Global meta-analysis reveals urban-associated behavioral differences among wild populations

Urbanization is causing fundamental changes to natural environments, effecting rapid and substantial adaptive phenotypic change in wild populations. While a large body of work has investigated how urbanization may shape interspecific variation in behavioral traits, such as via urban environmental filtering, no study has yet quantitatively assessed global patterns of urban-associated intraspecific behavioral variation. Here, we conducted a phylogenetic meta-analysis to assess urban-associated behavioral differences in wild populations of birds, mammals, amphibians, reptiles, and insects. We focused on four commonly measured behaviors (boldness, aggression, activity, and exploration) and extracted paired urban-nonurban effect size estimates for phenotypic means and variances (k = 278), behavioral repeatability (k = 13), and behavioral correlations (k = 14) from 80 studies. We found clear evidence that urban populations exhibit heightened average boldness, aggression, exploration, and activity compared to nonurban conspecifics, a result that was robust among species, geographic region, and ecological niche. Further, our results suggested that "generalist" species have the strongest behavioral responses. Conversely, we did not find strong evidence linking urbanization to changes in phenotypic variation, behavioral repeatability, or behavioral correlations. Our results summarize data from a rapidly evolving field in urban ecology and demonstrate geographically and taxonomically widespread differences in behavior between urban and nonurban populations, suggesting parallel natural selection across urban species and populations.