Dr. Munshi-South said the study emerged from a simple question: “What is a New York City rat, and where did it come from?”
He contacted researchers around the world to see if he could obtain DNA to compare with that of the rats he captures around New York City. To his surprise, he ended up with samples from hundreds of brown rats, from the Galápagos Islands to Brazil, from New Zealand to Japan.
Instead of simply asking where New York City’s brown rats came from, Dr. Munshi-South realized he might be able to figure out where the world’s brown rats came from.
Emily E. Puckett, a postdoctoral researcher in his lab, analyzed the DNA samples, sorting 314 brown rats from 30 countries into clusters of genetic relatives. Eventually, she was able to determine how different populations of the rats mixed together over time.
Dr. Puckett, Dr. Munshi-South and their colleagues published their findings last week in Proceedings of the Royal Society B Biological Sciences.
The brown rat is sometimes called the Norway rat, but the new research confirms that the name is a misnomer. Instead, brown rats originated in northern China or Mongolia. Before they became our companions, they fed on wild plants and small animals on cold, open plains.
Farming came relatively late to northern China, but at some point, native brown rats, finding a reliable food supply in their midst, switched to living on farms and in villages.
Dr. Puckett and her colleagues can’t say how long brown rats remained in northern China, but at some point, they started to expand their range. Their first migration, the study suggests, took them to southeastern Asia.
Much later, a wave of brown rats spread northeast, into Japan and Siberia. Another emigrated west, eventually reaching Europe in what appear to have been three major arrivals on the Continent. These rats may have traveled on overland routes, or perhaps hidden on ships that sailed along the coasts of Asia and Europe.
The new study suggests that brown rats were slower to spread around the globe than our other familiars, the black rat and the house mouse. Geography may be the reason: House mice originated in the Fertile Crescent, and black rats in India.
Farming societies and widespread trade arose in those places much earlier than in northern China, giving the black rat and the house mouse early opportunities to travel.
But in the past three centuries, the brown rat has more than made up for lost time.
Brown rats in Alaska and along the Pacific Coast of the United States and Canada can trace much of their ancestry to Russia, Dr. Puckett and her colleagues found. Their ancestors may have stowed away aboard ships that traveled to fur-trapping communities in the New World in the 1700s and early 1800s.
But the brown rats of Europe became the true globe-trotters. As Western European countries colonized other parts of the world, they took the rodents with them.
The brown rats of New York and other Eastern American cities trace their ancestry to those in Western Europe. So do brown rats in South America, Africa, New Zealand, and isolated islands scattered across the Atlantic and Pacific.
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