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How Many Rats Does New York City Have?

Quite a town we've got here.

Rats have been a fixture in New York City since its New Amsterdam days, but contrary to the city's reputation of having as many rats as people, new research appearing in the journal Significance indicates that there's four people for every rat in the five boroughs.

In a city full of nuisances, there is likely nothing more universally reviled in New York as the rats. We may all hate and fail to understand alternate-side parking rules, and agree that the rent is just too damn high, but no number of Taylor-Swift-penned ballads can put a romantic sheen on the knowledge that we're up to our ankles in rodents carrying heretofore unknown diseases.

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But while we all agree that there are too many rats, no one has been sure of just how many we're up against here. Jonathan Auerbach, author of the rat population study, traced the one-to-one ratio to a 1909 book by W. R. Boelter called The Rat Problem, "which assumed that there lived one rat per acre of land in England." Since the country had, at the time, both 40 million residents and 40 million acres, Boelter concluded that England had a rat for each person.

"The hypothesis was erroneously applied to New York City and is widely quoted to this day," Auerbach's paper states. Boelter probably wasn't right about England, and there's no reason to think what he found applies to New York.

The estimated number of rats has varied widely over the years: from 250,000 estimated in 1948—a ratio of 36 people for every rat—to twice as many rats as people, according to unnamed experts cited by the New York Post.

The city government isn't into putting a number on its rat residents, but it does track the number of properties that could be housing them. TheDepartment of Health and Mental Hygiene's Office of Pest Control Services reported that, in 2013, 10,800 property inspections had conditions able to harbour rats and 11,128 had active signs of rats.

Originally Auerbach wanted to catch, mark, and release rats, repeating the experiment after a number of months, in order to see how many marked rats they catch again, but "unfortunately, NYC's Department of Health and Mental Hygiene is unlikely to approve a large-scale rat-releasing experiment," he stated, adding, "I know, because I asked. So, instead, we have to rely on an alternative."

The alternative a much more hands-off estimation of the number of rats based on the number of rat-inhabited lots in the city.

With a figure of 40,500 rat-inhabited lots, we can give an estimate for the total number of rats in NYC. We know that 40–50 rats belong to a typical colony and that colonies are territorial; it is unlikely that two colonies will inhabit the same lot simultaneously. Now suppose, quite generously, that each rat-inhabited lot supports its own, unique colony of 50 rats. That would suggest that NYC has roughly 2 million rats (±150,000).

So congratulations, humans, there's a lot of you in New York. This will no doubt provide great lyrical fodder for the next big New York-related pop song, due sometime next spring.