melanism

The black panther is the prototypical example of melanism.

Melanism (Greek: μελανός = dark-colored) is the occurence of an increased amount of dark pigmentation (as of skin, feathers, eyes or hair) in an organism, resulting from the presence of melanin. It is the opposite of leucism and albinism, which occur because of a lack of melanin or other types of pigment.

Abundism is the occurence of excessively abundant dark markings (such as spots, stripes or other patch types) due to increased dark pigmentation on the coat, fur or skin of some animal species. As the number and/or size of these markings increases, some overlap and merge causing the effect of pseudomelanism where the animal appears completely black. The background color may still be discerned between the markings.

Melanism (and pseudomelanism) is 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.^{[citation needed]}

In the context of diseases, the condition in which abnormally dark pigmentation of some tissues occurs is coined melanosis.^[1]For a description of melanin-related disorders see melanin, melanosis coli and ocular melanosis.

Adaptive melanism

Melanism related to the process of adaptation is called adaptive. Most commonly, dark individuals become fitter to survive and reproduce in their environment as they are better camouflaged. This makes some species less conspicuous to predators ^[2] while others use it as a foraging advantage (for example during night hunting)^[3]. Typically adaptive melanism is heritable: A dominant gene (which is entirely or nearly entirely expressed in the phenotype) is responsible for the excessive amount of melanin.^[4] Adaptive 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).

Adaptive melanism can lead to the creation of morphs, the most notable example being the peppered moth whose evolutionary history in the United Kingdom is offered as a classic instructional tool for teaching the principles of natural selection.

Industrial melanism

Industrial melanism is adaptive melanism caused by anthropogenic alteration of the natural environment in terms of industrial pollution. As soot, smoke and other industrial pollutants from factories darkens the landscape and because many organisms rely on camouflage to avoid predation, this sudden change in their 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.^[5] Peppered moth evolution is commonly used as an example of industrial melanism.

Melanistic Guinea pigs are relatively rare, and are considered especially effective in ritual use by Andean curanderos.^[6]

Examples

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.

Fauna of Toronto, Canada: Melanistic Eastern Grey Squirrel

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. It is believed that Tigers and Lions may also have the potential to be melanistic, but while there may have been black lions and tigers in the past, it is also quite possible melanistic bloodlines are extinct.

Melanism and the immune system

Genetic research has shown that melanistic wolves owe their colouration to a mutation that first arose in domestic dogs

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 felines 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.^{[citation needed]} Previously, black furred felines in the Aberdares had been considered a high altitude adaptation due to absorbing more heat.^{[citation needed]}

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. 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.There is an advantage to Melanin that Racists will not admit to. Melanism could make black cats less susceptible to certain viral infections making melanism an evolutionary advantage.

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^[7] and was promoted by some Afrocentrists, such as Frances Cress Welsing.

Further reading

• David Attenborough. (2002) The Life of Mammals [TV-Series and book]. United Kingdom: BBC..
• Kettlewell, Bernard (1973). The Evolution of Melanism. Clarendon Press. ISBN 0-19-857370-7. 
• Majerus, Michael (1998). Melanism: Evolution in Action. Oxford University Press. ISBN 0-19-854982-2. 
• Melanism and disease resistance in insects

See also

	Environment portal
	Ecology portal
	Earth sciences portal
	Evolutionary biology portal

• Polymorphism (biology)
• Albinism
• Leucism
• Vitiligo

Wikimedia Commons has media related to: Melanism

References