Rats are closely associated with terrible events, partly because disasters, like rats, reveal the weakest points in man’s built environment—the cracks in the basements of our homes, in particular, and our cities, in general—and partly because we find rats terrifying, a measure of the breakdown of everything we think of when we think of civilization. So it is no surprise that the day after Hurricane Sandy hit New York City, people began to worry about rats. But rats rarely live up to headline writers fantasies. (“Rat Invasion? Will Subway Rodents Move Into Flooded NYC Neighborhoods?” asks Forbes.) It’s time to review what we know about rats in the city.

First of all, hordes of Norway rats, North America’s predominant rat species, do not live deep in the subway tunnels. This has long been known by pest control experts who operate on the essential rat truism: rats exist only where there is garbage—"food source,"  in their parlance. People make garbage, and in general, people do not hang out in the tunnels. The garbage and the people are on the platforms. (There definitely aren’t a lot of rats in the middle of the Holland or Midtown or Brooklyn-Battery tunnels.) In 2010, Robert “Bobby” Corrigan, New York City’s chief rodentologist, published a paper investigating the topic—officially busting the rat-filled tunnel “myth,” as he refers to it. And yet when you look into the dark of the tunnel, you can imagine why the myth persists.

Thus, while, yes, there are rats in the subways—plenty—there aren’t any more than might be in your local park, or in the cracks beneath the sidewalk on the streets. Or if you store food in your basement in Dumbo or Tribeca or have a shitty plastic garbage can with a hole in the bottom, your home. Many rats would likely have drowned when stations were flooded; some would have managed to find a way up and out. (More about numbers later.) A one-pound, foot-long rat would only need a hole or crack as big as its skull to enter any wall or sidewalk or basement to make a new home. In New York City this week, some rats will have swum—they are excellent swimmers—and some will have clambered up from the subway platforms, finding a home in a sewer catch basin that did not flood, like  the one in which I have been seeing rats at 34th Street and 8th Avenue. Some will have risen up from the watery sidewalk cracks and moved into a dry park, convenient to a garbage can, and some will have drowned. (Juvenile rats in nests will very likely have drowned.) We won’t know the score for weeks, and because some people won’t even notice the rats moving in anew in the alley behind their favorite restaurant or in a hole in the wall behind the local Trader Joe’s, we won’t even notice all the effects.

The type of disaster matters, too. In the case of Hurricane Sandy, people are expected to return to their homes in short order. After September 11, rat populations increased in lower Manhattan precisely because the area was cut off to people. Restaurants near the old World Trade Center that were abandoned were suddenly akin to rat farms because, yes, a rat will have sex up to twenty times a day, producing a lot more rats, offspring increasing in relation to food (garbage) supply. The lower Manhattan rat population increased dramatically, and, as a result, the city Department of Health built a ring of poison-filled bait stations around the abandoned areas, eventually beating down the population explosion. This week’s flooding just means we do what we always do with rats, which, like humans, are always looking to expand territory and relocate: Keep your food secure and be vigilant. 

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An exciting rat "fact" that is not true at all, despite how often it is quoted, is that there are 8 million rats in New York, one per person. You hear it all the time—when a rat is YouTubed on a subway or in a Taco Bell, or more significantly (if less likely to be covered) when Mayor Bloomberg cuts the number of workers who might be sent into a neighborhood to take care of a rodent infestation. You will likely hear it at some point this week, or at least some wild, unverifiable number.  This stat was disproved first and forever in 1949 by David E. Davis, a biologist from Johns Hopkins University who is the founding father of rat ecology, as well as urban ecology. Davis trapped rats throughout New York and estimated 250,000 rats, or one for every 36 people. But then the UN began quoting the one-rat-per-person statistic again, and within a short time, the city itself began quoting it. (When I published the history of the statistic in a New York Times Magazine piece in 2004, debunking it, the Times published a letter from a reader shortly thereafter, debunking me, citing a Joseph Mitchell story from 1944.) We despise it and we love it for what it purports to say about us, and the untrue fact, like a tough rat, survives.

Dave Davis’ work was pioneering in that he looked at cities as ecosystems. His work was sponsored by the federal government, concerned about food stores donated to a devastated Europe under the Marshall Plan. Prior to Davis, there was little hard science on what rats ate and how they lived in the urban environment, and little consideration of the ecology of cities. Today we have studies on how rats transmit various diseases, such as leptospirosis, hanta virus, and, in very rare cases, the plague. And we know that after a flood event, the water itself—contaminated with raw sewage, as well as petroleum products and all the not-as-terrifying pollutants that normally cover our streets—is potentially more of an issue in terms of spreading pathogens than rats. (After Irene and even Katrina, outbreaks of rat-associated viruses did not occur.) Again, sewer treatment plants in low-lying areas, tend not to be in high-income neighborhoods.

Our rat thinking is backwards, in other words. Rather than worry about armies of rats, we might better look at neighborhoods with evidence of high rat infestation, as reported by the city, that are near low water: These are the places where people are likely to have few resources to deal with disaster. The rat fleeing the sinking ship—this notion comes from the idea that rats have a special sense about when to flee. But in the ship, they flee first because they are in the hold, the storage place, the worst seats on the boat. They run first because their lives are first at risk, like any at-risk population. They run too because they do what they have to, also like humans. Dave Davis also showed us that the first Norway rats came to the U.S. in ships, at the moment the U.S. was born, most likely on Hessian ships that landed in New York. (The Norway rat was then making its way east across Europe, not yet in England but ubiquitous in Germany.) The British brought mercenaries and rats, and the devastated city, destroyed by fire just after the Continental Army abandoned the city, was a perfect habitat, rats burrowing in the beat-up, sanitation-poor city. 

If the British had been vigilant about pest control in revolutionary New York in 1776, we might only be worrying about pigeons and redesigning our city to survive the next flood—considering the continued restoration of historic tidal marshlands, for instance. Instead, the British command held a lot of fancy balls and cut down all the city’s trees and ruled with only a very small group of people in mind, not thinking at all about the critters or the rebel colonists that they considered vermin. In 1779, the natural disaster was a winter so cold that all the ports from the Carolinas up froze solid through 1780, thought to be the coldest winter ever on the East Coast. Suddenly, the British were susceptible to Continental troops on sleds. The vermin were again a factor.

Robert Sullivan is the author of numerous books, including "Rats: Observations on the City and It's Most Unwanted Inhabitants" and, most recently, "My American Revolution: Crossing the Delaware and I-78," a look at the relationship between the ecology of New York and its Revolutionary War history.