Melanism is an undue development of dark-colored pigment in the skin or its appendages, and the opposite of albinism. It is also the medical term for black jaundice.[2] The word 'melanism' is deduced from the Greek: μελανός, meaning black pigment.[3]
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.
Pseudo-melanism, also called abundism, is another variant of pigmentation, characterized by dark spots or enlarged stripes, which cover a big part of the body of the animal making it appear melanistic.[4]
A deficiency in or total absence of melanin pigments is called amelanism. In humans and many other animals, this most commonly manifests as albinism, although other forms exist, such as leucism.
The morbid deposition of black matter, often of a malignant character, causing pigmented tumors is called melanosis.[5] For a description of melanin-related disorders see melanin, melanosis coli and ocular melanosis.
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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, while others such as black panthers use it as a foraging advantage during night hunting.[6] 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. Adaptive melanism has been shown to occur in a variety of animals, including mammals such as squirrels, many felines and canids, and coral snakes. 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.[7]
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 darken 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.[8] Peppered moth evolution is commonly used as an example of industrial melanism.
Melanistic coat coloration occurs as a common polymorphism in 11 of 37 felid species and reaches high population frequency in some cases but never achieves complete fixation. The black panther, a melanic form of leopard is common in the equatorial rainforest of Malaya and the tropical rainforest on the slopes of some African mountains such as Mount Kenya. The serval also has melanic forms not uncommon in certain areas of East Africa. In the jaguarundi coloration varies from dark brown and gray to light reddish. Melanic forms of jaguar are apparently fairly common in certain parts of South America.[9] In 1938 and 1940, two melanistic bobcats were trapped alive in sub-tropical Florida.[10]
In 2003, the dominant mode of inheritance of melanism in jaguars was confirmed by performing phenotype transmission analysis in a 116-individual captive pedigree. Melanistic animals were found to carry at least one copy of a mutant MC1R sequence allele, bearing a 15-base pair inframe deletion. Ten unrelated melanistic jaguars were either homozygous or heterozygous for this allele. A 24-base pair deletion causes the incompletely dominant allele for melanism in the jaguarundi. Sequencing of the agouti signalling peptide in the agouti gene coding region revealed a 2-base pair deletion in black domestic cats. These variants were absent in melanistic individuals of Geoffroy’s cat, oncilla, pampas cat and Asian golden cat, suggesting that melanism arose independently at least four times in the cat family.[11]
Melanism in leopards is inherited as a Mendelian, monogenic recessive trait relative to the spotted form. Pairings of black animals inter se have a significantly smaller litter size than other possible pairings.[12] Between January 1996 and March 2009, leopards were photographed at 16 sites in the Malay Peninsula that had a sampling effort of more than 1000 trap nights. Of 445 photographs of melanistic leopards taken, 410 came from study sites south of the Isthmus of Kra, where the non-melanistic morph was never photographed. These data suggest the near fixation of the dark allele in the region. The expected time to fixation of this recessive allele due to genetic drift alone ranged from about 1,100 years to about 100,000 years.[13] Melanism in leopards has been hypothesized to be causally associated with a selective advantage for ambush.[14]
Another theory is that the black leopards have increased resistance to viruses, indicative of a potential immune benefit to melanism.
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.
Melanin has several physiological roles in maintaining health, but, notably, it affects the synthesis of vitamin D. Melanin is the primary determinant of the degree of skin pigmentation and protects the body from harmful ultraviolet radiation. Synthesis of 1,25-dihydroxyvitamin D3 in the skin, however, is dependent on ultraviolet B light. Highly pigmented skin, to the level found in people of African origin, abrogates almost all ultraviolet-induced 1,25-(OH)2D3 synthesis. Numerous animal models and clinical studies have underlined the essential role of vitamin D as a modulator of the different processes of the immune system. Evidence indicates that serum concentrations of 1,25-(OH)2D3 and the prevalence of autoimmune diseases in a certain population are associated with the latitude at which that population resides.[15]
Genes for melanism in felines may provide resistance to viral infections. 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 felines in the Aberdares had been considered a high altitude adaptation due to absorbing more heat.[16]
Studies reported in New Scientist magazine in 2003 also suggested that recessive-gene melanism is linked to disease resistance rather than altitude. Melanistic cats may have better resistance to disease than cats with "normal" color coats. This would explain why recessive melanism persists when melanistic individuals are disadvantaged due to being poorly camouflaged in open areas. The melanism mutations involve the same gene family as those involved in humans.
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[17] and was promoted by some Afrocentrists, such as Frances Cress Welsing.