Urbanization is changing the body size of animals, a Nature paper reports. Understanding the evolutionary relationship between species and their urban habitats is both timely and important as it will help us to predict the ecological and evolutionary effects of urbanization.

The higher temperatures associated with urban environments result in increased metabolic costs for resident animals and are expected to drive shifts towards smaller body sizes. Thomas Merckx and colleagues studied individuals from more than 700 species and 10 different taxonomic groups, including butterflies and spiders, that lived along a gradient of habitats in northern Belgium, which ranged from urban to non-urban. As expected, urban communities were generally found to consist of smaller species, but this was not always the case. In three of the taxa studied, the trend was towards bigger individuals. The authors suggest this is because bigger animals are sometimes better at dispersing and finding new habitats. With urbanization fragmenting natural habitats, this can be a useful evolutionary strategy.