Rats have long coexisted with human populations, adapting to various environments. Their presence is often associated with urban settings, infrastructure, and the movement of goods. This close association has shaped perceptions of these rodents.
Non-Native Status of Common Rats
The common rat species encountered in North America, primarily the Norway rat (Rattus norvegicus) and the black rat (Rattus rattus), are not indigenous to the continent. Both species originated in Asia, with the Norway rat believed to have come from northern China or Mongolia, and the black rat from the Indian subcontinent or Southeast Asia. These “Old World” rats spread globally by utilizing human transportation and settlements.
The Norway rat, also known as the brown rat or sewer rat, is robust with a blunt snout, small ears, and a tail shorter than its body. Its fur is brownish-grey. In contrast, the black rat, also called the roof rat or ship rat, has a more slender body, a pointed snout, large prominent ears, and a tail longer than its body. Black rats have black or dark brown fur with a lighter underside.
These species have adapted to live commensally with humans, benefiting from human environments, particularly in urban and agricultural areas. Their ability to consume varied foods and reproduce rapidly has allowed them to colonize most continents. They are highly adaptable, making homes in diverse habitats, from sewers and buildings to forests and open fields.
Historical Arrival in North America
The arrival of these non-native rat species in North America is directly linked to European colonization and transatlantic shipping. Black rats (Rattus rattus) were likely the first to reach the continent, arriving with early European explorers and settlers in the 16th century. They were stowaways on Spanish ships and arrived in what is now the United States with the Jamestown colonists in the early 1600s.
These early introductions occurred as rats found refuge and sustenance aboard sailing vessels, ideal for transport across oceans. The black rat’s proficiency as a climber made it a common inhabitant of ship rigging and upper decks, earning it the name “ship rat.” Once ashore, they spread rapidly, exploiting new human settlements and their associated food sources.
Norway rats (Rattus norvegicus) arrived later, establishing a presence in North America around the mid-18th century. They came via grain shipments brought by Hessian troops during the American Revolution. The Norway rat, being larger and more aggressive, often displaced the black rat in many northern and inland areas, though black rats persisted in warmer climates and coastal regions.
Native Rodents Often Confused with Rats
While the common “rats” found in North American cities are introduced species, the continent is home to numerous native rodent species, sometimes called rats. These include various species of woodrats or packrats, belonging to the genus Neotoma. Unlike the introduced Rattus species, woodrats are integral to North American ecosystems, inhabiting diverse natural environments.
Woodrats differ from Norway and black rats in appearance and behavior. They have softer fur, larger ears, and a bicolored, hairy tail, unlike the scaly, nearly hairless tails of Old World rats. Their habits also differ; woodrats are known for their distinctive “middens,” large nests constructed from sticks, leaves, and collected objects, often found in rocky outcrops, trees, or abandoned buildings.
These native rodents primarily consume plant material like leaves, stems, fruits, and seeds. Their ecological roles include seed dispersal and a food source for predators. They are less dependent on human habitation for survival compared to the introduced Rattus species. Understanding these distinctions clarifies that North America has its own diverse array of native rodents, even if the most commonly encountered “rats” are not among them.