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Aposematism (from apo- away, and sematic warning), is a means of natural defence employing conspicuous colours, sounds, or other methods by which an organism openly projects a warning signal to potential predators. The intent to become as noticeable as possible is the antithesis of the principles of natural camouflage.
Defense mechanism
Aposematism is a secondary defence mechanism – the organism's primary means of defence may be either:- Unpalatability:such as from the bitter taste arising from some insects such as the ladybird or tiger moth, or the noxious odour produced by the skunk, or:
- Danger : such as the poison glands of the poison dart frog, the sting of a velvet ant or neurotoxin in a Black widow spider.
In these particular examples, the organism advertises its capabilities via either bright colouration in the case of the ladybird, poison frog and spider or by conspicuous stripes in the case of the skunk. Various types of tiger moth advertise their unpalatability by either producing ultrasonic noises which warn bats to avoid them, or by warning postures which expose brightly-colored body parts. Velvet ants have both bright colours and produce audible noises when grabbed (via stridulation), which serve to reinforce the warning.
Prevalence
Aposematism is widespread in invertebrates, particularly insects, but less so in vertebrates, being mostly confined to a smaller number of reptile, amphibian and fish species. Some plants, such as Polygonum sagittatum, a species of knotweed, are thought to employ aposematism to warn herbivores of chemical (such as unpalatability) or physical defences (such as prickled leaves or thorns).The defence mechanism relies on the memory of the would-be predator; a bird that has once tried to eat a foul-tasting grasshopper will endeavour to avoid a repetition of the experience. Aposematism tends therefore to be confined to species that are attempting to defend themselves from predation by advanced species.
Origins of the theory
Alfred Russel Wallace, in response to an 1866 letter from Charles Darwin, was the first to suggest that aposematism could be an evolutionary mechanism. Darwin had proposed that conspicuous colouring could be explained in many species by means of sexual selection practices, but had realised that this could not explain the bright colouring of some species of caterpillar since they were not sexually active. Wallace responded with the suggestion that as the contrasting coloured bands of a hornet warned of its defensive sting, so could the bright colours of the caterpillar warn of its unpalatability.Aposematic mimicry
Aposematism is a sufficiently successful strategy that other organisms lacking the primary defence means will attempt to mimic the conspicuous markings of their genuinely aposematic counterparts. For example, the Aegeria moth is a mimic of the yellowjacket wasp, because it resembles the wasp, but is not capable of stinging. A predator who would thus avoid the wasp would similarly avoid the Aegeria. This form of mimicry, where the mimic lacks the defensive capabilities of its 'model', is known as Batesian mimicry, after Henry Walter Bates, a British naturalist who studied Amazonian butterflies in the second half of the nineteenth century.
Batesian mimicry finds greatest success when the ratio of mimic to mimicked is low, otherwise predators learn to recognise the imposters. Batesian mimics are known to adapt their mimicry to match the prevalence of aposematic organisms in their environment.
A second form of aposematism mimicry occurs when two organisms share the same anti-predation defence and mimic each other, to the benefit of both species. This form of mimicry is known as Müllerian mimicry, after Fritz Müller, a German naturalist who studied the phenomenon in the Amazonian in the late nineteenth century. For example, a yellowjacket wasp and a honeybee are Müllerian mimcs; their similar coloring teaches predators that a striped pattern is the pattern of a stinging insect. Therefore, a predator who has come into contact with either a wasp or a honeybee will likely avoid both in the future.
References
- The arts of Deception and Camouflage, Mongabay.com
- The Functionality and Evolution of Aposematic Coloration, Sterling, T.
"Aposematism"