The Eastern Gray Squirrel, Sciurus carolinensis, is common across North America, often observed in its familiar gray coat. A striking color variation—the black squirrel—frequently appears within the same populations. Black and gray squirrels are not distinct species; they are simply color variations of the same animal. Because they belong to the same species, they regularly interbreed and successfully produce fertile offspring.
Understanding the Biological Relationship
The black squirrel is classified as a melanistic morph of the Eastern Gray Squirrel, meaning it is the exact same species with only a difference in coat color. This black variant is not reproductively isolated from the gray variant. They share the same diet, habitat, social behaviors, and breeding cycles throughout their range.
Since they are the same species, black and gray squirrels readily mate, and their offspring are capable of reproducing themselves. Litters often contain a mix of gray, black, and sometimes brownish-black individuals, depending on the parents’ specific genetic makeup. This successful interbreeding demonstrates that the difference in coloration does not create a biological barrier to reproduction.
The Genetics of Coat Color
The black coat results from a genetic condition known as melanism, which causes an overproduction of the dark pigment eumelanin in the fur. This increased pigmentation is traced to a specific mutation in the melanocortin 1 receptor (MC1R) gene. Researchers have identified a 24 base-pair deletion within this gene, which alters how the cell signals for pigment production.
The inheritance of this trait is described as incompletely dominant relative to the standard gray coloration. A squirrel must inherit two copies of the wild-type allele to appear completely gray. If a squirrel inherits two copies of the mutated MC1R allele, it will display a jet-black coat. If an individual inherits one copy of the gray allele and one copy of the black allele, it will often exhibit a brownish-black or dark gray color. This pattern explains why a mating pair of a black and a gray squirrel can produce a variety of coat colors in a single litter.
Historical Origin and Geographic Spread
The melanism mutation is theorized to have arisen in the Fox Squirrel (Sciurus niger) and later transferred to the Eastern Gray Squirrel through occasional interspecies mating. The identical nature of the mutation in both species supports the hypothesis of this genetic exchange. Historically, the black morph may have been the more widespread coloration across North America when dense, shaded old-growth forests were prevalent.
The dark coat color provided superior camouflage in the deep shadows of these forests, conferring an adaptive advantage over the lighter gray variant. Furthermore, the black fur absorbs more solar radiation, offering a thermal advantage in the colder northern parts of the species’ range. Today, the highest concentrations of black squirrels are found in the northern regions, particularly around the Great Lakes Basin, including much of Ontario and Michigan. In some cities, such as Battle Creek, Michigan, and Kent, Ohio, black squirrels have become locally famous due to intentional reintroduction or protection efforts.