Rattus norvegicus
brown rat
(Also: Norway rat)

Norway rats (Rattus norvegicus) are originally native to northern China. Following a series of introductions, the species had found its way to Eastern Europe by the early eighteenth century. By the year 1800, they occurred in every European country. Records show the first sighting of R. norvegicus in the New World occur in the 1770's as ship stowaways. Today, Norway rats (also known as brown rats) can be found on every continent of the world except Antarctica. (Nowak and Paradiso, 1983; Silver, 1927)

In Asia, R. norvegicus was native to forests and brushy areas. Today, however, Norway rats find preferred habitat to be alongside the rapid expansion of the human population. Nearly every port city in the world has a substantial population of these rodents. They occupy a variety of habitats including garbage dumps, sewers, open fields and woodlands, basements, and nearly anywhere else that food and shelter might be found. Anywhere that humans are located, R. norvegicus will most likely follow. (Hamilton, 1998; Nowak and Paradiso, 1983; Parker, 1990)

Rattus norvegicus is a rather large member of the mouse family. On average, these rats reach nearly 400 mm nose-to-tail, and weigh 140 to 500 g. Males are usually larger than females. In natural populations, these rats are covered with coarse, brownish fur (sometimes splotched with black or white hairs) on their dorsal surface, which usually lightens to a gray or tan color nearing the underside. Various strains of these rats bred in captivity may be white, brown, or black. The ears and tail are bald. The length of the tail is shorter than the length of the body. Molars are lophodont and the dentary is 1/1-0/0-0/0-3/3. The ears of Norway rats are typically shorter than those of related species, and do not cover up the eyes when pulled down. Norway rats can be easily mistaken for black rats, however, the temporal ridges of the Norway rat are straight, whereas those of the black rat are curved. (Avalos and Callahan, 2001; Calhoun, 1962; Nowak and Paradiso, 1983; Parker, 1990)

The maximum lifespan of R. norvegicus is 4 years (in captivity). In the wild, it is assumed that they live for upwards of 2 years. (Nowak and Paradiso, 1983; Parker, 1990)

Mostly nocturnal or active at dusk, Norway rats go about digging burrows, foraging for food, and preparing nests during these hours. Often, these rats take up residence in areas near water. They are excellent swimmers, and are often referred to as "water rats." (Barnett, 1963; Calhoun, 1962; Hamilton, 1998; Nowak and Paradiso, 1983; Parker, 1990)

Foraging behaviors can take the rats on long nightly excursions to areas known to be rich in food resources via learned routes. There is much learning capacity in Norway rats. They are able to remember their way around complex sewer and burrow networks. Their ability to learn has been thoroughly studied by psychologists. (Barnett, 1963; Calhoun, 1962; Parker, 1990)

New packs are started when a couple establishes a nest in a previously unoccupied area. Typically, R. norvegicus live in large, male-dominated groups. The heirarchy of such groups is based on the size of an individual. Nests are constructed of any efficient foraged materials including leaves, garbage, twigs, etc. Burrows are usually complex; consisting of food storage, nesting and "last ditch" chambers. (Barnett, 1963; Calhoun, 1962; Hamilton, 1998; Parker, 1990)

Home Range

Territories can be 50 meters in diameter. (Nowak and Paradiso, 1983; Parker, 1990)

Like most mammals, Norway rats use a variety of communication avenues. They are vocal, and also use visual cues such as body postures when communicating. Norway rats have relatively good hearing and tactile capabilities. They are able to sense very minute vibrations in the ground, and feel their way through total darkness with their paws and whiskers. However, these rats would not have such an advantage over all other foragers if it were not for their spectacular sense of smell. Scent is the Norway rats' best sensory channel, and it is used to find food and distinguish between individuals of a group. (Parker, 1990)

Norway rats are excellent foragers. Using their sense of smell and touch, they are able to survive quite easily given that there is a steady supply of any type of food. In metropolitan areas, they survive mainly on discarded human food, and anything else that can be eaten without negative consequences. Some Norway rats living near the sea have been observed catching fish with their paws. Also preyed upon by Norway rats are chicks, mice, birds, and small lizards. They have even been known to attack infant human beings.

Examination of a wild R. norvegicus stomach in Germany revealed 4000 items, most of which were plants, although studies have shown that Norway rats prefer meat when given the option. (Nowak and Paradiso, 1983; Parker, 1990)

Rattus norvegicus is preyed on by any number of carnivorous mammals, birds, and reptiles. Humans also kill very large numbers of Norway rats as pests. (Hamilton, 1998; Nowak and Paradiso, 1983; Silver, 1927)

Norway rats are excellent competitors and will readily drive out competing rat species, such as Rattus rattus. Because of their foraging habits, Norway rats act as seed dispersers. Their burrows also tend to aerate the soil. As prey, they help to sustain predator populations. Norway rats are commensal species with humans. (Hamilton, 1998; Nowak and Paradiso, 1983; Parker, 1990; Silver, 1927)

Some consider Norway rats to be the greatest mammal pest of all time. They have caused more deaths than all the wars in history. Rat-borne diseases are thought to have killed more people in the last 1000 years than all of the wars and revolutions ever fought. They harbor lice and fleas that carry bubonic plague, typhus, trichinosus, tularemia, infectious jaundice, and many other serious diseases. These rats also cause considerable damage to property including crops, destroying and pollution of human food storage, and damage to insides and outsides of buildings. It is estimated that rats cause almost 1 billion dollars in damage in the United States each year. Rats kill poultry, domestic livestock, and game birds and are responsible for the endangerment or extinction of many species of wildlife, especially those found on islands. (Nowak and Paradiso, 1983; Silver, 1927)

Norway rats have been widely used in medical and genetic research. This research has led to important advances in physiology, genetics, immunology, pathology, and epidemiology. They are also popular pets and have been important in research on behavior because of their ability to learn quickly and because it is easy to keep them in laboratory settings. (Barnett, 1963; Calhoun, 1962)

These animals are not a conservation concern. In fact, humans spend a great deal of effort trying to eradicate them.