What’s new: Cities experiencing warmer temperatures, fewer green spaces and denser human populations are seeing a rise in rat numbers, a recent study shows. What the study says: Jonathan Richardson, a biology professor at the University of Richmond, U.S., and his colleagues wanted to check if anecdotal media claims about increasing urban rat (Rattus spp.) populations are true. Since there’s very little long-term scientifically collected data on city rats, the researchers used a proxy: publicly available data on public rat sightings and complaints, and pest inspection reports. They picked 16 cities — 13 in the U.S., as well as Tokyo, Toronto and Amsterdam — which had seven to 17 years’ worth of such data. In these cities, the data collection methods and sources were mostly consistent throughout the years. Only Tokyo, Louisville and New Orleans, saw declining rat numbers, the study found. Meanwhile, 11 cities, or 69%, saw a considerable surge in rat numbers over the years. Washington, D.C., San Francisco, Toronto, New York City and Amsterdam had the largest increases, followed by Oakland, Buffalo, Chicago, Boston, Kansas City and Cincinnati. The researchers noted wide variations in rat trends. For example, the increase in Washington, D.C., was three times larger than that in Boston. On exploring various factors that could explain the rat trends, the study found the strongest association with a city’s average temperature increase. “Cities that had a greater rise in temperature over time had larger increases in rat sightings,” the authors write. They speculate that since rat…This article was originally published on Mongabay