Consistent phenotypic shifts in a widespread urban butterfly across three cities

Abstract Urbanization is a major driver of global environmental change, exposing species to stressors such as elevated temperatures and habitat fragmentation. Multi‐region comparisons of urban and non‐urban populations can reveal whether functional trait responses to these pressures are consistent across regions. Here, we used the cabbage white butterfly, Pieris rapae , to test for repeated urban‐associated trait shifts related to thermoregulation and flight using 571 butterflies collected across three metropolitan regions in the eastern United States (Cleveland, OH, Philadelphia, PA, and Pittsburgh, PA). We found consistent reductions in overall body size in urban populations relative to non‐urban ones. Given that P. rapae body size plastically responds to temperature, our findings suggest that the temperature‐size rule applies to the urban heat island effect. We also found a negative relationship between relative thorax mass and the urbanization gradient, suggesting reduced mobility. Whereas these urban–non‐urban differences align with expectations from experimental studies, regional differences did not. Additionally, wing pigmentation did not show urban‐associated variation, though divergent responses between males and females resulted in novel sexual dimorphism in near‐infrared reflectance within urban populations. Together, these results demonstrate repeated phenotypic differences between urban and non‐urban butterfly populations across multiple trait axes and suggest that similar urban pressures may drive parallel responses across regions. They also offer hypotheses about the functional significance and potential fitness consequences of urban‐associated trait shifts while highlighting the need for future work to disentangle the ecological and evolutionary processes underlying urban phenotypic change.