to 24 hours in dark (above) or light (below).

Animal coloration may be the result of any combination of pigments, chromatophores, structural coloration and bioluminescence.[50]

Coloration by pigments

Flamingo rubro-Phoenicopterus ruber ruber
The red pigment in a flamingo's plumage comes from its diet of shrimps, which get it from microscopic algae.

Pigments are colored chemicals (such as melanin) in animal tissues.[50] For example, the Arctic fox has a white coat in winter (containing little pigment), and a brown coat in summer (containing more pigment), an example of seasonal camouflage (a polyphenism). Many animals, including mammals, birds, and amphibians, are unable to synthesize most of the pigments that color their fur or feathers, other than the brown or black melanins that give many mammals their earth tones.[51] For example, the bright yellow of an American goldfinch, the startling orange of a juvenile 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 color 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.[51]

Variable coloration by chromatophores

Melanophores with dispersed or aggregated melanosomes
Fish and frog melanophores are cells that can change color by dispersing or aggregating pigment-containing bodies.

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 color 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.[50] For example, cuttlefish and chameleons can rapidly change their appearance, both for camouflage and for signalling, as Aristotle first noted over 2000 years ago:[52]

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
Chromatophores
Squid chromatophores appear as black, brown, reddish and pink areas in this micrograph.

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.[53] 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.[51]

Structural coloration

Peacock feathers closeup
The brilliant iridescent colors of the peacock's tail feathers are created by Structural coloration.
Butterfly magnification series collage
Butterfly wing at different magnifications reveals microstructured chitin acting as diffraction grating.

While many animals are unable to synthesize carotenoid pigments to create red and yellow surfaces, the green and blue colors of bird feathers and insect carapaces are usually not produced by pigments at all, but by structural coloration.[51] Structural coloration means the production of color 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 colors, often iridescent.[50] For example, the blue/green gloss on the plumage of birds such as ducks, and the purple/blue/green/red colors of many beetles and butterflies are created by structural coloration.[54] Animals use several methods to produce structural color, as described in the table.[54]

Mechanisms of structural color production in animals
Mechanism Structure Example
Diffraction grating layers of chitin and air Iridescent colors of Butterfly wing scales, Peacock feathers[54]
Diffraction grating tree-shaped arrays of chitin Morpho butterfly wing scales[54]
Selective mirrors micron-sized dimples lined with chitin layers Papilio palinurus, emerald swallowtail butterfly wing scales[54]
Photonic crystals arrays of nano-sized holes Cattleheart butterfly wing scales[54]
Crystal fibres hexagonal arrays of hollow nanofibres Aphrodita, sea mouse spines[54]
Deformed matrices random nanochannels in spongelike keratin Diffuse non-iridescent blue of Ara ararauna, blue-and-yellow macaw[54]
Reversible proteins reflectin proteins controlled by electric charge Iridophore cells in Doryteuthis pealeii squid skin[54]

Bioluminescence

Bioluminescence emitted by comb jelly of genus Euplokamis
A Euplokamis comb jelly is bioluminescent.

Bioluminescence is the production of light, such as by the photophores of marine animals,[55] 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.[56] 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 colors. 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.[57] 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.[58] 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.[59]

See also

References

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Sources

External links

Abbott Handerson Thayer

Abbott Handerson Thayer (August 12, 1849 – May 29, 1921) was an American artist, naturalist and teacher. As a painter of portraits, figures, animals and landscapes, he enjoyed a certain prominence during his lifetime, and his paintings are represented in the major American art collections. He is perhaps best known for his 'angel' paintings, some of which use his children as models.

During the last third of his life, he worked together with his son, Gerald Handerson Thayer, on a book about protective coloration in nature, titled Concealing-Coloration in the Animal Kingdom. First published by Macmillan in 1909, then reissued in 1918, it may have had an effect on military camouflage during World War I. However it was roundly mocked by Theodore Roosevelt and others for its assumption that all animal coloration is cryptic.Thayer also influenced American art through his efforts as a teacher, training apprentices in his New Hampshire studio.

Animal Coloration (book)

Animal Coloration, or in full Animal Coloration: An Account of the Principal Facts and Theories Relating to the Colours and Markings of Animals, is a book by the English zoologist Frank Evers Beddard, published by Swan Sonnenschein in 1892. It formed part of the ongoing debate amongst zoologists about the relevance of Charles Darwin's theory of natural selection to the observed appearance, structure, and behaviour of animals, and vice versa.

Beddard states in the book that it contains little that is new, intending instead to give a clear overview of the subject. The main topics covered are camouflage, then called 'protective coloration'; mimicry; and sexual selection. Arguments for and against these aspects of animal coloration are intensively discussed in the book.

The book was reviewed in 1892 by the major journals including The Auk, Nature, and Science. The scientist reviewers Joel Asaph Allen, Edward Bagnall Poulton and Robert Wilson Shufeldt took up different positions on the book and accordingly praised or criticized Beddard's work.

Modern evaluation of the book is from a variety of perspectives, including the history of Darwinism, the history of the Thayer debate on the purpose of camouflage, the mechanisms of camouflage, sexual selection, and mimicry. Beddard is seen as having covered a wide swath of modern biology with both theory and experiment.

Animal reflectors

Animal reflectors or mirrors are important to the survival of many kinds of animal, and, in some cases, have been mimicked by engineers developing photonic crystals. Examples are the scales of silvery fish, and the tapetum lucidum that causes the eyeshine of dogs and cats. All these reflectors work by interference of light in multilayer structures with dimensions less than a wavelength, so can be classed as photonic crystals. Other animal photonic crystals have evolved to reflect narrow spectra, producing animal coloration.

Aposematism

Aposematism (from Ancient Greek ἀπό apo away, σῆμα sema sign) refers to the appearance of an animal that warns predators it is toxic, distasteful or dangerous. This warning signal is associated with the unprofitability of a prey item to potential predators. The unprofitability may consist of any defences which make the prey difficult to eat, such as toxicity, foul taste or smell, sharp spines, or aggressive nature. Aposematism always involves an advertising signal which may take the form of conspicuous animal coloration, sounds, odours or other perceivable characteristics. Aposematic signals are beneficial for both the predator and prey, since both avoid potential harm.

The term was coined by Edward Bagnall Poulton for Alfred Russel Wallace's concept of warning coloration. Aposematism is exploited in Müllerian mimicry, where species with strong defences evolve to resemble one another. By mimicking similarly coloured species, the warning signal to predators is shared, causing them to learn more quickly at less of a cost to each of the species.

A genuine aposematic signal that a species actually possesses chemical or physical defences is not the only way to deter predators. In Batesian mimicry, a mimicking species resembles an aposematic model closely enough to share the protection, while many species have bluffing deimatic displays which may startle a predator long enough to enable an otherwise undefended prey to escape.

Coloration

Coloration or colouration may refer to:

Color, the visual perceptual property corresponding in humans to the categories called red, green, blue and others, along with any variation, quality, or property thereof

Animal coloration, topic of research regarding animals' adaptive appearance

Cryptic coloration or camouflage, making animals or objects hard to see or disguising them

Color gradient, a range of position-dependent colors

Coloration effect, one of the phenomenal effects of watercolor illusions

Vowel coloration, an account for historical changes in vowel sounds according to Laryngeal theory

Coloration evidence for natural selection

Animal coloration provided important early evidence for evolution by natural selection, at a time when little direct evidence was available. Three major functions of coloration were discovered in the second half of the 19th century, and subsequently used as evidence of selection: camouflage (protective coloration); mimicry, both Batesian and Müllerian; and aposematism.

Charles Darwin's On the Origin of Species was published in 1859, arguing from circumstantial evidence that selection by human breeders could produce change, and that since there was clearly a struggle for existence, that natural selection must be taking place. But he lacked an explanation either for genetic variation or for heredity, both essential to the theory. Many alternative theories were accordingly considered by biologists, threatening to undermine Darwinian evolution.

Some of the first evidence was provided by Darwin's contemporaries, the naturalists Henry Walter Bates and Fritz Müller. They described forms of mimicry that now carry their names, based on their observations of tropical butterflies. These highly specific patterns of coloration are readily explained by natural selection, since predators such as birds which hunt by sight will more often catch and kill insects that are less good mimics of distasteful models than those that are better mimics; but the patterns are otherwise hard to explain.

Darwinists such as Alfred Russel Wallace and Edward Bagnall Poulton, and in the 20th century Hugh Cott and Bernard Kettlewell, sought evidence that natural selection was taking place. Wallace noted that snow camouflage, especially plumage and pelage that changed with the seasons, suggested an obvious explanation as an adaptation for concealment. Poulton's 1890 book, The Colours of Animals, written during Darwinism's lowest ebb, used all the forms of coloration to argue the case for natural selection. Cott described many kinds of camouflage, and in particular his drawings of coincident disruptive coloration in frogs convinced other biologists that these deceptive markings were products of natural selection. Kettlewell experimented on peppered moth evolution, showing that the species had adapted as pollution changed the environment; this provided compelling evidence of Darwinian evolution.

Concealing-Coloration in the Animal Kingdom

Concealing-Coloration in the Animal Kingdom: An Exposition of the Laws of Disguise Through Color and Pattern; Being a Summary of Abbott H. Thayer’s Discoveries is a book published ostensibly by Gerald H. Thayer in 1909, and revised in 1918, but in fact a collaboration with and completion of his father Abbott Handerson Thayer's major work.

The book, illustrated artistically by Abbott Thayer, sets out the controversial thesis that all animal coloration has the evolutionary purpose of camouflage. Thayer rejected Charles Darwin's theory of sexual selection, arguing in words and paintings that even such conspicuous animal features as the peacock's tail or the brilliant pink of flamingoes or roseate spoonbills were effective as camouflage in the right light.

The book introduced the concepts of disruptive coloration to break up an object's outlines, of masquerade, as when a butterfly mimics a leaf, and especially of countershading, where an animal's tones make it appear flat by concealing its self-shadowing.

The book was criticised by big game hunter and politician Theodore Roosevelt for its central assertion that every aspect of animal coloration is effective as camouflage. Roosevelt's detailed reply attacked the biased choice of examples to suit Abbott Thayer's thesis and the book's reliance on unsubstantiated claims in place of evidence. The book was more evenly criticised by zoologist and camouflage researcher Hugh Cott, who valued Thayer's work on countershading but regretted his overenthusiastic attempts to explain all animal coloration as camouflage. Thayer was mocked to a greater or lesser extent by other scientific reviewers.

Countershading

Countershading, or Thayer's law, is a method of camouflage in which an animal's coloration is darker on the upper side and lighter on the underside of the body. This pattern is found in many species of mammals, reptiles, birds, fish, and insects, both predators and prey, and has occurred since at least the Cretaceous period.

When light falls from above on a uniformly coloured three-dimensional object such as a sphere, it makes the upper side appear lighter and the underside darker, grading from one to the other. This pattern of light and shade makes the object appear solid, and therefore easier to detect. The classical form of countershading, discovered in 1909 by the artist Abbott Handerson Thayer, works by counterbalancing the effects of self-shadowing, again typically with grading from dark to light. In theory this could be useful for military camouflage, but in practice it has rarely been applied, despite the best efforts of Thayer and, later, in the Second World War, of the zoologist Hugh Cott.

The precise function of various patterns of animal coloration that have been called countershading has been debated by zoologists such as Hannah Rowland (2009), with the suggestion that there may be multiple functions including flattening and background matching when viewed from the side; background matching when viewed from above or below, implying separate colour schemes for the top and bottom surfaces; outline obliteration from above; and a variety of other largely untested non-camouflage theories. A related mechanism, counter-illumination, adds the creation of light by bioluminescence or lamps to match the actual brightness of a background. Counter-illumination camouflage is common in marine organisms such as squid. It has been studied up to the prototype stage for military use in ships and aircraft, but it too has rarely or never been used in warfare.

The reverse of countershading, with the belly pigmented darker than the back, enhances contrast and so makes animals more conspicuous. It is found in animals that can defend themselves, such as skunks. The pattern is used both in startle or deimatic displays and as a signal to warn off experienced predators. However, animals that habitually live upside-down but lack strong defences, such as the Nile catfish and the Luna moth caterpillar, have upside-down countershading for camouflage.

Dazzled and Deceived

Dazzled and Deceived: Mimicry and Camouflage is a 2009 book on camouflage and mimicry, in nature and military usage, by the science writer and journalist Peter Forbes. It covers the history of these topics from the 19th century onwards, describing the discoveries of Henry Walter Bates, Alfred Russel Wallace and Fritz Müller, especially their studies of butterflies in the Amazon. The narrative also covers 20th-century military camouflage, begun by the painter Abbot Thayer who advocated disruptive coloration and countershading and continued in the First World War by the zoologist John Graham Kerr and the marine artist Norman Wilkinson, who developed dazzle camouflage. In the Second World War, the leading expert was Hugh Cott, who advised the British army on camouflage in the Western Desert.

The book was well received by critics, both military historians and biologists, and won the 2011 Warwick Prize for Writing.

Decorator crab

Decorator crabs are crabs of several different species, belonging to the superfamily Majoidea (not all of which are decorators), that use materials from their environment to hide from, or ward off, predators. They decorate themselves by sticking mostly sedentary animals and plants to their bodies as camouflage, or if the attached organisms are noxious, to ward off predators through aposematism.

Dendronotus

Dendronotus is a genus of sea slugs, nudibranchs, marine gastropod molluscs in the superfamily Tritonioidea.This genus is within the clade Cladobranchia (according to the taxonomy of the Gastropoda by Bouchet & Rocroi, 2005).

Edward Bagnall Poulton

Sir Edward Bagnall Poulton, FRS HFRSE FLS (27 January 1856 – 20 November 1943) was a British evolutionary biologist who was a lifelong advocate of natural selection through a period in which many scientists such as Reginald Punnett doubted its importance. He invented the term sympatric for evolution of species in the same place, and in his book The Colours of Animals (1890) was the first to recognise frequency-dependent selection.

Poulton is also remembered for his pioneering work on animal coloration. He is credited with inventing the term aposematism for warning coloration, as well as for his experiments on 'protective coloration' (camouflage).

Poulton became Hope Professor of Zoology at the University of Oxford in 1893.

Frank Evers Beddard

Frank Evers Beddard FRS FRSE (19 June 1858 – 14 July 1925) was an English zoologist. He became a leading authority on annelids, including earthworms. He won the Linnean Medal in 1916 for his book on oligochaetes.

Ithomiini

Ithomiini is a butterfly tribe in the nymphalid subfamily Danainae. Some authors consider the group to be a subfamily (Ithomiinae). These butterflies are exclusively Neotropical, found in humid forests from sea level to 3000 m, from Mexico to Argentina. There are around 370 species in some 40–45 genera.

Martin Stevens (biologist)

Martin Stevens is a British sensory and evolutionary ecologist, known for his work on animal camouflage, especially disruptive coloration.

Productivity (ecology)

In ecology, productivity refers to the rate of generation of biomass in an ecosystem. It is usually expressed in units of mass per unit surface (or volume) per unit time, for instance grams per square metre per day (g m−2 d−1). The mass unit may relate to dry matter or to the mass of carbon generated. Productivity of autotrophs such as plants is called primary productivity, while that of heterotrophs such as animals is called secondary productivity.

Structural coloration

Structural coloration is 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 microscopic structure makes them also reflect blue, turquoise, and green light, and they are often iridescent.

Structural coloration was first observed by English scientists Robert Hooke and Isaac Newton, and its principle – wave interference – explained by Thomas Young a century later. Young described iridescence as the result of interference between reflections from two or more surfaces of thin films, combined with refraction as light enters and leaves such films. The geometry then determines that at certain angles, the light reflected from both surfaces interferes constructively, while at other angles, the light interferes destructively. Different colours therefore appear at different angles.

In animals such as on the feathers of birds and the scales of butterflies, interference is created by a range of photonic mechanisms, including diffraction gratings, selective mirrors, photonic crystals, crystal fibres, matrices of nanochannels and proteins that can vary their configuration. Some cuts of meat also show structural coloration due to the exposure of the periodic arrangement of the muscular fibres. Many of these photonic mechanisms correspond to elaborate structures visible by electron microscopy. In the few plants that exploit structural coloration, brilliant colours are produced by structures within cells. The most brilliant blue coloration known in any living tissue is found in the marble berries of Pollia condensata, where a spiral structure of cellulose fibrils produces Bragg's law scattering of light. The bright gloss of buttercups is produced by thin-film reflection by the epidermis supplemented by yellow pigmentation, and strong diffuse scattering by a layer of starch cells immediately beneath.

Structural coloration has potential for industrial, commercial and military application, with biomimetic surfaces that could provide brilliant colours, adaptive camouflage, efficient optical switches and low-reflectance glass.

The Colours of Animals

The Colours of Animals is a zoology book written in 1890 by Sir Edward Bagnall Poulton (1856–1943). It was the first substantial textbook to argue the case for Darwinian selection applying to all aspects of animal coloration. The book also pioneered the concept of frequency-dependent selection and introduced the term "aposematism".

The book begins with a brief account of the physical causes of animal coloration. The second chapter gives an overview of the book, describing the various uses of colour in terms of the advantages it can bring through natural selection. The next seven chapters describe camouflage, both in predators and in prey. Methods of camouflage covered include background matching, resemblance to specific objects such as bird droppings, self-decoration with materials from the environment, and the seasonal colour change of arctic animals. Two chapters cover warning colours, including both Batesian mimicry, where the mimic is edible, and Mullerian mimicry, where distasteful species mimic each other. A chapter then looks at how animals combine multiple methods of defence, for instance in the puss moth. Two chapters examine coloration related to sexual selection. Finally Poulton summarizes the subject with a fold-out table including a set of Greek derived words that he invented, of which "aposematic" and "cryptic" survive in biological usage.

The Colours of Animals was well received on its publication, although the book's support for sexual selection was criticised by Alfred Russel Wallace, and its Darwinism and critique of Lamarckism were attacked by Edward Drinker Cope. Wallace liked Poulton's experimental work but was critical of his opinions on sexual selection. The Neo-Lamarckian Cope criticised Poulton's support for Darwin but liked the book's many observations of animal coloration. Modern biologists respect Poulton's advocacy of natural selection and sexual selection, despite the lack at the time of an adequate theory of heredity, and his recognition of frequency-dependent selection.

Tim Caro

Tim Caro (c. 1952 – ) is an evolutionary ecologist known for his work on conservation biology, animal behaviour, anti-predator defences in animals, and especially the function of zebra stripes. He is the author of several textbooks on evolutionary ecology.

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