A graduate student in New York has studied the genetic makeup of the city's rat populations, and discovered a divide between uptown and downtown:
As a whole, Manhattan’s rats are genetically most similar to those from Western Europe, especially Great Britain and France. They most likely came on ships in the mid-18th century, when New York was still a British colony. Combs was surprised to find Manhattan’s rats so homogenous in origin. New York has been the center of so much trade and immigration, yet the descendants of these Western European rats have held on.
When [Fordham University graduate student Matthew] Combs looked closer, distinct rat subpopulations emerged. Manhattan has two genetically distinguishable groups of rats: the uptown rats and the downtown rats, separated by the geographic barrier that is midtown. It’s not that midtown is rat-free—such a notion is inconceivable—but the commercial district lacks the household trash (aka food) and backyards (aka shelter) that rats like. Since rats tend to move only a few blocks in their lifetimes, the uptown rats and downtown rats don’t mix much.
I think rats are cool. I know they carry diseases and they have generally unpleasant habits in many cases, but they're symbiotic with us and they're all over every urban environment in the world. Also they make great pets—not New York feral brown rats, mind you, but your typical Norway lab rat is a pretty chill companion.