Melanism
Congenital dark pigmentation from high melanin levels.
Melanism is a congenital condition where an organism produces an unusually high amount of melanin, leading to dark pigmentation. A related but distinct variation is pseudomelanism, also called abundism, in which an animal develops large dark spots or enlarged stripes that cover most of its body, creating a melanistic appearance. Melanosis refers to hyperpigmentation from increased melanin that occurs in various conditions; some are benign, like freckles, while others are or can become malignant, and these conditions may be either congenital or acquired.
When melanism helps an organism adapt, it is called adaptive melanism. Dark individuals often become fitter for survival and reproduction because they are better camouflaged—some become less visible to predators, while others, such as leopards, gain an advantage when hunting at night. Adaptive melanism is usually heritable, typically through a dominant allele that is fully or nearly fully expressed in the phenotype. However, in the stonefly *Zelandoperla fenestrata*, adaptive melanism linked to Batesian mimicry is controlled by a recessive allele at the ebony locus. A well-documented example of human-driven change occurred when repeated selection against melanic *Z. fenestrata* phenotypes followed widespread deforestation in New Zealand.
Industrial melanism is an evolutionary effect seen in insects like the peppered moth (*Biston betularia*) in areas with industrial pollution. Darker individuals are favored by natural selection, likely because they are better camouflaged against soot-covered backgrounds. When pollution later decreased, lighter forms regained the advantage, and melanism became less common. Other proposed explanations include that melanin boosts immune function or provides a thermal benefit from darker coloration.
In cats, melanism has been documented in several species. Two melanistic bobcats were trapped alive in subtropical Florida in 1938 and 1940. In jaguars, a 2003 study of a captive pedigree confirmed that melanism is inherited as a dominant trait, linked to a mutant MC1R allele with a 15-base pair inframe deletion; ten unrelated melanistic jaguars were either homozygous or heterozygous for this allele. In the jaguarundi, a 24-base pair deletion causes an incompletely dominant melanism allele.
- Field
- Biology, Genetics, Evolutionary Biology
- Known for
- Congenital high melanin causing dark pigmentation; adaptive and industrial melanism; independent evolution in cats
- Types
- Adaptive melanism, industrial melanism, pseudomelanism (abundism), melanosis
- Inheritance
- Dominant or recessive alleles depending on species; e.g., dominant in jaguars, recessive in leopards and Zelandoperla fenestrata stoneflies
- Examples
- Peppered moth, black jaguar, melanistic leopard, Ayam Cemani chicken, alpine salamander
Lore & Background
Melanism is observed in numerous animal groups. In cats, melanism in jaguars is caused by a dominant mutant MC1R allele with a 15-base pair deletion, while in leopards it is a Mendelian recessive trait. Melanism arose independently at least four times in the cat family. In birds, the Ayam Cemani chicken has a dominant gene causing hyperpigmentation (Fibromelanosis), making it entirely black, including internal organs. The alpine salamander has a completely black subspecies, Salamandra atra atra, which lacks xanthophores that produce yellow spots in other subspecies.
Reader's Guide
Melanism is significant in evolutionary biology as a clear example of natural selection. Industrial melanism in the peppered moth (Biston betularia) shows how pollution favored darker individuals, and how reduced pollution later favored lighter forms. Adaptive melanism provides camouflage or foraging advantages, as seen in leopards and stoneflies. Human-induced shifts, such as selection against melanic Zelandoperla fenestrata stoneflies after deforestation, demonstrate rapid evolutionary change. Melanism also has implications for disease resistance and thermal regulation, though some explanations remain debated. In humans, true melanism (completely dark skin from a mutation) does not exist; skin color is polygenic.
Did You Know?
- Melanism in leopards is inherited as a Mendelian, monogenic recessive trait, and pairings of black animals have significantly smaller litter sizes.
- In the Malay Peninsula, south of the Kra Isthmus, the non-melanistic leopard morph was never photographed, suggesting near fixation of the dark allele.
- Eastern tree frogs (Hyla orientalis) living within the Chornobyl Exclusion Zone have darker dorsal skin than those outside, possibly due to selection from ionizing radiation.
- The chicken breed Ayam Cemani has a dominant gene causing hyperpigmentation (Fibromelanosis), making it entirely black including feathers, beak, and internal organs.
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