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 colours, 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 colours are a bluff, not an honest signal.
Motion dazzle Edit
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. 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. Another theory is that the zebra’s stripes could provide some protection from flies and biting insects.
Physical protection Edit
Further information: Biological pigment
Many animals have dark pigments such as melanin in their skin, eyes and fur to protect themselves against sunburn (damage to living tissues caused by ultraviolet light).
Some frogs such as Bokermannohyla alvarengai, which basks in sunlight, lighten their skin colour when hot (and darkens when cold), making their skin reflect more heat and so avoid overheating.
Some animals are coloured purely incidentally because their blood contains pigments. For example, amphibians like the olm that live in caves may be largely colourless as colour 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 coloured riboflavin in their skin. Human albinos and people with fair skin have a similar colour for the same reason.
Animal coloration may be the result of any combination of pigments, chromatophores, structural coloration and bioluminescence.
Pigments are coloured chemicals (such as melanin) in 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), an example of seasonal camouflage (a polyphenism). 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, 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 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.
Chromatophores are special pigment-containing cells that can change their size, thus varying the colour and pattern of the animal. The voluntary control of chromatophores is known as metachrosis. For example, cuttlefish and chameleons can rapidly change their appearance, both for camouflage and for signalling, as Aristotle first noted over 2000 years ago:
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.
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. Animals use several methods to produce structural colour, as described in the table.
Bioluminescence is the production of light, such as by the photophores 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, especially when stressed; when disturbed, they secrete an ink which luminesces in the same colours. 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. 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. 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.