Limited cranial shifts in city-dwelling rodents after a century of urbanization.

Urbanization and climate change can have unexpected effects on organisms that share space with humans. Examining these effects is important for understanding how wildlife have so far adapted to human modifications on the environment, and can aid in mitigating damage to the well-being of local populations going forward. Properly documenting these effects requires samples covering long, evolutionarily-relevant time spans. Most studies are not designed to span more than a few years, but natural history collections provide ready-made long-term samples, accumulated through the continuing efforts of field collectors. Here, we used museum specimens of two ecologically distinct rodent species collected over 100 years in the greater Chicago region to examine the effects of urbanization and climate change on cranial morphology. We quantified the shape of the skulls of eastern chipmunks (Tamias striatus) and eastern meadow voles (Microtus pennsylvanicus) using linear and geometric morphometrics, and investigated the effects of urbanization and climate (represented by annual mean temperature) on skull variation through time. Spatiotemporal urbanization levels were incorporated in the analyses to account for uneven urbanization growth. Overall, we recovered a significant albeit limited effect of urbanization and, to a lesser degree, climate on driving skull shifts in both species. We attributed these limited shifts to a non-directional selective pressure caused by a heterogeneous urbanization growth and oscillating climate. We further recovered different responses between the two rodent species. Chipmunks showed a weak association of cranial changes with urbanization and climate, although their size increased throughout the study era. Interestingly, they also showed a decrease in toothrow length, indicating a possible dietary shift. On the other hand, vole cranial morphology was stronger associated with degree of urbanization and showed a more obvious shift in morphospace occupation between urbanization categories. Voles from highly urbanized sites displayed a reduction in shape diversity and flatter skulls. These different patterns between chipmunks and voles reveal species-specific responses to the same human-induced habitat changes and the need for nuanced conservation plans in the face of continuing change. Our study emphasizes the importance of analyzing long temporal series to assess urban selection on phenotypic traits.