Warning color

Aposematism (from Greek ἀπό apo away, σῆμα sema sign) is a term coined by Edward Bagnall Poulton for Alfred Russel Wallace's concept of warning coloration. It describes a family of antipredator adaptations in which a warning signal is associated with the unprofitability of a prey item to potential predators. The unprofitability may consist of any defences which make a prey difficult to eat, such as toxicity, foul taste or smell, or aggressive nature. Aposematism always involves an advertising signal which may take the form of conspicuous animal coloration, sounds, odours or other perceivable characteristics. Aposematic signals are beneficial for both the predator and prey, since both avoid potential harm.

Aposematism is exploited in Müllerian mimicry, where species with strong defences evolve to resemble one another. By mimicking similarly coloured species, the warning signal to predators is shared, causing them to learn more quickly at less of a cost to each of the species.

A genuine aposematic signal that a species actually possesses chemical or physical defences is not the only way to deter predators. In Batesian mimicry, a mimicking species resembles an aposematic model closely enough to share the protection, while many species have bluffing deimatic displays which may startle a predator long enough to enable an otherwise undefended prey to escape.

The term aposematism was coined by the English zoologist Edward Bagnall Poulton in his 1890 book The Colours of Animals. He based the term on the Ancient Greek words ἀπό apo away, ση̑μα sēma sign, referring to signs that warn other animals away.

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. This is in contrast to deimatic displays, which attempt to startle a predator with a threatening appearance but which are bluffing, unsupported by any strong defences.

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