Crypsis aposematism and mimicry

An animal may be less obvious to a predator if it matches its background, or possesses a pattern that disrupts its outline, or resembles an inedible feature of its environment. Straightforward examples of such crypsis are the green coloration of many grasshoppers and caterpillars, and the transparency of many planktonic animals that inhabit the surface layers of oceans and lakes. More dramatic cases are the sargassum fish (Histrio pictus), whose body outline mimics the sargassum weed in which it is found, or the caterpillar of the viceroy butterfly (Limenitis archippus) that resembles a bird dropping. Cryptic animals may be highly palatable, but their morphology and color (and their choice of the appropriate background) reduce the likelihood that they will be used as a resource.

animal defenses one man's poison is another man's meat crypsis

Whilst crypsis may be a defense strategy for a palatable organism, noxious or dangerous animals often seem to advertize the fact by bright, conspicuous colors and patterns. This phenomenon is referred to as aposematism. The monarch butterfly, discussed above, is aposematically colored, as is its caterpillar, which actually sequesters the defensive cardiac glucosinolates from its food. The usual evolutionary argument for this runs as follows: conspicuous coloration will be favored because noxious prey will be recognized (memorized) as such by experienced predators, and thus will be protected, whereas the costs of 'educating' the predator will have been shared amongst the whole population of conspicuous prey. This argument, however, leaves unanswered the question of how conspicuous, noxious prey arose in the first place, since when initially rare, they seem likely to be repeatedly eliminated by naive (i.e. 'uneducated') predators (Speed & Ruxton, 2002). One possible answer is that predators and prey have coevolved: in each generation - from an original mixture of conspicuous and inconspicuous, noxious and edible prey - conspicuous edible prey are eliminated, and, with conspicuous prey therefore becoming disproportionately noxious, predators evolve an increased wariness for conspicuous prey (Sherratt, 2002).

The adoption of memorable body patterns by distasteful prey also immediately opens the door for deceit by other species, because there will be a clear evolutionary advantage to a palatable prey, 'the mimic', if it looks like an unpalatable species, 'the model' (Batesian mimicry). Developing the story of the monarch butterfly a little further, the adult of the palatable viceroy butterfly mimics the distasteful monarch, and a blue jay that has learned to avoid mon-archs will also avoid viceroys. There will also be an advantage to aposematically colored, distasteful prey in looking like one another (Mullerian mimicry), though many unanswered questions remain as to where exactly Batesian mimicry ends and Mullerian mimicry begins, in part because there are more theoretical viewpoints than impeccable data sets that might distinguish between them (Speed, 1999).

By living in holes (e.g. millipedes and moles) animals may avoid stimulating the sensory receptors of predators, and by 'playing dead' (e.g. the opossum Didelphis virginiana and African ground squirrels) animals may fail to stimulate a killing response. Animals that withdraw to a prepared retreat (e.g. rabbits and prairie dogs to their burrows, snails to their shells), or which roll up and protect their vulnerable parts by a tough exterior (e.g. armadillos and pill millipedes), reduce their chance of capture but stake their lives on the chance that the attacker will not be able to breach their defenses. Other animals seem to try to bluff themselves out of trouble by threat displays. The startle response of moths and butterflies that suddenly expose eye-spots on their wings is one example. No doubt the most common behavioral response of an animal in danger of being preyed upon is to flee.

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