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See R. A. Carr, Protective Coloration and Mimicry (1972); M. Edmunds, Defence in Animals (1974).
A strategy that organisms use to avoid or deflect the attacks of predators by misleading the latter's visual senses.
Protective coloration can be classified according to whether the functioning or malfunctioning of the vertebrate visual system is exploited. Exploiting the malfunction of the system means simply “not being seen”: the prey fails to attract the attention of the predator, usually because it is the same color as the general background or because it fails to cast a shadow. The organism avoids producing shadow by flattening itself against the substrate, or by countershading, in which the lower parts of a cylindrical prey such as a caterpillar are more lightly colored than the upper parts. As shadows normally form on the underside of cylinders, the shading cancels the shadow and makes the caterpillar optically flat. Animals that match their background often have an ability to select the appropriate background to rest on, or much less frequently can change their own color to match (as in the case of the chameleon).
Exploiting the functioning of the vertebrate perceptual system takes many forms. The vertebrate visual cortex decodes the image on the retina in a hierarchical process starting with the detection of edges. A moth may counter this by possessing strikingly contrasted patches of color on its wings, arranged in a random way. The outline of the moth is thus broken up, and the predator cannot decode it as a significant shape. The prey may also exploit the learning capacity of the predator. For example, insectivorous birds see leaves but do not attack them because they have learned (or perhaps know innately) that these are not edible. Resemblances to leaves, twigs, thorns, flowers, parts of flowers, and more bizarre objects like fresh turds (usually bird droppings) are very widespread. This type of camouflage is termed mimetic camouflage. Camouflage in general is often termed cryptic coloration.
Coloration may be considered mimetic if protection is achieved by a resemblance to some other existing object, which is recognized by the predator but not associated in its mind with feeding. Usually this negative or neutral association is learned, but in a minority of instances it is almost certainly innate. Small birds have an innate flight response to large eyes in close-up (which normally indicate that a cat or a predatory bird is dangerously close). This reaction is exploited by many moths and other insects, which have eyelike markings, sometimes very convincing in their shading and highlighting, on concealed parts of the wings. Attack by a bird causes such a moth to change its posture rapidly to reveal the fake eyes, thus frightening away the attacker. Motmots (birds which habitually prey on snakes) have a similar innate fear of the red, black, and yellow striping patterns of the deadly coral snakes. These patterns are mimicked by various nonvenomous snakes, and even some caterpillars.
Flash coloration describes the phenomenon in which the prey is cryptic when at rest, but reveals brilliantly colored parts while escaping. This behavior seems to function simply by startling the predator. Very small eye marks at the tips of the wings, or a false head at the wrong end of the body (shown by some coral reef fish, for example) may cause the predator to misdirect its attack.
Protection through the possession of a chemical or physical defense that is dangerous to one's potential predator, accompanied by a strikingly conspicuous pattern known as warning coloration (often black, red, yellow, and white), is widespread---the ensemble of defense and color is termed aposematic. The actual defense ranges from toxic venoms through stings (in wasps, for example), to the oozing of noxious foams or hemolymph (as in ladybirds), to the possession of toxic chemicals (cyanides, cardiac glycosides, alkaloids) that will poison the predator or simply produce a revolting taste. The function of the warning color is to remind the predator of its previous unpleasant experience.
Sometimes the term mimicry is restricted to resemblances between edible species and actively defended and warningly colored models (as opposed to inedible objects such as thorns). Much is known about the evolution of this kind of mimicry in butterflies (and to a lesser extent, in bees and flies). If the mimic is entirely edible, the relationship is parasitic; the mimic benefits from the resemblance, but as every encounter with a mimic reduces the predator's aversion, the model suffers some increase in the rate of attack. Such mimicry has traditionally been termed Batesian mimicry. Alternatively, the mimic may be almost or fully as defended as the model, leading to a mutualistic relationship known as Müllerian mimicry, in which both the model and the mimic species suffer a decreased rate of predation.