Abstract
Urbanization implies a dramatic impact on ecosystems, which may lead to drastic phenotypic differences between urban and nonurban individuals. For instance, urbanization is associated with increased metabolic costs, which may constrain body size, but urbanization also leads to habitat fragmentation, which may favor increases in body mass when for instance it correlates with dispersal capacity. However, this apparent contradiction has rarely been studied. This is particularly evident in China where the urbanization process is currently occurring at an unprecedented scale. Moreover, no study has addressed this issue across large geographical areas encompassing locations in different climates. In this regard, Barn Swallows (Hirundo rustica) are a suitable model to study the impact of urbanization on wild animals because they are a widely distributed species tightly associated with humans. Here, we collected body mass and wing length data for 359 breeding individuals of Barn Swallow (H. r. gutturalis) from 128 sites showing different levels of urbanization around the whole China. Using a set of linear mixed‐effects models, we assessed how urbanization and geography influenced body size measured using body mass, wing length, and their regression residuals. Interestingly, we found that the impact of urbanization was sex‐dependent, negatively affecting males’ body mass, its regression residuals, and females’ wing length. We also found that northern and western individuals were larger, regarding both body mass and wing length, than southern and eastern individuals. Females were heavier than males, yet males had slightly longer wings than females. Overall, our results showed that body mass of males was particularly sensitive trait to urbanization, latitude, and longitude, while it only showed a weak response to latitude in females. Conversely, while wing length showed a similar geographical pattern, it was only affected by urbanization in the case of females. Further research is needed to determine whether these phenotypic differences are associated with negative effects of urbanization or potential selective advantages.
Highlights
Urbanization drives a dramatic change in environmental conditions, eliciting a broad variety of phenotypic and genetic responses by living organisms (Alberti, 2015; Johnson & Munshi-South, 2017)
This suggests that different factors linked to species’ ecology and evolutionary history may result in divergent patterns of body size change across urbanization gradients
28.36° in latitude), we found that urbanization exerted a negative impact on body size of Barn Swallows
Summary
Urbanization drives a dramatic change in environmental conditions, eliciting a broad variety of phenotypic and genetic responses by living organisms (Alberti, 2015; Johnson & Munshi-South, 2017). In line with Bergmann's rule and due to the potential negative effect of high temperature on nestling development (Andrew et al, 2018; Ashton, 2002), we predicted a positive relationship between latitude and body size These patterns, namely a decrease in body size toward highly urbanized and hot areas, could be promoted by geographical variation in the urban heat-island effect, which in China is stronger in southern than in northern cities (Zhou et al, 2004, 2016), and by the relatively high degree of urbanization of East China compared to West China. Given that males experience stronger sexual selection and may have higher energetic demands than females, the negative effect of urbanization on body size could be more intense in males than in females
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