Melanistic canids, jaguars, and squirrels

Melanism in North American wolves is an ancient trait, that according to 1 genetic study, originated from an hybridization event between Eurasian wolves (Canis lupus) and primitive dogs (Canis familiaris).  This occurred at least 46,000 years ago, predating the actual domestication of dogs.  At this early date primitive dogs probably varied from wolves in their preference for living in close proximity to human habitation.  The differences between dogs and wolves then were even less than they are today, and hybridization was much more likely to occur.  Eurasian wolves crossed the Bering landbridge and brought this trait for melanism with them, but curiously, wolves in Eurasia with the trait for melanism died out.  The only population of wolves in Europe today that carry the trait for melanism live in Italy, and this is thought to derive from a much more recent hybridization with dogs.  In North America hybridization between western wolves and dogs no longer occurs in the wild, or at least that’s what the evidence suggests.

Black-coated predators are harder for prey to see in deep shady forests.  Therefore, forested environments provide a natural selection mechanism that favors melanistic individuals.  The incidence of melanistic canids is greater in deep forests than in open environments.  In Alaska dark wolves are rare in open tundra but common in adjacent boreal forests–a great example of natural selection.  Melanism in coyotes (Canis latrans) is extremely rare in western North America but common in the east, especially in the south.  Having darker colored fur is a beneficial mutation for a predator living in the more heavily forested southeast.  

Taxidermic mount of a melanistic coyote killed near Cedartown, Georgia.  This looks identical to the so-called red wolves that were formerly found in Florida.  Is this evidence the wild canids formerly living in the south were not completely extirpated and have bred this melanistic gene into the western coyotes that have colonized the region within the last 50 years?  I believe the wolves that used to live in the south were merely eastern coyote x primitive dog hybrids.

Melanism in eastern coyotes also is derived from hybridization with dogs.  In the past  male dogs mated with  female coyotes and the hybrids backcrossed into the coyote population.  This occurred prior to when coyotes recolonized the south about 50 years ago.  I suspect the southeastern wolves in North America during the time of European colonization were merely hybrids between eastern coyotes and the primitive dogs brought over by Indians about 13,000 years ago.  These primitive dogs, known as the American dingo, readily revert to a wild state, and some likely bred with eastern coyotes during the late Pleistocene/early Holocene.  These hybrids took over the ecological niche left vacant when dire wolves became extinct.  Evidence from 500 year old wolf remains found in Ontario, Canada show the eastern wolf had DNA sequences from both dogs and coyotes but not from western gray wolves.  Yet, morphologically they resembled wolves.  Eastern Canadian wolves probably had a similar origin as southeastern wolves.

Melanism is also common in 2 species of big cats that favor heavily forested environments.  In jaguars (Panthera onca)melanism is conferred through a dominant allele, while in leopards (Panthera pardus) it is conferred through a recessive allele.  When a spotted jaguar mates with another spotted jaguar, the cubs are always spotted.  Cubs from a pairing of a spotted jaguar and a black jaguar can be either spotted or black.  Melanism is also associated with beneficial mutations in the immune system.  This probably also explains the increased incidence of black jaguars in jungles where tropical diseases are prevalent.  Melanistic margays, ocelots, and jaguarundis are known to occur, but no proven specimen of a melanistic cougar has ever been confirmed.  However, cougars in south Florida are grayish, and this coloration affords them a similar advantage enjoyed by completely melanistic cats. 

I hypothesize that Smilodon fatalis, the saber-tooth cat, may have had some melanistic individuals in regions with deep forests.  It was an ambush predator, and a black coat would have made them difficult to spot, especially at night.  Artists rarely portray them with a black coat.  There is enough genetic material at the La Brea tarpits museum to determine whether or not Smilodon carried this trait, but so far, no scientist has thought to look for it.

Melanistic jaguar, showing that markings are visible

Melanistic jaguar–a real black panther.

Fox squirrels (Sciurus niger) are frequently found in melanistic color phases.  One study found that the higher incidence of black color phases in fox squirrels correlates positively with the higher frequency of lightning-induced wildfires in longleaf pine savannahs.  Apparently, the scorched black ground provides camouflage for fox squirrels from 1 of their most common predators, the red-tailed hawk.  Longleaf pine savannahs are often burned annually, and the ground stays black for long enough periods to select for melanistic individuals. 

Melanistic fox squirrel and deer fawn.

Melanistic gray squirrels(Sciurus carolinensis) were formerly abundant when much of North America was covered by shady deep forests.  Today, the black phase of the gray squirrel is still locally common in some midwestern states and CanadaThe black fur conveys an advantage in colder climates, helping the animals warm in the sun.  Individuals of this color phase have been widely relocated, and some populations exist as far south as Kentucky and Washington DC.

References:

Adams, J.R.; and Leonard Waits

“Widespread Occurrence of Domestic Dog Mitochondrial DNA Haplotype in Southeastern, USA, Coyotes”

Molecular Ecology 12 2003

Anderson, J.R.; et. al.

“Molecular and Evolutionary History of Melanism in North American Gray Wolves”

Science 323 2009

Kiltie, R.A.

“Wildfire and the Evolution of Dorsal Melansim in Fox Squirrels, Sciurus niger

Journal of Mammalogy 70 1989

Mowry, Christopher; and Justin Edge

“Melanistic Coyotes in Northwest Georgia”

Southeastern Naturalist  13 (2) 2014

Rutledge, Linda; et. al.

“Genetic and Morphometric Analysis of 16th Century Canis Skull Fragments: Implications for Historic Eastern Gray Wolf Distribution in North America”

Conservation Genetics 2010

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