How rats took over North America | Trending Viral hub

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How rats took over North America

Rat remains from shipwrecks and excavation sites show how two rodent species clashed in eastern North America.

Juvenile brown rat (Rattus norvegicus) emerging from drain on pavement in city street

De Meester Johan/Arterra Picture Library/Alamy Stock Photo

He black death It first spread across Europe in the mid-15th century and killed tens of millions of people. But it didn’t stop there: the bacteria that causes the plague Yersinia pestis hidden inside the bodies of black rats (Rattus rattus) and continued to cause smaller outbreaks when fleas transmitted it from rats to humans for hundreds of years afterward. Then, in the mid-18th century, these outbreaks largely ceased. The moment coincides with the introduction of brown rats (norwegian rat), which spread from Asia and overtook black rats almost as soon as they set their little clawed paws on the European continent.

This brown rat takeover occurred not only in Europe but also in America, when these pests stowed away on ships crossing the Atlantic Ocean. Today, the brown rat is most prevalent in eastern North America, but it is also beginning to gain a foothold in the western part of the continent.

“Brown rats show up and it’s like a ghost town for the black rats,” says Eric Guiry, a molecular zooarchaeologist at the University of Leicester in England. But although the two rodent species are ubiquitous pests and critical disease vectors, scientists know very little about how they took over North America. To piece together this story, Guiry and his colleagues performed molecular analyzes on 311 samples of rat bones from archaeological sites in eastern and southeastern North America dating from the 1550s to the early 1900s. Their findings, published Nov. April in Scientific advances, show fundamental differences between the two species and provide clues as to how one species came to dominate the other.


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Until now, reconstructing a timeline of rats has been difficult. From historical accounts we know that brown rats arrived from Europe in 1776, around the time of American independence. But although archaeologists have unearthed large quantities of rat bones at various dig sites dating back even further, they cannot say with certainty when those rats lived. Radiocarbon dating is too imprecise to be useful in this period of history, Guiry explains. And brown rats have the uncomfortable habit of burrowing in the ground, so their presence at an archaeological site could be due to subsequent contamination.

To solve this problem, the researchers collected many samples of shipwrecked rats. “If you find a black rat on a shipwreck, you know it has to date from that period,” Guiry says. This is “a pretty innovative approach,” says Johannes Krause, an archaeogeneticist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, who has studied the history of the brown rat’s introduction to Europe and has encountered similar problems by brown burrowing rats. in his own work.

Some of the sunken samples came from the remains of the The beauty, a ship that ran aground off the coast of Texas in 1686. Among the remains, which were rediscovered in 1995, archaeologists found three bronze cannons, pottery and jewelry, a human skeleton and “a really large amount of rat remains,” says the study. Co-author Susan deFrance, a zooarchaeologist at the University of Florida who studied the The beauty shipwreck in the 1990s. Samples unearthed from seven ships that sank between 1559 and 1760 allowed researchers to determine that brown rats first arrived on the continent earlier in the 1700s than historical records indicated.

Next, to understand how these rat species spread and contended on land, researchers analyzed remains from coastal sites from Louisiana to Nova Scotia, including the 17th-century Jamestown Colony and a site in New Orleans that may have inspired the popular tune “House of the Rising Sun.” First, the researchers identified brown and black rats using a relatively new technique called collagen peptide mass fingerprinting that tracks small differences in the structure of collagen in the bones of different species. This technique is less expensive and simpler than DNA analysis, says deFrance: “If ancient DNA (analysis) had been done for all this, the cost would have been simply astronomical.”

Then, to determine the rats’ eating habits over the centuries, researchers looked at the varieties of carbon and nitrogen found in their bones. There are more likely to be different isotopes of these elements in an animal’s body depending on its position in the food web. While the information scientists can glean here is limited, the researchers were able to conclude that brown rats and black rats had “fundamental differences” in their dietary preferences: Brown rats ate more animal protein than black rats.

Their different feeding habits mean that the two species were probably not competing for the same ecological niche. “Yet the (black rat) continues to disappear,” Krause says. “That was the biggest surprise” about the molecular analysis results, she says.

Why the brown rat “smashed” the black rat “remains a completely open question,” Guiry says. One leading theory is that the brown rat’s aggressive nature and larger size helped it outcompete the black rat. But this cannot explain everything: there are places in the world where the black rat dominates, says Guiry. He would love to see a similar study done in a single city, which could give scientists more granular information about how this transition occurred, at least in one place.

And there is no shortage of rat bones for that to happen, says Guiry. “I was able to find most of these rats through rumors,” he says, adding that there are countless more samples around the world. He is currently developing a similar analysis of sites in Europe to trace how these populations arrived from Asia. “I think there is much more to find here. In reality, this is just a sample of what happens.”

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