Patterns of Raccoon Craniodental Morphology under Urbanization

Urbanization is one of the fastest and most extreme processes creating novel conditions and affecting persistence of animals in the Anthropocene. Documenting behavioral and morphological changes in urban wildlife is difficult because few studies account for non-urban sources of variation. As a result, multiple hypotheses have emerged. These hypotheses implicate the influence of cognitive demands (cognitive buffer hypothesis), human-wildlife interactions (self-domestication hypothesis), and diet (diet composition hypothesis) on morphology under urban conditions. We investigated craniodental morphological change in raccoons (Procyon lotor) with 98 specimens collected across the continental USA based on these hypotheses. Raccoons occur in rural and urban habitats, facilitating the exploration of these hypotheses in a single species under multiple conditions. Using modeling of linear measures, as well as 3-dimensional meshes and geometric analysis, and stable isotope diet estimation, we found that the occlusal area of the first molars and mandibular fourth premolar, in addition to the cranial vault shape, were most responsive to urbanization. Rostral shape showed limited variation across the urban and ecological gradients. Some structures, while variable, did not respond to any of the predictors tested. Our results provide the highest support to the self-domestication hypothesis, although multiple other pressures co-occur, including effects of sex, age, individual size, and geography. Our results also suggest that some craniodental structures may be evolutionarily fixed and promote the raccoon's generalist ecology.