From Soil to Squirrel: The Legacy of Lead Pollution & Its Effects on Urban Wildlife Behavior

Abstract

Urbanization increasingly threatens wildlife through the introduction of novel threats and pollution. Animals can use behavior to adapt to urban environments and urban pollutants drive changes in behavior, leading to populations of urban wildlife with some behaviors that are distinct from rural populations. Lead (Pb) pollution is ubiquitous in urban areas, but there can be significant local variation in soil lead levels. In this study, I examined the relationship between soil lead, hair lead, docility, and aggressive and social behavior in eastern gray squirrels (Sciurus carolinensis) and eastern chipmunks (Tamias striatus) in Minneapolis and Saint Paul, Minnesota. Docility was measured through a struggle test and aggressive and social responses were measured in a mirror image stimulation trial. Soil lead and hair lead were significantly positively correlated in gray squirrels and chipmunks, and there was no difference in hair lead based on species or sex. Chipmunks had significantly longer struggle times than gray squirrels, but struggle time did not vary based on hair lead or sex. Only six of 235 animals displayed aggression in the mirror image stimulation trial, and frequency of contacting the mirror in a nonaggressive way was not correlated with hair lead, species, or sex. These results provide correlational evidence of lead transfer from soils to gray squirrels and chipmunks, though the magnitude of lead accumulation does not seem to depend on the distinct life histories of these two species. At the levels currently present in these urban environments, lead does not affect aggressive or social behavior in gray squirrels or chipmunks. Future studies should examine aggression through direct observation rather than a mirror image stimulation trial to better quantify aggression in these species that have low territorial aggression.