What Is Industrial Melanism?

Industrial melanism is a term that describes how some animals change color in response to environmental changes caused by pollution. The term was coined just after the Industrial Revolution when coal was used to power factories in cities such as London and New York. Industrial melanism was discovered in 1900 by geneticist William Bateson, and various naturalists have observed the phenomenon over time. While the reason for industrial melanism wasn't immediately obvious, researchers found that it was an evolutionary response to a changing environment.

Why Industrial Melanism Occurs

Many animals, such as chameleons, change color in response to their environments. Those exhibiting industrial melanism live in highly industrialized areas, and these color changes camouflage the animals so they aren't seen by predators. This phenomenon is explained by Darwin's "survival of the fittest" theory; animals that are closest to their background color and thus better-camouflaged are able to survive long enough to reproduce. As a result, they pass their ability to change color to their offspring so they, too, can survive.

In a sooty city, darker-colored moths and butterflies fare better than their lighter-colored cousins. Of course, if the industrial waste is cleaned up and the environment becomes lighter, the darker-colored animals become more visible and vulnerable to attack. Those who are lighter, in this scenario, will be able to survive longer and pass along their lighter genes to their offspring.

While this explanation makes sense for some examples of industrial melanism, some animals such as snakes and beetles don't seem better camouflaged as a result of changing pigmentation; these species have other reasons for changing color.

Examples of Industrial Melanism

There are quite a few examples of industrial melanism. The best-known and most common is moths living in industrialized cities.

Peppered Moths

Peppered moths are commonly found in England; originally, they were light-colored moths living on the light-colored lichens that cover the trees. Their light color effectively camouflaged them from predators.

During the Industrial Revolution, coal-powered plants emitted both sulfur dioxide and soot. The sulfur dioxide killed off much of the lichen, while the soot darkened light-colored trees and stones. Light-colored peppered moths stood out brightly against the now-darkened background and were easily picked off by birds. Meanwhile, the darker colored moths lived longer and reproduced; in fact, the darker peppered moths had 30% greater fitness advantage as compared with light-color moths. By 1895, over 90% of peppered moths were dark-colored.

Over time, new environmental laws in the United States and Britain radically reduced soot and sulfur dioxide emissions. Almost all peppered moths in Pennsylvania and Michigan were dark-colored in 1959, but by 2001 only 6% were dark. They had responded to cleaner air, lighter surfaces, and healthier light-colored lichens.

Sea Snakes

Turtle-headed sea snakes live in the South Pacific ocean, where they originally sported bands of light and dark coloring. Some populations of these snakes, however, are nearly black. Researchers were intrigued by the differences in coloration and worked together to better understand why and how the differences occurred.

The researchers had collected hundreds of sea snakes over the years from industrial and non-industrial sites in New Zealand and Australia. They had also collected sloughed-off snake skins. After testing, they discovered that:

• black skins were more common in snakes living in industrial areas;
• black skins contained elements like zinc and arsenic, which are used in industry;
• banded snakes were more common in snakes living in cleaner areas;
• the darker bands of the banded snakes contained more zinc and arsenic than their lighter bands;
• darker colored snakes are more likely to slough their skins.

Unlike the peppered moths, sea snakes don't seem to gain any adaptive advantage as a result of changed coloring. So why the change? Darker snakes slough their skins more often, which may mean that they rid themselves of pollutants more often. This hypothesis has been tested but not yet proven.

Two-Spot Ladybugs

Two-spot ladybugs came in two color patterns: red with black spots and black with red spots. Over time, however, researchers have found that the majority are red with black spots. This seems to be an adaptive advantage; red bugs are easier to see and look less appetizing to predators because of their color, which makes them less likely to be eaten.

Unlike pepper moths and sea snakes, two-spot ladybugs don't seem to be responding directly to industrial impacts. The area of the study (in Norway) has been consistently warming, and researchers believe the ladybugs are most likely responding to climate change.