How many of the NYC rats survived hurricane Sandy? This question has been asked in the wake of Sandy's flooding of lower and east Manhattan. See, for example, articles in Huffington Post Green, Forbes, National Geographic, Business Insider, Mother Nature Network and NYMag.

The short answer is: some rats drowned, some survived.

The complicated question, how many drowned and how many survived, is probably impossible to answer. But we can speculate using the information and knowledge we have in our possession. But things we really need to know, we don't - information is just not available (and some of it never will be).

How many rats are in NYC?

Nobody knows. Nobody seems to even be attempting to estimate.

Beware of the myth that there is one rat per person. That is a very old myth. It started in 1909 when W.R.Boelter published a study of rats in England. He asked farmers (but never bothered to look in the cities) to estimate how many rats they have in their fields. From that informal survey, Boelter came up with an average of one rat per acre (yes, of agricultural land). At that time, there were 40 million cultivated acres in England. From that, he estimated the total population of rats on agricultural land to be about 40 million. Completely coincidentally, England in 1909 also had a population of 40 million people. So, the 1:1 ratio stuck. And it has been repeated for more than a century, by media, by scientists, by United Nations, by pest control companies, by health departments, and apparently everyone else.

In 1949, Dave Davis did a systematic study of rats, by trapping and capturing them, and estimated that rat population in New York City was only about 250,000. Not even close to 8 million.

An aside - I have an indirect personal connection to Davis. For a while he was a professor in the Department of Zoology at NCSU, that is, in my own department. At the time he was ready to retire, in the 1970s, he was actively working on daily and seasonal rhythms in various animals. He used to work with Curt Richter before, at Johns Hopkins, and Curt is one of the pioneers of chronobiology. David sent some woodchucks on a ship from Philadelphia to Australia. While on the ship, rats kept EST time, but quickly re-entrained to the Australian local time once they arrived there and were exposed to ambient light. Although the field was still very young, Davis' work made the rest of the department aware of it (they did not think it was Biorrhythms silliness, as many assumed at the time), so they were interested in hiring a replacement who was doing something similar. So they hired this bright, young lad from Texas in his spot - two Science papers already published and he took only 3.5 years to get both MS and PhD. The new faculty's name was Herbert Underwood. Two decades later I joined the Underwood lab. The rest is history.

Anyway, back to rat population. Estimates vary wildly, to as high as 32 million. Nobody really knows.

New York City is old. It was built and rebuilt. New buildings were built on top of the old ones. There are old, buried tunnels, rooms, chambers, now not accessible to humans but perfectly accessible to rats. Gradually, the city dug out more and more sewers, more and more various pipes, more subways and other tunnels. Thus more places for rats to nest. We gradually built comfortable homes for more and more rats.

The rat population is not evenly distributed either. They tend to be where poor people live, and where the restaurants are. That's where there is food.

And not all rats go to the surface. Rats are pretty loyal to the place of birth, and rarely venture more than about 60 feet from it, throughout their lives. If displaced, they can find their way home from as far as 4 miles, but for a foot-long animal, that is an extremely long distance.

If they can get food down under, e.g., from subway passengers throwing out uneaten food onto the tracks (which they do), rats never need to go up to the surface. They never get captured and counted in surface surveys.

Can rats swim?

Yes, rats are strong swimmers. They can even dive for a little while - see this video: if a domesticated rat can be trained to dive (and enjoy it), I assume that a wild rat can do it when its life is threatened:

The thing is, swimming in a water maze in the lab, or on the surface of a body of water is one thing. Swimming upward, against the powerful stream of water streaming downward is a completely different thing. They may be strong swimmers, but they are not Johnny Weissmullers.

There are many ways up to the surface, but they all go up. And if the water was mainly gushing into the tunnels from above, from the streets as Sandy was flooding, they would have had to swim or dive up narrow pipes, essentially vertically up against the water. No way. Those guys drowned.

To go up to the surface, rats need to know the way to the surface. Rats know their own territory very well. But rats that never go to the surface do not know how to get there. They may still want to instinctually go up, but they don't know the way so would have to get lucky to actually find the stairs and then fight their way up against the gushing water.

Rats already on the surface would probably be fine. The water and wind from Battery would carry them north until they reach the dry ground. They can certainly stay on the surface. Salty water is denser than fresh water, so they would find it even easier to stay on the surface, though their eyes may not like all of the salt.

What was flooded, when and how?

Right now, we do not know exactly where, when and how the water entered the subway tunnels, sewers, etc. MTA site does not provide much information. New York Times does not either - they are concerned with information useful to people, e.g., when will the subway open again, not where, when and how the subway initially flooded. Most likely the water came from above, from the flooded streets after sea water rose high at the Battery and the East side. This is important. It is easier for rats to float on the surface of water rising from below, than to fight against the water falling from above.

Also, most of Manhattan (and rest of NYC) did not flood at all. Most of the rats probably survived just fine where they were.

Who lived, who died?

So, from above, we can speculate that many rats survived. Some were never affected by flooding. Some were on the surface already and managed to run or swim to the higher ground. Some knew their way out to the surface and made it there. Rats are smart and crafty - if they can find a way to hide or go out, they will.

But some rats certainly drowned. Those are the rats that live deep inside holes we never know about, let alone visit. Rats that never go up to the surface. Rats that had the misfortune to have to try to escape essentially vertically up against strong gushing water.

There is a rule of thumb - if you see a rat on the surface during the daylight time, this means that the underground population is enormous. And I see them every month I go up to New York. When the rats are crowded, dominant rats take the best spots. If the population forages on the surface, dominant rats forage during the night. Subdominant (or submissive) rats are temporally displaced to the daytime shift.

This is important. If Sandy started to flood the tunnels during the day (and nobody knows, or makes public, this information as the subway was already closed to people by then), it will be the non-dominant rats who are on the surface, and thus more likely to survive. If the flooding started at night, it will be dominant rats on the surface, floating away into safety. Dominant rats are more likely to be able to relocate and survive in other places where they have to compete with locals. Non-dominant rats would have a much harder time finding a new home.

So, my guess is that most of the rats survived. But quite a large number of rats drowned - depending on exact location, depth, how much they know how to get to the surface at all, their exact route to the surface, and their status in the social hierarchy.

You can learn much more about New York City rats from Rats: Observations on the History and Habitat of the City's Most Unwanted Inhabitants by Robert Sullivan, one of the most wonderful popular science books I have read over the past decade.

I will also be doing a HuffPoLive segment about this at 1pm EDT, will post the link in the comments once I have it.

Update: More from The Urban Scientist, Jezebel, Tha Daily Beast, Live Science, Forbes.