Animal colouration
Encyclopedia
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 pigment
s, chromatophore
s, 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
, 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 at least six separate reasons why animal coloration may evolve:
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, 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.
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." 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.
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).
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.
Müller's argument runs basically (using a simple example rather than equations) as follows:
Colour is often used in 'deimatic' displays that have evolved to scare off predators.
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. 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).
Many Noctuid
moth
s, 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 Orthoptera
ns 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.
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.
Different forms of sexual selection are possible, including rivalry among males, and selection of females by males.
in their skin
, eye
s and fur
to protect themselves against sunburn
(damage to living tissues caused by ultraviolet
light).
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
.
) deposited into the animal tissues. For example, the Arctic fox
has a white coat in winter (containing little pigment), and a brown coat in summer (containing more pigment).
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.
and chameleon
s can rapidly change their appearance, both for camouflage and for signalling, as first noted by Aristotle
over 2000 years ago.
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.
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.
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.
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
. 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.
, such as by the photophore
s of marine animals, 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.
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.
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.
General reading
Children's books
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
from its surfaces. The mechanisms for colour production in animals include pigment
Pigment
A pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.Many materials selectively absorb...
s, chromatophore
Chromatophore
Chromatophores are pigment-containing and light-reflecting cells found in amphibians, fish, reptiles, crustaceans, and cephalopods. They are largely responsible for generating skin and eye colour in cold-blooded animals and are generated in the neural crest during embryonic development...
s, structural coloration, and bioluminescence
Bioluminescence
Bioluminescence is the production and emission of light by a living organism. Its name is a hybrid word, originating from the Greek bios for "living" and the Latin lumen "light". Bioluminescence is a naturally occurring form of chemiluminescence where energy is released by a chemical reaction in...
.
Animal coloration has been a topic of interest and research
Research
Research can be defined as the scientific search for knowledge, or as any systematic investigation, to establish novel facts, solve new or existing problems, prove new ideas, or develop new theories, usually using a scientific method...
in biology
Biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...
for well over a century. According to Charles Darwin
Charles Darwin
Charles Robert Darwin FRS was an English naturalist. He established that all species of life have descended over time from common ancestry, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection.He published his theory...
's 1859 theory of natural selection
Natural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....
, features such as coloration evolved
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
by providing individual animals with a reproductive advantage. For example, an individual with slightly better camouflage than others of the same species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
would on average leave more offspring.
There are at least six separate reasons why animal coloration may evolve:
- Camouflage, enabling an animal to remain hidden from view
- Warning, signalling to other animals not to attack
- Mimicry, taking advantage of another species' warning coloration
- Sexual selection, signalling sexual status to other members of the same species
- Physical protection, such as having pigments to protect against sunburn
- Incidental coloration, such as having red blood because, as it happens, haem (needed to carry oxygen) is red.
Camouflage
One of the pioneers of research into animal coloration, Edward Bagnall Poulton classified the forms of protective coloration including camouflage in a way which is still helpful:- Protective resemblance
- Special: the whole animal looks like some other object, for example when a caterpillar resembles a twig or a bird dropping. This is now called Mimesis
- General: the animal's texture blends with the background, for example when a moth's colour and pattern blend in with tree bark. This is now called CrypsisCrypsisIn ecology, crypsis is the ability of an organism to avoid observation or detection by other organisms. It may be either a predation strategy or an antipredator adaptation, and methods include camouflage, nocturnality, subterranean lifestyle, transparency, and mimicry...
- Aggressive resemblance
- Special: a predatorPredationIn ecology, predation describes a biological interaction where a predator feeds on its prey . Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the death of its prey and the eventual absorption of the prey's tissue through consumption...
(or parasite) looks like something else, luring the prey to approach, for example when a flower mantisFlower MantisFlower Mantis is a common name given to various species of praying mantis that mimic flowers including:*Acromantis formosana *Blepharopsis mendica...
resembles a particular kind of flower, such as an orchid - General: a predator or parasite blends in with the background, for example when a leopardLeopardThe leopard , Panthera pardus, is a member of the Felidae family and the smallest of the four "big cats" in the genus Panthera, the other three being the tiger, lion, and jaguar. The leopard was once distributed across eastern and southern Asia and Africa, from Siberia to South Africa, but its...
is hard to see in long grass.
- 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.
- Variable protective resemblance: an animal such as a chameleonChameleonChameleons are a distinctive and highly specialized clade of lizards. They are distinguished by their parrot-like zygodactylous feet, their separately mobile and stereoscopic eyes, their very long, highly modified, and rapidly extrudable tongues, their swaying gait, the possession by many of a...
, flatfish, squid or octopusOctopusThe octopus is a cephalopod mollusc of the order Octopoda. Octopuses have two eyes and four pairs of arms, and like other cephalopods they are bilaterally symmetric. An octopus has a hard beak, with its mouth at the center point of the arms...
changes its skin pattern and colour using special chromatophoreChromatophoreChromatophores are pigment-containing and light-reflecting cells found in amphibians, fish, reptiles, crustaceans, and cephalopods. They are largely responsible for generating skin and eye colour in cold-blooded animals and are generated in the neural crest during embryonic development...
cells to resemble whatever background it is currently resting on (as well as for signalling). See also :Category:Animals that can change color.
The main mechanisms to create the resemblances described by Poulton – whether in nature or in military applications – are:
- CrypsisCrypsisIn ecology, crypsis is the ability of an organism to avoid observation or detection by other organisms. It may be either a predation strategy or an antipredator adaptation, and methods include camouflage, nocturnality, subterranean lifestyle, transparency, and mimicry...
, blending into the background so as to become hard to see (this covers both special and general resemblance) - Disruption, using colour and pattern to break up the animal's outline (this relates mainly to general resemblance)
- Mimicry (in a narrow sense: other types of mimicry are described separately), resembling other objects of no special interest to the observer (this relates to special resemblance)
- CountershadingCountershadingCountershading, or Thayer's Law, is a form of camouflage. Countershading, in which an animal’s pigmentation is darker dorsally, is often thought to have an adaptive effect of reducing conspicuous shadows cast on the ventral region of an animal’s body...
, using graded colour to create the illusion of flatness (this relates mainly to general resemblance) - Counterillumination, producing light to match the background, notably in some species of SquidSquidSquid are cephalopods of the order Teuthida, which comprises around 300 species. Like all other cephalopods, squid have a distinct head, bilateral symmetry, a mantle, and arms. Squid, like cuttlefish, have eight arms arranged in pairs and two, usually longer, tentacles...
.
Countershading was first described by the American artist Abbott Handerson Thayer
Abbott Handerson Thayer
Abbott Handerson Thayer was an American artist, naturalist and teacher. As a painter of portraits, figures, animals and landscapes, he enjoyed a certain prominence during his lifetime, as indicated by the fact that his paintings are part of the most important U.S. art collections...
, 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, 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.
Warning coloration
- distasteful, for example a Cinnabar mothCinnabar mothThe Cinnabar moth is a brightly coloured arctiid moth, found in Europe and western and central Asia. It has been introduced into New Zealand, Australia and North America to control poisonous ragwort, which its larvae feed on. The moth is named after the red mineral cinnabar because of the red...
caterpillar has bitter-tasting chemicals in its blood - foul-smelling, for example the skunkSkunkSkunks are mammals best known for their ability to secrete a liquid with a strong, foul odor. General appearance varies from species to species, from black-and-white to brown or cream colored. Skunks belong to the family Mephitidae and to the order Carnivora...
can eject a liquid with a long-lasting and powerful odour - poisonous, for example a waspWaspThe term wasp is typically defined as any insect of the order Hymenoptera and suborder Apocrita that is neither a bee nor an ant. Almost every pest insect species has at least one wasp species that preys upon it or parasitizes it, making wasps critically important in natural control of their...
can deliver a painful sting, while a viper can deliver a fatal bite
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.
Mimicry
Natural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....
to drive slight, chance, resemblance to progressively more perfect mimicry. There are numerous possible mechanisms, of which by far the best known are:
- Batesian mimicryBatesian mimicryBatesian mimicry is a form of mimicry typified by a situation where a harmless species has evolved to imitate the warning signals of a harmful species directed at a common predator...
, 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 mimicryMüllerian mimicryMüllerian mimicry is a natural phenomenon when two or more harmful species, that may or may not be closely related and share one or more common predators, have come to mimic each other's warning signals...
, the mutual resemblances among distasteful animals, most commonly insects such as wasps and bees (hymenopteraHymenopteraHymenoptera is one of the largest orders of insects, comprising the sawflies, wasps, bees and ants. There are over 130,000 recognized species, with many more remaining to be described. The name refers to the heavy wings of the insects, and is derived from the Ancient Greek ὑμήν : membrane and...
)
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).
Müllerian mimicry was first described by pioneering naturalist Fritz Müller
Fritz Müller
Johann Friedrich Theodor Müller , better known as Fritz Müller, and also as Müller-Desterro, was a German biologist and physician who emigrated to southern Brazil, where he lived in and near the German community of Blumenau, Santa Catarina...
. 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.
Müller's argument runs basically (using a simple example rather than equations) as follows:
- Suppose there are, say, 100 wasps of rare species A and 1000 wasps of common species B in a place.
- Suppose that wasps are eaten by young inexperienced birds, which quickly learn after one trial, by getting stung in the mouth, not to eat wasps again.
- Suppose there are 10 young birds in the place.
- If species A does not resemble B (to the birds), then each young bird must eat one A and one B to learn to avoid them. 10 out of 100 wasps of species A, and 10 out of 1000 wasps of species B perish in the training process. Note that in this example, A is 10 times rarer than B, and therefore suffers 10 times as heavily.
- Now suppose that species A resembles B perfectly, so the young birds cannot distinguish them. Each young bird now needs only to eat one wasp - A or B, it doesn't matter - to learn to avoid both of them.
- The advantage gained by species A of resembling species B is that where before 10 individuals of species A perished, now only about 1 perishes, as most of the wasps sampled by the young birds at random will belong to the commoner species B. Note that there is a large gain from the resemblance in the rare species, and a small gain (1 fewer individual out of 1000 perishes) in the common species: in fact, A's gain is 100 times as much as B's, comparing before and after.
- Therefore there is a powerful selective pressure favouring progressively closer resemblance of species A to species B.
Startle
Colour is often used in 'deimatic' displays that have evolved to scare off predators.
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
Eyespot (mimicry)
An eyespot is an eye-like marking. They are found on butterflies, reptiles, birds and fish. In members of the Felidae family , the white circular markings on the backs of the ears are termed ocelli, and they are functionally similar to eyespots in other animals.Eyespots may be a form of...
, and startling the predator to hesitate, increasing the butterfly's chances of escape. 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).
Many Noctuid
Noctuidae
The Noctuidae or owlet moths are a family of robustly-built moths that includes more than 35,000 known species out of possibly 100,000 total, in more than 4,200 genera. They constitute the largest family in the Lepidoptera....
moth
Moth
A moth is an insect closely related to the butterfly, both being of the order Lepidoptera. Moths form the majority of this order; there are thought to be 150,000 to 250,000 different species of moth , with thousands of species yet to be described...
s, such as the Large Red Underwing
Red Underwing
The Red Underwing moth, Catocala nupta, is a moth of Noctuidae family.This is a large nocturnal European species which, like most noctuids, is drably coloured to aid concealment during the day...
, 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 Orthoptera
Orthoptera
Orthoptera is an order of insects with paurometabolous or incomplete metamorphosis, including the grasshoppers, crickets and locusts.Many insects in this order produce sound by rubbing their wings against each other or their legs, the wings or legs containing rows of corrugated bumps...
ns 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.
Sexual selection
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.
Different forms of sexual selection are possible, including rivalry among males, and selection of females by males.
Physical protection
Many animals have dark pigments such as melaninMelanin
Melanin is a pigment that is ubiquitous in nature, being found in most organisms . In animals melanin pigments are derivatives of the amino acid tyrosine. The most common form of biological melanin is eumelanin, a brown-black polymer of dihydroxyindole carboxylic acids, and their reduced forms...
in their skin
Skin
-Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...
, eye
Eye
Eyes are organs that detect light and convert it into electro-chemical impulses in neurons. The simplest photoreceptors in conscious vision connect light to movement...
s and fur
Fur
Fur is a synonym for hair, used more in reference to non-human animals, usually mammals; particularly those with extensives body hair coverage. The term is sometimes used to refer to the body hair of an animal as a complete coat, also known as the "pelage". Fur is also used to refer to animal...
to protect themselves against sunburn
Sunburn
A sunburn is a burn to living tissue, such as skin, which is produced by overexposure to ultraviolet radiation, commonly from the sun's rays. Usual mild symptoms in humans and other animals include red or reddish skin that is hot to the touch, general fatigue, and mild dizziness. An excess of UV...
(damage to living tissues caused by ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
light).
Incidental coloration
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 haemHeme
A heme or haem is a prosthetic group that consists of an iron atom contained in the center of a large heterocyclic organic ring called a porphyrin. Not all porphyrins contain iron, but a substantial fraction of porphyrin-containing metalloproteins have heme as their prosthetic group; these are...
in their blood cells, needed to carry oxygen, happens to be red.
Mechanisms of colour production in animals
Biological pigment
Biological pigments, also known simply as pigments or biochromes are substances produced by living organisms that have a color resulting from selective color absorption. Biological pigments include plant pigments and flower pigments...
, Chromatophores, Structural coloration and Bioluminescence
Bioluminescence
Bioluminescence is the production and emission of light by a living organism. Its name is a hybrid word, originating from the Greek bios for "living" and the Latin lumen "light". Bioluminescence is a naturally occurring form of chemiluminescence where energy is released by a chemical reaction in...
.
Coloration by pigments
Pigments are coloured chemicals (such as melaninMelanin
Melanin is a pigment that is ubiquitous in nature, being found in most organisms . In animals melanin pigments are derivatives of the amino acid tyrosine. The most common form of biological melanin is eumelanin, a brown-black polymer of dihydroxyindole carboxylic acids, and their reduced forms...
) deposited into the animal tissues. For example, the Arctic fox
Arctic fox
The arctic fox , also known as the white fox, polar fox or snow fox, is a small fox native to Arctic regions of the Northern Hemisphere and is common throughout the Arctic tundra biome. The Greek word alopex, means a fox and Vulpes is the Latin version...
has a white coat in winter (containing little pigment), and a brown coat in summer (containing more pigment).
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.For example, the bright yellow of an American Goldfinch
American Goldfinch
The American Goldfinch , also known as the Eastern Goldfinch and Wild Canary, is a small North American bird in the finch family...
, the startling orange of a juvenile Red-spotted Newt, the deep red of a Cardinal bird
Cardinal (bird)
The Cardinals or Cardinalidae are a family of passerine birds found in North and South America. The South American cardinals in the genus Paroaria are placed in another family, the Thraupidae ....
and the pink of a Flamingo
Flamingo
Flamingos or flamingoes are gregarious wading birds in the genus Phoenicopterus , the only genus in the family Phoenicopteridae...
are all produced by Carotenoid
Carotenoid
Carotenoids are tetraterpenoid organic pigments that are naturally occurring in the chloroplasts and chromoplasts of plants and some other photosynthetic organisms like algae, some bacteria, and some types of fungus. Carotenoids can be synthesized fats and other basic organic metabolic building...
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
Chlorophyll
Chlorophyll is a green pigment found in almost all plants, algae, and cyanobacteria. Its name is derived from the Greek words χλωρος, chloros and φύλλον, phyllon . Chlorophyll is an extremely important biomolecule, critical in photosynthesis, which allows plants to obtain energy from light...
. Animals that eat green plants do not become green, however, as chlorophyll does not survive digestion.
Variable coloration by chromatophores
Chromatophores are special pigment-containing cells that can change their size, so varying the colour and pattern of the animal. For example, cuttlefishCuttlefish
Cuttlefish are marine animals of the order Sepiida. They belong to the class Cephalopoda . Despite their name, cuttlefish are not fish but molluscs....
and chameleon
Chameleon
Chameleons are a distinctive and highly specialized clade of lizards. They are distinguished by their parrot-like zygodactylous feet, their separately mobile and stereoscopic eyes, their very long, highly modified, and rapidly extrudable tongues, their swaying gait, the possession by many of a...
s can rapidly change their appearance, both for camouflage and for signalling, as first noted by Aristotle
Aristotle
Aristotle was a Greek philosopher and polymath, a student of Plato and teacher of Alexander the Great. His writings cover many subjects, including physics, metaphysics, poetry, theater, music, logic, rhetoric, linguistics, politics, government, ethics, biology, and zoology...
over 2000 years ago.
When Cephalopod
Cephalopod
A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...
molluscs like squid
Squid
Squid are cephalopods of the order Teuthida, which comprises around 300 species. Like all other cephalopods, squid have a distinct head, bilateral symmetry, a mantle, and arms. Squid, like cuttlefish, have eight arms arranged in pairs and two, usually longer, tentacles...
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.
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.
Structural coloration
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
Iridescence
Iridescence is generally known as the property of certain surfaces which appear to change color as the angle of view or the angle of illumination changes...
. 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
Bioluminescence is the production of lightLight
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
, such as by the photophore
Photophore
A photophore is a light-emitting organ which appears as luminous spots on various marine animals, including fish and cephalopods. The organ can be simple, or as complex as the human eye; equipped with lenses, shutters, color filters and reflectors...
s of marine animals, and the tails of glow-worms and fireflies.
Bioluminescence, like other forms of metabolism
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
, releases energy derived from the chemical energy of food. A pigment, luciferin
Luciferin
Luciferins are a class of light-emitting biological pigments found in organisms that cause bioluminescence...
is catalysed by the enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
luciferase
Luciferase
Luciferase is a generic term for the class of oxidative enzymes used in bioluminescence and is distinct from a photoprotein. One famous example is the firefly luciferase from the firefly Photinus pyralis. "Firefly luciferase" as a laboratory reagent usually refers to P...
to react with oxygen, releasing light.
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.
Some Angler fish of the deep sea, where it is too dark to hunt by sight, contain symbiotic
Symbiosis
Symbiosis is close and often long-term interaction between different biological species. In 1877 Bennett used the word symbiosis to describe the mutualistic relationship in lichens...
bacteria in the 'bait' on their 'fishing rods'. These emit light to attract prey.
Further reading
Pioneering books- Bates, H. W.Henry Walter BatesHenry Walter Bates FRS FLS FGS was an English naturalist and explorer who gave the first scientific account of mimicry in animals. He was most famous for his expedition to the Amazon with Alfred Russel Wallace in 1848. Wallace returned in 1852, but lost his collection in a shipwreck...
(1857) The Naturalist on the River Amazons (2 volumes). John Murray, London. - Cott, H. B. (1940) Adaptive Coloration in Animals. Methuen, London.
- Darwin, C.Charles DarwinCharles Robert Darwin FRS was an English naturalist. He established that all species of life have descended over time from common ancestry, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection.He published his theory...
(1874) The Descent of Man. London. - Darwin, C.Charles DarwinCharles Robert Darwin FRS was an English naturalist. He established that all species of life have descended over time from common ancestry, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection.He published his theory...
(1859) On the Origin of Species. London. Reprinted 1985, Penguin Classics, Harmondsworth. - Poulton, E. B. (1890) The colours of Animals. London.
- Thayer, A. H.Abbott Handerson ThayerAbbott Handerson Thayer was an American artist, naturalist and teacher. As a painter of portraits, figures, animals and landscapes, he enjoyed a certain prominence during his lifetime, as indicated by the fact that his paintings are part of the most important U.S. art collections...
and G. H. Thayer (1909) Concealing Colouration in the Animal Kingdom. New York. - Wallace, A. R. (1895) Natural Selection and Tropical Nature. London.
General reading
- Forbes, P. (2009) Dazzled and Deceived: Mimicry and Camouflage. Yale, New Haven and London.
- Wickler, W. (1968) Mimicry in plants and animals. McGraw-Hill, New York.
Children's books
- Kalman, B. and J. Crossingham. What are Camouflage and Mimicry?. Crabtree Publishing. (ages 4-8)
- Maze, S. (2006) Beautiful Moments in the Wild: Animals and Their Colors. Moonstone Press, LLC. (ages 4–8)
- Mettler, R. (2001) Animal Camouflage. Moonlight Publishing.