Road mortality contributes to the evolution of an urban-rural cline in squirrel coat color

Cities impose unique selection pressures on wildlife and generate clines in phenotypic traits along urban-rural gradients. Roads, as a ubiquitous feature of human dominated landscapes, are a significant source of direct mortality for wildlife, but their role as a selective force in producing phenotypic trait variation along urban-rural clines is not known. This study tested the hypothesis that roads influence natural selection of coat color in the eastern gray squirrel ( Sciurus carolinensis ), a species for which vehicular collisions is a significant source of mortality. This species has two distinct coat colors: a gray morph that is common in all areas, and a melanic morph more prevalent in urban areas than in rural ones. We conducted standardized road cruise surveys to compare the proportion of each color morph among road-killed and living squirrels along an urbanization gradient in Syracuse, New York, USA. We also examined the prevalence of each color morph on and off road surfaces in a range-wide compilation of > 100,000 photographs of S. carolinensis . The prevalence of melanism among living squirrels in Syracuse was negatively related to distance from the city center, whereas there was no urban-rural cline in melanism among road-killed squirrels. The melanic morph was underrepresented among road-killed squirrels by up to 30% along the urbanization gradient. We also found the melanic morph was underrepresented among road-killed squirrels in the image database. Our study highlights vehicular collisions as an important cause of natural selection on phenotypic traits that can contribute to the maintenance of urban-rural clines.