The black squirrel is not a separate species, but a color variation of the common eastern gray squirrel, Sciurus carolinensis. This unique coloration results from melanism, a genetic event causing an overproduction of dark pigment in the fur. While black squirrels share the same natural range as their gray counterparts, the black morph appears at significantly higher frequencies in specific geographic pockets.
Primary North American Range
The black squirrel population is most densely concentrated within the northern portion of the eastern gray squirrel’s native North American range. This area centers primarily around the Great Lakes Basin, including southern central Canada and the northern United States. Large populations are frequently observed in the Canadian province of Ontario, particularly in cities like Toronto and London, where the black morph can form the majority of the local squirrel population.
In the United States, high-frequency populations are found in states bordering the Great Lakes, such as Michigan, Ohio, and parts of New York. The prevalence of black squirrels in this northern region is linked to a thermal advantage provided by their dark fur. This greater cold tolerance gives the black morph a selective advantage over the gray morph in environments between the 41st and 45th parallels north.
The distribution of this color morph forms a gradient, with the frequency of black squirrels decreasing as one moves south. Historically, the black morph may have been more widespread when the continent was covered in dense, old-growth forests that offered darker camouflage. Extensive deforestation and habitat changes likely shifted the camouflage advantage toward the lighter gray coat in many areas south of the Great Lakes.
Global Presence and Introduced Colonies
Beyond their primary native range, black squirrels are also found in isolated populations that were often established through human intervention. The most notable example is in the United Kingdom, where eastern gray squirrels, including the melanistic variant, were introduced in the late 19th and early 20th centuries. These populations originated from squirrels that either escaped from private zoos or were intentionally released into the wild.
In the UK, the black morph is concentrated in specific English counties, such as Hertfordshire, Bedfordshire, and parts of Cambridgeshire. These localized populations continue to expand their territory at a rate of approximately a half-mile per year.
In North America, other isolated populations have been established in urban areas outside the main Great Lakes region. Washington, D.C., for instance, has a black squirrel population dating back to the early 1900s. Other cities and towns, such as Marysville, Kansas, and Kenton, Ohio, are known for their high local abundance of the black morph, often due to deliberate introductions or escapes from captivity.
Understanding Melanism
The dark fur of the black squirrel is a manifestation of melanism, an increased presence of the dark pigment eumelanin in the animal’s hair. This trait results from a genetic mutation involving the melanocortin 1 receptor gene. The mutation is a 24 base-pair deletion in the gene, creating a distinct allele associated with the dark coat color.
This melanic allele acts as an incompletely dominant trait, meaning color expression depends on the number of copies a squirrel inherits. A squirrel with one copy of the melanic allele and one wild-type gray allele typically has a brown-black or brownish-gray coat. A jet-black squirrel has inherited two copies of the melanic allele, resulting in the maximum production of the dark pigment.
Genetic analysis suggests the melanic allele did not originate in the eastern gray squirrel, but was likely passed from the fox squirrel (Sciurus niger) through interspecies mating. This genetic exchange is an example of adaptive introgression, where a beneficial trait is transferred between different species.
Survival Advantages of Dark Fur
The persistence and high frequency of the black morph in northern climates are a consequence of the survival benefits associated with their dark pigmentation. The most significant advantage is a thermal one, where the black fur absorbs more solar radiation compared to the lighter gray coat. This enhanced absorption helps the squirrels maintain a higher body temperature, which is especially beneficial for survival during the harsh, cold winters common in the Great Lakes region.
The dark coat may also provide a degree of camouflage, though this advantage is context-dependent. Historically, in dense, shaded forests, the black color offered better concealment from predators. Today, in urban and suburban areas, the dark fur may still offer a slight advantage when moving among the dark bark of trees, despite higher visibility in open, snowy environments.