The Norway rat (Rattus norvegicus) is a highly adaptable rodent species. Often referred to as the brown rat, common rat, or sewer rat, it has become a familiar presence in diverse environments worldwide. Despite its common name, it does not originate from Norway. Its widespread distribution today is a testament to its resilience and its close association with human activity.
Unraveling Its Origins
The scientific understanding of the Norway rat’s true geographical origins points primarily to Central Asia or Northern China. Genetic studies and archaeological findings suggest its ancestral home lies in the plains of northern China and Mongolia, where wild rats continue to live in burrows. This species diverged from its closest relative, the Himalayan field rat (Rattus nitidus), around 620,000 to 644,000 years ago. Some of the oldest archaeological evidence of Rattus norvegicus has been found in the Sichuan-Guizhou province of China, dating back to the early Neolithic period, coinciding with the development of agriculture.
The genus Rattus itself emerged within the Muridae family approximately 3.5 to 6 million years ago, with significant speciation events occurring much later. The ancestors of the Norway rat and the black rat (Rattus rattus) diverged from each other about 2 million years ago.
The Global Spread and Key Arrivals
The Norway rat’s global dispersal was not a single event but a gradual process largely facilitated by human activities, particularly maritime trade and colonization. While earlier theories suggested its arrival in Europe in the 18th century, some archaeological evidence indicates small populations may have been present as early as the 14th century, though widespread presence is more reliably documented later. The species is thought to have arrived in Europe in the 16th or 17th century, significantly later than the black rat. By the early 18th century, it was establishing itself across Eastern Europe, with reliable accounts placing it in Ireland by 1722, England by 1730, and Germany by 1750.
The Norway rat made its way to North America between 1750 and 1775, arriving on ships carrying European settlers. Once introduced, it spread throughout the contiguous 48 states. This westward migration often saw the Norway rat displacing the black rat, which had arrived in the New World earlier. The Norway rat can be found on every continent except Antarctica.
Why It Thrived in New Territories
The Norway rat’s remarkable success in establishing and proliferating in new environments stems from a combination of biological and behavioral traits. It possesses an omnivorous diet, allowing it to consume nearly any type of food available, from cereal grains and meats to discarded human waste. This dietary flexibility enables it to thrive in diverse settings, from urban centers to agricultural areas.
A high reproductive rate further contributes to its rapid population growth. Female Norway rats can reach sexual maturity in as little as 3 months and can produce an average of 4 to 6 litters per year, with each litter typically consisting of 6 to 12 young. The gestation period is short, only 21 to 23 days, and females can mate again within a day of giving birth, allowing for continuous breeding throughout the year in suitable conditions.
The Norway rat’s adaptability to various climates and habitats, including its proficiency at burrowing, also plays a significant role in its success. While they can climb, they tend to inhabit lower floors of buildings and excel at creating extensive burrow systems under structures, along stream banks, and in garbage dumps. This species has developed a strong commensal relationship with humans, preferring to live in close proximity to human settlements where food and shelter are readily available.