Melanism
Encyclopedia
Pigment
A pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.Many materials selectively absorb...
in the skin
Skin
-Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...
or its appendages, and the opposite of albinism
Albinism
Albinism is a congenital disorder characterized by the complete or partial absence of pigment in the skin, hair and eyes due to absence or defect of an enzyme involved in the production of melanin...
. It is also the medical term for black jaundice.
The word is deduced from the , meaning black pigment.
It is the occurrence of an increased amount of dark pigmentation of skin, feathers, eyes or hair in an organism, resulting from the presence of melanin
Melanin
Melanin is a pigment that is ubiquitous in nature, being found in most organisms . In animals melanin pigments are derivatives of the amino acid tyrosine. The most common form of biological melanin is eumelanin, a brown-black polymer of dihydroxyindole carboxylic acids, and their reduced forms...
. It is the opposite of leucism, which occur because of a lack of melanin or other types of pigment.
Abundism is the occurrence 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
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
. 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
Mutation
In molecular biology and genetics, mutations are changes in a genomic sequence: the DNA sequence of a cell's genome or the DNA or RNA sequence of a virus. They can be defined as sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic...
, but can result from other stimuli, such as exposure to abnormal temperature changes during gestation which transiently alter gene transcription
Transcription (genetics)
Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...
or translation
Translation (genetics)
In molecular biology and genetics, translation is the third stage of protein biosynthesis . In translation, messenger RNA produced by transcription is decoded by the ribosome to produce a specific amino acid chain, or polypeptide, that will later fold into an active protein...
.
In the context of diseases, the condition in which abnormally dark pigmentation of some tissues occurs is coined melanosis. For a description of melanin-related disorders see melanin
Melanin
Melanin is a pigment that is ubiquitous in nature, being found in most organisms . In animals melanin pigments are derivatives of the amino acid tyrosine. The most common form of biological melanin is eumelanin, a brown-black polymer of dihydroxyindole carboxylic acids, and their reduced forms...
, melanosis coli
Melanosis coli
Melanosis coli, also pseudomelanosis coli, is a disorder of pigmentation of the wall of the colon, often identified at the time of colonoscopy. It is benign, and may have no significant correlation with disease...
and ocular melanosis
Ocular melanosis
Ocular melanosis , also known as ocular melanocytosis or melanosis oculi, is a congenital disease of the eye which affects about 1 in every 5000 people and is a risk factor for uveal melanoma. In dogs is found almost exclusively in the Cairn Terrier, where until recently it was known as pigmentary...
.
The opposite of melanism is amelanism
Amelanism
Amelanism is a pigmentation abnormality characterized by the lack of pigments called melanins, commonly associated with a genetic loss of tyrosinase function. Amelanism can affect fish, amphibians, reptiles, birds, and mammals including humans. The appearance of an amelanistic animal depends on...
, a deficiency in or total absence of melanin pigments. In humans and many other animals, this most commonly manifests as albinism
Albinism
Albinism is a congenital disorder characterized by the complete or partial absence of pigment in the skin, hair and eyes due to absence or defect of an enzyme involved in the production of melanin...
, although other forms exist, such as leucism.
Adaptive melanism
Melanism related to the process of adaptationAdaptation
An adaptation in biology is a trait with a current functional role in the life history of an organism that is maintained and evolved by means of natural selection. An adaptation refers to both the current state of being adapted and to the dynamic evolutionary process that leads to the adaptation....
is called adaptive. Most commonly, dark individuals become fitter
Fitness (biology)
Fitness is a central idea in evolutionary theory. It can be defined either with respect to a genotype or to a phenotype in a given environment...
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 panther
Black panther
A black panther is typically a melanistic color variant of any of several species of larger cat. Wild black panthers in Latin America are black jaguars , in Asia and Africa they are black leopards , and in North America they may be black jaguars or possibly black cougars A black panther is...
s use it as a foraging advantage during night hunting. Typically adaptive melanism is heritable
Heritability
The Heritability of a population is the proportion of observable differences between individuals that is due to genetic differences. Factors including genetics, environment and random chance can all contribute to the variation between individuals in their observable characteristics...
: A dominant gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
, which is entirely or nearly entirely expressed in the phenotype
Phenotype
A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...
is responsible for the excessive amount of melanin. Adaptive melanism has been shown to occur in a variety of animals, including mammals such as squirrel
Squirrel
Squirrels belong to a large family of small or medium-sized rodents called the Sciuridae. The family includes tree squirrels, ground squirrels, chipmunks, marmots , flying squirrels, and prairie dogs. Squirrels are indigenous to the Americas, Eurasia, and Africa and have been introduced to Australia...
s, many felines
Felidae
Felidae is the biological family of the cats; a member of this family is called a felid. Felids are the strictest carnivores of the thirteen terrestrial families in the order Carnivora, although the three families of marine mammals comprising the superfamily pinnipedia are as carnivorous as the...
and canids, and coral snake
Coral snake
The coral snakes are a large group of elapid snakes that can be subdivided into two distinct groups, Old World coral snakes and New World coral snakes...
s. Adaptive melanism can lead to the creation of morphs
Polymorphism (biology)
Polymorphism in biology occurs when two or more clearly different phenotypes exist in the same population of a species — in other words, the occurrence of more than one form or morph...
, the most notable example being the peppered moth
Peppered moth
The peppered moth is a temperate species of night-flying moth. Peppered moth evolution is often used by educators as an example of natural selection.- Distribution :...
whose evolutionary history in the United Kingdom
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
is offered as a classic instructional tool for teaching the principles of natural selection
Natural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....
.
Industrial melanism
Industrial melanism is adaptive melanism caused by anthropogenicAnthropogenic
Human impact on the environment or anthropogenic impact on the environment includes impacts on biophysical environments, biodiversity and other resources. The term anthropogenic designates an effect or object resulting from human activity. The term was first used in the technical sense by Russian...
alteration of the natural environment
Natural environment
The natural environment encompasses all living and non-living things occurring naturally on Earth or some region thereof. It is an environment that encompasses the interaction of all living species....
in terms of industrial pollution
Pollution
Pollution is the introduction of contaminants into a natural environment that causes instability, disorder, harm or discomfort to the ecosystem i.e. physical systems or living organisms. Pollution can take the form of chemical substances or energy, such as noise, heat or light...
. As soot
Soot
Soot is a general term that refers to impure carbon particles resulting from the incomplete combustion of a hydrocarbon. It is more properly restricted to the product of the gas-phase combustion process but is commonly extended to include the residual pyrolyzed fuel particles such as cenospheres,...
, smoke and other industrial pollutants from factories darken the landscape and because many organisms rely on camouflage to avoid predation
Predation
In ecology, predation describes a biological interaction where a predator feeds on its prey . Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the death of its prey and the eventual absorption of the prey's tissue through consumption...
, this sudden change in their environment makes them highly vulnerable to predators. This creates a strong selective
Natural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....
pressure which will see any organism with a darker colour much more likely to survive and contribute to the gene pool
Gene pool
In population genetics, a gene pool is the complete set of unique alleles in a species or population.- Description :A large gene pool indicates extensive genetic diversity, which is associated with robust populations that can survive bouts of intense selection...
of the next generation. Rare mutations are hence selected for and over time the population will adjust to a new equilibrium. Peppered moth evolution
Peppered moth evolution
The evolution of the peppered moth over the last two hundred years has been studied in detail. Originally, the vast majority of peppered moths had light colouration, which effectively camouflaged them against the light-coloured trees and lichens which they rested upon...
is commonly used as an example of industrial melanism.
Melanism in the Cat Family
Melanistic coat coloration occurs as a common polymorphismPolymorphism (biology)
Polymorphism in biology occurs when two or more clearly different phenotypes exist in the same population of a species — in other words, the occurrence of more than one form or morph...
in 11 of 37 felid species and reaches high population frequency in some cases but never achieves complete fixation
Fixation (population genetics)
In population genetics, fixation is the change in a gene pool from a situation where there exist at least two variants of a particular gene to a situation where only one of the alleles remains...
. The black panther
Black panther
A black panther is typically a melanistic color variant of any of several species of larger cat. Wild black panthers in Latin America are black jaguars , in Asia and Africa they are black leopards , and in North America they may be black jaguars or possibly black cougars A black panther is...
, a melanic form of leopard
Leopard
The leopard , Panthera pardus, is a member of the Felidae family and the smallest of the four "big cats" in the genus Panthera, the other three being the tiger, lion, and jaguar. The leopard was once distributed across eastern and southern Asia and Africa, from Siberia to South Africa, but its...
is common in the equatorial rainforest of Malaya
Malaya
-Geographic/former political entities:* The Malay Peninsula* Malay Archipelago, the broader macroregion.-Malaya including Singapore:* British Malaya , a loose collection of British-controlled states...
and the tropical rainforest
Tropical rainforest
A tropical rainforest is an ecosystem type that occurs roughly within the latitudes 28 degrees north or south of the equator . This ecosystem experiences high average temperatures and a significant amount of rainfall...
on the slopes of some African mountains such as Mount Kenya
Mount Kenya
Mount Kenya is the highest mountain in Kenya and the second-highest in Africa, after Kilimanjaro. The highest peaks of the mountain are Batian , Nelion and Point Lenana . Mount Kenya is located in central Kenya, just south of the equator, around north-northeast of the capital Nairobi...
. The serval
Serval
The serval , Leptailurus serval or Caracal serval, known in Afrikaans as Tierboskat, "tiger-forest-cat", is a medium-sized African wild cat. DNA studies have shown that the serval is closely related to the African golden cat and the caracal...
also has melanic forms not uncommon in certain areas of East Africa
East Africa
East Africa or Eastern Africa is the easterly region of the African continent, variably defined by geography or geopolitics. In the UN scheme of geographic regions, 19 territories constitute Eastern Africa:...
. In the jaguarundi
Jaguarundi
The jaguarundi is a small-sized wild cat native to Central and South America. In 2002, the IUCN classified the jaguarundi as Least Concern as it is likely that no conservation units, with the probable exception of the mega-reserves of the Amazon basin could sustain long-term viable populations. It...
coloration varies from dark brown and gray to light reddish. Melanic forms of jaguar
Jaguar
The jaguar is a big cat, a feline in the Panthera genus, and is the only Panthera species found in the Americas. The jaguar is the third-largest feline after the tiger and the lion, and the largest in the Western Hemisphere. The jaguar's present range extends from Southern United States and Mexico...
are apparently fairly common in certain parts of South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...
. In 1938 and 1940, two melanistic bobcat
Bobcat
The bobcat is a North American mammal of the cat family Felidae, appearing during the Irvingtonian stage of around 1.8 million years ago . With twelve recognized subspecies, it ranges from southern Canada to northern Mexico, including most of the continental United States...
s were trapped alive in sub-tropical Florida
Florida
Florida is a state in the southeastern United States, located on the nation's Atlantic and Gulf coasts. It is bordered to the west by the Gulf of Mexico, to the north by Alabama and Georgia and to the east by the Atlantic Ocean. With a population of 18,801,310 as measured by the 2010 census, it...
.
In 2003, the dominant mode of inheritance of melanism in jaguars was confirmed by performing phenotype
Phenotype
A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...
transmission analysis in a 116-individual captive pedigree
Purebred
Purebreds, also called purebreeds, are cultivated varieties or cultivars of an animal species, achieved through the process of selective breeding...
. Melanistic animals were found to carry at least one copy of a mutant MC1R sequence allele
Allele
An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...
, bearing a 15-base pair
Base pair
In molecular biology and genetics, the linking between two nitrogenous bases on opposite complementary DNA or certain types of RNA strands that are connected via hydrogen bonds is called a 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
Agouti signalling peptide
Agouti signalling peptide, a product of the Agouti gene, is a peptide consisting of 131 amino acids. Its discovery was published in 1994 in the scientific journal Nature where its functional properties were described...
in the agouti gene
Agouti gene
The Agouti gene is responsible for determining whether a mammal's coat is banded or of a solid color . The chief product of the Agouti gene is Agouti signalling peptide , but there are a number of alternative splice products....
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
Pampas Cat
The Colocolo is a small spotted and striped cat native to the west Andean slope in central and northern Chile. Until recently it included the more widespread Pampas Cat and Pantanal Cat , and some maintain these as subspecies of the Colocolo...
and Asian golden cat, suggesting that melanism arose independently at least four times in the cat family.
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. Between January 1996 and March 2009, leopard
Indochinese leopard
The Indochinese leopard is a leopard subspecies native to mainland Southeast Asia and into southern China, classified as “Near Threatened” in 2008 by IUCN. In Indochina, leopards are threatened primarily by habitat loss due to deforestation, as well as poaching for the illegal trade...
s were photographed at 16 sites in the Malay Peninsula
Malay Peninsula
The Malay Peninsula or Thai-Malay Peninsula is a peninsula in Southeast Asia. The land mass runs approximately north-south and, at its terminus, is the southern-most point of the Asian mainland...
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
Genetic drift
Genetic drift or allelic drift is the change in the frequency of a gene variant in a population due to random sampling.The alleles in the offspring are a sample of those in the parents, and chance has a role in determining whether a given individual survives and reproduces...
alone ranged from about 1,100 years to about 100,000 years. Melanism in leopards has been hypothesized to be causally associated with a selective advantage for ambush.
Another theory is that the black leopards have increased resistance to virus
Virus
A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...
es, 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.
Melanism and the immune system
Java
Java is an island of Indonesia. With a population of 135 million , it is the world's most populous island, and one of the most densely populated regions in the world. It is home to 60% of Indonesia's population. The Indonesian capital city, Jakarta, is in west Java...
and Malaysia, and the relatively high incidence of black leopard
Leopard
The leopard , Panthera pardus, is a member of the Felidae family and the smallest of the four "big cats" in the genus Panthera, the other three being the tiger, lion, and jaguar. The leopard was once distributed across eastern and southern Asia and Africa, from Siberia to South Africa, but its...
s and black serval
Serval
The serval , Leptailurus serval or Caracal serval, known in Afrikaans as Tierboskat, "tiger-forest-cat", is a medium-sized African wild cat. DNA studies have shown that the serval is closely related to the African golden cat and the caracal...
s 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.
Studies reported in New Scientist
New Scientist
New Scientist is a weekly non-peer-reviewed English-language international science magazine, which since 1996 has also run a website, covering recent developments in science and technology for a general audience. Founded in 1956, it is published by Reed Business Information Ltd, a subsidiary of...
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" colour coats. This would explain why recessive melanism persists when melanistic individuals are disadvantaged due to being poorly camouflaged in open areas. According to Eduardo Eizirik and Stephen O'Brien of the United States National Cancer Institute
National Cancer Institute
The National Cancer Institute is part of the National Institutes of Health , which is one of 11 agencies that are part of the U.S. Department of Health and Human Services. The NCI coordinates the U.S...
in Maryland, the melanism mutations involve the same gene family as those involved in humans.
In the United States National Cancer Institute
National Cancer Institute
The National Cancer Institute is part of the National Institutes of Health , which is one of 11 agencies that are part of the U.S. Department of Health and Human Services. The NCI coordinates the U.S...
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. 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 usenetUsenet
Usenet is a worldwide distributed Internet discussion system. It developed from the general purpose UUCP architecture of the same name.Duke University graduate students Tom Truscott and Jim Ellis conceived the idea in 1979 and it was established in 1980...
, internet forum
Internet forum
An Internet forum, or message board, is an online discussion site where people can hold conversations in the form of posted messages. They differ from chat rooms in that messages are at least temporarily archived...
s and blog
Blog
A blog is a type of website or part of a website supposed to be updated with new content from time to time. Blogs are usually maintained by an individual with regular entries of commentary, descriptions of events, or other material such as graphics or video. Entries are commonly displayed in...
s to mean an African-American social movement
Social movement
Social movements are a type of group action. They are large informal groupings of individuals or organizations focused on specific political or social issues, in other words, on carrying out, resisting or undoing a social change....
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 and was promoted by some Afrocentrists, such as Frances Cress Welsing
Frances Cress Welsing
Frances Cress Welsing Frances Cress Welsing Frances Cress Welsing (born March 18, 1935 in Chicago is an African American psychiatrist practicing in Washington, D.C.. She is noted for her "Cress Theory of Color Confrontation", which explores the practice of white supremacy...
.
See also
- AlbinismAlbinismAlbinism is a congenital disorder characterized by the complete or partial absence of pigment in the skin, hair and eyes due to absence or defect of an enzyme involved in the production of melanin...
- Leucism
- VitiligoVitiligoVitiligo is a condition that causes depigmentation of sections of skin. It occurs when melanocytes, the cells responsible for skin pigmentation, die or are unable to function. The cause of vitiligo is unknown, but research suggests that it may arise from autoimmune, genetic, oxidative stress,...