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 pigmented
tumors, is called
melanosis.[3]
Adaptation
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
leopards, use it as a foraging advantage during night hunting.[4] Typically, adaptive melanism is
heritable: A
dominantallele, 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
cats 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.[5]
Industrial melanism is an
evolutionary effect in insects such as the peppered moth, Biston betularia in areas subject to
industrial pollution. Darker pigmented individuals are favored by
natural selection, apparently because they are better
camouflaged against polluted backgrounds. When pollution was later reduced, lighter forms regained the advantage and melanism became less frequent.[6][7][8][9][10][11] Other explanations have been proposed, such as that the melanin pigment enhances function of immune defences,[12] or a thermal advantage from the darker coloration.[13][14][15]
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.[18]
Melanism in leopards is inherited as a
Mendelian,
monogenicrecessive trait relative to the spotted form. Pairings of black animals have a significantly smaller litter size than other possible pairings.[19] Between January 1996 and March 2009,
Indochinese leopards were photographed at 16 sites in the
Malay Peninsula in a sampling effort of more than 1000 trap nights. Of 445 photographs of melanistic leopards, 410 were taken south of the
Kra 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 to
genetic drift alone ranged from about 1,100 years to about 100,000 years.[20]
Melanism in leopards has been hypothesized to be causally associated with a selective advantage for ambush.[21] 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.[22]
In birds
The chicken breeds
Silkie and
Ayam 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.
In April 2015, an extremely rare black
flamingo was spotted on the Mediterranean island of
Cyprus.[23]
In amphibians
The
alpine salamander, Salamandra atra, has one subspecies (S. atra atra) that is completely black.[24] The pigment comes from a specific cell called a melanophore, which produce the compound melanin.[25][26]
There are four other subspecies of this salamander,[27] and they have varying levels of melanin pigmentation.[26][28][29] The subspecies have yellow spots in different concentrations or proportions. The
pigment-producing cells that contribute to the yellow spots of some sub-species are called xanthophores.[28] It appears that the fully-black phenotypes do not ever develop these xanthophores.[29] Alpine salamanders produce a toxin from their skin, and both fully melanistic, black salamanders and spotted individuals produce the compound.[30]
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.[29]
In humans
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 white people do not have as many as black people.[31] Melanin is the primary determinant of the degree of skin pigmentation and protects the body from harmful
ultraviolet radiation. The same ultraviolet radiation is essential for the synthesis of
vitamin 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 from parts of Africa,
South Asia,
Southeast Asia, and Australia may have very dark skin, but this is not melanism.
Melanosis, hyperpigmentation via increased melanin
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^Grant, B. S., Wiseman L. L., 2002. Recent history of melanism in American peppered moths. Journal of Heredity 93:86-90.
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^Grant, B. S., Cook, A. D., Clarke, C. A., & Owen, D. F. (1998). Geographic and temporal variation in the incidence of melanism in peppered moth populations in America and Britain. Journal of Heredity, 89(5), 465–471.
^Mikkola, K., & Rantala, M. J. (2010). Immune defence, a possible nonvisual selective factor behind the industrial melanism of moths (Lepidoptera). Biological Journal of the Linnean Society, 99(4), 831–838.
^Mikkola, K., Albrecht, A., 1988. The melanism of Adalia-bipunctata around the Gulf of Finland as an industrial phenomenon (Coleoptera, Coccinellidae). Annales Zoologici Fennici 25:177–85.
^Muggleton, J., Lonsdale, D., Benham, B. R., 1975. Melanism in Adalia-bipunctata L (ColCoccinellidae) and its relationship to atmospheric pollution. Journal of Applied Ecology 2:451–464.
^De Jong, P. W., Verhoog, M. D., Brakefield, P. M., 1992. Sperm competition and melanic polymorphism in the 2-spot ladybird, Adalla bipunctata (Coleoptera, Coccinellidae). Journal of Heredity 70:172–178.
^Searle, A. G. (1968) Comparative Genetics of Coat Colour in Mammals. Logos Press, London
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