The common rat species found throughout the Americas are invasive species that originated in Eurasia. Their presence is a direct result of European exploration and subsequent colonization. These rodents are “commensal” species, thriving in close association with people and exploiting human transportation and infrastructure. The story of their arrival involves two distinct species, each with a unique journey across the Atlantic Ocean.
The First Invader: The Black Rat
The first of the Old World rats to establish a foothold in the Americas was Rattus rattus, commonly known as the Black Rat, Ship Rat, or Roof Rat. This species, originally from the Indian subcontinent, was widespread throughout Europe by the time of the Age of Exploration. It is highly likely the Black Rat arrived with Christopher Columbus’s initial voyages to the Caribbean as early as 1492, hitching rides on the early Spanish colonial vessels.
The Black Rat rapidly proliferated in the Caribbean islands and along the coastal port cities of Central and South America. It is a more agile climber than its later counterpart, which allowed it to inhabit the upper decks, rigging, and dry storage areas of ships and coastal buildings. Early settlements across North America, particularly in New England, reported significant rat populations as early as the 1600s. This species prefers warmer climates, explaining its early success in the southern and coastal regions of the continent.
The Later Arrival: The Norway Rat
The second and now most common species to arrive was Rattus norvegicus, known as the Norway Rat, Brown Rat, or Sewer Rat, despite originating in northern China. This larger, stockier rodent arrived significantly later than the Black Rat, primarily associated with later trade routes, likely those of Northern European and British shipping. Historical records traditionally placed its arrival in the United States around the time of the American Revolution (approximately 1775), though recent molecular analysis suggests an earlier establishment date, possibly as early as 1731.
The Norway Rat is a semi-burrowing rodent that excels at living at ground level, often inhabiting sewers, basements, and waterfront areas. It is larger and more aggressive than the Black Rat, often displacing the earlier species in temperate and northern urban environments. This displacement was rapid, occurring in only a few decades in many coastal cities, likely due to the Brown Rat’s superior adaptation to burrowing and its ability to monopolize food resources. The shift from older, wooden structures to brick and tiled buildings also favored the burrowing habits of the Norway Rat over the climbing nature of the Black Rat.
The Mechanism of Transport: Maritime Travel
The successful transatlantic journey of both rat species depended upon the conditions and structure of early sailing vessels. These ships functioned as self-contained, floating ecosystems that provided the resources necessary for the rodents’ survival. Rats would easily board vessels while they were docked, often climbing the mooring lines from the pier or stowing away within loaded cargo.
The wooden hulls of these ships contained numerous gaps between planks, creating secure passageways for the rats. Cargo holds were dark, quiet, and offered abundant nesting material, such as wool bales, cotton bundles, and coiled ropes. Food sources were plentiful, consisting of stored provisions like grain sacks, barrels of flour, livestock feed, and hardtack biscuits. The availability of food, water, and shelter allowed pregnant females to produce multiple litters during the weeks-long ocean crossing, ensuring a viable population was ready to disembark upon arrival.
Establishing a Foothold and Continental Spread
Once the rats disembarked in coastal port cities, their spread inland was facilitated by the developing human transportation infrastructure. The initial coastal settlements provided a new, rich habitat with stored food and ample shelter, allowing the populations to explode. Their subsequent migration across the continent mirrored the expansion of trade and settlement.
Early canals and river traffic acted as primary distribution channels, carrying the rodents further into the interior. Later, the expansion of road networks and the growth of the railroad system provided the perfect “superhighways” for their continental spread. Human activities, such as the establishment of farms, granaries, and growing cities, offered concentrated resources necessary for permanent establishment in new territories. Wherever human settlement expanded, the rats followed closely, exploiting the man-made environments for food and shelter.