You see them ducking under the bushes, scurrying along subway tracks, or crawling around the bags of trash that are stinking up the curb. Legend has it that there’s a rat for every person, more than 8 milllions rats in New York City. But this urban myth was already debunked. So how many rats live in the city, where do they come from and are they ready to take over the Big Apple?

rats in new york city
Are there really 8 milllion rats in New York City?

There are approximately 2 million rats in New York City

For over a hundred years, nearly everyone has believed New York has about 8 million rats, a ratio of one human to one rat. This theory began in the early 1900s, when author and rat expert W.R. Boelter hypothesized that in England there was a ratio of one human to one rat.

In 2014 statistician Jonathan Auerbach decided to test that idea. Auerbach used a method called capture-recapture estimation, developed by ecologists. The simplest version of this technique, called the two-sample version, would have required the large-scale release of rats into the population. Catch a number (say, 10,000) from a certain area, mark them, and release them back into the wild. Then after catching another 10,000 animals from the same area, 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.

Unfortunately, NYC’s Department of Health and Mental Hygiene is unlikely to approve a large-scale rat-releasing experiment (I know, because I asked).Jonathan Auerbach

He turned to data maintained by the city’s Department of Health — particularly a data set on rat sightings, by location, reported by dialing 311. While this method doesn’t allow the marking of individual rats, Auerbach wrote, it does allow the locations in which they were reported to be classified and marked by city lot — of which there are 842,000 in New York.

We first observe the number of lots that reported a rat sighting during the first half of 2010. These lots constitute our first sample and are our “marked” lots. Then we observe the number of lots that reported a rat sighting during the first half of 2011. These lots constitute our second sample. Some of them are “marked” in that they were also identified within our first sample. A “marked” lot that appears in the second sample period has been ‘recaptured’. If we assume that a recaptured lot is as likely to be reported as any other rat-inhabited lot, the proportion of “recaptured” lots in the second sample period will then provide an estimate of the total number of rat-inhabited lots.Jonathan Auerbach
rats in new york city
“Just” 2 million rats in New York City

The numbers, when Auerbach ran them, were far lower than 8 million, the stuff of urban legend. His number: a paltry 2 million.

Audie Cornish speaks with Jonathan Auerbach.

Where are New York City rats coming from?

Rats that inhabit New York City are all the same species: Rattus norvegicus or the Norwegian rat. This is the same species as pet rats and laboratory rats. Other names include: the brown rat, the sewer rat, and the alley rat. They grow to approximately 16 inches long and weigh about a pound.

Brown rat

The brown rat (Rattus norvegicus), also referred to as the common rat, street rat, sewer rat, Hanover rat, Norway rat, Norwegian rat, or wharf rat is
Since the late 1700s, Norwegian rats have haunted New York City’s alleys, parks, and basements. They came on ships from France and England, and then they never left. Rats stay close to their burrows, which are often at ground level in apartment floorboards, alleyways, sidewalks or basements. They are nocturnal and survive by mastering their surroundings, sticking to familiar areas within 450 feet of home.
Rats live in colonies of around 40–50 members and can be seen travelling in herds, passing down successful feeding paths to younger generations. Solitary, wandering rats, especially those found during the day, have often been displaced from their burrows. They have a phenomenal rate of reproduction, mating up to 20 times in 6 hours. A female rat produces 4–7 litters of around 10 rats each year. Rat populations fluctuate based on factors such as food availability and the weather. Rats are hard to kill. They are neophobic, meaning they avoid new objects introduced to their environment. They generally avoid traps and can smell poison, avoiding bait unless other food sources are unavailable.

Downtown and uptown rats

Manhattan has two genetically distinguishable groups of rats: the uptown rats and the downtown rats, separated by the geographic barrier that is midtown. The midtown break appears to be based on the layout of the sewer system and population density. Midtown in particular is filled with buildings and not many residential buildings. That translates o fewer habitats for rats.  Since rats tend to move only a few blocks in their lifetimes, the uptown rats and downtown rats don’t mix much. Uptown rats are genetically different than downtown rats — and there’s also a distinction between “West Village” and “East Village”.

rats in new york city
Uptown rats are genetically different than downtown rats

Disease, allergies, asthma, etc.

The greatest danger to humans is from diseases rats can transmit. City-dwelling rats carry pathogens that can cause diarrhea and vomiting in humans. Disease-causing bacteria commonly carried by rats include E. coli, Clostridium difficile, and Salmonella. The bacteria can be spread by contact with rat feces, saliva, or urine. Viral diseases spread by rats include rat-bite fever and hemorrhagic fevers caused by Seoul hantavirus. While at least 18 of the viruses found are known to cause disease in humans, it is unclear how infectious the rats are to residents. The NYC Health Department recommends that people bitten by a rat seek immediate medical attention, as bacteria from the rat’s teeth can cause tetanus as well as rat bite fever, which can be fatal.

New York City rodent complaints can be made online, by filling out the New York City Rodent Complaint Formor by dialing 3-1-1.

Rats are intelligent little critters

Rats are highly intelligent rodents. They are natural students who excel at learning and understanding concepts. Rats are considerably smaller than dogs, but they are at least as capable of thinking about things and figuring them out as dogs are! And, while rats are much smaller than elephants, they have excellent memories. Although their eyesight is poor, once rats learn a navigation route, they never forget it. They are also extremely social animals. They become attached to other rats and recognize their own family members.

They communicate with each other using high-frequency sounds that we can’t hear without instrumentation. They play together, wrestle, and love sleeping curled up together. Much like us, if they do not have companionship, they can become lonely, anxious, depressed, and stressed.

Rats have clearly demonstrated empathy. In one ethically questionable study, the vast majority of the rats tested chose to help another rat who was being forced to tread water, even when they were offered the opportunity to help themselves to a chocolate treat instead. Rats can recognize expressions of pain on other rats’ faces and react to them.

Pest control in New York City

The New York City Department of Health handles enforcement of rat infestation problems in New York City. Local authorities in New York have long recognized that eliminating rats from the city is unrealistic, but have made various efforts to control their prevalence. The approach has traditionally been reactive: after receiving complaints of infestation, officials would commence control efforts at that site by placing rodent poison, traps, or contraceptives.

In recent years, the city adopted a more proactive approach to rodent control known as integrated pest management, which focuses on preventive measures. Such efforts include developing a rodent control map using geotagging to focus countermeasures more systematically, instituting a “Rodent Control Academy” that trains city employees on rat behavior and control, and emphasizing building integrity and garbage disposal.

rats in nyc
New York City rodent complaints can be made online or by dialing 3-1-1

In 2013, it was announced that New York municipal authorities would implement a plan for mass sterilization of the city’s rats, using a chemical to neutralize the reproductive systems of female rats. Bait stations loaded with the chemical were to be deployed. The chemical’s effects were to gradually shrink the number of kittens a female rat can have in a litter, eventually rendering them infertile.

The Upper West Side and the Upper East Side logged the most rat complaints to the Health Department from 2010 to mid-2014.In 2014 the Health Department hired 9 new inspectors to augment its staff of 45. With $611,000 in funding, the new squad was tasked with targeting major infestations in the South Bronx and Manhattan.

A private group, Ryders Alley Trencher-fed Society, conducts rat hunts in New York using dogs.





There are 8 million rats in New York City. Really?
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