Populations of fox squirrels (Sciurus niger) of the southeastern United States often include melanistic individuals. The genetic determinants of melanism in fox squirrels of the lower Mississippi River drainage (Louisiana and Mississippi) appear to differ from those for melanism in fox squirrels of the rest of the southeastern U.S. coastal plain, but functional causes of the melanism could be the same in both regions. I examine the hypothesis that melanistic polymorphism may have resulted from alternating cryptic superiority of light and dark individuals as a result of intermittent blackening of substrates by wildfires. I show that frequencies of all wildfires and of lightning-caused wildfires (both normalized per unit area) over the total range of the squirrels are correlated positively with the incidence of melanistic individuals. However, when just the southeastern coastal-plain states are considered, only frequency of lightning-caused fires shows significant correlation with melanism; in the lower Mississippi River drainage region, neither measure of fire frequency shows significant positive correlations. Other climatic variables have little potential for explaining maintenance of the polymorphism. Weak sex and size correlations with melanism may be pleiotropic side-effects of genetic systems determining coloration rather than clues to the functional significance of the trait.

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