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)."
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The model (Eastern Coral Snake) above.
The mimic (Scarlet Kingsnake) below. JCM
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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.
Labels: imperfect mimics, lampropeltis, micrurus, mimicry
