BI Answers: How many rats are there in New York City?
Legend has it that there’s a rat in New York City for every person, more than eight million in total.
That’s quite a lot of rats — too many, in fact, according to a recent prize-winning statistical analysis by Jonathan Auerbach, a Ph.D. student at Columbia University.
But it’s not an easy thing to measure. After all, as Auerbach writes, “animals are terrible survey respondents.”
With surveys out of the question, the most effective statistical method used to count an animal population is to catch a number (say, 10,000), mark them, and release them back into the wild. Then after catching another 10,000 animals, you can see what percentage of those are marked and use that to estimate the total population. If 1% of the newly captured rats were marked, that would imply that 1% of the total population was marked, meaning a million rats total.
But unfortunately the city wasn’t cool with capturing 10,000 rats and then unleashing them on the city again. Auerbach knows, because he asked.
So instead, he looked at another measure. Land in New York City is divided up into city lots — there are 842,000 in total. So Auerbach checked the number of reported rat sightings in these lots at two separate periods of time. He concluded that there were 40,500 vermin-populated city lots. That means that rats were reported in less than 5% of those lots, which seems low, but it’s the data that exists.
Auerbach also acknowledges that some neighborhoods may be more likely to report rat sightings than others, but he writes that this was at least partially controlled for by comparing sightings within neighborhoods as well as city lots.
Rats are territorial, so Auerbach doesn’t think it’s likely more than one colony occupies a city lot. If each colony has 40 to 50 rats, a number he calls generous, that would mean approximately two million of the creatures, plus or minus 150,000.
Note: This does ignore the number of rats that live below ground in the city, but Auerbach says that the New York Department of Health states that the idea there are underground “cities of rats” is a myth.
But who knows what lurks down there?
Auerbach isn’t the first to estimate that there are fewer than eight million rats. An ecologist in 1949 estimated one rat for every 36 people, though the city has grown and changed since then. And in 1998, a researcher classified between 45% and 86% of the city as “medium to high risk” for rodent bites. In 2013, the city found 11,128 buildings with active signs of the creatures.
Eight million is more of an urban legend, something people claim because it fits some idea they have of New York. “Everybody loves the idea of eight million rats,” Robert Sullivan, author of “Rats: Observations on the History and Habitat of the City’s Most Unwanted Inhabitants,” told the New York Times. “The one-rat-per-person scenario is too good.”
And why is that?
Maybe we think we deserve it. As Sullivan explains in his book, “I think of rats as our mirror species, reversed but the same, thriving or suffering in the very cities where we do the same.”
This post is part of a continuing series that answers all of your “why” questions related to science. Have your own question? Email [email protected] with the subject line “Q&A”; tweet your question to @BI_Science; or post to our Facebook page.
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