If you have squirrels with gray-colored fur in your neighborhood, you’ve probably correctly identified them as eastern gray squirrels. Eastern gray squirrels are the type of squirrel most commonly found in urban and suburban environments.1 Less widely known is the fact that black “suburban” squirrels are also eastern gray squirrels. So why are these supposedly “gray” squirrels black?
The hair of all mammals, including squirrels and humans, contains a pigment called melanin. Mammals produce two different types of melanin, a light-colored version and a dark-colored version.2 The amount of these light and dark pigments present in a squirrel’s fur is what gives the squirrel its distinctive color.
In normal gray squirrels, both light- and dark-colored melanin is deposited in the hair in layers.2 The result is an overall gray appearance. However, some gray squirrels have a mutation that makes their fur black.2 The hair of these squirrels contains much higher quantities of the darker shade of melanin.
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References
1 Haupt, L. (2013). The urban bestiary: encountering the everyday wild. New York, NY: Little, Brown and Company.
2 McRobie, H., Thomas, A., & Kelly, J. (2009). The genetic basis of melanism in the gray squirrel (Sciurus carolinensis). Journal of Heredity. Retrieved from http://jhered.oxfordjournals.org/content/early/2009/07/30/jhered.esp059.full