Aposematism

Aposematism (from Ancient Greek ἀπό apo away, σῆμα sema sign) refers to the appearance of an animal that warns predators it is toxic, distasteful, or dangerous. This warning signal is associated with the unprofitability of a prey item to potential predators. The unprofitability may consist of any defences which make the prey difficult to eat, such as toxicity, foul taste or smell, sharp spines, 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. The term was coined by Edward Bagnall Poulton for Alfred Russel Wallace's concept of warning coloration. 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. 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 lighting, are highly chromatic, and provide distance dependent camouflage. Some forms of warning coloration provide this distance dependent camouflage by having an effective pattern and colour combination that do not allow for easy detection by a predator from a distance, but are warning-like from a close proximity, allowing for an advantageous balance between camouflage and aposematism. 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, broadly understood, 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 (actually parasitic wasps) such as Dasymutilla occidentalis both have bright colours and produce audible noises when grabbed (via stridulation), which serve to reinforce the warning. Among mammals, predators can be dissuaded when a smaller animal is aggressive and able to defend itself, as for example in honey badgers. Aposematism is widespread in insects, but less so in vertebrates, being mostly confined to a smaller number of reptile, amphibian, and fish species, and some foul-smelling or aggressive mammals. Pitohuis, red and black birds whose toxic feathers and skin apparently comes from the poisonous beetles they ingest, could be included. It has been recently proposed that aposematism played a significant role in human evolution.