One of the largest muroids, it is a brown or grey rodent with a body length of up to 28 cm (11 in) long, and a tail slightly shorter than that.

Thought to have originated in northern China and neighbouring areas, this rodent has now spread to all continents except Antarctica, and is the dominant rat in Europe and much of North America.

However, the English naturalist John Berkenhout, author of the 1769 book Outlines of the Natural History of Great Britain, is most likely responsible for popularizing the misnomer.

[7] Berkenhout gave the brown rat the binomial name Rattus norvegicus, believing it had migrated to England from Norwegian ships in 1728.

[8] By the early to the middle part of the 19th century, British academics believed that the brown rat was not native to Norway, hypothesizing (incorrectly) that it may have come from Ireland, Gibraltar or across the English Channel with William the Conqueror.

[10] The British novelist Charles Dickens acknowledged this in his weekly journal, All the Year Round, writing: It is frequently called, in books and otherwise, the 'Norway rat', and it is said to have been imported into this country in a ship-load of timber from Norway.

Stories of rats attaining sizes as big as cats are exaggerations, or misidentifications of larger rodents, such as the coypu and muskrat.

[21] The brown rat is nocturnal and is a good swimmer, both on the surface and underwater, and has been observed climbing slim round metal poles several feet in order to reach garden bird feeders.

As pups, young rats use different types of ultrasonic cries to elicit and direct maternal search behavior,[24] as well as to regulate their mother's movements in the nest.

[30] Rats may also emit short, high frequency, ultrasonic, socially induced vocalization during rough and tumble play, before receiving morphine, or mating, and when tickled.

In research studies, the chirping is associated with positive emotional feelings, and social bonding occurs with the tickler, resulting in the rats becoming conditioned to seek the tickling.

The most commonly heard in domestic rats is bruxing, or teeth-grinding, which is most usually triggered by happiness, but can also be 'self-comforting' in stressful situations, such as a visit to the vet.

[36] Some colonies along the banks of the Po River in Italy dive for mollusks,[37][38] a practice demonstrating social learning among members of this species.

When lactating, female rats display a 24-hour rhythm of maternal behavior, and will usually spend more time attending to smaller litters than large ones.

Possibly originating from the plains of northern China and Mongolia, the brown rat spread to other parts of the world sometime in the Middle Ages.

[63] The brown rat may have been present in Europe as early as 1553, a conclusion drawn from an illustration and description by Swiss naturalist Conrad Gesner in his book Historiae animalium, published 1551–1558.

[64] Though Gesner's description could apply to the black rat, his mention of a large percentage of albino specimens—not uncommon among wild populations of brown rats—adds credibility to this conclusion.

[65] Reliable reports dating to the 18th century document the presence of the brown rat in Ireland in 1722, England in 1730, France in 1735, Germany in 1750, and Spain in 1800,[65] becoming widespread during the Industrial Revolution.

The position focuses on instituting policies measures to curb the population such as garbage regulation and additional rat trapping.

In addition to sewers, rats are very comfortable living in alleyways and residential buildings, as there is usually a large and continuous food source in those areas.

High rat populations in the UK are often attributed to the mild climate, which allow them higher survival rates during the winter.

It is thought that the first individuals arrived on the southernmost island, Suðuroy, via the wreck of a Norwegian ship that had stranded on the Scottish Isle of Lewis on its way from Trondheim to Dublin.

The Northern islands were invaded by the brown rat more than 100 years later, after Norwegians built and operated a whaling station in the village of Hvannasund on Borðoy from 1898 to 1920.

Although it is a major agricultural area, Alberta is far from any seaport and only a portion of its eastern boundary with Saskatchewan provides a favorable entry route for rats.

Brown rats cannot survive in the wild boreal forest to the north, the Rocky Mountains to the west, nor can they safely cross the semiarid High Plains of Montana to the south.

[89] In the first year of the rat control program, 64 t (71 short tons) of arsenic trioxide were spread throughout 8,000 buildings on farms along the Saskatchewan border.

Before an eradication effort was launched in 2001, the sub-Antarctic Campbell Island had the highest population density of brown rats in the world.

In the United Kingdom, brown rats are an important reservoir for Coxiella burnetii, the bacterium that causes Q fever, with seroprevalence for the bacteria found to be as high as 53% in some wild populations.

[106] They can also be responsible for transmitting Angiostrongylus larvae to humans by eating raw or undercooked snails, slugs, molluscs, crustaceans, water and/or vegetables contaminated with them.

[113] Like mice, these rats are frequently subjects of medical, psychological and other biological experiments, and constitute an important model organism.