Aposematism (from Greek ἀπό apo away, σ̑ημα sema sign, coined by Edward Bagnall Poulton), perhaps most commonly known in the context of warning coloration, describes a family of antipredator adaptations where a warning signal is associated with the unprofitability of a prey item to potential predators. 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 or other perceivable characteristics. Aposematic signals are beneficial for both the predator and prey, both of which 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.
The function of aposematism is to prevent attack, by warning potential predators that the prey animal has defences such as being unpalatable or poisonous. The easily-detected warning is a primary defence mechanism, and the non-visible defences are secondary. Aposematic signals are primarily visual, using bright colours and high-contrast patterns such as stripes. Warning signals are honest indications of noxious prey, because conspicuousness evolves in tandem with noxiousness. Thus, the brighter and more conspicuous the organism, the more toxic it usually is. The most common and effective colours are red, yellow, black and white. These colours provide strong contrast with green foliage, resist changes in shadow and luminescence, have luminescence contrast, are highly chromatic, and provide distance dependent camouflage. Warning coloration evolves in response to background, light conditions, and predator vision. Visible signals may be accompanied by odours, sounds or behaviour to provide a multi-modal signal which is more effectively detected by predators.
Unpalatability can be created in a variety of ways. Some insects such as the ladybird or tiger moth contain bitter-tasting chemicals, while the skunk produces a noxious odour, and the poison glands of the poison dart frog, the sting of a velvet ant or neurotoxin in a black widow spider make them dangerous or painful to attack. Tiger moths advertise their unpalatability by either producing ultrasonic noises which warn bats to avoid them, or by warning postures which expose brightly coloured body parts (see Unkenreflex), or exposing eyespots. Velvet ants both have bright colours and produce audible noises when grabbed (via stridulation), which serve to reinforce the warning.
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). Many insects, such as cinnabar moth caterpillars, acquire toxic chemicals from their host plants. Sharply contrasting black-and-white skunks and zorillas are examples within mammals. Some brightly coloured birds with contrasting patterns may also be aposematic.
The defence 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. As a consequence, 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, as they have little need for speed and agility. Instead, their morphology is frequently tough and resistant to injury, thereby allowing them to escape once the predator gets a bad taste or sting before the kill.
Origins of the theory
Alfred Russel Wallace, in response to an 1866 letter from Charles Darwin, was the first to suggest that the conspicuous colour schemes of some insects might have evolved through natural selection as a warning to predators. Darwin had proposed that conspicuous colouring could be explained in many species by means of sexual selection, 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 could be used to 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. The term aposematism was introduced by Wallace's friend Edward Bagnall Poulton in The Colours of Animals (1890).
Aposematism is paradoxical in evolutionary terms, as it makes individuals conspicuous to predators, so they may be killed and the trait eliminated before predators learn to avoid it. A possible explanation is that predators fear unfamiliar forms (neophobia) long enough for them to become established, but this is likely to be only temporary. A second possibility is dietary conservatism, in which predators avoid new prey because it is an unknown quantity, a longer-lasting effect than neophobia. Dietary conservatism has been demonstrated experimentally in some species of birds. A third possibility is gregariousness, where prey animals cluster tightly enough to enhance the warning signal. If the species was already unpalatable, predators might learn to avoid the cluster, protecting gregarious individuals with the new aposematic trait. Gregariousness assists predators to learn to avoid unpalatable, gregarious prey. Aposematism may also be favoured in dense populations even if these are not gregarious. A fourth possibility is that a gene for aposematism may be recessive and located on the X chromosome. If so, predators learn to associate the colour with unpalatability from males with the trait, while heterozygous females carry the trait until it becomes common and predators understand the signal. Well-fed predators may also ignore aposematic morphs, preferring other prey species.
Aposematism is a sufficiently successful strategy that other organisms lacking the same secondary defence means may come to mimic the conspicuous markings of their genuinely aposematic counterparts. For example, the Aegeria moth is a mimic of the yellow jacket wasp; it resembles the wasp, but is not capable of stinging. A predator which 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 19th century. Batesian mimicry finds greatest success when the ratio of 'mimic' to 'mimicked' is low; otherwise, predators learn to recognise the impostors. 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 antipredation 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 Amazon in the late 19th century. For example, a yellow jacket 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.
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- Ruxton, G. D.; Speed, M. P.; Sherratt, T. N. (2004). Avoiding Attack. The Evolutionary Ecology of Crypsis, Warning Signals and Mimicry. Oxford: Oxford University Press. ISBN 0-19-852860-4