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.[1] 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[2] 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[3][4] for Alfred Russel Wallace's concept of warning coloration.[5] 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.

Honey badger
The honey badger's reverse countershading makes it conspicuous, honestly signalling its ability to defend itself through its aggressive temperament, and its sharp teeth and claws.

Etymology

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.[3][4]

Defence mechanism

Metasepia pfefferi 1
Flamboyant cuttlefish colours warn of toxicity

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.[6] 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.[7] Thus, the brighter and more conspicuous the organism, the more toxic it usually is.[7][8] 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.[9]

The most common and effective colours are red, yellow, black and white.[10] These colours provide strong contrast with green foliage, resist changes in shadow and lighting, are highly chromatic, and provide distance dependent camouflage.[10] 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.[11] Warning coloration evolves in response to background, light conditions, and predator vision.[12] Visible signals may be accompanied by odours, sounds or behaviour to provide a multi-modal signal which is more effectively detected by predators.[13]

Hycleus lugens, Meloidae
Hycleus lugens, an aposematically coloured beetle

Unpalatability, broadly understood, can be created in a variety of ways. Some insects such as the ladybird or tiger moth contain bitter-tasting chemicals,[14] 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,[15] 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.[16] Among mammals, predators can be dissuaded when a smaller animal is aggressive and able to defend itself, as for example in honey badgers.[17]

Prevalence

Striped skunk Florida
The skunk, an aposematic mammal

In terrestrial ecosystems

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.[18] It has been recently proposed that aposematism played a significant role in human evolution.[19]

Perhaps the most numerous aposematic vertebrates are the poison dart frogs (family: Dendrobatidae).[20] These neotropical anuran amphibians exhibit a wide spectrum of coloration and toxicity.[21] Some species in this poison frog family (particularly Dendrobates, Epipedobates, and Phyllobates) are conspicuously colored and sequester one of the most toxic alkaloids among all living species. Within the same family, there are also cryptic frogs (such as Colostethus, Mannophryne, and Nephelobates) that lack these toxic alkaloids.[22] Although these frogs display an extensive array of coloration and toxicity, there is very little genetic difference between the species.[23] Evolution of their conspicuous coloration is correlated to traits such as chemical defense, dietary specialization, and increased body mass.[24]

Some plants are thought to employ aposematism to warn herbivores of unpalatable chemicals or physical defences such as prickled leaves or thorns.[25] Many insects, such as cinnabar moth caterpillars, acquire toxic chemicals from their host plants.[26] Among mammals, skunks and zorillas advertise their foul-smelling chemical defences with sharply contrasting black-and-white patterns on their fur, while the similarly-patterned badger and honey badger advertise their sharp claws, powerful jaws, and aggressive natures.[27] Some brightly coloured birds such as passerines with contrasting patterns may also be aposematic, at least in females; but since male birds are often brightly coloured through sexual selection, and their coloration is not correlated with edibility, it is unclear whether aposematism is significant.[28]

Phyllidia
There is evidence that nudibranchs like Phyllidia varicosa are aposematic.

In marine ecosystems

Crown of Thorns-jonhanson
Conspicuous colours of crown-of-thorns starfish spines may warn of strong toxins within.[29][30]

The existence of aposematism in marine ecosystems is controversial.[31] Many marine organisms, particularly those on coral reefs, are brightly coloured or patterned, including sponges, corals, molluscs and fishes, with little or no connection to chemical or physical defenses. Caribbean reef sponges are brightly coloured, and many species are full of toxic chemicals, but there is no relationship between the two factors.[32]

Nudibranch molluscs are the most commonly cited examples of aposematism in marine ecosystems, but the evidence for this has been contested,[33] mostly because (1) there are few examples of mimicry among species, (2) many species are nocturnal or cryptic, and (3) bright colours at the red end of the colour spectrum are rapidly attenuated as a function of water depth. For example, the Spanish Dancer nudibranch (genus Hexabranchus), among the largest of tropical marine slugs, potently chemically defended, and brilliantly red and white, is nocturnal and has no known mimics. Mimicry is to be expected as Batesian mimics with weak defences can gain a measure of protection from their resemblance to aposematic species.[34] Other studies have concluded that nudibranchs such as the slugs of the family Phyllidiidae from Indo-Pacific coral reefs are aposematically coloured.[35] Müllerian mimicry has been implicated in the coloration of some Mediterranean nudibranchs, all of which derive defensive chemicals from their sponge diet.[36]

The crown-of-thorns starfish, like other starfish such as Metrodira subulata, has conspicuous coloration and conspicuous long, sharp spines, as well as cytolytic saponins, chemicals which could function as an effective defence; this evidence is argued to be sufficient for such species to be considered aposematic.[29][30] It has been proposed that aposematism and mimicry is less evident in marine invertebrates than terrestrial insects because predation is a more intense selective force for many insects, which also disperse as adults rather than as larvae and have much shorter generation times.[31] Further, there is evidence that fish predators such as blueheads may adapt to visual cues more rapidly than do birds, making aposematism less effective.[37]

Variable ring patterns on mantles of the blue-ringed octopus Hapalochlaena lunulata
Iridescent blue rings on the mantles of the venomous octopus Hapalochlaena lunulata are considered by some to be aposematic.

Blue-ringed octopuses are venomous. They spend much of their time hiding in crevices whilst displaying effective camouflage patterns with their dermal chromatophore cells. However, if they are provoked, they quickly change colour, becoming bright yellow with each of the 50-60 rings flashing bright iridescent blue within a third of a second.[38] It is often stated this is an aposematic warning display,[39][40][41][42] but the hypothesis has rarely if ever been tested.[43]

Behaviour

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. 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 is warned off. Aposematic species do not need to hide or stay still as cryptic organisms do, so aposematic individuals benefit from more freedom in exposed areas and can spend more time foraging, allowing them to find more and better quality food.[44] Aposematic individuals can similarly make use of conspicuous mating displays.[45]

Origins of the theory

Lygeaus kalmii nymphs
Gregarious nymphs of an aposematic milkweed bug, Lygaeus kalmii

Wallace, 1867

In a letter to Alfred Russel Wallace dated 23 February 1867 Charles Darwin wrote "On Monday evening I called on Bates & put a difficulty before him, which he could not answer, & as on some former similar occasion, his first suggestion was, 'you had better ask Wallace'. My difficulty is, why are caterpillars sometimes so beautifully & artistically coloured?"[46] Darwin was puzzled because his theory of sexual selection (where females choose their mates based on how attractive they are) could not apply to caterpillars since they are immature and hence not sexually active.

Wallace replied the next day with the suggestion that since some caterpillars "...are protected by a disagreeable taste or odour, it would be a positive advantage to them never to be mistaken for any of the palatable catterpillars [sic], because a slight wound such as would be caused by a peck of a bird’s bill almost always I believe kills a growing catterpillar. Any gaudy & conspicuous colour therefore, that would plainly distinguish them from the brown & green eatable catterpillars, would enable birds to recognise them easily as at a kind not fit for food, & thus they would escape seizure which is as bad as being eaten."[47]

Since Darwin was enthusiastic about the idea, Wallace asked the Entomological Society of London to test the hypothesis.[48] In response, the entomologist John Jenner Weir conducted experiments with caterpillars and birds in his aviary, and in 1869 he provided the first experimental evidence for warning coloration in animals.[49] The evolution of aposematism surprised 19th century naturalists because the probability of its establishment in a population was presumed to be low, since a conspicuous signal suggested a higher chance of predation.[50]

Poulton, 1890

The Colours of Animals Classified According to Their Uses from Poulton 1890 (first use of Aposematic)
First edition of Edward Bagnall Poulton's The Colours of Animals, 1890, introduced a set of new terms for animal coloration including "aposematic".

Wallace coined the term "warning colours" in an article about animal coloration in 1877.[5] In 1890 Edward Bagnall Poulton renamed the concept aposematism in his book The Colours of Animals.[4] He described the derivation of the term as follows:

The second head (Sematic Colours) includes Warning Colours and Recognition Markings: the former warn an enemy off, and are therefore called Aposematic [Greek, apo, from, and sema, sign][51]

Evolution

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. If warning coloration puts the first few individuals at such a strong disadvantage, it would never last in the species long enough to become beneficial.[52]

Supported explanations

There is evidence for explanations involving dietary conservatism, in which predators avoid new prey because it is an unknown quantity;[53] this is a long-lasting effect.[53][54][55] Dietary conservatism has been demonstrated experimentally in some species of birds and fish.[56][53][55][57] Further, birds recall and avoid objects that are both conspicuous and foul-tasting longer than objects that are equally foul-tasting but cryptically coloured[58]. This suggests that Wallace's original view, that warning coloration helped to teach predators to avoid prey thus coloured, was correct.[59] However, some birds (inexperienced starlings and domestic chicks) also innately avoid conspicuously coloured objects, as demonstrated using mealworms painted yellow and black to resemble wasps, with dull green controls. This implies that warning coloration works at least in part by stimulating the evolution of predators to encode the meaning of the warning signal, rather than by requiring each new generation to learn the signal's meaning.[59] All of these results contradict the idea that novel, brightly coloured individuals would be more likely to be eaten or attacked by predators.[53][60]

Alternative hypotheses

Other explanations are possible. Predators might innately fear unfamiliar forms (neophobia)[61] long enough for them to become established, but this is likely to be only temporary.[52][61][62]

Alternatively, prey animals might be sufficiently gregarious to form clusters tight 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.[63][64] Gregariousness would assist predators to learn to avoid unpalatable, gregarious prey.[65] Aposematism could also be favoured in dense populations even if these are not gregarious.[53][61]

Another possibility is that a gene for aposematism might be recessive and located on the X chromosome.[66] If so, predators would 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.[66] Well-fed predators might also ignore aposematic morphs, preferring other prey species.[52][67]

A further explanation is that females might prefer brighter males, so sexual selection could result in aposematic males having higher reproductive success than non-aposematic males if they can survive long enough to mate. Sexual selection is strong enough to allow seemingly maladaptive traits to persist despite other factors working against the trait.[20]

Once aposematic individuals reach a certain threshold population, for whatever reason, the predator learning process would be spread out over a larger number of individuals and therefore is less likely to wipe out the trait for warning coloration completely.[68] If the population of aposematic individuals all originated from the same few individuals, the predator learning process would result in a stronger warning signal for surviving kin, resulting in higher inclusive fitness for the dead or injured individuals through kin selection.[69]

A theory for the evolution of aposematism posits that it arises by reciprocal selection between predators and prey, where distinctive features in prey, which could be visual or chemical, are selected by non-discriminating predators, and where, concurrently, avoidance of distinctive prey is selected by predators. Concurrent reciprocal selection (CRS) may entail learning by predators or it may give rise to unlearned avoidances by them. Aposematism arising by CRS operates without special conditions of the gregariousness or the relatedness of prey, and it is not contingent upon predator sampling of prey to learn that aposematic cues are associated with unpalatability or other unprofitable features.[70]

Mimicry

Micrurus tener
A venomous and genuinely aposematic coral snake
Red milk snake
The harmless red milk snake, a Batesian mimic of the coral snake

Aposematism is a sufficiently successful strategy to have had significant effects on the evolution of both aposematic and non-aposematic species.

Non-aposematic species have often evolved to mimic the conspicuous markings of their aposematic counterparts. For example, the hornet moth is a deceptive mimic of the yellowjacket wasp; it resembles the wasp, but has no sting. A predator which avoids the wasp will to some degree also avoid the moth. This is known as Batesian mimicry, after Henry Walter Bates, a British naturalist who studied Amazonian butterflies in the second half of the 19th century.[71] Batesian mimicry is frequency dependent: it is most effective when the ratio of mimic to model is low; otherwise, predators will encounter the mimic too often.[72][73]

A second form of mimicry occurs when two aposematic organisms share the same anti-predator adaptation and non-deceptively mimic each other, to the benefit of both species, since fewer individuals of either species need to be attacked for predators to learn to avoid both of them. 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.[74][75] Many species of bee and wasp that occur together are Müllerian mimics; their similar coloration teaches predators that a striped pattern is associated with being stung. Therefore, a predator which has had a negative experience with any such species will likely avoid any that resemble it in the future. Müllerian mimicry is found in vertebrates such as the mimic poison frog (Ranitomeya imitator) which has several morphs throughout its natural geographical range, each of which looks very similar to a different species of poison frog which lives in that area.[76]

See also

References

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Anti-predator adaptation

Anti-predator adaptations are mechanisms developed through evolution that assist prey organisms in their constant struggle against predators. Throughout the animal kingdom, adaptations have evolved for every stage of this struggle, namely by avoiding detection, warding off attack, fighting back, or escaping when caught.

The first line of defence consists in avoiding detection, through mechanisms such as camouflage, masquerade, apostatic selection, living underground, or nocturnality.

Alternatively, prey animals may ward off attack, whether by advertising the presence of strong defences in aposematism, by mimicking animals which do possess such defences, by startling the attacker, by signalling to the predator that pursuit is not worthwhile, by distraction, by using defensive structures such as spines, and by living in a group. Members of groups are at reduced risk of predation, despite the increased conspicuousness of a group, through improved vigilance, predator confusion, and the likelihood that the predator will attack some other individual.

Some prey species are capable of fighting back against predators, whether with chemicals, through communal defence, or by ejecting noxious materials. Many animals can escape by fleeing rapidly, outrunning or outmanoeuvring their attacker.

Finally, some species are able to escape even when caught by sacrificing certain body parts: crabs can shed a claw, while lizards can shed their tails, often distracting predators long enough to permit the prey to escape.

Arctiinae (moth)

The Arctiinae (formerly called the family Arctiidae) are a large and diverse subfamily of moths, with around 11,000 species found all over the world, including 6,000 neotropical species. This group includes the groups commonly known as tiger moths (or tigers), which usually have bright colours, footmen, which are usually much drabber, lichen moths, and wasp moths. Many species have "hairy" caterpillars that are popularly known as woolly bears or woolly worms. The scientific name of this subfamily refers to this hairiness (Gk. αρκτος = a bear). Some species within the Arctiinae have the word “tussock” in their common name due to people misidentifying them as members of the Lymantriinae based on the characteristics of the larvae.

Coloration evidence for natural selection

Animal coloration provided important early evidence for evolution by natural selection, at a time when little direct evidence was available. Three major functions of coloration were discovered in the second half of the 19th century, and subsequently used as evidence of selection: camouflage (protective coloration); mimicry, both Batesian and Müllerian; and aposematism.

Charles Darwin's On the Origin of Species was published in 1859, arguing from circumstantial evidence that selection by human breeders could produce change, and that since there was clearly a struggle for existence, that natural selection must be taking place. But he lacked an explanation either for genetic variation or for heredity, both essential to the theory. Many alternative theories were accordingly considered by biologists, threatening to undermine Darwinian evolution.

Some of the first evidence was provided by Darwin's contemporaries, the naturalists Henry Walter Bates and Fritz Müller. They described forms of mimicry that now carry their names, based on their observations of tropical butterflies. These highly specific patterns of coloration are readily explained by natural selection, since predators such as birds which hunt by sight will more often catch and kill insects that are less good mimics of distasteful models than those that are better mimics; but the patterns are otherwise hard to explain.

Darwinists such as Alfred Russel Wallace and Edward Bagnall Poulton, and in the 20th century Hugh Cott and Bernard Kettlewell, sought evidence that natural selection was taking place. Wallace noted that snow camouflage, especially plumage and pelage that changed with the seasons, suggested an obvious explanation as an adaptation for concealment. Poulton's 1890 book, The Colours of Animals, written during Darwinism's lowest ebb, used all the forms of coloration to argue the case for natural selection. Cott described many kinds of camouflage, and in particular his drawings of coincident disruptive coloration in frogs convinced other biologists that these deceptive markings were products of natural selection. Kettlewell experimented on peppered moth evolution, showing that the species had adapted as pollution changed the environment; this provided compelling evidence of Darwinian evolution.

Crucifix toad

The crucifix toad or holy cross frog (Notaden bennettii) is an Australian, fossorial frog. It is one of the few Australian frogs to display aposematism. It is native to western New South Wales, and south western Queensland.

Crypsis

In ecology, crypsis is the ability of an animal to avoid observation or detection by other animals. It may be a predation strategy or an antipredator adaptation. Methods include camouflage, nocturnality, subterranean lifestyle and mimicry. Crypsis can involve visual, olfactory (with pheromones), or auditory concealment. When it is visual, the term cryptic coloration, effectively a synonym for animal camouflage, is sometimes used, but many different methods of camouflage are employed by animals.

Decorator crab

Decorator crabs are crabs of several different species, belonging to the superfamily Majoidea (not all of which are decorators), that use materials from their environment to hide from, or ward off, predators. They decorate themselves by sticking mostly sedentary animals and plants to their bodies as camouflage, or if the attached organisms are noxious, to ward off predators through aposematism.

Disruptive coloration

Disruptive coloration (also known as disruptive camouflage or disruptive patterning) is a form of camouflage that works by breaking up the outlines of an animal, soldier or military vehicle with a strongly contrasting pattern. It is often combined with other methods of crypsis including background colour matching and countershading; special cases are coincident disruptive coloration and the disruptive eye mask seen in some fishes, amphibians, and reptiles. It appears paradoxical as a way of not being seen, since disruption of outlines depends on high contrast, so the patches of colour are themselves conspicuous.

The importance of high-contrast patterns for successful disruption was predicted in general terms by the artist Abbott Thayer in 1909 and explicitly by the zoologist Hugh Cott in 1940. Later experimental research has started to confirm these predictions. Disruptive patterns work best when all their components match the background.

While background matching works best for a single background, disruptive coloration is a more effective strategy when an animal or a military vehicle may have a variety of backgrounds.

Conversely, poisonous or distasteful animals that advertise their presence with warning coloration (aposematism) use patterns that emphasize rather than disrupt their outlines. For example, skunks, salamanders and monarch butterflies all have high-contrast patterns that display their outlines.

Edward Bagnall Poulton

Sir Edward Bagnall Poulton, FRS HFRSE FLS (27 January 1856 – 20 November 1943) was a British evolutionary biologist who was a lifelong advocate of natural selection through a period in which many scientists such as Reginald Punnett doubted its importance. He invented the term sympatric for evolution of species in the same place, and in his book The Colours of Animals (1890) was the first to recognise frequency-dependent selection.

Poulton is also remembered for his pioneering work on animal coloration. He is credited with inventing the term aposematism for warning coloration, as well as for his experiments on 'protective coloration' (camouflage).

Poulton became Hope Professor of Zoology at the University of Oxford in 1893.

Heliconius

Heliconius comprises a colorful and widespread genus of brush-footed butterflies commonly known as the longwings or heliconians. This genus is distributed throughout the tropical and subtropical regions of the New World, from South America as far north as the southern United States. The larvae of these butterflies eat passion flower vines (Passifloraceae). Adults exhibit bright wing color patterns to signal their distastefulness to potential predators.

Brought to the forefront of scientific attention by Victorian naturalists, these butterflies exhibit a striking diversity and mimicry, both amongst themselves and with species in other groups of butterflies and moths. The study of Heliconius and other groups of mimetic butterflies allowed the English naturalist Henry Walter Bates, following his return from Brazil in 1859, to lend support to Charles Darwin, who had found similar diversity amongst the Galapagos finches.

Life That Glows

Life That Glows is a 2016 British nature documentary programme made for BBC Television, first shown in the UK on BBC Two on 9 May 2016. The programme is presented and narrated by Sir David Attenborough.

Life That Glows films the biology and ecology of bioluminescent organisms, that is, capable of creating light. The programme features fireflies, who use light as a means of sexual attraction, luminous fungi, luminous marine bacteria responsible for the Milky seas effect, the flashlight fish, the aposematism of the Sierra luminous millipede, earthworms, the bioluminescent tides created by blooms of dinoflagellates in Tasmania, as well as dolphins swimming in the bloom in the Sea of Cortez, the defensive flashes of brittle stars and ostracods, sexual attraction in ostracods, prey attraction by luminous click beetles in Cerrado,Brazil and the Arachnocampa gnats in New Zealand.

The programme then introduces many luminous deep sea animals, including the vampire squid, the polychaete worm Tomopteris that generates yellow light, the jellyfish Atolla, the comb jelly Beroe, the viper fish, pyrosomes, a dragonfish, and the polychaete worm Flota. Then, the programme discusses specialised adaptations in the eyes of particular animals to see bioluminescence, such as the barreleye fish and the cock-eyed squid. Lastly, they feature the mass spawning event of the firefly squid in Japan.

Müllerian mimicry

Müllerian mimicry is a natural phenomenon in which two or more unprofitable (often, distasteful) species, that may or may not be closely related and share one or more common predators, have come to mimic each other's honest warning signals, to their mutual benefit, since predators can learn to avoid all of them with fewer experiences. It is named after the German naturalist Fritz Müller, who first proposed the concept in 1878, supporting his theory with the first mathematical model of frequency-dependent selection, one of the first such models anywhere in biology.Müllerian mimicry was first identified in tropical butterflies that shared colourful wing patterns, but it is found in many groups of insects such as bumblebees, and other animals including poison frogs and coral snakes. The mimicry need not be visual; for example, many snakes share auditory warning signals. Similarly, the defences involved are not limited to toxicity; anything that tends to deter predators, such as foul taste, sharp spines, or defensive behaviour can make a species unprofitable enough to predators to allow Müllerian mimicry to develop.

Once a pair of Müllerian mimics has formed, other mimics may join them by advergent evolution (one species changing to conform to the appearance of the pair, rather than mutual convergence), forming mimicry rings. Large rings are found for example in velvet ants. Since the frequency of mimics is negatively correlated with survivability, rarer mimics are likely to adapt to resemble commoner models, favouring both advergence and larger Müllerian mimicry rings. Where mimics are not strongly protected by venom or other defences, honest Müllerian mimicry grades into bluffing Batesian mimicry.

Ophidiasteridae

The Ophidiasteridae (Greek ophidia, Οφιδια, "of snakes", diminutive form) are a family of sea stars with about 30 genera. Occurring both in the Indo-Pacific and Atlantic Oceans, ophidiasterids are greatest in diversity in the Indo-Pacific. Many of the genera in this family exhibit brilliant colors and patterns, which sometimes can be attributed to aposematism and crypsis to protect themselves from predators. Some ophidiasterids possess remarkable powers of regeneration, enabling them to either reproduce asexually or to survive serious damage made by predators or forces of nature (an example for this is the genus Linckia). Some species belonging to Linckia, Ophidiaster and Phataria shed single arms that regenerate the disc and the remaining rays to form a complete individual. Some of these also reproduce asexually by parthenogenesis.The name of the family is taken from the genus Ophidiaster, whose limbs are slender, semitubular and serpentine.

Pachliopta hector

Pachliopta hector, the crimson rose, is a large swallowtail butterfly belonging to the genus Pachliopta (roses) of the red-bodied swallowtails.

Peacock-pheasant

The peacock-pheasants are a bird genus, Polyplectron, of the family Phasianidae, consisting of eight species. They are colored inconspicuously, relying on heavily on crypsis to avoid detection. When threatened, peacock-pheasants will alter their shapes using specialised plumage that when expanded reveals numerous iridescent orbs. The birds also vibrate their plume quills further accentuating their aposematism. Peacock-pheasants exhibit well developed metatarsal spurs. Older individuals may have multiple spurs on each leg. These kicking thorns are used in self-defense.

The systematics of the genus are somewhat unclear. Molecular research has revealed that peacock-pheasants are not genetically related to pheasants and only distantly to peafowl. Their closest allies are the Asiatic spurfowl and the crimson-headed partridge, endemic to Borneo. These three genera share the curious tendency for multiple metatarsal spurs. Though they are somewhat divergent morphologically, their skeletons are nearly identical.

The species of Polyplectron diverged at some time between, roughly, the Early Pliocene and the Middle Pleistocene, or 5–1 million years ago. Polyplectron malacense and P. schleiermacheri form a basal radiation around the southern South China Sea together with P. napoleonis, as is confirmed by comparison of biogeography and mtDNA cytochrome b and D-loop as well as the nuclear ovomucoid intron G.The relationships of the other forms are more poorly understood. P. germaini and P. bicalcaratum are similar in morphology and are nearly parapatric; the molecular data suggests that the latter is a symplesiomorphy. It would appear that P. germaini and P. katsumatae represent an early offshoot of the aforementioned basal radiation. The two montane-adapted species P. chalcurum and P. inopinatum are not derived from a single isolation event, and appear to have acquired more subdued coloration independently. A trend in this genus to lose—not gain—pronounced sexual dimorphism is better supported by biogeographical and molecular data than the alternate scenario.In 2010 the IOC World Bird List listed the Hainan peacock-pheasant as a species. Following Jean Théodore Delacour, this species has historically been listed as a subspecies of P. bicalcaratum. Prior to reclassification by Delacour, the Hainan peacock-pheasant has been considered a distinct species by several ornithologists. Indeed, when it was first described to science by Katsumata it was considered a distinct species. Prominent organizations including the World Bird List have recently concurred, and species status is currently under review by the Oriental Bird Club. It is considered of utmost importance to have the Hainan peacock-pheasant recognized as a full species due to its endangered status. The Hainan peacock-pheasant is endemic to the island of Hainan, where its population density is very low in its tropical forest habitat on the island and the wild population is declining, making it now severely endangered and among the rarest species in the order Galliformes in China.

Phyllobates

Phyllobates is a genus of poison dart frogs native to Central and South America, from Nicaragua to Colombia.

Phyllobates contains the most poisonous species of frog, the golden poison frog (P. terribilis). They are typical of the poison dart frogs, in that all species have bright warning coloration (aposematism), and have varying degrees of toxicity. Only species of Phyllobates are used by natives of South American tribes as sources of poison for their hunting darts. The most toxic of the many poisonous alkaloids these frogs emit from their skins is batrachotoxin, alongside a wide variety of other toxic compounds.

Pieris brassicae

Pieris brassicae, the large white, also called cabbage butterfly, cabbage white, cabbage moth (erroneously), or in India the large cabbage white, is a butterfly in the family Pieridae. It is a close relative of the small white, Pieris rapae.

The large white is common throughout Europe, North Africa and Asia.

Poison dart frog

Poison dart frog (also known as dart-poison frog, poison frog or formerly known as poison arrow frog) is the common name of a group of frogs in the family Dendrobatidae which are native to tropical Central and South America. These species are diurnal and often have brightly colored bodies. This bright coloration is correlated with the toxicity of the species, making them aposematic. Some species of the family Dendrobatidae exhibit extremely bright coloration along with high toxicity, while others have cryptic coloration with minimal to no amount of observed toxicity. The species that have great toxicity derive this from their diet of ants, mites and termites. Other species however, that exhibit cryptic coloration and low to no amounts of toxicity, eat a much larger variety of prey. Many species of this family are threatened due to human infrastructure encroaching the places they inhabit.

These amphibians are often called "dart frogs" due to the Amerindians' indigenous use of their toxic secretions to poison the tips of blowdarts. However, of over 170 species, only four have been documented as being used for this purpose (curare plants are more commonly used), all of which come from the genus Phyllobates, which is characterized by the relatively large size and high levels of toxicity of its members.

Thomas N. Sherratt

Thomas N. Sherratt, known as Tom, is a professor of evolutionary ecology at Carleton University, Canada. He is known for his research on camouflage, aposematism and mimicry.

Tymbal

The tymbal (or timbal) is the corrugated exoskeletal structure used to produce sounds in insects. In male cicadas, the tymbals are membranes in the abdomen, responsible for the characteristic sound produced by the insect. In tiger moths, the tymbals are modified regions of the thorax, and produce high-frequency clicks. In lesser wax moths the left and right tymbals emit high frequency pulses that are used as mating calls.The paired tymbals of a cicada are located on the sides of the abdominal base. The "singing" of a cicada is not stridulation as in many other familiar sound-producing insects like crickets (where one structure is rubbed against another): the tymbals are regions of the exoskeleton that are modified to form a complex membrane with thin, membranous portions and thickened "ribs". These membranes vibrate rapidly, and enlarged chambers derived from the tracheae make the cicada's body serve as a resonance chamber, greatly amplifying the sound. Some cicadas produce sounds louder than 106 dB (SPL), among the loudest of all insect-produced sounds. They modulate their noise by positioning their abdomens toward or away from the substrate.

The tymbals of a tiger moth are specialized regions on the metathoracic episterna, normally corrugated such that sound is produced when the entire tymbal surface is buckled by muscular contraction and then released, producing a series of extremely rapid "clicks" as the corrugations flex back into place. These sounds are only occasionally audible to humans, and are used in both acoustic aposematism (the moths are advertising to bats that they are toxic), and as mating signals. A recent study demonstrates that these sounds are used by some moths to "jam" the sonar of moth-eating bats.

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