Black squirrels, with their striking dark coats, are a common sight in many regions, often sparking curiosity about their unique coloration. These animals are not a separate species but rather a color variation, known as a melanistic subgroup, of more common squirrel species. Melanism occurs most frequently in the eastern gray squirrel (Sciurus carolinensis) and the fox squirrel (Sciurus niger).
The Genetic Basis of Black Fur
The dark fur of black squirrels is a result of melanism, a genetic condition characterized by an increased production of melanin, the dark pigment responsible for fur, skin, and eye color. This heightened pigmentation stems from a mutation in the melanocortin 1 receptor (MC1R) gene. In eastern gray squirrels, the black coloration is linked to a 24 base-pair deletion in the MC1R gene, often referred to as MC1RΔ24, which alters its function and promotes the production of dark pigment.
The MC1RΔ24 allele exhibits incomplete dominance. Squirrels inheriting two copies of this mutated gene typically display a jet-black coat. If a squirrel inherits one copy of the MC1RΔ24 allele and one wild-type (gray) allele, its fur will often appear brownish-black. Research indicates that this specific variant pigment gene likely originated in fox squirrels and subsequently transferred to eastern gray squirrels through interspecies breeding.
Survival Advantages of Dark Fur
The dark fur of melanistic squirrels offers several advantages, particularly in specific environments. One advantage centers on thermoregulation, as black fur absorbs more solar radiation, beneficial in colder climates. Studies suggest that black squirrels can generate approximately 11% more body heat and retain it up to 18% better than gray squirrels, providing a thermal edge in northern regions. This helps them stay active during winter months.
Another advantage relates to camouflage, particularly in historical habitats. Dark fur provided better concealment in dense, shaded old-growth forests that once covered much of eastern North America. In contemporary urban environments, black squirrels might blend more effectively with shadows and dark surfaces, aiding in predator avoidance. Interestingly, in urban settings, black squirrels are also more visible against light-colored pavement, potentially leading drivers to spot and avoid them more easily than gray squirrels. This increased visibility could contribute to lower road mortality rates for black squirrels in cities.
Geographic Spread and Historical Context
Black squirrels are found across North America, though their prevalence varies significantly by region. They are particularly common in the northern parts of the eastern gray squirrel’s range, especially around the Great Lakes Basin, encompassing southern Canada and the northern U.S. In provinces like Ontario and the state of Michigan, black morphs constitute the majority of the squirrel population. They are also observed more often in urban areas compared to rural forests.
Historically, black squirrels were more widespread across eastern North America before European settlement, when dense old-growth forests were prevalent. Their populations have since expanded due to natural spread and human introduction. For instance, in 1915, John Harvey Kellogg released 400 eastern gray squirrels, including some black morphs, in Battle Creek, Michigan; similarly, black squirrels were introduced to Kent, Ohio, from Ontario in the 1960s. Beyond North America, black squirrels were also brought to the United Kingdom during the 19th and 20th centuries. Many of these UK populations originated from animals that escaped from zoos or private collections.