There Aren't as Many Rats as Humans in NYC. Stop Saying That

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A recent study estimates that there are about 2 million rats in New York City, thus busting the urban myth that there are as many rats as people -- around 8 million -- in the Big Apple.

Tired of hearing the one-rat-per-person statistic, Jonathan Auerbach, a doctoral statistics student at Columbia University, began analyzing rat-related 311 complaint calls to the New York City Department of Health and Mental Hygiene from 2010 to 2011.

Ecologists often use a survey method called "capture-recapture estimation" to determine animal populations. In the case of rats, the method would entail capturing many rats, marking and releasing them back into the general population, and then capturing a second group to take stock of how many previously marked rats reappear in the second set.

"Unfortunately," Auerbach wrote in his research, the health department "is unlikely to approve a large-scale rat-releasing experiment (I know, because I asked)." So the 26-year-old statistician went with his city "lot-comparison" method instead.

More than simply getting new numbers, however, Auerbach had another statistical end in mind.

"My goal from the start was to demonstrate the value of open data," he tells the Voice. "New York City has gradually been releasing its data to the public over the last few years. Surprisingly, little work has been done relating this information to the services government provides us."

Of the roughly 842,000 city lots, Auerbach identified about 40,500 that were rat-inhabited. Working with the assumption that there are 40 to 50 rats in a typical colony, and that a full colony occupies each rat-inhabited lot, Auerbach estimated the rat population at 2 million. And even that number, he says, is "generous".

The health department, however, doesn't seem too convinced or impressed with Auerbach's methods and findings. DOH spokesperson Levi Fishman acknowledges that the department found his research "interesting," but adds, "[T]here are no scientific methods for being able to accurately count the number of rats in New York or any large city.

"Rat populations fluctuate due to weather, breeding, and local environmental conditions," Fishman tells the Voice via email.

The DOH also cautions that putting too much faith in 311 data can lead to inaccuracies, as complaint statistics can vary by neighborhood. "There are many factors that affect the variance of the 311 calls, and there are perhaps even more uncontrollable biological and non-biological variables that affect urban rat populations," Fishman says. "Some rat sightings go unreported; other sightings are reported by multiple callers."

A much more "reliable" source of information, the department says, is its inspection results, which are available on the city's rat information portal. In 2013, 10,896 property inspections had conditions able to harbor rats and 11,196 had active signs of rats.

Remember that rat on the A train in April?

New York City is mostly infested by the Rattus norvegicus -- or the Norwegian, a/k/a the brown, sewer, or alley rat. "Norwegian rats have a short lifespan and are affected by numerous diseases," says Richard Reynolds of RATS, the Ryders Alley Trencher-fed Society.

The group hunts rats for the sake of the dogs they own -- terriers and dachshunds -- which are bred for vermin control. While hunting a few rats is good exercise for the dogs, it does very little in terms of putting a dent in the city's overall rat population, Reynolds admits.

Norwegian rats "die young and often, but have a reproductive cycle that ensures the ultimate survival of the species," he says. "They may lose a battle, but they will surely win the war, as they have for centuries."

In August, the city launched its aggressive pilot program to curb the rat infestation by sealing rat "reservoirs," notorious areas where rats congregate and breed.

Rather than estimate the number of rats in the city, the health department tracks the number of properties amenable to rats and properties with signs of rat activity.

"The precise number of rats would not influence how the city and property owners should respond to signs of rats," says Fishman.

For his research, which he released in October, Auerbach won the young statisticians writing award from London's Royal Statistical Society. He's not the first researcher to attempt to debunk the one-rat-per-person theory. After trapping rats in East Harlem apartments in 1949, ecologist David Davis estimated that there was one rat to every 36 humans in the city.

According to Auerbach's study, the origins of the one-rat-per-person myth can be traced to a book in 1909 based on deductions in England that were later "erroneously" applied to NYC.

Critics of Auerbach's findings, unwilling to let go of the legend, cite concerns with unreported rat sightings, especially considering how ubiquitous the New York rats are. How were those details taken into consideration?

"Readers should be skeptical of any statistic they read," Auerbach says. "The method I used assumes (among other things) that, within a neighborhood, rats are reported in a consistent manner. It does not rely on the assumption that every rat is reported."

Despite their misgivings, perhaps the Department of Health will come to realize that in its war on rats, it needs all the assistance it can get.

"Rats are a threat to public health," Auerbach says. "So knowing the size and distribution of NYC's rat population is crucial in order to evaluate the scope of the threat."

Read the full study below:

Does New York City really have as many rats as people?

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