Aposematism

Aposematism is the advertising by an animal, whether terrestrial or marine, to potential predators that it is not worth attacking or eating. This unprofitability may consist of any defenses that make the prey difficult to kill and eat, such as toxicity, venom, foul taste or smell, sharp spines, or aggressive nature. These advertising signals may take the form of conspicuous coloration, sounds, odours, or other perceivable characteristics. Aposematic signals are beneficial for both predator and prey, because both avoid potential harm.

The term was coined in 1877 by Edward Bagnall Poulton for Alfred Russel Wallace's concept of warning coloration. Aposematism is exploited in Müllerian mimicry, wherein species with strong defences evolve to resemble one another. By mimicking similarly coloured species the warning signal to predators is shared, causing the predators to learn more quickly at less cost.

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 that 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' and σῆμα 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 defenses such as being unpalatable or poisonous. The easily detected warning is a primary defense mechanism, and the non-visible defenses 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 are attempts to startle predators with a threatening appearance but which are bluffing, i.e., 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 color combination that does not allow for easy detection by a predator from a distance but is warning-like up close, thus providing 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 odors, sounds, or behavior to provide a multi-modal signal that 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 odor, 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. Giant velvet mites (Dinothrombium spp.) have a foul taste and are also protected by a tough integument that is difficult to puncture. 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, in which it is confined primarily to a smaller number of reptile, amphibian, and fish species, as well as some foul-smelling or aggressive mammals. Pitohuis, red and black birds whose toxic feathers and skin apparently comes from the poisonous beetles they ingest, may also be included. It has been proposed that aposematism played a role in human evolution, with body odour carrying a warning to predators of large hominins able to defend themselves with weapons.