This is

is Scientific Americans 60 Second Science.

I'm Jason Goldman.

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 assess the body size of more than 95,000 individual critters.

They measured butterflies and beetles, weevils, ground spiders, web-building spiders, moths, and grasshoppers.

They also tested a handful of more obscure invertebrates, like a group of microscopic shrimp-like critters called ostracods, and a group of aquatic crustaceans known as water fleas. On average, urban communities contain smaller individuals than rural ones.

It's not that cities are causing animals to evolve smaller bodies, at least not necessarily.

What the 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.

But some groups of city dwellers were actually bigger than their countryside counterparts.

For three of our groups, for butterflies, m moths and for grasshoppers, we actually saw the completely

reversed pattern. These three groups out of the ten groups that we tested, these were the only groups where large species are also the most mobile ones.

Catholic University of Levine Ecologist Thomas Merckes.

These are animals that need wide spaces, something that's in short supply in cities, where

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