Brown rats used shipping “superhighways” to conquer North American cities, study says

In new York, they forced the city to hire a “rat czar.” In Chicago, they caused the deployment of wild cats. In New Orleansthey literally eat police evidence.

Today, researchers are trying to understand why Norway rats are the undisputed winners of the real rat race.

The new study suggests they crawled out of ships arriving in North America earlier than previously thought and outcompeted their rodent rivals – something that infuriated and disgusted generations of city dwellers and became so ubiquitous that they are known as common rats, street rats. or sewer rats.

It didn’t take long for them to ward off the black rats that had likely arrived with Columbus and were thriving in colonial cities.

“Rodent Rivals”

After first appearing on the continent before 1740, Norway rats succeeded black rats on the East Coast “within just a few decades,” said Michael Buckley, one of the authors of a study published Wednesday in the journal Science Advances .

Norway rats are larger and more aggressive than black rats — and they want to be close to human populations, said Matthew Frye, a researcher and community educator with the New York State Integrated Pest Management Program at Cornell University.

Through this research, “we know the more exact time they arrived and what they were doing once they got here,” said Frye, who was not involved in the study. “Having this picture of the rat population helps us better understand what they are doing and perhaps how we can manage them.”

Neither rat species is native to North America, said Buckley, of the University of Manchester in the United Kingdom. Scientists thought Norway rats arrived around 1776. The new study pushes that date back more than 35 years.

Buckley and his colleagues analyzed rodent bones previously excavated by archaeologists. The remains came from 32 colonies in eastern North America and the Gulf of Mexico, dating from the founding of Jamestown in 1607 to the early 1900s. Other samples came from seven wrecks dating from around 1550 to 1770.

The bones analyzed came from New Orleans, Charleston, the Chesapeake Bay (Virginia and Maryland), Quebec, the Canadian Maritimes and Newfoundland.

“Rat highways”

The data suggests that shipping networks across the Atlantic Ocean “essentially functioned like rat superhighways,” with brown rats gaining a foothold in early coastal shipping centers, said Ryan Kennedy, an author of the study at the Indiana University which studies animal remains at archaeological sites.

One likely reason for their dominance, researchers say, is that they were eating food that black rats would have otherwise consumed – which may have reduced reproduction among black rats. Historical anecdotes support this discovery, describing the virtual disappearance of black rats from cities in the 1830s.

Today, both types of rats exist in North American cities, although Norway rats are more common. Certain urban centers are particularly invaded. New York City, for example, last year hired a “rat czar” to tackle a growing problem there. City Mayor Eric Adams inherited a city that has seen a 71% increase in rat sightings since 2020, according to a city council member.

In New Orleans, officials say the police evidence room is being invaded by rodents. “The rats are eating our marijuana. They’re all high,” NOPD Chief Anne Kirkpatrick testified at a meeting of the city’s criminal justice committee last month.

The biggest problem ? Rats can carry diseases. Norway rats are known to spread a bacterial disease called leptospirosis, which is caused by bacteria found in the urine of infected animals. They can also help spread murine typhus and foodborne germs like salmonella.

Experts said knowing which type of rat is leading the pack helps cities control pests — even if it sometimes doesn’t seem like it.

For example, Norway rats like to hang out on or near the ground rather than in trees or other high places, where roof rats often prefer to stay.

Both black and brown rats are omnivores, but Norway rats are particularly fond of animal products – meaning that reducing those present in food waste “should have the greatest chance of reducing the value of urban habitats for rat populations,” Buckley said.

According to the study, “limiting the Norway rat’s access to animal protein sources is expected to have the greatest impact on restricting this species’ preferred niche.”

Frye said any efforts to reduce available food waste are helpful.

“Availability of food is the main reason Norway rats are present,” he said. “Any effort to prevent rats from accessing food sources is an effective measure.”