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The Spread of the Brown Rat

Through genetic analysis, researchers have traced the origin of brown rats to northern China, the New York Times reports.

Fordham University's Jason Munshi-South tells the Times' Carl Zimmer that the idea for the study came from wondering: "What is a New York City rat, and where did it come from?"

As Munshi-South and his colleagues report in the Proceedings of the Royal Society B, they analyzed 314 rats from 30 countries using 32,127 SNPs to tease out their global phylogeography. Rather than originating in Norway as their name, Rattus norvegicus, suggests, they are instead from northern China or Mongolia, Zimmer notes. The rats spread from there to southeastern Asia and, later, to Japan and Siberia. They then headed toward Europe, likely by boat. New York City rats, the researchers say, are from Western Europe.

Munshi-South tells the Times that rats are still arriving at the world's ports, but that his team noted little admixture between New York and other city rats with migrants — the established rat population just doesn't let them in. This might be protective, Zimmer says, as the diseases that the new rats carry then don't spread.

"If that's true, then the new study also contains a strange paradox," he writes. "As much as we dislike brown rats, they may be our staunchest defenders."