Rats have been in New York City since the 1700s and they're never leaving


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.

Matthew Combs, a graduate student at Fordham University, didn’t just want to learn about the history of those rats. He also investigated how their families move around the city, and found that certain areas had more genetic diversity. The Lower East Side and East Harlem had more movement of genes between rats, while Midtown (especially west) had less. The residential areas of the city provide better homes for the rats than the more touristy areas. Better habitats mean more rat babies, which leads to the more frequent DNA mutations that Combs measured.

The Norwegian rat, or brown rat, paradoxically originated in Asia, then spread through the Middle East to Europe and Africa. Combs found that a population from England or France made their way to New York City sometime between 1750 and 1770. Even after the U.S. started to trade with Asia and Africa, other rodent species were unable to gain a foothold.

Robert Corrigan, an urban rodentologist and consultant to the city, says he was most relieved to learn that new kinds of rodents aren’t still arriving on ships. “There was always a question if we should worry about the ports,” he says. “There may be some arriving, but they’re not able to fight their way in here and establish a new genetic peg.” Corrigan says that the results of Combs’s study allow the city to better allocate resources. The De Blasio administration dedicated $32 million to reduce rat activity by up to 70 percent in the three most infested areas of the city. But Corrigan says that won’t be enough. “My experience is that we rarely give these local populations a knockout punch,” he says. “We’re really good at getting 70 percent eradication. Everyone feels good because that feels simple. But the population dynamics of this species shows that you've got to get 90 percent education.”

Combs now has a deep understanding of the rat problem in New York City. He and his army of undergraduates walked across the entire island of Manhattan beginning at the northern tip, leaving traps. They collected the dead rats and chopped off a piece of their tails, the most convenient sample for genetic analysis. “It’s a useful piece of tissue,” says Combs. “We also could have taken an organ or a toe.”

Urban fieldwork is not always easy. Combs said that sometimes people would steal traps, but he enjoyed the opportunity to ask for tips about where the best rat collection spots are. “Almost every time you say you’re studying rats to someone in New York City, they have stories for you,” he says.

Now that Combs has figured out that the ancestors of the NYC’s modern rats arrived on a boat at about the same time as Alexander Hamilton, he wants to know why. A combination of rat and human behavior probably comes into play. “Rats are territorial. But that's not necessarily the whole story,” he says. “It also might be that trade doesn’t happen in New York City anymore.” He wants to investigate genetic ports in New Jersey and other areas of the country to compare results. Corrigan says he is excited to see the future of the research. “Let me tell you something about the rats on the Upper West Side,” he says. “They would probably have some secrets for us. Because they’re all related, and they go way, way back.”

More on rats around the world:

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Rats in action help to save lives in Africa
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Rats in action help to save lives in Africa
A rat handler carries on his arm an African giant pouched rat at APOPO's training facility in Morogoro on June 17, 2016. APOPO trains the rats to sniff for traces of landmine explosives at its facility. Every year landmines kill or maim thousands of people worldwide. The trained rats sniff for explosive and so are able to detect the presence of landmines far faster than conventional methods which involve metal detection. Metal detection is longer and more laborious because detection equipment picks up all metal traces in the ground including scrap metal. APOPO deploy the trained rats to work in mine affected areas like Cambodia, Lao, Vietnam and Mozambique. To date APOPO has helped countries detect 69,269 landmines which have been destroyed by the countries authorities. Land mine clearance is also crucial to farmers and citizens reclaiming and using land which was previously unavailable due to landmine risk. / AFP / CARL DE SOUZA (Photo credit should read CARL DE SOUZA/AFP/Getty Images)
An African giant pouched rat sniffs for traces of landmine explosives at APOPO's training facility in Morogoro on June 17, 2016. APOPO trains the rats to detect both tuberculosis and landmines at its facility. Every year landmines kill or maim thousands of people worldwide. The trained rats sniff for explosive and so are able to detect the presence of landmines far faster than conventional methods which involve metal detection. Metal detection is longer and more laborious because detection equipment picks up all metal traces in the ground including scrap metal. APOPO deploy the trained rats to work in mine affected areas like Cambodia, Lao, Vietnam and Mozambique. To date APOPO has helped countries detect 69,269 landmines which have been destroyed by the countries authorities. Land mine clearance is also crucial to farmers and citizens reclaiming and using land which was previously unavailable due to landmine risk. / AFP / CARL DE SOUZA (Photo credit should read CARL DE SOUZA/AFP/Getty Images)
An African giant pouched rat sniffs for traces of landmine explosives at APOPO's training facility in Morogoro on June 17, 2016. APOPO trains the rats to detect both tuberculosis and landmines at its facility. Every year landmines kill or maim thousands of people worldwide. The trained rats sniff for explosive and so are able to detect the presence of landmines far faster than conventional methods which involve metal detection. Metal detection is longer and more laborious because detection equipment picks up all metal traces in the ground including scrap metal. APOPO deploy the trained rats to work in mine affected areas like Cambodia, Lao, Vietnam and Mozambique. To date APOPO has helped countries detect 69,269 landmines which have been destroyed by the countries authorities. Land mine clearance is also crucial to farmers and citizens reclaiming and using land which was previously unavailable due to landmine risk. / AFP / CARL DE SOUZA (Photo credit should read CARL DE SOUZA/AFP/Getty Images)
A photo shows an African giant pouched rat at APOPO's training facility in Morogoro on June 16, 2016. APOPO trains rats to detect both tuberculosis and landmines at its facility. Rats are given sputum samples, some of which contain tuberculosis traces and some which don't, the rats indicate that believe they have detected the disease by pausing for longer at a sample, the sample is then marked for further testing and confirmation. The rats are as effective as conventional lab screening of samples but can screen 100 samples in 20 minutes, a workload which would take a lab technician 4 days to complete. In November 2015, the World Health Organization named Tuberculosis as the worlds top infectious disease killer, citing more annual deaths from TB than from HIV. 1 in 3 HIV deaths is related to TB. Tuberculosis is caused by bacteria and spreads from person to person through the air. Despite its reputation as a deadly and highly contagious disease, tuberculosis it is completely curable and preventable if detected. AFP PHOTO/Carl de Souza / AFP / CARL DE SOUZA (Photo credit should read CARL DE SOUZA/AFP/Getty Images)
Lab technicians work with an African giant pouched rat at APOPO's training facility in Morogoro on June 16, 2016. APOPO trains rats to detect both tuberculosis and landmines at its facility. Rats are given sputum samples, some of which contain tuberculosis traces and some which don't, the rats indicate that believe they have detected the disease by pausing for longer at a sample, the sample is then marked for further testing and confirmation. The rats are as effective as conventional lab screening of samples but can screen 100 samples in 20 minutes, a workload which would take a lab technician 4 days to complete. In November 2015, the World Health Organization named Tuberculosis as the worlds top infectious disease killer, citing more annual deaths from TB than from HIV. 1 in 3 HIV deaths is related to TB. Tuberculosis is caused by bacteria and spreads from person to person through the air. Despite its reputation as a deadly and highly contagious disease, tuberculosis it is completely curable and preventable if detected. / AFP / CARL DE SOUZA (Photo credit should read CARL DE SOUZA/AFP/Getty Images)
A photo shows an African giant pouched rat in a cage at APOPO's training facility in Morogoro on June 16, 2016. APOPO trains rats to detect both tuberculosis and landmines at its facility. Rats are given sputum samples, some of which contain tuberculosis traces and some which don't, the rats indicate that believe they have detected the disease by pausing for longer at a sample, the sample is then marked for further testing and confirmation. The rats are as effective as conventional lab screening of samples but can screen 100 samples in 20 minutes, a workload which would take a lab technician 4 days to complete. In November 2015, the World Health Organization named Tuberculosis as the worlds top infectious disease killer, citing more annual deaths from TB than from HIV. 1 in 3 HIV deaths is related to TB. Tuberculosis is caused by bacteria and spreads from person to person through the air. Despite its reputation as a deadly and highly contagious disease, tuberculosis it is completely curable and preventable if detected. AFP PHOTO/Carl de Souza / AFP / CARL DE SOUZA (Photo credit should read CARL DE SOUZA/AFP/Getty Images)
A photo shows African giant pouched rats in cages at APOPO's training facility in Morogoro on June 16, 2016. APOPO trains rats to detect both tuberculosis and landmines at its facility. Rats are given sputum samples, some of which contain tuberculosis traces and some which don't, the rats indicate that believe they have detected the disease by pausing for longer at a sample, the sample is then marked for further testing and confirmation. The rats are as effective as conventional lab screening of samples but can screen 100 samples in 20 minutes, a workload which would take a lab technician 4 days to complete. In November 2015, the World Health Organization named Tuberculosis as the worlds top infectious disease killer, citing more annual deaths from TB than from HIV. 1 in 3 HIV deaths is related to TB. Tuberculosis is caused by bacteria and spreads from person to person through the air. Despite its reputation as a deadly and highly contagious disease, tuberculosis it is completely curable and preventable if detected. AFP PHOTO/Carl de Souza / AFP / CARL DE SOUZA (Photo credit should read CARL DE SOUZA/AFP/Getty Images)
A lab technician handles sputum samples at APOPO's training facility in Morogoro on June 16, 2016. APOPO trains African giant pouched rats to detect both tuberculosis and landmines at its facility. Rats are given sputum samples, some of which contain tuberculosis traces and some which don't, the rats indicate that believe they have detected the disease by pausing for longer at a sample, the sample is then marked for further testing and confirmation. The rats are as effective as conventional lab screening of samples but can screen 100 samples in 20 minutes, a workload which would take a lab technician 4 days to complete. In November 2015, the World Health Organization named Tuberculosis as the worlds top infectious disease killer, citing more annual deaths from TB than from HIV. 1 in 3 HIV deaths is related to TB. Tuberculosis is caused by bacteria and spreads from person to person through the air. Despite its reputation as a deadly and highly contagious disease, tuberculosis it is completely curable and preventable if detected. / AFP / CARL DE SOUZA (Photo credit should read CARL DE SOUZA/AFP/Getty Images)
A lab technician holds an African giant pouched rat at APOPO's training facility in Morogoro on June 16, 2016. APOPO trains rats to detect both tuberculosis and landmines at its facility. Rats are given sputum samples, some of which contain tuberculosis traces and some which don't, the rats indicate that believe they have detected the disease by pausing for longer at a sample, the sample is then marked for further testing and confirmation. The rats are as effective as conventional lab screening of samples but can screen 100 samples in 20 minutes, a workload which would take a lab technician 4 days to complete. In November 2015, the World Health Organization named Tuberculosis as the worlds top infectious disease killer, citing more annual deaths from TB than from HIV. 1 in 3 HIV deaths is related to TB. Tuberculosis is caused by bacteria and spreads from person to person through the air. Despite its reputation as a deadly and highly contagious disease, tuberculosis it is completely curable and preventable if detected. / AFP / CARL DE SOUZA (Photo credit should read CARL DE SOUZA/AFP/Getty Images)
Chimoio, MOZAMBIQUE: Picture taken 24 June 2005 shows, one of the nine African rats, helping Mozambique to sniff out landmines. Twelve years after the end of a brutal civil war, Mozambique is still dealing with a 'critical situation' from landmines in areas where more than one million people live. Apopo, a Belgian de-mining research group train rats and there handlers, saying that the animals have a highly developped sense of smell, are easy to tame and train and cheap and easy to maintian. AFP PHOTO / Alexander JOE (Photo credit should read ALEXANDER JOE/AFP/Getty Images)
TO GO WITH AFP STORY BY ADRIEN BARBIER A lab technician opens access to a human sputum sample for a giant rat to detect whether it is infected by tuberculosis at Apopo research center in Eduardo Mondlane University in Maputo, Mozambique, on February 25, 2015. Giant rats may strike fear and disgust into the hearts of homeowners worldwide, but researchers in impoverished Mozambique are improbably turning some of them into heroes. At Eduardo Mondlane University in the capital Maputo, nine giant rats are busy at work -- sniffing out tuberculosis-causing bacteria from rows of sputum samples. AFP PHOTO/ADRIEN BARBIER (Photo credit should read ADRIEN BARBIER/AFP/Getty Images)
A juvenile African giant pouched rat carries a metal tea egg containing traces of TNT, at APOPO's training facility in Morogoro on June 17, 2016. APOPO trains the rats to detect both tuberculosis and landmines at its facility. Every year landmines kill or maim thousands of people worldwide. The trained rats sniff for explosive and so are able to detect the presence of landmines far faster than conventional methods which involve metal detection. Metal detection is longer and more laborious because detection equipment picks up all metal traces in the ground including scrap metal. APOPO deploy the trained rats to work in mine affected areas like Cambodia, Lao, Vietnam and Mozambique. To date APOPO has helped countries detect 69,269 landmines which have been destroyed by the countries authorities. Land mine clearance is also crucial to farmers and citizens reclaiming and using land which was previously unavailable due to landmine risk. / AFP / CARL DE SOUZA (Photo credit should read CARL DE SOUZA/AFP/Getty Images)
A rat handler carries on his shoulder an African giant pouched rat at APOPO's training facility in Morogoro on June 17, 2016. APOPO trains the rats to sniff for traces of landmine explosives at its facility. Every year landmines kill or maim thousands of people worldwide. The trained rats sniff for explosive and so are able to detect the presence of landmines far faster than conventional methods which involve metal detection. Metal detection is longer and more laborious because detection equipment picks up all metal traces in the ground including scrap metal. APOPO deploy the trained rats to work in mine affected areas like Cambodia, Lao, Vietnam and Mozambique. To date APOPO has helped countries detect 69,269 landmines which have been destroyed by the countries authorities. Land mine clearance is also crucial to farmers and citizens reclaiming and using land which was previously unavailable due to landmine risk. / AFP / CARL DE SOUZA (Photo credit should read CARL DE SOUZA/AFP/Getty Images)
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