July 19, 2024

When Giant Rats Conquered North America

When Giant Rats Conquered North America

In archaeological studies worldwide, a specific set of remains often goes unnoticed: those of rodents. Rats, coexisting (and perishing) alongside humans for millennia, have left behind their small skeletons throughout history. These tiny remnants of the past have received little attention from researchers compared to more captivating discoveries. However, a recent study delves into the intricacies of rat bones discovered at settlement sites and recovered from shipwrecks across eastern North America. It reveals evidence indicating that one highly invasive rat species arrived much earlier than previously believed, swiftly dominating over another to establish itself in U.S. and Canadian cities. Published on April 3 in the journal Science Advances, the new paper represents the collaborative work of biomolecular archaeologists, zooarchaeologists, and other scientists. They examined remains from over 300 rodents discovered at 32 locations along the Eastern and Gulf coasts of the U.S., as well as the Maritime and St. Laurence regions of Canada. These sites, dating from 1559 to the early 1900s, include early ports, settlements, and even seven shipwrecks explored through various methods like damming, dredging, and diving. “Archaeological rats have been understudied,” says Eric Guiry, lead author of the study and a biomolecular archaeologist at Trent University. Due to this lack of research, Guiry mentions that he and his team made several surprising discoveries about the types of rats present throughout different historical periods. This research could significantly enhance our scientific comprehension of one of history’s most formidable pests. “It’s a truly intriguing amalgamation of data,” remarks Jonathan Richardson, an integrative biologist at the University of Richmond who was not involved in the study. He studies urban rats and adds that black and brown rats exhibit distinct behaviors, carry different zoonotic diseases, and have varying impacts on humans. Therefore, understanding the emergence timeline of each species in North America is crucial for comprehending urban ecology and human development. Richardson notes, “It’s both biologically and historically fascinating.”

Rat Competition

The term “rat” encompasses 56 known species, but the black rat (Rattus rattus) and the brown rat or Norway rat (Rattus norvegicus) stand out as the most widespread, originally hailing from different regions of Asia and now invasive across the globe. Through historical records, it has long been established that black rats were the first to arrive in North America, hitching rides on the ships of Columbus and other European colonists to the Caribbean in the 15th Century, from where they spread. Brown rats appeared in the Americas later, though the exact timeline remained unresolved. Guiry mentions that many historical accounts suggest a brown rat arrival date around the time of U.S. independence in 1776. However, the new research indicates that brown rats were present in North America much earlier than previously thought. Dating brown rat remains accurately at archaeological sites poses challenges because these rodents burrow (unlike black rats that climb), potentially allowing more recently living brown rats to infiltrate older sites. Additionally, radiocarbon dating lacks precision for objects less than 300 years old. Nonetheless, the shipwreck data conclusively proves that brown rats were being transported across the Atlantic by 1760 at the latest. Multiple findings from terrestrial sites further suggest that the species established itself in North America as early as 1731.

Dietary Disparities

Brown rats generally exhibit larger sizes and more aggression compared to black rats. In many parts of the world, they tend to displace black rats, although this dynamic is not universal and local ecology plays a crucial role. Richardson notes that in New Zealand, for example, black rats are dominant, illustrating an “ecological mystery” behind these varying outcomes. In North America, Guiry and his team proposed that differences in diet and competition for food might explain why brown rats swiftly claimed territory. Utilizing carbon and nitrogen isotope analysis, they outlined a rough dietary picture of rats across the studied sites, revealing variations by location and species. Rat bones from northern areas showed lower delta-C-13 ratios, indicating a diet rich in corn and other drought-resistant plants. Meanwhile, rat remains from the mid-Atlantic and southern regions contained relatively higher levels of 13C, suggesting a diet heavier in corn or warm-weather plants, aligning with human agricultural practices. Moreover, brown rat bones exhibited higher delta-N-15 ratios compared to black rats across sites, indicating a diet with more animal protein than their smaller counterparts. This protein preference difference likely contributed to the brown rats’ success in overtaking their territory. “It’s possible that the overlap between the two species involved the animal protein in the black rats’ diet,” suggests Guiry. Black rats seemed to consume less meat, eggs, and dairy overall, and when faced with competition from aggressive brown rats for these resources, they likely reduced their intake even further. Guiry explains, “That portion of their diet could have been particularly crucial for reproductive success,” implying that black rats may have produced fewer offspring when resources were scarce. The victory of the brown rats likely resulted from a combination of factors. Competition for nest space, human-induced changes to the environment, and potential inter-species predation may have also played roles, according to Guiry. Further research and sample analyses are necessary for conclusive answers, but fortunately, the archaeological record contains a wealth of rat remains yet to be studied. Guiry and his team continue to unravel the insights these rodent bones hold for us.

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