Camouflage is a method of concealment that allows an otherwise visible animal, military vehicle, or other object to remain unnoticed by blending with its environment. Examples include a leopard's spotted coat, the battledress of a modern soldier and a leaf-mimic butterfly. Camouflage is a form of visual deception; the term probably comes from camouflet, a French term meaning smoke blown in someone's face as a practical joke.[1] Military camouflage is part of a broad area of deception and concealment from all means of detection including sound and radar, and involving non-camouflage techniques such as use of decoys and electronic jamming.[2][3]
According to Charles Darwin's theory of Natural Selection, characteristics such as camouflage that help an animal to survive will tend to evolve in any population.[4]
Camouflage, whether in animals or in military use, can be achieved in different ways, including the apparent opposites Mimesis - being seen, but resembling something else, and Crypsis - being hidden. [5] In both cases, however, camouflage is achieved not by actual invisibility, but by not being noticed. A third approach, Dazzle, found military application in the 20th century.
Camouflage is not the only form of Animal coloration that helps animals to survive. Other adaptations include Warning coloration, non-concealing forms of Mimicry (as when a harmless Hoverfly resembles a stinging Wasp), the use of bright colours in Sexual selection, and the use of pigment in the skin to protect against sunburn.
Contents |
In Mimesis (also called Masquerade), the whole animal (or piece of military equipment) looks like some other object, which is of no special interest to the observing animal or enemy. [6]:512,513 Mimesis is common in prey animals, for example when a Peppered Moth caterpillar mimics a twig, or a grasshopper mimics a dry leaf.[5]:151
Mimesis is also employed by some predators (or parasites) to lure their prey, for example, a flower mantis mimics a particular kind of flower, such as an orchid[5]:134
A different, non-camouflage strategy is Mimicry, where an animal boldly resembles another animal, typically one that is poisonous or distasteful: it is then easily seen, but avoided.[5]:6-42
Crypsis means blending with the background, making the animal (or military equipment) hard to see (or to detect in other ways, such as by sound or scent: for details, see Crypsis). Camouflage - visual crypsis - can be achieved in many different ways, including: [5]
These ways of achieving crypsis are described below.
Some animals' colors and patterns resemble a particular natural background, for example the Peppered Moth adult blends in with tree bark.[7]
Some animals, whether predators or prey, have disruptive patterns that help to achieve crypsis by breaking up their outlines with strongly-contrasting markings, for example in the Leopard.[8] Disruptive patterns "are characterized by high-contrast light and dark patches, in a nonrepetitive configuration, that also provide camouflage by disrupting the recognizable shape or orientation of the animal".[9]
Disruptive patterning is now common in military usage, both for uniforms and for military vehicles. Disruptive patterning, however, does not always achieve crypsis on its own, as an animal or a military target may be given away by other factors including shape, shine, and shadow.[10]
Some animals, such as the Horned Lizards of North America, have evolved elaborate measures to eliminate shadow: their bodies are flattened, with the sides thinning to an edge; the animals habitually press their bodies to the ground; and their sides are fringed with white scales which "break up and camouflage any dark shadow line that might fall along the body's edge."[11] The theory that the body shape of the Horned Lizards which live in open desert is adapted to minimize shadow is supported by the one species which lacks fringe scales, "perhaps the exception that proves the rule": the Roundtail Horned Lizard, which lives in rocky areas and resembles a rock. "When threatened, it enhances this resemblance by hunching up its back, an act that displays rather than hides its three-dimensionality."[11]
"Elimination of shadow" was identified as a principle of military camouflage during the Second World War.[12]
Some animals actively seek to make themselves cryptic by using materials from their environment, such as twigs, sand, or pieces of shell to conceal their outlines, for example when a Caddis Fly larva builds a decorated case, or when a Decorator Crab covers its back with seaweed, sponges and stones.[5] Most other forms of crypsis also require some animal behaviour, e.g., keeping still, lying flat, as in the Flat-tail Horned Lizard,[13] or swaying as if rippled by wind or water currents, as in the Leafy Sea Dragon.[14]
Similar principles can be applied for military purposes, for example when a Sniper wears a Ghillie suit designed to be further camouflaged by decoration with materials such as tufts of grass from the sniper's immediate environment.
Animals such as chameleon, flatfish, squid or octopus actively and rapidly change their skin patterns and colours using special chromatophore cells to resemble whatever background they are currently resting on (as well as for signalling).[5][15]
On a longer timescale, some animals like the Arctic hare, Arctic fox, Stoat (also called Ermine), and Ptarmigan change their coat colour (by moulting and growing new fur or feathers) from brown or grey in the summer to white in the winter; the Arctic fox is the only species in the dog family (Canidae) to do so.[16] However, Arctic hares which live in the far north of Canada, where summer is very short, remain white all year round.[16][17]
Again, similar principles can be applied for military purposes.
Countershading uses graded colour to create the illusion of flatness. Shadow makes an animal lightest on top, darkest below; countershading 'paints in' tones which are darkest on top, lightest below, making the countershaded animal nearly invisible against a matching background.[18] American artist Abbott Handerson Thayer's observation that "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.[19]
Countershading is widely used by both terrestrial and marine animals. Examples include deer and sharks.
Countershading is relatively seldom used for military camouflage, despite Second World War experiments that demonstrated its effectiveness. English Zoologist Hugh B. Cott encouraged the use of techniques including countershading to provide effective concealment, observing that soldiers viewed camouflage netting as "some kind of invisibility cloak: just throw it over the truck and now you don't see it", as Peter Forbes comments.[20] At the same time in Australia, zoologist William John Dakin advised soldiers to copy animals' methods, using their instincts for wartime camouflage.[21]
Counterillumination means producing light to match the background, notably in some species of Squid, such as the Sparkling Enope Squid (Watasenia scintillans) and the Midwater Squid (Abralia veranyi). Abralia has light-producing organs (photophores) scattered all over its underside; these create a sparkling glow that prevents the animal from appearing as a dark shape when seen from below.[22]
Counterillumination is the likely function of the bioluminescence of many marine organisms, though light is also produced to attract prey and for signalling.
Most forms of camouflage are made ineffective by movement: a deer or grasshopper may be highly cryptic when motionless, but instantly seen when it moves. But one form of 'camouflage' works only when in motion: dazzle patterning.[23]
Dazzle patterning superficially resembles disruptive patterning, but has a different purpose. It was used on ships during the First World War, not to make vessels hard to see, but to make their speed, size, range and direction difficult to ascertain by eye.[24] Dazzle patterning is therefore arguably (by definition) not camouflage, though it has been called camouflage since the First World War.[25][24] Non-aligning dazzle patterns may have helped to confuse gunners using optical rangefinders, where two halves of the image had to be aligned by eye to estimate the range to the target ship. However the evidence for its success in naval warfare is mixed.[24] Remarkably, some United States Navy camouflage schemes in World War II attempted to combine disruptive camouflage and dazzle.[26]
Motion dazzle is caused by rapidly-moving bold patterns of contrasting stripes, as when Zebras run from a lion. Motion dazzle may degrade predators' ability to estimate the prey's speed and direction accurately, giving the prey an improved chance of escape. [25] Motion dazzle distorts speed perception, and is most effective at high speeds; stripes can also distort perception of size (and so, perceived range to the target).[23] Since dazzle patterns (such as a zebra's stripes) make animals more difficult to accurately locate when moving, but easier to visually fix when stationary, there is an evolutionary trade-off between dazzle and crypsis. [25]
|
||||||||