What do they look like?

On average, Norway rats have lengths from nose to tip of tail of about 399 mm while the tail itself is 187 mm. Norway rats have coarse-fur, large furless ears, and a scaly tail. Norway rats tend to be brown or dark gray with scattered black hairs, while their bellies are pale gray or grayish brown.

Where in the world do they live?

Although Norway rats were originally from northern China, they can now be found in nearly every part of the world as a human commensal. They were native only to the Palearctic region, but have been introduced to all other biogeographic regions except Antarctica. They are most commonly found in urban areas.

What kind of habitat do they need?

Norway rats probably originally lived in forested habitats. They are now found almost exclusively in areas near human settlements. They live wherever there is an abundance of food and shelter, from the subways and crowded buildings of cities, to the corn and grain fields of farm country, to the salt marshes of the Atlantic Coast.

How do they reproduce?

If there is enough food and shelter Norway rats will breed throughout the year, although more births occur in spring and autumn. Females can have between 1 and 12 litters each year, though they usually have 6 to 8 litters each year. Females are pregnant for 21 to 26 days. Each litter usually contains 7 babies but some litters can have between 2 and 22 young! The young are born blind, naked, and helpless at birth; the eyes open in 14-17 days, and the young are weaned when 3 weeks old. Sexual maturity is reached at 3 months, although males must wait longer until they are able to challenge the dominant male.

Young are cared for and nursed in their mother's nest until they are weaned. Soon after that they leave the nest and establish their own territory or home range.

How long do they live?

In captivity Norway rats may live to be 4 years old. Their average lifespan is probably about 2 years.

How do they behave?

Norway rats are active year-round and are mostly nocturnal, only leaving their homes as night approaches. Norway rats are shy and will hide or run away if a human approaches. Norway rats generally travel on the ground, but they are also good swimmers, jumpers, and climbers. Norway rats are usually found living together in large groups with the biggest male as the leader. As the leader of the group the male gets first pick of food, water, and shelter.

What do they eat?

Norway rats are true omnivores. Norway rats eat a huge variety of foods including everything from soap to candy, milk, meat, vegetables, poultry, eggs and all grains, nuts and fruits. Norway rats are also very big eaters. They can eat a third of their weight in a day. Norway rats are capable of catching fish and small rodents, such as mice, and they readily eat dead animals. In cities, Norway rats enjoy human food that is uneaten or spoiled.

What eats them and how do they avoid being eaten?

Norway rats typically construct burrows or build their nests in protected places, such as sewers and basements. They are active at night and they swim and dive well. All of these factors contribute to avoiding predators.

What roles do they have in the ecosystem?

Where they are abundant, Norway rats are an important component of the ecosystem because they consume foods that would otherwise be available to other animals. The typical ecosystem of Norway rats, currently, is an urban one so their presence most directly influences humans. In these urban environments there are few predators to take advantage of the large numbers of rats available as prey.

Do they cause problems?

Most people think that Norway rats cause more harm than good. Many consider these 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 and have been the source of 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. There is also some evidence of rats attacking helpless human beings, especially infants.

How do they interact with us?

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.

Are they endangered?

Humans are not looking to conserve Norway rats; rather, they would like to reduce their numbers. Humans often try to control the numbers of rats by trapping and poisoning.

Some more information...

Norway rats were introduced to the Great Lakes region a long time ago. These rats are not originally from Norway, as its name would lead us to believe. The species originated in Asia, reached Europe by the mid-1500's and arrived in North America about 1775 on ships from England.