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As a melanistic variety of the eastern grey squirrel, individual black squirrels can exist wherever grey squirrels live. Grey mating pairs cannot produce black offspring. Grey squirrels have 2 copies of a normal pigment gene and black squirrels have either 1 or 2 copies of a mutant pigment gene. If a black squirrel has 2 copies of the mutant gene it will be jet black. If it has 1 copy of a mutant gene and 1 normal gene it will be brown-black. In areas with high concentrations of black squirrels, mixed litters are common. The black subgroup seems to have been predominant throughout North America prior to the arrival of Europeans in the 16th century, since their dark colour helped them hide in old growth forests which tended to be very dense and shaded. As time passed, hunting and deforestation led to biological advantages for grey coloured individuals. Today, the black subgroup is particularly abundant in the northern part of the eastern grey squirrel's range. This is likely due to the significantly increased cold tolerance of black squirrels which lose less heat than greys. Black squirrels also enjoy concealment advantages in denser northern forests.
Large natural populations of black squirrels can be found throughout Ontario and in several parts of Ohio, Maryland, Michigan, Indiana, Virginia, Washington, D.C., Wisconsin, Minnesota, and Pennsylvania. Populations of grey squirrels in which the black subgroup is predominant can be found in these six areas as well as in smaller enclaves in Missouri, New Jersey, southern New York, Illinois and Connecticut. Outside areas of North America where black squirrels occur naturally in abundance, there are several notable introduced populations of black squirrels:
In the United States, the city of Kent, Ohio, developed a significant black squirrel population after 10 were legally imported from Canada in February 1961 by biologist Ralph W. Dexter to study whether they would upset the ecosystem on Northeast Ohio. They have driven out native squirrels in many areas, though they peacefully coexist with most other rodent wildlife.
Black squirrels are well established in the Quad Cities area along the Iowa-Illinois boundary. According to one story, recounted in the book "The Palmers", they were first introduced on the Rock Island Arsenal Island. Some of them then escaped by jumping across ice floes on the Mississippi River when it was frozen and populated other areas in Rock Island. In Iowa, their population extends to the west, including Council Bluffs, where it is the town mascot. From there they have spread in increasing abundance into eastern Nebraska by making their way across the Missouri River and into Omaha, Lincoln, and the surrounding areas.
Black squirrels are abundant in Battle Creek, Michigan, and, according to legend, were first introduced there by Will Keith Kellogg, founder of the Kellogg Company, in an effort to destroy the local population of red squirrels. The story continues that this same population of squirrels was further introduced to the campus of Michigan State University by John Harvey Kellogg for the same purpose.
Black squirrels were introduced to Stanley Park in Westfield, Massachusetts, in 1948, having been brought from Michigan as a gift to a local business man. The squirrels are thriving in the park as of 2011.
Marysville, Kansas, has a notable population of black squirrels which legend claims arrived there by escaping from a travelling circus. The city of Hobbs, New Mexico attempted to introduce black squirrels from Marysville in 1973. However, the new population of black squirrels did not survive, likely having been killed by local fox squirrels shortly after their introduction.
Eighteen Canadian black squirrels were released at the National Zoo in Washington, D.C., near the beginning of the 20th century during President Theodore Roosevelt's administration. Since their introduction, the population of black squirrels in and near Washington have slowly but steadily increased, and black squirrels now account for up to half of the squirrel population in certain locations, such as the grounds of the Washington National Cathedral.
Vancouver, British Columbia, has a growing population of black squirrels after they were introduced to the Stanley Park Peninsula before 1914. The squirrels have thrived and spread throughout the Vancouver area.
Black squirrels can also be found in Britain, where grey squirrels were first introduced from North America at the end of the 19th century. The origin of the UK's black individuals has been a topic of dispute, with initial research indicating that melanistic individuals are descendants of black zoo escapees. Regardless of their origins, the melanistic population in the UK continues to grow, and around the towns of Letchworth, Stevenage and Hitchin, as well as nearby villages such as Shillington and Meppershall in England, black squirrels are now as abundant as grey individuals. Black squirrels have been present and studied in Cambridgeshire since the 1990s; in the village of Girton three quarters of the squirrel population is black.
There is a heavy population of black squirrels in the Stuyvesant Town–Peter Cooper Village development in Manhattan, New York City. Residents of the large residential development have taken to nicknaming the black squirrel a "sqrat".
Though black squirrels are common or predominant in many areas of North America, their overall rarity (perhaps as few as 1 in 10,000) has caused many towns, cities, colleges, and universities to take special pride in their populations of black squirrels. Several cities and towns in the United States and one in Canada make efforts to publicly promote their local populations of black squirrels.
Several colleges and universities in the United States promote the black squirrel as an official or unofficial mascot: