Colour variation in the peppered moth Biston betularia was long accepted to be under strong natural selection. Melanics were believed to be fitter than pale morphs because of lower predation at daytime resting sites on dark, sooty bark. Melanics became common during the industrial revolution, but since 1970 there has been a rapid reversal, assumed to have been caused by predators selecting against melanics resting on today's less sooty bark. Recently, these classical explanations of melanism were attacked, and there has been general scepticism about birds as selective agents. Experiments and observations were accordingly carried out by Michael Majerus to address perceived weaknesses of earlier work. Unfortunately, he did not live to publish the results, which are analysed and presented here by the authors. Majerus released 4864 moths in his six-year experiment, the largest ever attempted for any similar study. There was strong differential bird predation against melanic peppered moths. Daily selection against melanics (s ≃ 0.1) was sufficient in magnitude and direction to explain the recent rapid decline of melanism in post-industrial Britain. These data provide the most direct evidence yet to implicate camouflage and bird predation as the overriding explanation for the rise and fall of melanism in moths.
Insect body color polyphenisms enhance survival by producing crypsis in diverse backgrounds. While color polyphenisms are often indirectly induced by temperature, rearing density, or diet, insects can benefit from immediate crypsis if they evolve polyphenisms directly induced by exposure to the background color, hence immediately deriving protection from predation. Here, we examine such a directly induced color polyphenism in caterpillars of the geometrid peppered moth (Biston betularia). This larval color polyphenism is unrelated to the genetic polymorphism for melanic phenotypes in adult moths. B. betularia caterpillars are generalist feeders and develop body colors that closely match the brown or green twigs of their host plant. We expand on previous studies examining the proximal cues that stimulate color development. Under controlled rearing conditions, we manipulated diets and background reflectance, using both natural and artificial twigs, and show that visual experience has a much stronger effect than does diet in promoting precise color matching. Their induced body color was not a simple response to reflectance or light intensity but instead specifically matched the wavelength of light to which they were exposed. We also show that the potential to change color is retained until the final (sixth) larval instar. Given their broad host range, this directly induced color polyphenism likely provides the caterpillars with strong protection from bird predation.
The hypothesis that dimorphically coloured, cryptic moths select appropriate rest sites by comparing their body scales to substrate reflectance was tested using typical and melanic morphs of the peppered moth, Biston betuiaria (L.). Experiments designed to block the individual's inspection of its inherited colour phenotype do not support Kettlewell's contrast/conflict (self-inspection) hypothesis. Instead, tracking of marked moths over successive days revealed individual differences in rest-site selection which were not related to treatments, experience (imprinting), nor closely to a moth's inherited colour pattern. Differences between family broods indicate that some genetic bias in background selection exists. The production of artificially selected lines with consistent but opposing preferences will allow us to investigate the co-evolution of pattern and behaviour.
Industrial melanism in peppered moths has been studied most intensively in Britain. The first melanic phenotype (effectively solid black) was recorded near Manchester in 1848. By 1895 about 98% of the specimens near Manchester were melanic, and this once rare phenotype had spread across regions of the country blackened by industrial soot. In rural, unpolluted regions, well away from industrial centers, the pale phenotype (peppered with white and black scales) remained the predominant form. During the latter half of the 20th century, following legislation designed to improve air quality, melanics began to decline in frequency and are now rare where once they had been common. Similar evolutionary changes have occurred elsewhere, but records from outside Britain are fragmentary. We have extended previous surveys of American peppered moth populations and present a composite picture of the recent decline in melanism in northern industrial states-Michigan and Pennsylvania-where melanic phenotypes decreased from more than 90% in 1959 to 6% by 2001. We contrast these changes to the near absence of melanism in a southern state-Virginia-during that same period. As in Britain, the decline in melanism in American peppered moths followed clean air legislation.
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