Mimicry can be Imperfect but Effective

British naturalist and explorer, Henry W. Bates is known for his 1848 collecting expedition to the Amazon with Alfred Russel Wallace. Wallace lost most of what he collected in a shipwreck on his way home in 1852. Bates, however, did not return until 1859 and by that date had sent most of the 14,000 species he collected home. Bates' work on Amazonian butterflies led him to develop the concept of mimicry. In 1861, he wrote, "The process by which a mimetic analogy is brought about in nature is a problem which involves that of the origin of all species and all adaptations." Today, Batesian mimicry is the term used to describe the situation when a palatable species looks like (or mimics) an unpalatable or noxious species. One criticism of mimicry theory is that the mimics are often imperfect. Now, David W. Kikuchi and David W. Pfennig at the University of North Carolina have addressed this issue with field experiments, using the venomous Eastern Coral Snake (Micrurus fulvius) and its harmless mimic the Scarlet Kingsnake (Lampropeltis elapsoides). They designed a field experiment to determine if predator cognitive abilities could explain imperfect coral snake mimicry. The Scarlet Kingsnake is an imprecise mimic of the Eastern Coral Snakes.  Both species possess brightly colored rings of red, yellow, and black encircling their bodies, but the rings differ in order: the coral snake has a black-yellow-red-yellow ring order, while the kingsnake has a black-yellow-black-red ring order.  Therefore, the well known rhyme for distinguishing coral snakes models from kingsnakes mimics, “red on yellow, kill a fellow; red on black, venom lack”. They chose a study site in southeastern North Carolina where the two species distributions overlap. To measure selection on different snake phenotypes, they used clay replicas of snakes bearing three different color patterns. The authors asked, would predators avoid perfect mimics (the coral snake) and prey on imperfect mimics (the kingsnake), or a very poor mimic (a kingsnake model that differs from the coral snake in ring order and relative proportions of red and black)? The contrast between predation on the poor mimic and on the good and perfect mimics served as a control. A previous study demonstrated that the poor mimic is attacked significantly more often than the good mimic in an area where the two species overlap. In the field, Kikuchi and Pfennig arranged the replicas in threes (consisting of one of each phenotype) and placed them in transects. These were separated from adjacent sets by about 75 m. Eighteen such transects were placed in natural areas where mimics and snake predators are abundant. After 5 weeks in the field the replicas were collected and scored as having been attacked if it bore a marks suggesting a beak, claw, or carnivore bite marks. Or, if the model was carried off completely. Markings consistent with rodent or insect activity were ignored because they do not pose a threat to snakes. Of 537 replicas available for analysis 66 (12.3%) were attacked. Of these, 10 were attacked by birds and 21 by carnivorous mammals and 35 could not be assigned to a specific predator group. Replicas of the good mimics (based on the kingsnake) were no more likely to be attacked by predators than were replicas of the model (the coral snake). The authors suggest two hypotheses that might explain why selection does not favor improvement in mimicry. First, predators might generalize aposematic signals of models due to an increasingly high probability of incorrectly identifying prey as mimics grow more similar to models in phenotype – this is a widely supported idea. Secondly, with a highly toxic model (like the coral snake) risk taking by predators is disfavored. Consequently, predators should avoid a wide range of trait values, thereby maintaining imprecise mimics. Evidence for the second hypothesis was found in this study. The authors suggest that the difference in predation rates on good and poor mimics can best be reconciled if mimics exploit a limitation in predator cognition. In summary they wrote, "If only certain traits are required to deceive predators, then mimics need not resemble their model exactly.... The fact that good mimics did not suffer any greater predation than perfect mimics...suggests that good mimics achieved complete protection by resembling the model in color proportions alone (or, for deterring attacks by mammalian predators that might lack color vision..., good mimics achieved complete protection by resembling the model in proportions of different shades of gray)."
The model (Eastern Coral Snake) above. 
The mimic (Scarlet Kingsnake) below. JCM



Literature
Bates H. W. 1862. Contributions to an insect fauna of the Amazon Valley. Lepidoptera: Heliconidae. Transactions of the Entomological Society, 23:495-566.

Kikuchi D. W. and David W. Pfennig. 2010. Predator Cognition Permits Imperfect Coral Snake Mimicry. The American Naturalist 176:830-834.

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