Most critters seem better able to survive big cities if they’re smaller than usual. But a few others are better adapted to urban areas if they’re larger. That's the result of a study of more than 700 types of invertebrates from across 10 different taxonomic groups living in a variety of habitats in northern Belgium. European ecologists were interested in understanding how animals adapt to urbanization. So they set up a variety of traps in both urban and rural areas, and assessed the body size of more than 95,000 individual critters. They measured butterflies, beetles, weevils, ground spiders, web-building spiders, moths and grasshoppers. They also tested a handful of more obscure invertebrates, like a group of microscopic shrimplike critters called ostracods, and a group of aquatic crustaceans known as water fleas. On average, urban communities contained smaller individuals than rural ones. It's not that cities are causing animals to evolve smaller bodies, at least not necessarily. What this study found is that animals that are already smaller seem better suited to city living. The researchers think that has to do with what’s called the urban heat island effect. Animals expend more energy going about their daily lives in warmer areas, and cities tend to be warmer than more natural areas. Smaller body sizes can compensate for that heat effect.