In evolutionary biology, mimicry[1] is the similarity of one species to another which protects one or both.[2] This similarity can be in appearance, behaviour, sound, scent and even location, with the mimics found in similar places to their models.[3]
Mimicry occurs when a group of organisms,[4] the mimics, evolve to share common perceived characteristics with another group, the models.[5] The evolution is driven by the selective action of a signal-receiver, or dupe.[6] For example, birds that use sight to identify palatable insects (the mimics), whilst avoiding the noxious models.
Collectively, this situation is known as a mimicry complex.[6] The model is usually another species except in cases of automimicry. The signal-receiver is typically another intermediate organism like the common predator of two species, but may actually be the model itself, such as a moth resembling its spider predator.[7] As an interaction, mimicry is in most cases advantageous to the mimic and harmful to the receiver, but may increase, reduce or have no effect on the fitness of the model depending on the situation. Models themselves are difficult to define in some cases, for example eye spots may not bear resemblance to any specific organism's eyes, and camouflage often cannot be attributed to a particular model.
Camouflage, in which a species resembles its surroundings, is essentially a form of visual mimicry. In between camouflage and mimicry is mimesis, in which the mimic takes on the properties of a specific object or organism, but one to which the dupe is indifferent.[8] The lack of a true distinction between the two phenomena can be seen in animals that resemble twigs, bark, leaves or flowers, in that they are often classified as camouflaged (a plant constitutes its "surroundings"), but are sometimes classified as mimics (a plant is also an organism).[3]p51 Crypsis is a broader concept which encompasses all forms of avoiding detection, such as mimicry, camouflage, hiding etc.[9]
Though visual mimicry is most obvious to humans, other senses such as olfaction (smell) or hearing may be involved, and more than one type of signal may be employed.[3] Mimicry may involve morphology, behavior, and other properties. In any case, the signal always functions to deceive the receiver by preventing it from correctly identifying the mimic. In evolutionary terms, this phenomenon is a form of co-evolution usually involving an evolutionary arms race.[9]p161 It should not be confused with convergent evolution, which occurs when species come to resemble one another independently by adapting to similar lifestyles.
Mimics may have different models for different life cycle stages, or they may be polymorphic, with different individuals imitating different models. Models themselves may have more than one mimic, though frequency dependent selection favors mimicry where models outnumber mimics. Models tend to be relatively closely related organisms,[10] but mimicry of vastly different species is also known. Most known mimics are insects,[3] though many other animal mimics including mammals are known. Plants and fungi may also be mimics, though less research has been carried out in this area.[11][12][13]
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Use of the word mimicry dates back to 1637. It is derived from the Greek term mimetikos, "imitative," in turn from mimetos, the verbal adjective of mimeisthai, "to imitate." Originally used to describe people, it was only applied to other forms of life after 1851.[14]
Many types of mimicry have been described. An overview of each follows, highlighting the similarities and differences between the various forms. Classification is often based on function with respect to the mimic (e.g. avoiding harm). Some cases may belong to more than one class, e.g. automimicry and aggressive mimicry are not mutually exclusive, as one describes the species relationship between model and mimic, while the other describes the function for the mimic (obtaining food).
Defensive or protective mimicry takes place when organisms are able to avoid encounters that would be harmful to them by deceiving enemies into treating them as something else. The first three such cases discussed here entail mimicry of organisms protected by warning colouration: Batesian mimicry, where a harmless mimic poses as harmful; Müllerian mimicry, where two or more harmful species mutually advertise themselves as harmful; and Mertensian mimicry, where a deadly mimic resembles a less harmful but lesson-teaching model. The fourth case, Vavilovian mimicry, where weeds resemble crops, is important for several reasons; and humans are the agent of selection.
In Batesian mimicry the mimic shares signals similar to the model, but does not have the attribute that makes it unprofitable to predators (e.g. unpalatability). In other words, a Batesian mimic is a sheep in wolf's clothing. It is named after Henry Walter Bates, an English naturalist whose work on butterflies in the Amazon rainforest (including Naturalist on the River Amazons) was pioneering in this field of study.[16][17] Mimics are less likely to be found out when in low proportion to their model, a phenomenon known as negative frequency dependent selection which applies in most other forms of mimicry as well. This is not the case in Müllerian mimicry however, which is described next. Examples:
Müllerian mimicry describes a situation where two or more species have very similar warning or aposematic signals and both share genuine anti-predation attributes (e.g. being unpalatable). At first Bates could not explain why this should be so; if both were harmful why did one need to mimic another? The German naturalist Fritz Müller put forward the first explanation for this phenomenon: If two species were confused with one another by a common predator, individuals in both would be more likely to survive.[22][23] This type of mimicry is unique in several respects. Firstly, both the mimic and the model benefit from the interaction, which could thus be classified as mutualism in this respect. The signal receiver is also advantaged by this system, despite being deceived regarding species identity, as it avoids potentially harmful encounters. The usually clear identity of mimic and model are also blurred. In cases where one species is scarce and another abundant, the rare species can be said to be the mimic. When both are present in similar numbers however it is more realistic to speak of each as comimics than of a distinct 'mimic' and 'model' species, as their warning signals tend to converge toward something intermediate between the two.[24] Also, the two species may exist on a continuum from harmless to highly noxious, so Batesian mimicry grades smoothly into Müllerian convergence.[25][26]
Examples:
Emsleyan[8] or Mertensian mimicry describes unusual cases where deadly prey mimic a less dangerous species. It was first proposed by Emsley[31] as a possible answer for the problem of Coral Snake mimicry in the New World. It was elaborated on by the German biologist Wolfgang Wickler in a chapter of Mimicry in Plants and Animals,[3] who named it after the German herpetologist Robert Mertens.[32] Sheppard points out that Hecht and Marien put forward a similar hypothesis ten years earlier.[33][34]
This scenario is a little more difficult to understand, as in other types of mimicry it is usually the most harmful species that is the model. But if a predator dies, it cannot learn to recognize a warning signal, e.g. bright colors in a certain pattern. In other words, there is no advantage in being aposematic for an organism that is likely to kill any predator it succeeds in poisoning; such an animal would rather profit from being camouflaged, to avoid attacks altogether. If, however, there is some other species that is harmful but not deadly as well as aposematic, the predator may learn to recognize its particular warning colors and avoid such animals. A deadly species will then profit by mimicking the less dangerous aposematic organism, if this results in less attacks than camouflage would.
The exception here, ignoring any chance of animals learning by watching a conspecific die (see Jouventin et al. for a discussion of observational learning and mimicry),[35] is the possibility of not having to learn that it is harmful in the first place: instinctive genetic programming to be wary of certain signals. In this case, other organisms could benefit from this programming, and Batesian or Müllerian mimics of it could potentially evolve. In fact, it has been shown that some species do have an innate recognition of certain aposematic warnings. Hand-reared Turquoise-browed Motmots (Eumomota superciliosa), avian predators, instinctively avoid snakes with red and yellow rings.[36] Other colors with the same pattern, and even red and yellow stripes with the same width as rings, were tolerated. However, models with red and yellow rings were feared, with the birds flying away and giving alarm calls in some cases. This provides one alternative explanation to Mertensian mimicry. See Greene and McDiarmid for a review of the subject.[37]
Examples:
Wasmannian mimicry refers to cases where the mimic resembles a model along with which it lives (inquiline) in a nest or colony. Most of the models here are social insects such as ants, termites, bees and wasps.[38]
Vavilovian mimicry describes weeds which come to share characteristics with a domesticated plant through artificial selection.[8] It is named after Russian botanist and geneticist Nikolai Vavilov.[39] Selection against the weed may occur either by manually killing the weed, or separating its seeds from those of the crop. The latter process, known as winnowing, can be done manually or by a machine.
Vavilovian mimicry presents an illustration of unintentional (or rather 'anti-intentional') selection by man. While some cases of artificial selection go in the direction desired, such as selective breeding, this case presents the opposite characteristics. Weeders do not want to select weeds that look increasingly like the cultivated plant, yet there is no other option. A similar problem in agriculture is pesticide resistance: farmers do not wish to select for weeds that have increasingly similar resistance to pesticides as the crop itself, yet that is the inevitable effect. Vavilovian mimics may eventually be domesticated themselves, and Vavilov called these weeds-cum-crops secondary crops.
It can be classified as defensive mimicry in that the weed mimics a protected species. This bears strong similarity to Batesian mimicry in that the weed does not share the properties that give the model its protection, and both the model and the dupe (in this case people) are harmed by its presence. There are some key differences, though; in Batesian mimicry the model and signal receiver are enemies (the predator would eat the protected species if could), whereas here the crop and its human growers are in a mutualistic relationship: the crop benefits from being dispersed and protected by people, despite being eaten by them. In fact, the crop's only 'protection' relevant here is its usefulness to humans. Secondly, the weed is not eaten, but simply destroyed. The only motivation for killing the weed is its effect on crop yields. Finally, this type of mimicry does not occur in ecosystems unaltered by humans.
One case is Echinochloa oryzoides, a species of grass which is found as a weed in rice (Oryza sativa) fields. The plant looks similar to rice and its seeds are often mixed in rice and difficult to separate. This close similarity was enhanced by the weeding process which is a selective force that increases the similarity of the weed in each subsequent generation.[40]
Unlike the above forms of mimicry, Gilbertian mimicry involves only two species. The potential host/prey drives away its parasite/predator by mimicking it, the reverse of host-parasite aggressive mimicry. It was coined by Pasteur as a term for such rare mimicry systems,[8] and is named after the American ecologist Lawrence E. Gilbert.[41]
This form of protective mimicry occurs in the genus Passiflora. The leaves of this plant contain toxins which deter herbivorous animals, however some Heliconius butterfly larvae have evolved enzymes which break down these toxins, allowing them to specialize on this genus. This has created further selection pressure on the host plants, which have evolved stipules that mimic mature Heliconius eggs near the point of hatching. These butterflies tend to avoid laying eggs near each existing ones, which helps avoid exploitative intraspecific competition between caterpillars—those that lay on vacant leaves provide their offspring with a greater chance of survival. Additionally, most Heliconius larvae are cannibalistic, meaning those leaves with older eggs will hatch first and eat the new arrivals. Thus, it seems such plants have evolved egg dummies due to these grazing herbivore enemies. The decoy eggs are also nectaries though, attracting predators of the caterpillars such as ants and wasps. The extent of their mimetic function is therefore slightly more difficult to assess.[10]
The use of eggs is not essential to this system, only the species composition and protective function. Many other forms of mimicry also involve eggs, such as cuckoo eggs mimicking those of their host (the reverse of this situation), or plants seeds (often those with an elaiosome) being dispersed by ants, who treat them as they would their own eggs.
Browerian mimicry,[8] named after Lincoln P. Brower and Jane Van Zandt Brower,[42][43] is a form of automimicry; where the model belongs to the same species as the mimic. This is the analogue of Batesian mimicry within a single species, and occurs when there is a palatability spectrum within a population. One example is Monarch Butterflies (Danaus plexippus), which feed on milkweed species of varying toxicity. This species stores toxins from its host plant, which are maintained even in the adult (imago) form. As the levels of toxin will vary depending on diet during the larval stage, some individuals will be more toxic than others. The less palatable organisms will therefore be mimics of the more dangerous individuals, with their likeness already perfected. This need not be the case however; in sexually dimorphic species one sex may be more of a threat than the other, which could mimic the protected sex. Evidence for this possibility is provided by the behavior of a monkey from Gabon, which regularly ate male moths of the genus Anaphe, but promptly stopped after it tasted a noxious female.[44]
Aggressive mimicry describes predators (or parasites) which share the same characteristics as a harmless species, allowing them to avoid detection by their prey (or host). The mimic may resemble the prey or host itself, or another organism which is either neutral or beneficial to the signal receiver. In this class of mimicry the model may be affected negatively, positively or not at all. Just as parasites can be treated as a form of predator,[45] host-parasite mimicry is treated here as a subclass of aggressive mimicry.
The mimic may have a particular significance for duped prey. One such case is spiders, amongst which aggressive mimicry is quite common in both luring prey and stealthily approaching predators.[46] One case is the Golden Orb Weaver (Nephila clavipes), which spins a conspicuous golden colored web in well-lit areas. Experiments show that bees are able to associate the webs with danger when the yellow pigment is not present, as occurs in less well-lit areas where the web is much harder to see. Other colors were also learned and avoided, but bees seemed least able to effectively associate yellow pigmented webs with danger. Yellow is the color of many nectar bearing flowers, however, so perhaps avoiding yellow is not worth while. Another form of mimicry is based not on color but pattern. Species such as Argiope argentata employ prominent patterns in the middle of their webs, such as zigzags. These may reflect ultraviolet light, and mimic the pattern seen in many flowers known as nectar guides. Spiders change their web day to day, which can be explained by bee's ability to remember web patterns. Bees are able to associate a certain pattern with a spatial location, meaning the spider must spin a new pattern regularly or suffer diminishing prey capture.[47]
Another case is where males are lured towards what would seem to be a sexually receptive female; the model in this situation being the same species as the dupe. Beginning in the 1960s, James E. Lloyd's investigation of female fireflies of the genus Photuris revealed they emit the same light signals that females of the genus Photinus use as a mating signal.[48] Further research showed male fireflies from several different genera are attracted to these "femmes fatales", and are subsequently captured and eaten. Female signals are based on that received from the male, each female having a repertoire of signals matching the delay and duration of the female of the corresponding species. This mimicry may have evolved from non-mating signals that have become modified for predation.[49]
The listrosceline katydid Chlorobalius leucoviridis of inland Australia is capable of attracting male cicadas of the Tribe Cicadettini by imitating the species-specific reply clicks of sexually receptive female cicadas. This example of acoustic aggressive mimicry is similar to the Photuris firefly case in that the predator's mimicry is remarkably versatile – playback experiments show that C. leucoviridis is able to attract males of many cicada species, including Cicadettine cicadas from other continents, even though cicada mating signals are species-specific.[50]
Some carnivorous plants may also be able to increase their rate of capture through mimicry.[51]
Luring is not a necessary condition however, as the predator will still have a significant advantage by simply not being identified as such. They may resemble a mutualistic symbiont or a species of little relevance to the prey.
A case of the latter situation is a species of cleaner fish and its mimic, though in this example the model is greatly disadvantaged by the presence of the mimic. Cleaner fish are the allies of many other species, which allow them to eat their parasites and dead skin. Some allow the cleaner to venture inside their body to hunt these parasites. However, one species of cleaner, the Bluestreak cleaner wrasse (Labroides dimidiatus), is the unknowing model of a mimetic species, the Sabre-toothed blenny (Aspidontus taeniatus). This wrasse, shown to the left cleaning a grouper of the genus Epinephelus, resides in coral reefs in the Indian and the Pacific Oceans, and is recognized by other fishes who then allow it to clean them. Its imposter, a species of blenny, lives in the Indian Ocean and not only looks like it in terms of size and coloration, but even mimics the cleaner's 'dance'. Having fooled its prey into letting its guard down, it then bites it, tearing off a piece of its fin before fleeing the scene. Fish grazed upon in this fashion soon learn to distinguish mimic from model, but because the similarity is close between the two they become much more cautious of the model as well, such that both are affected. Due to victim's ability to discriminate between foe and helper, the blennies have evolved close similarity, right down to the regional level.[52]
Another interesting example that does not involve any luring is the Zone-tailed Hawk, which resembles the Turkey Vulture. It flies amongst the vultures, suddenly breaking from the formation and ambushing its prey.[53] Here the hawk's presence is of no evident significance to the vultures, affecting them neither negatively or positively.
Parasites can also be aggressive mimics, though the situation is somewhat different from those outlined above.
Some of the predators described have a feature that draws prey, and parasites can also mimic their host's natural prey, but are eaten themselves, a pathway into their host. Leucochloridium, a genus of flatworm, matures in the digestive system of songbirds, their eggs then passing out of the bird via the feces. They are then taken up by Succinea, a terrestrial snail. The eggs develop in this intermediate host, and then must find of a suitable bird to mature in. As the host birds do not eat snails, so the sporocyst has another strategy to reach its host's intestine. They are brightly colored and move in a pulsating fashion. A sporocyst-sac pulsates in the snail's eye stalks,[54][55] coming to resemble an irresistible meal for a songbird. In this way, it can bridge the gap between hosts, allowing it to complete its life cycle.[3] A nematode (Myrmeconema neotropicum) changes the colour of the abdomen of workers of the canopy ant Cephalotes atratus to make it appear like the ripe fruits of Hyeronima alchorneoides. It also changes the behaviour of the ant so that the gaster (rear part) is held raised. This presumably increases the chances of the ant being eaten by birds. The droppings of birds are collected by other ants and fed to their brood, thereby helping to spread the nematode.[56]
In an unusual case, planidium larvae of some beetles of the genus Meloe will form a group and produce a pheromone that mimics the sex attractant of its host bee species; when the male bee arrives and attempts to mate with the mass of larvae, they climb onto his abdomen, and from there transfer to a female bee, and from there to the bee nest to parasitize the bee larvae.[57]
Host-parasite mimicry is a two species system where a parasite mimics its own host. Cuckoos are a canonical example of brood parasitism, a form of kleptoparasitism where the mother has its offspring raised by another unwitting organism, cutting down the biological mother's parental investment in the process. The ability to lay eggs which mimic the host eggs is the key adaptation. The adaptation to different hosts is inherited through the female line in so-called gentes. Cases of intraspecific brood parasitism, where a female lays in conspecific's nest, as illustrated by the Goldeneye duck (Bucephala clangula),[58] do not represent a case of mimicry.
Reproductive mimicry occurs when the actions of the dupe directly aid in the mimic's reproduction. This is common in plants, which may have deceptive flowers that do not provide the reward they would seem to. Other forms of mimicry have a reproductive component, such as Vavilovian mimicry involving seeds, and brood parasitism, which also involves aggressive mimicry.
Bakerian mimicry, named after Herbert G. Baker,[59] is a form of automimicry where female flowers mimic male flowers of their own species, cheating pollinators out of a reward. This reproductive mimicry may not be readily apparent as members of the same species may still exhibit some degree of sexual dimorphism. It is common in many species of Caricaceae.[60]
Like Bakerian mimicry, Dodsonian mimicry is a form of reproductive floral mimicry, but the model belongs to a different species than the mimic. The name refers to Calaway H. Dodson.[61] By providing similar sensory signals as the model flower, it can lure its pollinators. Like Bakerian mimics, no nectar is provided. Epidendrum ibaguense of the family Orchidaceae resembles flowers of Lantana camara and Asclepias curassavica, and is pollinated by Monarch Butterflies and perhaps hummingbirds.[62] Similar cases are seen in some other species of the same family. The mimetic species may still have pollinators of its own though, for example a lamellicorn beetle which usually pollinates correspondingly colored Cistus flowers is also known to aid in pollination of Ophrys species that are normally pollinated by bees.[63]
Pseudocopulation occurs when a flower mimics a female of a certain insect species, the males of which try to copulate with it. This is much like the aggressive mimicry in fireflies described above, but with a much more benign outcome for the pollinator. This form of mimicry has been called Pouyannian mimicry,[8] after Pouyanne, who first described the phenomenon.[64][65] It is most common in orchids which mimic females of the order Hymenoptera (generally bees and wasps), and may account for around 60% of pollinations.[66] Depending on the morphology of the flower, a pollen sac called a pollinia is attached to the head or abdomen of the male. This is then transferred to the stigma of the next flower the male tries to inseminate, resulting in pollination. Visual mimicry is the most obvious sign of this deception for humans, but the visual aspect may be minor or non-existent. It is the senses of touch and olfaction that are most important.[66]
Inter-sexual mimicry occurs when individuals of one sex in a species mimic members of the opposite sex. An example is the three male forms of the marine isopod, Paracerceis sculpta. Alpha males are the largest and guard a harem of females. Beta males mimic females and manage to enter the harem of females without being detected by the alpha males allowing them to mate. Gamma males are the smallest males and mimic juveniles. This also allows them to mate with the females without the alpha males detecting them.[67] Some male Australian Giant Cuttlefish also mimic females, allowing them to mate undetected by other males.
Automimicry or intraspecific mimicry occurs within a single species, one case being where one part of an organism's body resembles another part. Examples include snakes in which the tail resembles the head and show behavior such as moving backwards to confuse predators and insects and fishes with eyespots on their hind ends to resemble the head. The term is also used when the mimic imitates other morphs within the same species. When males mimic females or vice versa this may be referred to as sexual mimicry.
Examples:
Some forms of mimicry do not fit easily within the classification given above.
Owl butterflies (genus Caligo) bear eye-spots on the underside of their wings; if turned upside-down, their undersides resemble the face of an owl (such as the Short-eared Owl or the Tropical Screech Owl) for which in turn the butterfly predators – small lizards and birds – would be fooled.[70] Thus it has been supposed that the eye-spots are a form of Batesian mimicry. However, the pose in which the butterfly resembles an owl's head is not normally adopted in life. Research suggests that eye-spots are not a form of mimicry and do not deter predators because they look like eyes. Rather the conspicuous contrast in the patterns on the wings deter predators.[71]
Another case is floral mimicry induced by the discomycete fungus Monilinia vaccinii-corymbosi.[72] In this unusual case, a fungal plant pathogen infects leaves of blueberries, causing them to secrete sugary substances including glucose and fructose, in effect mimicking the nectar of flowers. To the naked eye the leaves do not look like flowers, yet strangely they still attract pollinating insects like bees. As it turns out, the sweet secretions are not the only cues—the leaves also reflect ultraviolet, which is normally absorbed by the plant's leaves. Ultraviolet light is also employed by the host's flowers as a signal to insects, which have visual systems quite capable of picking up this low wavelength (300–400 nm) radiation. The fungus is then transferred to the ovaries of the flower where it produces mummified, inedible berries, which overwinter before infecting new plants. This case is unusual in that the fungus benefits from the deception, but it is the leaves which act as mimics, being harmed in the process. It bears similarity to host-parasite mimicry, but the host does not receive the signal. It also has a little in common with automimicry, but the plant does not benefit from the mimicry, and the action of the pathogen is required to produce it.
It is widely accepted that mimicry evolves as a positive adaptation. The lepidopterist and writer Vladimir Nabokov argued that although natural selection might stabilize a "mimic" form, it would not be necessary to create it. It may be that much of insect mimicry, including the Viceroy/Monarch mimicry, results from similar self-organizing processes, and thus the tendency for convergence by chance would be high.[73]
The most widely accepted model used to explain the evolution of mimicry in butterflies is the two-step hypothesis. In this model the first step involves mutation in modifier genes that regulate a complex cluster of linked genes associated with large changes in morphology. The second step consists of selections on genes with smaller phenotypic effects and this leading to increasing closeness of resemblance. This model is supported by empirical evidence that suggests that there are only a few single point mutations that cause large phenotypic effects while there are numerous others that produce smaller effects. Some regulatory elements are now known to be involved in a supergene that is involved in the development of butterfly color patterns. Computational simulations of population genetics have also supported this idea.[74]
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