Aposematism
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Aposematism (from apo- away, and sematic warning), is a means of natural defence employing conspicuous colours or body parts by which an organism publicly displays a warning signal to potential predators. The intent to become as noticeable as possible is the antithesis of the principles of natural camouflage.
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Defence 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 the noxious odour produced by the skunk, or:
- Danger
- such as the poison glands of the poison dart frog.
In these particular examples, the organism advertises its capabilities via either bright colouration in the case of the ladybird and poison frog, or by conspicuous stripes in the case of the skunk.
Prevalence
Aposematism is a 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 unpalatibility) 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. The Viceroy butterfly, for example, imitates the distinctive orange and black patterned wings of the unpleasant-tasting Monarch butterfly. 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 phenomenom in the Amazonian in the late nineteenth century.
References
- The arts of Deception and Camouflage, Mongabay.com
- The Functionality and Evolution of Aposematic Coloration, Sterling, T.
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- Rubino, D. & McCarthy, B. (2004). Presence of aposematic (warning) coloration in vascular plants of southeastern Ohio. Journal of the Torrey Botanical Society 131(3): 252–256.
