How big rats took over North America

At archaeological sites around the world, a certain subset of remains is usually overlooked: rodent bones. Rats have lived (and died) alongside people for thousands of years, leaving small skeletons behind throughout history. Few researchers have examined these diminutive bits of the past, in favor of more charismatic finds. But a new study digs into details of rat bones unearthed at settlement sites and collected from shipwrecks across eastern North America. It uncovers evidence that one hyper-invasive rat species arrived decades earlier than previously thought and rapidly dominated over another to colonize U.S. and Canadian cities.

In the new paper, published April 3 in the journal Science Advances, a team of biomolecular archaeologists, zooarchaeologists, and other scientists analyzed remains from more than 300 rodents previously found at 32 locations along the U.S.’s Eastern and Gulf coasts and the Maritime and St. Laurence regions of Canada. The sites, spanning in age from 1559 to the early 1900s, include early ports and settlements as well as seven shipwrecks explored through damming, dredging, and diving. 

“So little work has been done with archaeological rats,” says lead study author Eric Guiry, a biomolecular archaeologist at Trent University. As a result of this vacuum of information, Guiry says he and his colleagues were able to make several surprising finds about the types of rats present throughout time. The research could better inform our scientific understanding of one of history’s greatest pests.

“It’s a really interesting combination of data,” says Jonathan Richardson, an integrative biologist uninvolved in the new research who studies urban rats at the University of Richmond. Black and brown rats behave differently, carry different zoonotic diseases, and have different impacts on people, he adds, so knowing how and when each species emerged in North America is relevant for understanding urban ecology and human development. “It’s interesting biologically and also historically,” Richardson says.  

Rat fight

The term “rat” encompasses 56 known species, but two are more widespread than any other: the black rat (Rattus rattus) and the brown rat or Norway rat (Rattus norvegicus)—both originally native to different regions of Asia and both now invasive worldwide. Through historical records, it’s long been known that black rats were the first to arrive in North America, stowing away on the ships of Columbus and other European colonists to the Caribbean in the 15th Century, and spreading from there. Brown rats showed up in the Americas later, though exactly how much later has gone unresolved. 

Many historical accounts indicate an arrival date sometime around U.S. independence in 1776, says Guiry. Yet the new research suggests brown rats were in North America much sooner than that. It can be difficult to accurately date brown rat remains at archaeological sites because the rodents burrow (in contrast, black rats climb), and so more recently living brown rats can end up infiltrating older sites. Plus, radiocarbon dating isn’t particularly precise for things less than 300 years old. But the shipwreck data provides clear proof that brown rats were being carried across the Atlantic by 1760 at the latest. Numerous findings from the terrestrial sites further suggest the species established in North America as early as 1731.

Once here, brown rats rapidly took over black rats’ turf, dominating in just a few decades, per the study. 

To distinguish between the historical remains of black and brown rats, the researchers used a molecular analysis protocol called ZooMS that identifies different species based on the amino acid makeup of collagen proteins inside bone. They found that, by the mid-1700s, black rats went from the sole or dominant species in site samples to rare compared to their brown counterparts. Only five black rat specimens were identified from samples after 1760, and just two samples out of 108 showed black rats occuring after 1800. Meanwhile, brown rat samples proliferated over the same time period. The findings provide firm scientific support for anecdotal evidence brown rats had become dominant, outcompeting black rats in most early North American coastal cities by the 1800s. Today, brown rats account for the vast majority of rats in North American cities, with few exceptions

Dietary differences

Brown rats are generally larger and more aggressive than black rats. In many parts of the world, they displace black rats. Though this doesn’t hold true everywhere, and local ecology plays a role. In New Zealand, for instance, black rats reign supreme, says Richardson. What accounts for the different outcomes in different locations is an “ecological mystery”, he adds. 

In North America, Guiry and his colleagues hypothesized that differences in diet and competition for food could be part of why brown rats so quickly won the territory battle. Using carbon and nitrogen isotope analysis, they sketched a rough picture of what rats across their study sites were eating, and found variation by location and species. 

Farther north, the rat bones had lower delta-C-13 ratios, an isotope signature associated with consuming corn and other plants evolved to resist drought. In mid-Atlantic and farther south, rat remains contained relatively higher amounts of ¹³C on average, indicating a diet heavier in corn or warm-weather plants and following human agricultural trends.

Between the species, brown rat bones had a higher delta-N-15 ratio than black rats across sites, suggesting the brown rats were eating more animal protein than their smaller competitors. This difference in protein preference could be part of the reason why black rats failed to hold their ground. 

“It’s possible that where there was overlap between the two [species], it involved the animal protein in the black rats’ diet,” says Guiry. Black rats seemed to eat less meat, eggs, and dairy overall, and when faced with fighting aggressive brown rats for those resources, they likely ate even less, he explains. “That portion of their diet could have been particularly important for reproductive success,” he adds. Maybe, where they had to fight for their protein, black rats were simply able to produce fewer offspring. 

Likely, the brown rats’ victory was the result of a combination of factors. Competition over nest space, human changes to the landscape, and even inter-species predation may have also played a role, suggests Guiry. More research and more sample analysis is needed to know for sure, he says. But thankfully for science, the archaeological record is full of still un-studied rat remains. Guiry and his colleagues are continuing to untangle what this cache of rodent bones can tell us.

Rats past and present

“Archaeology represents a big trove of potential information for ecology, and information that has potential relevance to people today,” says Guiry. “It’s more than just understanding what people did in the past, it [informs the present].”

Richardson agrees. He studies the gene flow of the rats that live alongside humans in cities currently, to track how the pests move between places. The new archaeological work provides clearer context for parsing some of the patterns he’s observed in his work. “We really need that [historical] baseline to be able to understand what’s happening today,” he says. 

The story of rats is also the story of human civilization. From food security to disease risks, rodents play a big role in history and the modern day. “We’re never going to completely exterminate rats. They’re always going to be in and outside of cities,” says Richardson, “so we better do everything we can to understand them.”