melanosis n : a condition characterized by abnormal deposits of melanin (especially in the skin) [syn: melanism]

Melanism [Gr. μέλας] is an increased amount of black or nearly black pigmentation (as of skin, feathers, or hair) of an organism, resulting from the presence of melanin. It is the opposite of albinism, which occurs due to lack of melanin. More technically, it refers to a phenotype in which the pigmentation of an organism is entirely, or nearly entirely, expressed. A synonym for this condition used in the context of human disease is melanosis. Abundism is an increase in dark pigmentation in patterned coats or skins which causes an increase in the number or size of pigmented spots, stripes or other patch types. Abundism which is sufficiently extreme to appear like melanism, such as when the stripes of a striped animal increase in width sufficiently to overlap, is known as pseudo-melanism. Melanism and abundism are often the result of genetic mutation, but can result from other stimuli, such as exposure to abnormal temperature changes during gestation which transiently alter gene transcription or translation. Melanism or abundism triggered by human modification of the environment is known as industrial melanism; the history of this phenomenon in the peppered moth in the United Kingdom is a classic instructional tool for teaching the principles of natural selection.

Melanism has been shown to occur in a variety of animals, including mammals (squirrels, many felines, many canids); reptiles (coral snakes); and insects (peppered moth).

Many examples of melanism are known among felines. Melanism is due to changes in the agouti gene which controls banding of black and light areas on the hair shaft. Leopards and jaguars with this condition are often called black panther (although cougars are also known as panthers, there are no verified cases of melanism in that species). However, the leopard, the jaguar, the lion and the tiger are all members of the Panthera genus. One good example of melanism expressed within a certain animal community is that of the leopard population in Malaysia, South East Asia, in which case up to 50% of the population has melanism. That is apparently due to them being more cryptic in their dusky rainforest habitat. Better resistance to viruses may also explain the greater prevalence of black leopards in those areas. In the Jaguar, melanism is due to a dominant gene mutation meaning that black jaguars may produce spotted offspring. In the leopard, melanism is due to a recessive gene mutation meaning that two spotted leopards carrying the gene may produce black cubs, but black leopards will breed true when mated together.

Industrial melanism

Melanism is a phenomenon caused by anthropogenic alteration of the natural environment where industrial pollution turns vegetation a dark sooty colour. Because many organisms rely on camouflage to avoid predation, the sudden change in environment makes them highly vulnerable to predators. This creates a strong selective pressure which will see any organism with a darker colour much more likely to survive and contribute to the gene pool of the next generation. Rare mutations are hence selected for and over time the population will adjust to a new equilibrium.

Melanism and the immune system

Melanism has been found to be linked to beneficial changes in the immune system. "The Smithsonian Answer Book: Cats" notes that genes for melanism in felids may provide resistance from viral infections and that a viral epidemic may explain the prevalence of black leopards in Java and Malaysia, and the relatively high incidence of black leopards and black servals in the Aberdares region of Africa. Previously, black furred felids in the Aberdares had been considered a high altitude adaptation due to absorbing more heat.

Studies reported in New Scientist magazine in 2003 also suggested that recessive-gene melanism is linked to disease resistance rather than altitude. According to Eduardo Eizirik and Stephen O'Brien of the United States National Cancer Institute in Maryland, the melanism mutations involve the same gene family as those involved in human diseases such as AIDS. Melanistic cats may therefore have better resistance to disease than cats with "normal" colour coats. This would explain why recessive melanism persists when melanistic individuals are disadvantaged due to being poorly camouflaged in open areas.

In the United States National Cancer Institute studies, black cats were found to have changes to a gene known as MC1R. MC1R is a member of a family of genes that includes the human gene CCR5 which codes for a protein on the cell membrane. This protein is a key allowing in various viruses, including HIV. Melanism could make black cats less susceptible to certain viral infections making melanism an evolutionary advantage.

Pseudo-melanism and abundism

In animal species that normally have black markings on a paler background colour, excessively abundant markings (abundism) which merge or overlap produce an effect called pseudo-melanism. The background colour may still be discerned between the markings, but to the casual observer, or from a distance, the animal appears to be black.

Melanism as a socio-political movement

The term melanism has been used on usenet, internet forums and blogs to mean an African-American social movement holding that dark-skinned humans are in some measures superior to those of other skin colour. The term melanism has been used in this context as early as the mid-1990s and was promoted by some Afrocentrists, such as Frances Cress Welsing.

Further reading

• The Evolution of Melanism
• Melanism: Evolution in Action
• Melanism and disease resistance in insects

See also

• Albinism
• Leucism
• Vitiligo

References