Where Did Brown Rats Come From? How They Spread Worldwide

The brown rat’s original home

The brown rat (Rattus norvegicus) evolved in northern Asia. Genetic studies suggest that its ancestors split from the lineage of the black rat (Rattus rattus) roughly 3 million years ago, and that its closest living relatives include rats of the Himalayan region.

A large genetic study published in 2016 by Emily Puckett and colleagues compared brown rats from around the world. It found that the species most likely originated in northern China and Mongolia, where wild brown rats still live in fields, scrub and along rivers, often far from people. From there, rats appear to have spread first into Southeast Asia and later westward toward the Middle East and Europe.

At some point, probably thousands of years ago, brown rats began living around farms, villages and grain stores. This close association with people, called commensalism, is what allowed them to travel the world.

How did brown rats reach Europe?

The exact route and date are still uncertain. The Swiss naturalist Conrad Gessner described a large rat in the 1550s that some historians think may have been a brown rat, but the evidence is weak. Reliable records begin in the 18th century:

Rats probably arrived by several routes at once, carried overland with trade goods from Russia and the east and by sea in ships’ holds. An often-repeated story by the German naturalist Peter Simon Pallas tells of huge numbers of rats swimming across the Volga River near Astrakhan in 1727, but historians treat the details with caution. By 1800 the brown rat lived in nearly every country in Europe.

Why is it called the Norway rat?

The name is a historical mistake. In 1769 the English naturalist John Berkenhout gave the brown rat the scientific name Mus norvegicus, meaning “Norwegian mouse”, because he believed it had come to England on Norwegian timber ships around 1728. The name stuck when the species was later moved to the genus Rattus.

In fact, brown rats were common in England before they were known in Norway. Other old names reflect politics more than biology: some 18th-century English opponents of the royal House of Hanover called it the “Hanover rat”, blaming the new German-born kings for the pest. Today it is also known as the common rat, sewer rat and wharf rat.

Spreading around the world

Once established in European ports, brown rats spread on ships to almost every coast. They reached North America in the mid-1700s, probably from Britain and other parts of Western Europe, and arrived in Australia, New Zealand, South America and Africa with European ships during the late 18th and 19th centuries. Some rats in western North America and on Pacific islands appear to descend partly from Asian populations, carried by later shipping.

Islands suffered badly. In the Aleutian Islands of Alaska, brown rats reached Hawadax Island, long called Rat Island, after a Japanese shipwreck in the 1780s, and wiped out many breeding seabirds. Rats spread to New Zealand before 1800 and to sub-Antarctic islands such as Campbell Island and South Georgia, where they became serious threats to native birds until large eradication projects removed them.

Brown rats vs black rats

Europe already had rats before the brown rat arrived. The black rat, which originated in southern Asia, reached Europe by Roman times and was the rat of medieval towns. The newcomer gradually replaced it across most of temperate Europe and North America.

Brown rats are larger and more aggressive, dig burrows and cope better with cold. As towns switched from wooden houses and thatched roofs to brick, stone and underground sewers, conditions suited them better than the climbing black rat. In many warm regions and port cities, however, black rats are still common, and the two species can live side by side, with black rats in roofs and brown rats in cellars and sewers.

Did brown rats cause the plague?

No. The Black Death of 1347–1351 swept Europe centuries before brown rats arrived. Historians have linked medieval plague to the black rat and its fleas, although human fleas and lice may also have played a large role. An old idea that brown rats drove out plague by replacing black rats is no longer accepted, because the timing of the two events does not match.

Where are brown rats today?

Brown rats now live on every continent except Antarctica and are the dominant rat in Europe and much of North America. A few large areas remain rat-free, including the Canadian province of Alberta, which has kept brown rats from establishing since the 1950s. For more on how rats survive in cities and how long they live there, read how long do rats live?