Aposematism

Aposematism (from apo- away, and sematic sign/meaning), perhaps most commonly known in the context of warning colouration, describes a family of antipredator adaptations where a warning signal is associated with the unprofitability of a prey item to potential predators.^[1] It is one form of "advertising" signal, with many others existing such as the bright colours of flowers which lure pollinators. The warning signal may take the form of conspicuous colours, sounds, odours^[2] or other perceivable characteristics. Aposematic signals are beneficial for both the predator and prey, who both avoid potential harm.

This tendency to become highly noticeable and distinct from harmless organisms is the antithesis of crypsis, or avoidance of detection. Aposematism has been such a successful adaptation that harmless organisms have repeatedly evolved to mimic aposematic species, a pattern known as Batesian mimicry. Another related pattern is Müllerian mimicry, where aposematic species come to resemble one another.  

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Defense mechanism

Aposematism is a secondary defense mechanism that warns potential predators of the existence of another primary defensive mechanism. The organism's primary means of defense may include:

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^[3], or by warning postures which expose brightly-coloured body parts (see Unkenreflex). 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). Sharply contrasting black-and-white skunks are an example within mammals.

Behavior

  The defense mechanism relies on the memory of the would-be predator; a bird that has once experienced a foul-tasting grasshopper will endeavour to avoid a repetition of the experience. One consequence of this is that aposematic species are often gregarious. Before the memory of a bad experience attenuates, the predator may have the experience reinforced through repetition, or else leave all the remaining and similarly coloured prey alone and safe. Aposematic organisms often move in a languid fashion foregoing the need for speed and agility. Instead their morphology is frequently tough and resistant to injury thereby allowing the insect escape if the predator gets a bad taste or sting before killing it.

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. He also pointed out that John Jenner Weir had observed that birds in his aviary would not attempt to catch or eat a certain common white moth, and that a white moth at dusk would be as conspicuous as a brightly coloured caterpillar during the day. After Darwin responded enthusiastically to the suggestion, Wallace made a request at a meeting of the Entomological Society of London for data that would test the hypothesis. In response, John Jenner Weir conducted experiments with caterpillars and birds in his aviary for two years. The results he reported in 1869 provided the first experimental evidence for warning colouration in animals.^[4]

Mimicry

Further information: Mimicry

Aposematism is a sufficiently successful strategy that other organisms lacking the primary defence means may come to mimic the conspicuous markings of their genuinely aposematic counterparts. For example, the Aegeria moth is a mimic of the yellowjacket wasp; 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 mimics; their similar colouring 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.

There are other forms of mimicry not related to aposematism, though these two forms are among the best known and most studied.

See also

• Unkenreflex
• Handicap principle

Notes

References

• The arts of Deception and Camouflage, Mongabay.com
• The Functionality and Evolution of Aposematic Coloration, Sterling, T.

• Komarek, S. (1998). Mimicry, Aposematism and Related Phenomena in Animals & Plants. Vesmir. ISBN 80-85977-15-X. 

• 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.

• Slotten, Ross (2004), , New York: Columbia University Press, ISBN 0-231-13010-4