Not all cities are the same: variation in animal phenotypes across cities within urban ecology studies

The sustained expansion of urban environments has been paralleled by an increase in the number of studies investigating the phenotypic changes of animals driven by urbanization. Most of these studies have been confined to only one urban center. However, as the types and strength of anthropogenic stressors differ across cities, a generalizable understanding of the effects of urbanization on urban-dwelling species can only be reached by comparing the responses of urban populations from the same species across more than one city. We conducted phylogenetic meta-analyses on data for animal species (including both invertebrates and vertebrates) for which measures about any morphological, physiological, or behavioral trait were reported for two or more cities. We found that morphological, physiological and behavioral traits of urban animals all differ similarly across cities, and that such phenotypic differences across cities increase as the more cities were investigated in any given study. We also found support for phenotypic differences across cities being more pronounced as the farther away cities are from each other. Our results clearly indicate that separate urban populations of the same species can diverge phenotypically, and support previous pleas from many researchers to conduct urban studies across several urban populations. We particularly recommend that future studies choose cities in different biomes, as urban adaptations may differ substantially in cities sited in different ecological matrices. Ultimately, a generalized knowledge about how organisms are affected by urbanization will only be possible when comprehensive biological patterns are similarly studied across separate and distinct cities.