Wait a second.
What exactly are the rats in the Underland?
No, seriously. I just realized something: the Brown Rat, aka Norway Rat, and the Black Rat, are both not native to North America. They were accidentally introduced by European settlers some time around the 1700s, but the Underland's rats were presumably already there when Sandwich showed up in the early 1600s.
Unless an ancestral population of Norway Rats managed to cross the Bering Land Bridge and proceeded to find their way across the continent to New York and then have all the surface dwelling ones go extinct, the Gnawers can't use be giant, sapient Norway Rats. They have to be some kind of woodrat / packrat that just happens to have convergently evolved social behavior in addition to size and intelligence.
Similarly, the Nibblers have to be descended from some kind of Deer Mouse.
The Crawlers are also an enigma because apparently American Cockroaches aren't actually native to the Americas and were also introduced, and I think the native cockroach species in the area of New York are either wingless or only the males have full wings.
