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3 Coat Color and Squirrel Genetics
- Johns Hopkins University Press
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What species are the black squirrels I see? If you are in the eastern United States, it is probably the melanistic form of the eastern gray squirrel (Sciurus carolinensis). The black color morph of the eastern gray squirrel occurs most commonly in northern latitudes, but it has been introduced into more southern areas across the United States and has established itself successfully on college campuses and in urban and suburban communities. Individual variation in coat color is well known in squirrels. In urban and suburban areas, there are frequently several color forms of the eastern gray squirrel. In the vicinity of Washington, D.C., we have black squirrels, albino squirrels, the normal gray squirrels, reddish squirrels, and a silvery squirrel, which can look albino at a distance, but close up can be seen to have dark eyes and silvery, not white, hair. All these color forms belong to the same species and subspecies, Sciurus carolinensis pennsylvanicus, and can be seen to breed with one another. The melanistic squirrels found in the Washington, D.C., area have an interesting history. In 1902 and 1906, a total of 18 black squirrels, because they were considered “unusual,” were brought to the National Zoo from Canada, where all-black squirrels are common. In 1923, it was noted in the Mammals of the District of Columbia that the black coloration had spread and that black squirrels could be found well outside the zoo grounds in Cleveland Park. In the Smithsonian collection there are several specimens documenting this, all collected by the famous anthropologist, Aleš Hrdlička, who was probably picking up roadkill squirrels on his way to work. When Thorington came to Washington in 1969, he saw black squirrels regularly, Chapter 3 Coat Color and Squirrel Genetics but always inside the Washington Beltway, and he never heard any reports of them in Virginia, across the Potomac River. In the 1980s, he began to see black squirrels outside the Beltway in Maryland, and he began to receive phone calls from long-term Virginia residents, inquiring about the black squirrels that they were seeing for the first time. Now, the black squirrels are common in both places, but we do not know how far outside the Beltway or how far south in Virginia black squirrels can be seen. We have received reports of black squirrels 30 miles to the north around Fort Meade, Maryland, 30 miles to the east in Annapolis, Maryland, and 25 miles to the south in Prince William County, Virginia. They are not common in these areas, however, and it is unclear if their presence there results from the natural spread of the gene for melanism from the original introduction at the National Zoo. Melanism can occur naturally in wild populations, and additional introductions are also possible explanations. The obvious question is why melanism is so successful in urban and suburban environments, such as Washington, D.C.? One hypothesis is that they survive better because motorists are more likely to try to avoid them. Katherine and Richard Thorington tested this hypothesis by counting the numbers of black and gray squirrels they saw living in different neighborhoods in 1990, and the number that they saw dead on the roads. The frequency with which they found dead black squirrels was the same as Two eastern gray squirrels eat at a stump in Thorington’s backyard: on the left, a normal colored one, and on the right, a melanistic one. Photo © Caroline Thorington Coat Color and Squirrel Genetics 39 [44.197.251.102] Project MUSE (2024-03-19 05:15 GMT) 40 Squirrels: The Animal Answer Guide their frequency in the population. So the Thoringtons concluded that black squirrels were just as likely to be run over by motorists as the gray squirrels. Another hypothesis is that natural predators are less common in the cities and suburbs than in the countryside and that this allows the black form to survive better there. This is a reasonable idea, because black squirrels are much more conspicuous than the gray squirrels in many situations, but it needs to be tested. Nevertheless, there still needs to be some advantage to being black, even if predators are not selecting against them. This leads us to the question of why melanistic squirrels are most common in the northern latitudes. One hypothesis is that their black coloring allows the squirrels to absorb more heat when they sun themselves in the winter and that this promotes survival. This hypothesis has been tested, but unfortunately the...