Aggressive mimicry
Aggressive mimicry is a form of mimicry in which predators, parasites, or parasitoids share similar signals, using a harmless model, allowing them to avoid being correctly identified by their prey or host. Zoologists have repeatedly compared this strategy to a wolf in sheep's clothing. In its broadest sense, aggressive mimicry could include various types of exploitation, as when an orchid exploits a male insect by mimicking a sexually receptive female, but will here be restricted to forms of exploitation involving feeding. For example, indigenous Australians who dress up as and imitate kangaroos when hunting would not be considered aggressive mimics, nor would a human angler, though they are undoubtedly practising self-decoration camouflage. Treated separately is molecular mimicry, which shares some similarity; for instance a virus may mimic the molecular properties of its host, allowing it access to its cells. An alternative term, Peckhamian mimicry, has been suggested, but it is seldom used.
Aggressive mimicry is opposite in principle to defensive mimicry, where the mimic generally benefits from being treated as harmful. The mimic may resemble its own prey, or some other organism which is beneficial or at least not harmful to the prey. The model, i.e. the organism being 'imitated', may experience increased or reduced fitness, or may not be affected at all by the relationship. On the other hand, the signal receiver inevitably suffers from being tricked, as is the case in most mimicry complexes.
Aggressive mimicry often involves the predator employing signals which draw its potential prey towards it, a strategy which allows predators to simply sit and wait for prey to come to them. The promise of food or sex are most commonly used as lures. However, this need not be the case; as long as the predator's true identity is concealed, it may be able to approach prey more easily than would otherwise be the case. In terms of species involved, systems may be composed of two or three species; in two-species systems the signal receiver, or "dupe", is the model.
In terms of the visual dimension, the distinction between aggressive mimicry and camouflage is not always clear. Authors such as Wickler have emphasized the significance of the signal to its receiver as delineating mimicry from camouflage. However, it is not easy to assess how 'significant' a signal may be for the dupe, and the distinction between the two can thus be rather fuzzy. Mixed signals may be employed: aggressive mimics often have a specific part of the body sending a deceptive signal, with the rest being hidden or camouflaged.
Contrast with defensive mimicry
Aggressive mimicry stands in semantic contrast with defensive mimicry, where it is the prey that acts as a mimic, with predators being duped. Defensive mimicry includes the well-known Batesian and Müllerian forms of mimicry, where the mimic shares outward characteristics with an aposematic or harmful model. In Batesian mimicry, the mimic is modeled on a dangerous species, while in Müllerian mimicry both species are harmful, and act as comimics, converging on a common set of signals and sharing the burden of 'educating' their predators. Included in defensive mimicry is the lesser known Mertensian mimicry, where the mimic is more harmful than the model, and Vavilovian mimicry, where weeds come to mimic crops through unintentional artificial selection. In defensive mimicry, the mimic benefits by avoiding a harmful interaction with another organism that would be more likely to take place without the deceptive signals employed. Harmful interactions might involve being eaten, or pulled out of the ground as a weed. In contrast, the aggressive mimic benefits from an interaction that would be less likely to take place without the deception, at the expense of its target.File:Aggressive Mimicry.svg|thumb|center|upright=4|Aggressive mimicry compared to a defensive form, Batesian mimicry. The mechanism is often called "Wolf in sheep's clothing". The model for an aggressive mimic can be a harmless species, in which case the 3 roles are separate, or the model can be the prey itself, in which case there are only 2 species involved.
Classification
Luring prey
In some cases the dupe is lured toward the mimic. This involves mimicry of a resource that is often vital to the prey's survival such as nutrition or a mate. If the bait offered is of little value to prey they would not be expected to take such a risk. For example, in all known cases of sexual signal mimicry it is always the male sex that is deceived. In these cases the predator need not move about foraging for prey, but may simply stay still and allow prey to come to it.Some studies suggest that the northern shrike sings in winter often imitating small passerines that may be preyed upon when lured within reach.
There has been one report of a margay using mimicry of the cry of an infant pied tamarin to try to lure an adult tamarin within striking distance.
Appearance of food
Many aggressive mimics use the promise of nourishment as a way of attracting prey. The alligator snapping turtle is a well-camouflaged ambush predator. Its tongue bears a conspicuous pink extension that resembles a worm and can be wriggled around; fish that try to eat the "worm" are themselves eaten by the turtle. Similarly, some snakes employ caudal luring or lingual luring to entice small vertebrates into striking range.Aggressive mimicry is common amongst spiders, both in luring prey and stealthily approaching predators. One case is the golden orb weaver, which spins a conspicuous golden coloured 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 colours too were learned and avoided, but bees seemed least able to effectively associate yellow pigmented webs with danger. Yellow is the colour of many nectar bearing flowers, however, so perhaps avoiding yellow is not worthwhile. Another form of mimicry is based not on colour 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 bees' 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.
Spiders can be the prey of aggressive mimics. The assassin bug Stenolemus bituberus preys on spiders, entering their web and plucking its silk threads until the spider approaches. This vibrational aggressive mimicry matches a general pattern of vibrations which spiders treat as prey, having a similar temporal structure and amplitude to leg and body movements of typical prey caught in the web.
Larvae of the ground beetle Epomis move their mandibles one after another to lure amphibians toward them and then prey on them. Their body structure allows them to bite and feed on the amphibians even when they are ingested by larger prey such as frogs.
Although plants are better known for defensive mimicry, there are exceptions. For example, many flowers use mimicry to attract pollinators, while others may trick insects into dispersing their seeds. Nonetheless, most mimicry in plants would not be classified as aggressive, as although luring pollinators is similar to cases above, they are certainly not eaten by the plant. However some carnivorous plants may be able to increase their rate of capture through mimicry. For example, some have patterns in the ultraviolet region of the electromagnetic spectrum, much like the spider webs described above.
Bipolar mimicry systems
Mimicry systems involving only two species are known as bipolar. Only one bipolar arrangement is possible here, namely where the dupe is itself the model. There are two such variants on this arrangement of mimic imitating its target. In one case, Kirbyan mimicry, the model is the host of a brood parasite. In the other case, termed Batesian-Wallacian mimicry after Henry Walter Bates and Alfred Russel Wallace, the model is the prey species.Kirbyan or brood parasite mimicry
Host-parasite mimicry is a situation where a parasite mimics its own host. As with mimicry of the female sex outlined previously, only two species are involved, the model and mimic being of the same species. Brood parasitism, a form of kleptoparasitism where the mother has its offspring raised by another unwitting organism, is one such situation where host-parasite mimicry has evolved. Georges Pasteur terms this form of aggressive-reproductive mimicry Kirbyan mimicry, after the English entomologist William Kirby, who noticed that the young of syrphid hoverflies are raised by bumblebees.Batesian-Wallacian or prey mimicry
In Batesian-Wallacian mimicry, the model is a sexually receptive female, which provides a strong attractive effect on males. Some spiders use chemical rather than visual means to ensnare prey. Female bolas spiders of the genus Mastophora lure male moth-flies by producing analogues of the fly species' sex pheromones. Each species of spider appears to specialize in a particular species of prey in the family Psychodidae. Juveniles use their front pair of legs to capture prey, such as flies. Older spiders use a different strategy however, swinging a sticky ball known as a bolas suspended by a silk thread at moths. But both old and juvenile are able to lure prey via this olfactory signal; even young spiderlings have been shown to attract prey species.The listroscelidine 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. The evolution of versatile mimicry in C. leucoviridis may have been facilitated by constraints on song evolution in duetting communication systems in which reply signals are recognizable only by their precise timing in relation to the male song.
Female fireflies of the genus Photuris emit the same light signals that females of the genus Photinus use as a mating signal. Male fireflies from several different genera are attracted to these mimics, 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.