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In truth, the city's rat population is probably closer to 2
million, said Jonathan Auerbach, a Columbia doctoral student who
wrote an essay on the subject published in Significance
magazine.
The urban lore that there are as many rats as citizens dates
back at least a century, Auerbach says. It may have endured in
part because reliably estimating the city's rat population is
difficult even though the creatures are hardly invisible, as
most New Yorkers who see them skittering about the subway tracks
or hear them rustling through trash piles will attest.
"Animals are terrible survey respondents," he wrote in the
article, which was the winning entry in a young statisticians
writing competition organized by London's Royal Statistical
Society.
Auerbach did not let the difficulties deter him, arguing that
more precise estimates would be useful given that the rodents
spread disease, start fires by chewing on electric cables and
occasionally bite people.
His initial plan was to use a method that involves capturing a
random sample of rats, marking them, releasing them, and then
capturing another random sample of rats.
But the city's health department, which is responsible for
dealing with rats, was not enthralled with the idea, Auerbach
wrote.
Instead, he used complaints from the public about rat sightings,
which the city tracks and publishes online. Combining the data
with a number of assumptions, he was able to extrapolate the
number of rat-occupied lots to about 40,500 across the city, or
less than 5 percent of the total.
If each inhabited lot is home to a typical colony of 50 rats,
that would mean there are about 2 million rats in the city.
In a statement, the health department called Auerbach's
"interesting," but added that there was simply no valid method
for counting any large city's rat population, nor would it be
particularly useful if there were.
"The precise number of rats would not influence how the city and
property owners should respond to signs of rats," the department
said.
(Reporting by Jonathan Allen; Editing by Frank McGurty and Will
Dunham)
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