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'Throughline': How rats became one of Earth's most successful mammalian colonizers

LEILA FADEL, HOST:

Aristotle once theorized that nature abhors a vacuum. And as many of us retreated indoors during the pandemic, rats were great at filling that vacuum, especially in cities. Lawrence Wu is a New York City-based producer on NPR's history podcast, Throughline. And in this report, he looks at how rats became one of the planet's most successful mammalian colonizers.

JASON MUNSHI-SOUTH: You know, you kind of see the landscape, and you're looking for signs of rats everywhere. Are there burrows there? Is there a rat feeding in that corner? So yeah, they're just, to me, like, part of the city.

LAWRENCE WU, BYLINE: This is Dr. Jason Munshi-South. He's a professor of biology at Fordham University in the Bronx, where he leads his own research lab.

MUNSHI-SOUTH: And since about 2008, when I moved to New York City, I've been studying the effects of urbanization on wild animals and also pest species like rats.

WU: Jason's lab focuses on understanding how humans and cities affect wild animal populations in those places. So I call him up to get a little more insight into what is up with New York City's rats.

MUNSHI-SOUTH: They're primarily nocturnal. They live in burrows. So they'll burrow into soil and spend, you know, most of the day down there. And they build these colonies...

(SOUNDBITE OF RATS SQUEAKING)

MUNSHI-SOUTH: ...Almost like villages of related rats. They're highly social. They spend a lot of time with other rats. They have to be somewhere near water sources. And they are, you know, territorial to some degree. But over time, they'll add more tunnels, and they'll start to connect. They'll sort of overlap with neighboring burrows. And so it becomes this big tangle.

WU: Like a subway but for rats.

MUNSHI-SOUTH: So I've seen them, you know, in, like, New York City parks where there wasn't a lot of control going on, where you could count, like, 300 holes. And you could just watch them coming in and out all day.

WU: Seeing all those rats coming in and out of those rat holes sparked a question.

MUNSHI-SOUTH: What's going on with rats in New York City? How did these animals get here?

WU: And Jason decided to build a whole study around it. The first thing he discovered was that New York City is actually overrun by just one kind of rat - the brown rat.

MUNSHI-SOUTH: Their Latin name is Rattus norvegicus, which would translate to the Norway rat. But that's a misnomer. They did not originate in Norway. We don't exactly know why they have that name.

WU: Jason and his team decided that in order to find the actual origin of the New York City rat, they had to compare its DNA to other rats in the world to find a match, kind of like an ancestry.com or 23andMe but for brown rats. So he and his team started calling and asking labs around the world to send them DNA samples of their brown rats.

MUNSHI-SOUTH: We ended up with, you know, like, 500 samples all around the world. And we just decided, OK, let's do this properly and try to understand what major groups of rats exist everywhere and use that as context to understand what rats are in New York City.

WU: And what they found was that all the signs were pointing to a place thousands of miles east of Norway.

(SOUNDBITE OF FOREST AMBIENCE)

MUNSHI-SOUTH: As far as we know, they originated in East Asia.

WU: Likely in a region between northern China and Mongolia a couple million years ago.

MUNSHI-SOUTH: It's likely that originally they were living along, like, streams, sort of grassy savannah areas where there was water sources. And they were probably eating all sorts of things - seeds, fruits, insects, you know, snails. They've even been found in coastal areas to eat, like, mussels and things.

WU: And for a long time, the brown rat kind of did its own thing. So the question is, how and when did our paths get so intertwined?

MUNSHI-SOUTH: When did they become commensal with humans? Commensal is this Latin term that basically means eating from the same table. And they probably began utilizing human foods when agriculture began in China. And that was, you know, 11,000 years ago. It seems like brown rats kind of stayed for a while, and they didn't really spread out for a long time, for hundreds of years, maybe thousands of years. And then boom, something happens. Our date suggests less than a thousand years ago they got to Southeast Asia. By the time they got there, you know, humans had more advanced ships, and you were starting to see, like, regional trade through the Indian Ocean and even up into Europe. Cities start building up. Human populations are expanding. And you see the brown rat just getting everywhere.

(SOUNDBITE OF MUSIC)

WU: But the great brown rat migration didn't end there. In fact, in order for the brown rat to take over the world, they needed to hitch a ride with humans looking to take over the world. Rats would hang around ports, waiting to board ships that were stocked with all kinds of foods perfect for rats to feast on. And it turned out in the 17th and 18th centuries, there were a lot of ships moving around the globe. It was the age of conquest.

MUNSHI-SOUTH: The British Empire, the Dutch, the Spanish, the French - they were all moving rats all over the place in North and South America, in Africa, in New Zealand, in Australia.

WU: And at some point, one of those ships crossed the Atlantic and made its way to the United States.

MUNSHI-SOUTH: And once they were in all those ports, they just moved inland across continents. And so they hitched a ride with humans. And, you know, we can look at their history as kind of a proxy for human history 'cause humans moved them around around the world.

WU: Rats have been our companions for a long, long time. And you know what they say about couples. The longer you're together, the more you start to resemble one another.

MUNSHI-SOUTH: They're very intelligent. They're very adaptable, just like humans. You know, they moved around with us because they can live in lots of different places and figure out how to survive. They also - you know, they're not lone wolves. They're like humans. They're very social. And part of their survival is going to be because they live in these groups.

FADEL: That was associate professor Jason Munshi-South of Fordham University, speaking with Lawrence Wu of NPR's history podcast, Throughline. Transcript provided by NPR, Copyright NPR.