Müllerian mimicry is a natural phenomenon in which two or more poisonous species, that may or may not be closely related and share one or more common predators, have come to mimic each other's warning signals. It is named after the German naturalist Fritz Müller, who first proposed the concept in 1878.
The phenomenon can be understood by imagining two poisonous species that do not resemble one another and are also prey to a common predator. Occasionally, individuals of the predatory third species will encounter one or the other type of noxious prey, and thereafter avoid it. Predators that avoid only one or the other type of harmful species provide no benefit to individuals of the species that is not avoided. Therefore, there is an advantage to be gained in the appearance of the two prey species. This is because a predator that learns to avoid either species in a pair of species exhibiting Müllerian mimicry learns, in effect, to avoid both.
This strategy is typically contrasted with Batesian mimicry, in which one harmless species adopts the appearance of another, harmful species to gain the advantage of predators' avoidance. However, because comimics may have differing degrees of protection, the distinction between Müllerian and Batesian mimicry is not absolute, and there can be said to be a spectrum between the two forms. Additionally, a species may be a Batesian mimic to one predator and a Müllerian mimic to another. While Batesian and Müllerian mimicry are commonly given examples of mimicry, there is often little or no mention of other forms. There are many other types of mimicry however, some very similar in principle, others far separated. For example in aggressive mimicry a predator might mimic the food of its prey, luring them towards it and improving its foraging success.
Müllerian mimicry needs not involve visual mimicry; it may employ any of the senses. For example, many snakes share the same auditory warning signals, forming an auditory Müllerian mimicry ring. More than one common signal may show convergences by the parties. While model and mimic are often closely related species, Müllerian mimicry between very distantly related taxa also occurs.
Müllerian mimicry was proposed by the German zoologist and naturalist Johann Friedrich Theodor Müller (1821–1897), always known as Fritz. An early proponent of evolution, Müller offered the first explanation for resemblance between certain butterflies that had puzzled the English naturalist Henry Walter Bates, who, like Müller, spent a significant part of his life in Brazil. Müller had also seen these butterflies first hand, and collected specimens like Bates.
Understanding Müllerian mimicry is impossible without first understanding aposematism, or warning signals. Dangerous organisms with these aposematic signals are avoided by predators, who quickly learn after a bad experience not to pursue the same prey again. Learning is not actually necessary for animals which instinctively avoid certain prey; however, learning from experience is much more common. The underlying concept with predators that learn is that the warning signal makes the harmful organism easier to remember than if it remained as cryptic as possible (e.g. being still and silent, providing no scent, and blending in with the background). Aposematism and crypsis are in this way opposing concepts, but this does not mean they are mutually exclusive. Many animals remain inconspicuous until threatened, then suddenly employ warning signals, such as startling eyespots, bright colors on their undersides or loud vocalizations. In this way, they enjoy the best of both strategies. These strategies may also be employed differentially throughout stages of development. For instance, Large White butterflies are aposematic in color as larvae, but then display Mullerian mimicry once they emerge from the final stages of development as butterflies. 
Many different prey of the same predator may employ separate warning colors, but this makes no sense for any party. Surely if they could all get together and agree on a common warning signal, the predator would have fewer detrimental experiences, and the prey would lose fewer individuals educating it. But no such conference needs to take place, as a prey species that just so happens to look a little like a harmful species will be safer than its conspecifics, leading to a tendency toward a single warning language. This can lead to the evolution of both Batesian and Müllerian mimicry, depending on whether the prey is harmful, as well, or just a freerider. Multiple species can join this protective cooperative, expanding the mimicry ring.
Müller thus provided an explanation for 'Bates' paradox'; the mimicry was not a case of exploitation by one species, but rather a mutualistic arrangement.
Notes and references
- Meyer, A. (2006). "Repeating patterns of mimicry". PLoS Biol 4 (10): e341. doi:10.1371/journal.pbio.0040341. PMC 1617347. PMID 17048984.
- Müller, Fritz (1878). "Ueber die Vortheile der Mimicry bei Schmetterlingen". Zoologischer Anzeiger 1: 54–55.
- Müller, F. (1879). "Ituna and Thyridia; a remarkable case of mimicry in butterflies. (R. Meldola translation)". Proclamations of the Entomological Society of London 1879: 20–29.
- Brower, L. P.; Ryerson, W. N.; Coppinger, L. L.; Glazier, S. C. (1968). "Ecological chemistry and the palatability spectrum". Science 161 (3848): 1349–51. Bibcode:1968Sci...161.1349B. doi:10.1126/science.161.3848.1349.
- For example, aggressive mimicry is not mentioned in Poulton, E. B., Ford, E. B. 1968. Mimcry. In Encyclopædia Britannica, vol. 15, pp. 487-89. Chicago: William Benton, bicentennial ed.
- Ritland, D.; L. P. Brower (1991). "The viceroy butterfly is not a Batesian mimic". Nature 350 (6318): 497–498. Bibcode:1991Natur.350..497R. doi:10.1038/350497a0. Retrieved 2008-02-23. "Viceroys are as unpalatable as monarchs, and significantly more unpalatable than queens from representative Florida populations."
- Smith, S. M. (1975). "Innate Recognition of Coral Snake Pattern by a Possible Avian Predator". Science 187 (4178): 759–760. Bibcode:1975Sci...187..759S. doi:10.1126/science.187.4178.759. PMID 17795249.
- Wickler, Wolfgang (1998). “Mimicry”. Encyclopædia Britannica, 15th edition. Macropædia 24, 144–151. http://www.britannica.com/eb/article-11910
- Feltwell, John. Large White Butterfly: The Biology, Biochemistry, and Physiology of Pieris Brassicae (Linnaeus). The Hague: W. Junk, 1982. Print.
- Wickler, W. (1968) Mimicry in Plants and Animals (Translated from the German) McGraw-Hill, New York. ISBN 0-07-070100-8 Especially chapters 7 and 8.
- Ruxton, G. D.; Speed, M. P.; Sherratt, T. N. (2004). Avoiding Attack. The Evolutionary Ecology of Crypsis, Warning Signals and Mimicry. Oxford: Oxford University Press. ISBN 0-19-852860-4 Chapter 9 and 11 provide an overview of current understanding