0. 1em 0.0 1em No shape fits all: Trends in cranial morphology are not consistent with domestication syndrome across four urban-dwelling mammals

0. 1em 0.0 1em Conserving biodiversity in anthropogenic habitats necessitates unraveling patterns of wildlife responses to urbanization across spatial, temporal, and taxonomic scales. A central question in this endeavor concerns identifying the characteristics which enable some species to thrive in urban environments while others are excluded or displaced. Comparing cranial morphology between rural and urban habitats serves as a method to investigate common effects of urbanization among populations and species. For example, a link has been proposed between behavioral adaptations and skull features similar to those observed in some domesticated species (i.e., domestication syndrome), though the extent of this phenomenon remains unknown. In this study, we used museum specimens to quantify differences in skull shape between rural and urban environments, aiming to identify shared trends across species and whether these align with the expectations of domestication syndrome. We focused on four widespread, urban-dwelling mammals: Northern Raccoon ( Procyon lotor ), Virginia Opossum ( Didelphis virginiana ), Eastern Fox Squirrel ( Sciurus niger ), and Eastern Cottontail ( Sylvilagus floridanus ), assessing shape variation via a series of two-dimensional landmarks on skull photographs. The results indicated no universal trends across species at a broad geographic scale. In all but the squirrels, there were some significant shape differences between rural and urban habitat categories, although none of these differences followed predictions derived from domestication syndrome. Notably, the shape trends associated with advancing age classes in raccoons mirrored those found in more rural habitats, suggesting that age structure may significantly influence population-level morphological trends. These findings contribute to an expanding body of work revealing the complexity and diversity of urban ecosystems. While there is no “one size fits all” form or adaptive strategy that applies universally, understanding the context-dependent regularities among varied responses and their underlying factors is key to characterizing how wildlife will continue to be shaped by rapidly changing anthropogenic habitats.