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Mimicry

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Overview

Chrysotoxum cautum, a wasp-mimicking species of hoverfly.

Some species, such as wasps, have evolved protection against predators, such as stings, poison, or chemicals which cause them to taste bad. When a predator eats (or attempts to eat) a member of one of these species and discovers that it stings/makes them ill/tastes bad it remembers what it looks like and never eats another member of that species. To make it easier for predators to recognise these species, many have evolved bright colours, advertising their defenses. Because of this, many species, such as hoverflies, have evolved mimicry - they have the same bright colours advertising the defenses, but don't actually have the defenses themselves. Birds don't eat hoverflies (and many humans avoid them), because they look so much like wasps and bees. Mimicry evolved by means of natural selection as a method to protect prey or for predators to deceive prey specific to their natal environment. Therefore, it does not always work if the animals are put in foreign environments because the new predators did not evolve with the mimicked warning colorations and behaviors, and they therefore do not perceive them as such. There are four specific types of mimicry: molecular, evolving, aggressive, and host, and two theories of mimicry: Batesian and Mullerian.

Molecular mimicry refers to a parasitic organism mimicking a molecule of the host in order to escape recognition as a foreigner amongst the model species. Evolving mimicry exploits previously evolved methods of communication signals and responses between organisms. Aggressive mimicry differs from both molecular and evolving mimicry in that predators specifically use it; the predator mimics a behavioral signal that is attractive or deceptive to its prey. Host mimicry is an extension of aggressive mimicry but is specific to parasites, where the host is both the model and receiver.

The two theories of mimicry are Batesian and Mullerian. Batesian or pseudaposematic mimicry, introduced by British naturalist Henry Walter Bates in 1852, proposes that a member of a palatable species acquires a deceptive resemblance in appearance and behavior to a harmful, toxic, unrelated variety and is then more likely to escape predators, thereby furthering its success in survival and production of offspring that inherit the same appearance and behavior. The above cited example of hoverflies (Syrphidae) is an excellent example of a Batesian system, and of the syrphids, the Drone Fly Eristalis tenex is by far the most accomplished mimic, as it closely resembles a honeybee. Batesian mimics usually mimic the aposematic coloration and systematic displays of the model species, e.g., a species of sole, Solea vulgaris, mimics two species of weevers, Trachinus draco and T. vipera. The weevers are poisonous and the venom is located in the six spines of the first dorsal fin. The spines are connected to a black membrane that the weever projects above the sand that it buries itself in, as a warning signal to potential predators. Soles mimic this behavior of lying under the sand while projecting a conspicuous black object when threatened. The soles lie buried in the sand to which they match in color with the exception of the upper pectoral fin that is black in color. When threatened, the black fin becomes erect and is held so rigid that it becomes a conspicuous object. From this example, it can be seen that organisms must mimic not only the physical appearance of the model, but also the behavior to be effective. If the sole did not hold the fin erect, it could be mistaken for another species, instead of the harmful weever that it attempts to mimic.

Mullerian mimicry, proposed by German zoologist Fritz Muller, asserts that when one inedible (emetic) species is physically similar to a second emetic species living in the same locality, one of the two species will mimic the other, specifically in behavior, e.g., Limenitis archippus, the Viceroy, mimics Danaus plexippus, the Monarch, in coloration patterns and display behavior. Both butterflies are poisonous and this is the quality that both the monarch and the viceroy try to assert to potential predators by means of a very similar color pattern and display behaviors.

In the New World Tropics, Mullerian mimicry rings can involve literally hundreds of species. The reasoning behind the formation of such systems has been likened to a group of people all trying to crowd under one shared umbrella, since it offers the best protection from the elements. The basis of Mullerian mimicry lies in the response of the receiver; if the predator has the ability to learn to avoid a species due to previously failed attempts, then the mimic can gain protection against the predator by using the same warning signals as the species that the predator previously encountered. Therefore, by copying a similar pattern of warning coloration and markings, various emetic species can increase their protection against would-be predators by reducing the variation in warning patterns, which must be learned by these predators.

The reasoning behind the decrease in effectiveness of mimicry upon removing species from their natal environments into foreign environments, lies in the concept of protective coloration that is employed by the model species, and thus the mimic species as well. Protective coloration is "the coloration or color pattern of an animal that affords it protection from the observance of a predator or its prey". The most common form of protective coloration is cryptic resemblance, in which various effects that enhance the similarity of color between the animal and its surroundings allow the organism to remain undetected when in the background of its habitat. A form of coloration that is similar in purpose is disruptive coloration, in which "irregular patches of contrasting colors serve to distract the observer’s eye from the outline of the animal". A second type of protective coloration occurs in animals whose coloration or markings purposely contrast with their habitat in order to serve as a warning of the species' toxic qualities to predators or as a method of communication to members of its own species in the vicinity. The coloration used as a model in mimicry can then be described as concealing, revealing, or deceiving. These forms of coloration increase the statistical chance for survival of the species, which would therefore explain why they are employed by models. Mimics therefore try to copy these methods as much as possible, thereby increasing the presence of these physical features and behaviors in their own species over generations of time. However, it is not protective coloring alone that ensures the success of the mimicry; it is instead a combination of protective coloration and the specific behavior associated with the warning patterns. If the mimic does not incorporate the specific displays associated with the mimicked regions of the body than the receiver may not be impressed, and will determine that the mimic is a fake. Because protective coloration is usually based on qualities of the natal environments, if the species is placed into a foreign environment that bears little or no resemblance to the natal environment the protective coloration will not aid in the survival of the species; the animal may no longer be concealed or the behavior will not be recognized by new predators and neighbors as warnings.

Resources

http://www.cirrusimage.com/butterfly_viceroy.htm

http://dorakmt.tripod.com/evolution/mimicry.html

http://reference.allrefer.com/encyclopedia/M/mimicry.html

Images

Please post links to good images illustrating mimicry, camouflage, etc.

  1. http://www.eryptick.net/oz2001/chick-camouflage-011030.jpg
  2. http://www.lclark.edu/~krauss/hatchetweb/chapter15tomoko%20/images/camouflage.jpg
  3. http://eco.goo.ne.jp/wnn-x/unno/files/photo/mimicry/eda/image/tobim_j.jpg
  4. http://www.seaslugforum.net/images/mimic01.jpg

Nudibranch (top), Flatworm (bottom) (from the [www.seaslugforum.net/mimicry.htm Sea Slug Forum])

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