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In evolutionary biology, mimicry in vertebrates is defined by cases in which a vertebrate mimic appropriates some characteristic of a model organism or object (not necessarily a vertebrate) in order to deceive another animal. Mimicry is an adaptation that has evolved to be noticeable to other organisms, in contrast to camouflage, another common type of visual deception that animals use to remain hidden.

While there are relatively few well-studied examples, many of the basic types of mimicry persist in vertebrate systems. Visual, olfactory, auditory, biochemical, and behavioral modalities of mimicry have been documented in vertebrates. Human perception may be a limiting factor in our understanding of vertebrate mimicry, as humans are hyper-perceptive to visual mimicry systems, and find these the most abundant. However, olfactory, biochemical, electroreceptive, and other complex forms of mimicry are likely to be much more common than currently accounted for.

Classification of a mimicry system is largely based on its function (e.g. to defend against predators), and is not mutually exclusive. Though a majority of known mimics are insects, diverse forms of Müllerian, Batesian, predatory, parasitic, sexual, and anatomical mimicry have all been observed in various vertebrate groups, including mammals, reptiles, birds, amphibians, and fish.

Defensive
Defensive mimicry is used to avoid potentially dangerous or harmful encounters by deceiving potential predators or competition into treating an organism as something else. This is often done by using warning coloration.

Batesian
Batesian mimicry is a form of defense that allows a harmless species to mimic the appearance of a toxic, noxious, or harmful species to protect itself from predators. By mimicking the appearance of a harmful species, a predator is less likely to attack the mimic due to its awareness of the signal of warning color patterns. Batesian mimicry occurs in multiple vertebrates, but is less prevalent in mammals due to a relative rarity of well-marked harmful models. However, this form of mimicry is prevalent in snakes and frogs, where chemical and venomous defenses have co-evolved with distinct coloration.

A well documented example of this occurs in the scarlet kingsnake. This species resembles the venomous coral snake, sharing a pattern of red, black, and yellow bands. Although the order of the color rings differ between the two snakes, from a distance a predators often mistake the scarlet kingsnake for its venomous model.

Cheetah cubs may also copy the appearance of a sympatric species, the honey badger (Mellivora capensis). As cubs, cheetahs are roughly the same size and have the same reverse-countershading color pattern. Due to this conspicuous coloration, potential predators like lions and birds of prey are less likely to hunt cheetah cubs, as from a distance they appear to be honey badgers. Honey badgers make an effective model because their aggressive nature and noxious tail glands enable them to deter predators up to 10x its size.

Müllerian
Müllerian mimicry is another form of defensive mimicry, except the system involves species that are all potentially dangerous. These species develop similar appearances to collectively protect against predators by advertising their potency. Again, the relative lack of noxious models limits most examples to systems that involve reptiles or amphibians.

Many snakes, including pitvipers, exhibit this form of mimicry. All pit vipers are capable of delivering a life-threateningly venomous bite, and multiple species found throughout Asia have evolved separately to have a very similar appearance. Though each species evolved allopatrically, they have the same green coloration with reddish tail tip. Species that benefit from this system include Trimeresurus macrops, Trimeresurus septentrionalis, T. flavomaculatus and T. hageni.

Similarly, the mimic poison frog (Ranitomeya imitator) closely resembles three similarly poisonous frogs of the same genus that live in different areas (R. variabilis, R. fantastica, and R. ventrimaculatus). D. imitator replicates the different color patterns of all three species, with variations of black spots, a yellow back, and bluish green limbs.

The slow loris is the only known venomous primate, and appears to use Müllerian mimicry for protection through biochemical, behavioral, visual, and auditory modalities. It is hypothesized that its venom may have allowed it to develop this unique system of Müllerian mimicry with the Indian cobra. Slow lorises resemble the cobras with facial markings that closely mirror the eyespots and stripes of the cobra, and also make a noise that mimics the hiss of a cobra when provoked.

Aggressive
Aggressive mimicry is a form of mimicry, opposite in principle to defensive mimicry, that occurs in certain predators, parasites or parasitoids. These organisms benefit by sharing some of the characteristics of a harmless species in order to deceive their prey or host. Most examples of aggressive mimicry involve the predator employing a signal to lure its prey towards it under the promise of food, sex, or other rewards.

In predators
Some predator mimics pretend to be a prey species, while others mimic a third-party organism that the prey is known to interact with in a beneficial manner. In either situation, the mimicry increases the predator's chances of catching its prey.

One form of predatory mimicry, lingual luring, involves wriggling the tongue to attract prey, duping them into believing the tongue is a small worm. Multiple species of carniverous reptile have been documented using this technique, including the common puff adder, alligator snapping turtle, and aquatic garter snake. Another form of aggressive mimicry is caudal luring, which is notably exhibited in sidewinder rattlesnakes, puff adders, lanceheads, spider-tailed horned vipers, and multiple other ambush-predatory snakes to attract prey. By mimicking invertebrates, the predator attracts insectivorous prey such as frogs, lizards, and birds. Caudal luring is similar to lingual luring in that the mimic attempts to dupe its target into thinking there is a small worm or other prey, but instead of using their tongue, mimics use their tails as the disguise.

Complicated forms of aggressive mimicry have also been observed in fish, creating a system that resembles Batesian mimicry. The false cleanerfish, Aspidontus taeniatus, is a fin-eating blenny that has evolved to resemble a local species of cleaner wrasse, Labroides dimidiatus, which engages in mutualistic cleaning with larger fish. By closely mimicking the coloration and the cleaner fish's distinctive dancing display, false cleanerfish are able to remain in close quarters with large predatory reef fish, and gain easy access to their victims during foraging.

Some aggressive mimics switch rapidly between aggressive mimicry and defensive behavior depending on whether they are in the presence of a prey or a potential predator. For example, the sidewinder rattlesnake ceases caudal luring upon the arrival of a predatory toad to avoid attracting the potential threat.

In parasites
Host-parasite mimicry is a form of aggressive mimicry in which a parasite mimics its own host. Brood parasitism is a common form of parasitic aggressive mimicry that occurs in vertebrates, with cuckoos being a notable example. Parasitic mothers secretly lay their eggs, which match the color and pattern of their host's own eggs, in the nest of another bird. Hatchlings avoid detection by mimicking the acoustic and visual appearance of the host offspring. This allows the progeny to be nurtured without energy expenditure or parental care by the true parent. However, host-parasite systems are not always as precise. Great spotted cuckoos are brood parasites that lay eggs which can successfully dupe other birds such as the magpie, pied starling, and black crow, despite having different characteristics, like egg color, egg size, and offspring features.

Evidence also exists for other forms of parasitic mimicry in vertebrates. One such form is interspecific social dominance mimicry, a type of social parasitism where a subordinate species (usually determined by size) evolves over time to mimic its dominant ecological competitor, thereby competing with its previously socially dominant opponent. One such example is found in the tyrant flycatcher family, where species of similar appearance exist from six different genera. Smaller-bodied species from four genera have been found to mimic the appearance of the larger species of the other two genera, suggesting that an avian mimicry complex has contributed to convergent evolution, providing a competitive advantage in the same ecological niche.

Automimicry
Automimicry is a type of mimicry in which an individual mimics either a different member of its own species or a part of its own body. Automimicry is exhibited by a wide variety of vertebrates, and in some cases is considered a form of Batesian mimicry. Vertebrates use many of the same automimicry strategies that invertebrates classically exhibit, such as eyespots and false heads.

Sexual
In sexual mimicry, an organism mimics the behaviors or physical traits of the opposite sex within its own species. Sexual mimicry can be advantageous when members of the opposite sex have certain advantages or opportunities over the other, which can be exploited by any individual that mimics said sex. Spotted hyenas are one of the few vertebrate sexual mimics, as females have a pseudo-penis, which is highly erectile clitoral tissue, as well as a false scrotum. Females have also evolved to mimic the testosterone levels of males. This is advantageous because it lends females heightened aggression and dominance over males in a highly competitive environment. It also bestows an advantage upon sexually indistinguishable female cubs, which experience a high level of female-targeted infanticide.

A different form of sexual mimicry can be seen in flat lizards, where some males imitate female coloring to sneak around more dominant males and achieve copulation with females. Here, smaller and less dominant male flat lizards, which have low reproductive success when competing for females, can increase through sexual mimicry; female-appearing male lizards are not attacked by dominant males, and therefore may avoid detection when mating with real females.

Anatomical
Some vertebrate species self-mimic their own body parts, through the use of patterns or actual anatomy. Two widespread examples of this are eyespots and false heads, both of which can misdirect, confuse, or intimidate potential predators.

Eyespots are a form of automimicry in which an organism displays false eyes on a different part of its body, considered to be an aversion to predators who believe the prey animal has spotted them or is behaving aggressively, even when they are actually facing the other direction and unaware. In the case of attack, eyespots may also redirect damage away from the true head. Eyespots can be found across vertebrate taxa, from the four-eyed butterfly fish to pygmy owls.

False-head mimicry occurs when an organism displays a different body part that has evolved to look like a head, achieving the same scare tactic as eyespots, and also protecting the vulnerable and important real head in case of attack. Losing a section of a tail or other extremity that resembles a head through predator conflict is far less costly than sustaining injury or removal of the real head, which is fatal. For example, the defensive strategy of the rubber boa is to coil up and hide their vulnerable heads when confronted with threats, instead displaying their tails, which morphologically resemble their own head.

Evolutionary patterns
The evolution of mimicry, in vertebrates or otherwise, is widely hypothesized to follow patterns of directional selection, as mimic phenotypes are expected to shift towards advantageous alleles that increase the mimic's likeness to its model. However, it is argued that, while positive evolution might stabilize mimic forms, other evolutionary factors like random mutation create mimetic forms simply by coincidence.

While it had been historically postulated that mimicry systems evolve sympatrically, current theory states that mimics must only share a common signal reciever and not a geographic location. Allopatric mimicry systems can theoretically evolve, as long as they are connected by the same signal reciever (e.g. a migratory bird). This is likely the case with allopatric coral snake mimics, which share many wide-ranging predators, allowing them to extend over 1500km past the range of their model in some cases. Other likely causes of allopatry in mimics exists where the range of the model has retracted or a mimic's range has expanded sincethe evolution of the system.

Another evolutionary postulate that has since been disproven is the theory that mimics must be encountered less frequently than their models. This is based on the assumption that an unreliable signal will not be evolutionarily stable. In effect, a predator that encounters a non-harmful mimic more often than its dangerous model will evolve to ignore the signal. However, this is not always the case. Disproportionate populations that favor mimics can evolve in mimicry systems where the model is particularly dangerous (e.g. venemous snakes). For this reason, populous vertebrate Batesian mimics are most common, as vertebrate models are often particularly dangerous to predators or enemies. Still, the protective power of models lessens as the proportion of mimics increases, and every system will reach a critical mass after which the mimicry system becomes evolutionarily unstable.

Ecological differences between vertebrates and insects are assumed to be the source of the different quantitative and qualitative characteristics we observe between the mimicry systems of vertebrates and other animals. The primary difference between mimicry in vertebrates and in insects is a decreased diversity and frequency. The 50,000 extant vertebrates are dwarfed by the over 1 million known invertebrates. This might create a negative feedback loop for vertebrates whereby fewer examples of mimicry evolve due to a rarity of species to mimic, and may help to explain the relative scarcity of precise mimicry in vertebrates. Further, vertebrates seem to have multiple barriers to precise mimicry that invertebrates do not. Due to the drastic difference in average body size between the two phyla, vertebrates tend to mimic other living things, while invertebrates are much better able to mimic inanimate objects. Large size makes any imprecision much more noticeable to the naked eye, slowing or preventing the evolution of mimicry. However, when a potential prey is highly noxious, predators that avoid even poor mimics gain a strong selective advantage. Insects are rarely able to deliver enough toxin to threaten vertebrate predators, and therefore need precise mimicry to avoid detection.