In the animal kingdom, things are not always what they appear to be. Many animals use coloration or particular behaviors -- standing motionless, for instance -- to avoid being detected by other animals.
The monarch butterfly benefits from its reputation for being foul-tasting, even poisonous if eaten in large quantities. Most birds, having attempted to eat a monarch once, will never try again. They recognize instantly the orange-and-black pattern on the butterflies when they see it, and they know enough to stay away.
What these experienced birds don't know, however, is that some of the monarchs they so assiduously avoid could be imposters. The viceroy butterfly, an entirely nonpoisonous species, has evolved a striking resemblance to the monarch and has benefited greatly as a result. Had the monarch not established that an orange-and-black coloration means poison, the viceroy population would be a lot smaller than it is today.
Sometimes the environment plays an important role in the effectiveness of an animal's coloration. One important example is the peppered moth of England. Prior to industrialization and the pollution that came with it, the trees and rocks around Manchester were covered with a layer of very light-colored lichen. So too were the majority of peppered moths in the region very light in color prior to the mid-1800s.
By the late 1800s, however, the environment around Manchester was changing rapidly. Pollution from coal-fired power plants and factories was killing off the lichen and blackening the surfaces of trees and rocks. Soon, the light coloration of the peppered moths became a liability rather than an asset. Experiments showed that birds found and ate light-colored moths in much larger numbers than they did darker moths of the same species.
A mutation in some peppered moths causes darker coloration. Before pollution darkened the environment, the number of dark moths remained low -- in part because they were preyed upon more heavily than their light-colored, better-hidden cousins. In an environment covered in black soot, however, the dark moths fared quite well, so that by the end of the 1800s around Manchester, England, dark peppered moths formed more than 95 percent of the moth population.
The original research on this phenomenon was conducted in the 1950s. However, more recent studies have confirmed the early conclusions. Indeed, a study published in 1998 found that higher percentages of light-colored peppered moths were showing up again in populations around Manchester as levels of coal pollution diminished. The study also confirmed the validity of the original study's conclusion about the cause of the color change, stating that, "Differential bird predation of the [light] and [dark] forms, in habitats affected by industrial pollution to different degrees, is the primary influence on the evolution of melanism in the peppered moth."