Melanism dynamics in the Eastern Gray Squirrel ( Sciurus carolinensis ) over four centuries

Abstract The spatial patterning of gray-melanic color polymorphism of the Eastern Gray Squirrel (Sciurus carolinensis) has captivated naturalists for more than a century. We synthesized 400 years of historical records of occurrences, morph prevalence, and relative abundance of eastern gray squirrels to reconstruct the spatiotemporal dynamics of melanism and thereby provide insight into factors that shaped patterns of melanism observed today. Melanism declined from near fixation in the early 1800s in the Great Lakes–Northeast core area to <25% today, with the steepest losses tracking regional conversion of primary forest to cropland; later, secondary-forest recovery has not reversed these declines. Melanism occurrence has now expanded from its original Great Lakes–Northeast core area to multiple urban hotspots outside the original primary range of the melanic morph. Harvest rate analyses revealed dramatic declines in harvestable abundance for both morphs, with melanistic individuals declining 3 times faster than grays. A network was documented of 184 historical translocations of melanistic individuals mostly into cities (1900 to 1930 peak), with introduction sites associated with higher incidence of contemporary melanism. Through deforestation, intensive harvest, hunter selectivity, and widespread translocations into urban areas, humans have dramatically reshaped the evolutionary landscape of this prominent color polymorphism in eastern gray squirrels. These influences are likely to persist, with melanism potentially rebounding as hunting pressure declines and urbanization expands across the range of the species.