Black squirrels are not a separate species of rodent, but rather a color variation, or morph, of a common North American species, primarily the Eastern Gray Squirrel (Sciurus carolinensis). The striking black fur is a result of a genetic condition known as melanism, which causes an overproduction of the dark pigment melanin. While the gray morph dominates the species across the continent, the black morph’s commonality is highly localized. In some regions, they are the prevailing color, while in others, they are rarely encountered. This localized prevalence is determined by genetic history and environmental advantages.
The Genetic Origin of Melanism
The distinctive black coat color is directly linked to a mutation in the melanocortin 1 receptor (MC1R) gene, which is responsible for regulating the production of dark pigment in hair follicles. The specific variant is the MC1R∆24 allele, which involves a 24 base pair deletion. This deletion causes the receptor to become hyperactive, leading to an excessive deposit of melanin throughout the fur.
This genetic difference operates as an incompletely dominant trait, meaning a squirrel only needs to inherit one copy of the mutated gene to display the black coloration. The MC1R∆24 allele is believed to have originated in the black Fox Squirrel (Sciurus niger) and transferred to the Eastern Gray Squirrel population through interspecies mating. A black squirrel can be born to two gray parents if both carry the recessive black allele.
Geographic Distribution and Population Density
The commonality of black squirrels depends entirely on geography, as their populations are concentrated in specific areas. While they represent a small fraction of the total Eastern Gray Squirrel population in the United States, they form the majority in regions around the Great Lakes Basin. This prevalence is particularly evident in the Canadian province of Ontario and the US state of Michigan.
In Ontario, the black morph accounts for approximately 66% of the Eastern Gray Squirrel population, and in Michigan, the frequency is also high, around 56%. Populations decrease significantly south of the Great Lakes, though localized high-density pockets exist due to intentional introductions. Cities such as Kent, Ohio, and Council Bluffs, Iowa, have notable populations stemming from releases in the 19th and 20th centuries.
The black morph is more frequently observed in urban areas than in rural forests. This is partly because high-density populations are concentrated in parks and campuses within metropolitan areas. The highest frequency is found in the northern regions of the Eastern Gray Squirrel’s range, specifically between the 41st and 45th parallels north, correlating latitude and density.
Environmental Factors Shaping Their Range
The localized dominance of the black morph in northern latitudes is hypothesized to be a result of a selective advantage related to thermoregulation. The dark fur is more efficient at absorbing solar radiation compared to lighter gray fur, providing a thermal benefit in colder environments. This increased heat absorption may reduce the energy required to maintain body temperature during winter.
This thermal advantage is considered a primary reason for the higher frequency of black squirrels in the northern United States and Canada. Conversely, the trait might be a disadvantage in warmer southern climates, where the increased heat absorption could lead to overheating. Another factor is the historical environment, as the black coloration may have provided better camouflage in the dense, dark coniferous forests that were more widespread in the north centuries ago.
While the thermal advantage explains the northern distribution, the high prevalence in urban areas suggests human influence also plays a significant role. In cities, lower predation and consistent food sources allow the melanistic trait to persist. The overall distribution is a product of both genetic predisposition and environmental pressures.