The water flea Daphnia magna doesn’t grow as big in urban environs. JOACHIM MERGEAY

The body size of insects is evolving in urban zones. Researchers in Belgium found the water flea’s (Daphnia magna) city variant was 44 percent smaller than its rural cousin. Larger forms of this water flea, a natural water filter, are better at weeding out algae. In cities’ warmer temperatures, higher metabolic costs likely drive these and other creatures to adopt a smaller body size. Such changes can change the way ecosystems function while also giving a preview of the evolutionary outcomes of global warming.

T. Merckx et al., “Body-size shifts in aquatic and terrestrial urban communities,” Nature, doi: 10.1038/s41586-018-0140-0, 2018.

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