Melanism is thecongenital excess ofmelanin in an organism resulting in darkpigment.
Pseudomelanism, also called abundism, is another variant of pigmentation, identifiable by dark spots or enlarged stripes, which cover a large part of the body of the animal, making it appear melanistic.[2]
The morbid deposition of black matter, often of a malignant character causing pigmentedtumors, is calledmelanosis.[3]
Melanism related to the process ofadaptation is called adaptive. Most commonly, dark individuals becomefitter to survive and reproduce in their environment as they are better camouflaged. This makes some species less conspicuous to predators, while others, such asleopards, use it as a foraging advantage during night hunting.[4] Typically, adaptive melanism isheritable: Adominantallele, which is entirely or nearly entirely expressed in thephenotype, is responsible for the excessive amount of melanin. By contrast, adaptive melanism associated with Batesian mimicry inZelandoperla fenestratastoneflies is controlled by arecessive allele at the ebony locus.[5][6]
Adaptive melanism has been shown to occur in a variety of animals, including mammals such assquirrels, manycats andcanids, andcoral snakes. Adaptive melanism can lead to the creation ofmorphs, a notable example being thepeppered moth, whose evolutionary history in theUnited Kingdom is offered as a classic instructional tool for teaching the principles ofnatural selection.[7] A more replicated example of human-induced shifts inmelanism has arisen from repeated selection against melanicZelandoperla fenestrata stonefly phenotypes following widespreaddeforestation in New Zealand.[5][6]
Industrial melanism is anevolutionary effect in insects such as the peppered moth,Biston betularia in areas subject toindustrial pollution. Darker pigmented individuals are favored bynatural selection, apparently because they are bettercamouflaged against polluted backgrounds. When pollution was later reduced, lighter forms regained the advantage and melanism became less frequent.[8][9][10][11][12][13] Other explanations have been proposed, such as that the melanin pigment enhances function of immune defences,[14] or a thermal advantage from the darker coloration.[15][16][17]
Melanistic coat coloration occurs as a commonpolymorphism in 11 of 37felid species and reaches high population frequency in some cases but never achieves completefixation. Theblack panther, a melanisticleopard, is common in theequatorial rainforest ofMalaya and thetropical rainforest on the slopes of some African mountains, such asMount Kenya. Theserval also has melanistic forms in certain areas ofEast Africa. In thejaguarundi, coloration varies from dark brown and gray to light reddish. Melanic forms ofjaguar are common in certain parts ofSouth America.[18] In 1938 and 1940, two melanisticbobcats were trapped alive in sub-tropicalFlorida.[19]
In 2003, thedominant mode of inheritance of melanism in jaguars was confirmed by performingphenotype-transmission analysis in a 116-individual captivepedigree. Melanistic animals were found to carry at least one copy of a mutantMC1R sequenceallele, bearing a 15-base pair inframe deletion. Ten unrelated melanistic jaguars were eitherhomozygous orheterozygous for this allele. A 24-base pair deletion causes the incompletely dominant allele for melanism in the jaguarundi. Sequencing of theagouti signalling peptide in theagouti gene coding region revealed a 2-base pair deletion in blackdomestic cats. These variants were absent in melanistic individuals ofGeoffroy's cat,oncilla,pampas cat andAsian golden cat, suggesting that melanism arose independently at least four times in the cat family.[20]
Melanism in leopards is inherited as aMendelian,monogenicrecessive trait relative to the spotted form. Pairings of black animals have a significantly smaller litter size than other possible pairings.[21] Between January 1996 and March 2009,Indochinese leopards were photographed at 16 sites in theMalay Peninsula in a sampling effort of more than 1000 trap nights. Of 445 photographs of melanistic leopards, 410 were taken south of theKra Isthmus, 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 togenetic drift alone ranged from about 1,100 years to about 100,000 years.[22]Melanism in leopards has been hypothesized to be causally associated with a selective advantage for ambush.[23] Other theories are that genes for melanism in felines may provide resistance to viral infections, or a high-altitude adaptation, since black fur absorbs more light for warmth.[24]
In April 2015, an extremely rare blackflamingo was spotted on the Mediterranean island ofCyprus.[25]
The chicken breedsSilkie andAyam Cemani commonly exhibit this trait. Ayam Cemani is an uncommon and relatively modern breed of chicken from Indonesia. They have a dominant gene that causes hyperpigmentation (Fibromelanosis), making the chicken entirely black; including feathers, beak, and internal organs. Melanisticcommon pheasants are sometimes intentionally released as game birds.[citation needed]
Melanism in feralrock doves is quite common, especially if the species is abundant in the area. The amount of pigmentation varies, from a slightly darker pigmentation in the pigeon’s wings, to being almost completely black.[citation needed]
Thealpine salamander,Salamandra atra, has one subspecies (S. atra atra) that is completely black.[26] The pigment comes from a specific cell called a melanophore, which produce the compound melanin.[27][28]
There are four other subspecies of this salamander,[29] and they have varying levels of melanin pigmentation.[28][30][31] The subspecies have yellow spots in different concentrations or proportions. Thepigment-producing cells that contribute to the yellow spots of some sub-species are called xanthophores.[30] It appears that the fully-black phenotypes do not ever develop these xanthophores.[31] Alpine salamanders produce a toxin from their skin, and both fully melanistic, black salamanders and spotted individuals produce the compound.[32]
Studies done that traced DNA histories have suggested that the original alpine salamander phenotype was black with some yellow spots, meaning that the fully black color evolved over time and was thus selected for over many generations.[31]
Melanism, meaning a mutation that results in completely dark skin, does not exist in humans. In humans, the amount of melanin is determined by three dominant alleles (AABBCC), and different ethnicities have varying amounts.[33] Melanin is the primary determinant of the degree of skin pigmentation and protects the body from harmfulultraviolet radiation. The same ultraviolet radiation is essential for the synthesis ofvitamin D in skin, so lighter colored skin – less melanin – is an adaptation related to the prehistoric movement of humans away from equatorial regions, as there is less exposure to sunlight at higher latitudes. People fromSub-Saharan Africa,South Asia,Southeast Asia, Australia, andMelanesia may have very dark skin, but this is not melanism.
This raregenetic disorder is characterized by the development ofmacules withhyperpigmentation on the lips andoral mucosa (melanosis), as well as benign polyps in the gastrointestinal tract.[34]
The termmelanism has been used onUsenet,internet forums andblogs to mean anAfrican-Americansocial movement holding that dark-skinned humans are the original people from which those of otherskin color originate. The termmelanism has been used in this context as early as the mid-1990s[35] and was promoted by someAfrocentrists, such asFrances Cress Welsing.
