Batesian mimicry is a form ofmimicry where a harmless species has evolved to imitate the warning signals of a harmful species directed at apredator of them both. It is named after the English naturalistHenry Walter Bates, who worked onbutterflies in therainforests of Brazil.
Batesian mimicry is the most commonly known and widely studied of mimicry complexes, such that the word mimicry is often treated as synonymous with Batesian mimicry. There are many other forms however, some very similar in principle, others far separated. It is often contrasted withMüllerian mimicry, a form of mutually beneficial convergence between two or more harmful species. However, because the mimic may have a degree of protection itself, the distinction is not absolute. It can also be contrasted with functionally different forms of mimicry. Perhaps the sharpest contrast here is withaggressive mimicry where a predator or parasite mimics a harmless species, avoiding detection and improving itsforaging success.
The imitating species is called themimic, while the imitated species (protected by its toxicity, foul taste or other defenses) is known as themodel. The predatory species mediating indirect interactions between the mimic and the model is variously known as the[signal] receiver,dupe oroperator. By parasitising thehonest warning signal of the model, the Batesian mimic gains an advantage, without having to go to the expense of arming itself. The model, on the other hand, is disadvantaged, along with the dupe. If impostors appear in high numbers, positive experiences with the mimic may result in the model being treated as harmless. At higher frequency there is also a stronger selective advantage for the predator to distinguish mimic from model. For this reason, mimics are usually less numerous than models, an instance offrequency-dependent selection. Some mimetic populations have evolved multiple forms (polymorphism), enabling them to mimic several different models and thereby to gain greater protection. Batesian mimicry is not always perfect. A variety of explanations have been proposed for this, including limitations in predators'cognition.
While visual signals have attracted most study, Batesian mimicry can employdeception of any of thesenses; some moths mimic theultrasound warning signals sent by unpalatable moths tobat predators, constituting auditory Batesian mimicry, while some weaklyelectric fish appear to mimic theelectrolocation signals of strongly electric fish, probably constituting electrical mimicry.
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Henry Walter Bates (1825–1892) was an Englishexplorer-naturalist who surveyed theAmazon rainforest withAlfred Russel Wallace in 1848. While Wallace returned in 1852, Bates remained for over a decade. Bates's field research included collecting almost a hundred species ofbutterflies from the familiesIthomiinae andHeliconiinae, as well as thousands of other insects specimens. In sorting these butterflies into similar groups based on appearance, inconsistencies began to arise. Some appeared superficially similar to others, so much so that even Bates could not tell some species apart based only on wing appearance. However, closer examination of less obviousmorphological characters seemed to show that they were not even closely related. Shortly after his return to England, he read a paper on his theory of mimicry at a meeting of theLinnean Society of London on 21 November 1861, which was then published in 1862 as 'Contributions to an Insect Fauna of the Amazon Valley' in the society'sTransactions.[1] He elaborated on his experiences further inThe Naturalist on the River Amazons.[2]
Bates put forward the hypothesis that the close resemblance between unrelated species was anantipredator adaptation. He noted that some species showed very strikingcoloration and flew in a leisurely manner, almost as if taunting predators to eat them. He reasoned that these butterflies were unpalatable to birds and otherinsectivores, and were thus avoided by them. He extended that logic to forms that closely resembled such protected species and mimicked their warning coloration but not their toxicity.[1][2]
Thisnaturalistic explanation fitted well with the recent account ofevolution by Wallace andCharles Darwin, as outlined in his famous 1859 bookThe Origin of Species. Because theDarwinian explanation required no supernatural forces, it met with considerable criticism fromanti-evolutionists, both in academic circles and in the broadersocial realm.[3]
Most living things have predators and therefore are in a constantevolutionary arms race to developantipredator adaptations, while the predatoradapts to become more efficient at defeating the prey's adaptations. Some organisms have evolved to makedetection less likely, for example bynocturnality andcamouflage. Others have developed chemical defences such as the deadlytoxins of certain snakes and wasps, or the noxious scent of theskunk. Such prey often send clear andhonest warning signals to their attackers with conspicuousaposematic (warning) patterns. The brightness of such warning signs is correlated with the level of toxicity of the organism.[4]
In Batesian mimicry, the mimic effectively copies the coloration of an aposematic animal, known as the model, to deceive predators into behaving as if it were distasteful.[a] The success of this dishonest display depends on the level of toxicity of the model and the abundance of the model in the geographical area. The more toxic the model is, the more likely it is that the predator will avoid the mimic.[6] The abundance of the model species is also important for the success of the mimic because offrequency-dependent selection. When the model is abundant, mimics with imperfect model patterns or slightly different coloration from the model are still avoided by predators. This is because the predator has a strong incentive to avoid potentially lethal organisms, given the likelihood of encountering one.[7] However, in areas where the model is scarce or locally extinct, mimics are driven to accurate aposematic coloration. This is because predators attack imperfect mimics more readily where there is little chance that they are the model species.[8] Frequency-dependent selection may also have driven Batesian mimics to become polymorphic in rare cases where a single genetic switch controls appearance, as in the swallowtail butterflies (thePapilionidae) such as thepipevine swallowtail,[9] and in the New Zealand stoneflyZelandoperla fenestrata.[10]
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Batesian mimicry is a case of protective ordefensive mimicry, where the mimic does best by avoiding confrontations with the signal receiver. It is adisjunct system, which means that all three parties are from different species.[11] An example would be therobber flyMallophora bomboides, which is a Batesian mimic of itsbumblebee model and prey,B. americanorum (now more commonly known asBombus pensylvanicus), which is noxious to predators due to its sting.[12]
Batesian mimicry stands in contrast to other forms such asaggressive mimicry, where the mimic profits from interactions with the signal receiver. One such case of this is infireflies, where females of one species mimic the mating signals of another species, deceiving males to come close enough for them to eat. Mimicry sometimes does not involve a predator at all though. Such is the case indispersal mimicry, where the mimic once again benefits from the encounter. For instance, some fungi have their spores dispersed by insects by smelling likecarrion. In protective mimicry, the meeting between mimic and dupe is not such a fortuitous occasion for the mimic, and the signals it mimics tend to lower the probability of such an encounter.[3]
A case somewhat similar to Batesian mimicry is that of mimetic weeds, which imitate agricultural crops. In weed orVavilovian mimicry, the weed survives by having seeds whichwinnowing machinery identifies as belonging to the crop. Vavilovian mimicry is not Batesian, because humans and crops are not enemies.[3] By contrast, a leaf-mimicking plant, thechameleon vine, employs Batesian mimicry by adapting its leaf shape and colour to match that of its host to deter herbivores from eating its edible leaves.[13]
Another analogous case within a single species has been termedBrowerian mimicry[3] (afterLincoln P. Brower and Jane Van Zandt Brower[14][15]). This is a case ofautomimicry;[11] the model is the same species as its mimic. Equivalent to Batesian mimicry within a single species, it occurs when there is apalatability spectrum within a population of harmful prey. For example,monarch (Danaus plexippus) caterpillars feed onmilkweed species of varying toxicity. Some feed on more toxic plants and store these toxins within themselves. The more palatable caterpillars thus profit from the more toxic members of the same species.[14][16]
Another important form of protective mimicry isMüllerian mimicry, discovered by and named after the naturalistFritz Müller.[17][18] In Müllerian mimicry, both model and mimic are aposematic, so mimicry may be mutual, does not necessarily[b] constitute a bluff or deception and as in the wasps and bees may involve many species in a mimicry ring.[19][20]
In imperfect Batesian mimicry, the mimics do not exactly resemble their models. An example of this is the flySpilomyia longicornis, which mimicsvespid wasps. However, it is not a perfect mimic. Wasps have long black antennae and this fly does not. Instead, they wave their front legs above their heads to look like the antennae on the wasps.[21] Many reasons have been suggested for imperfect mimicry.Imperfect mimics may simply be evolving towards perfection.[22]They may gain advantage from resembling multiple models at once.[23]Humans may evaluate mimics differently from actual predators.[24] Mimics may confuse predators by resembling both model and nonmimic at the same time (satyric mimicry).[25] Kin selection may enforce poor mimicry.[26]The selective advantage of better mimicry may not outweigh the advantages of other strategies like thermoregulation or camouflage.[27]
Only certain traits may be required to deceive predators; for example, tests on thesympatry/allopatry border (where the two are in the same area, and where they are not) of the mimicLampropeltis elapsoides and the modelMicrurus fulvius showed that color proportions in these snakes were important in deceiving predators but that the order of the colored rings was not.[28]
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Batesian mimicry of ants appears to have evolved in certain plants, as a visualanti-herbivory strategy, analogous to a herbivorous insect's mimicking a well-defended insect to deter predators.[30]Passiflora flowers of at least 22 species, such asP. incarnata, have dark dots and stripes on their flowers thought to serve this purpose.[29]
Predators may identify their prey by sound as well as sight; mimics have accordingly evolved to deceive thehearing of their predators.Bats are nocturnal predators that rely onecholocation to detect their prey.[32] Some potential prey are unpalatable to bats, and produce an ultrasonic aposematic signal, the auditory equivalent of warning coloration. In response to echolocatingred bats andbig brown bats,tiger moths such asCycnia tenera produce warning sounds. Bats learn to avoid the harmful moths, but similarly avoid other species such as somepyralid moths that produce such warning sounds as well. Acoustic mimicry complexes, both Batesian and Müllerian, may be widespread in the auditory world.[31]
Theelectric eel,Electrophorus, is capable of delivering a powerful electric shock that can stun or kill its prey. Bluntnose knifefishes,Brachyhypopomus, create an electric discharge pattern similar to the low voltageelectrolocation discharge of the electric eel. This is thought to be Batesian mimicry of the powerfully protected electric eel.[33]
[Consider the case where one monarch caterpillar is feeding on cardenolide-containing milkweed, the other not], with one being completely potent with regard to cardiac glycoside toxicity, the second not. The first will fit all of the characteristics for warning coloration, the second not. In fact,the second butterfly is a harmless Batesian mimic of the first, even though both belong to the same species. L. Brower, J. Brower, and Corvino (1967) have termed this phenomenonautomimicry, though others have suggested thatBrowerian mimicry would be a better term (Pasteur, 1972; Bees, 1977; Rothschild, 1979). Note that all of the antagonisms raised by Batesian mimicry will arise, but now the model and the mimic are conspecific.
