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norway rat

Rattus norvegicus

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.

  • Sexual Dimorphism
  • male larger
  • Range mass
    200 to 500 g
    7.05 to 17.62 oz
  • Average length
    399 mm
    15.71 in
  • Average basal metabolic rate
    1.404 W

Where 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?

Norway rats don't have single mates, instead they breed with many others through their lifetime.

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 to 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.

  • How often does reproduction occur?
    Norway rats can breed many times in a year, up to 12 times.
  • Breeding season
    Norway rats breed throughout the year
  • Range number of offspring
    2.0 to 22.0
  • Average number of offspring
  • Average number of offspring
  • Range gestation period
    26.0 (high) days
  • Average weaning age
    22.0 days
  • Average age at sexual or reproductive maturity (female)
    3.0 months
  • Average age at sexual or reproductive maturity (female)
    Sex: female
    90 days
  • Average age at sexual or reproductive maturity (male)
    3.0 months
  • Average age at sexual or reproductive maturity (male)
    Sex: male
    70 days

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.

  • Range lifespan
    Status: captivity
    4.0 (high) years
  • Average lifespan
    Status: wild
    2.0 years
  • Average lifespan
    Status: captivity
    5 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.

How do they communicate with each other?

Like most mammals, Norway rats communicate in a variety of way. They make sounds and use body postures to communicate various moods and intents. They have good hearing and a very sensitive sense of touch. They are able to sense small vibrations in the ground and feel their way through total darkness with their paws and whiskers. They also have a 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.

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 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.

Species (or larger taxonomic groups) that are mutualists with this species
  • humans

Do they cause problems?

Some 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.

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. 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.

  • Ways that people benefit from these animals:
  • pet trade
  • research and education

Are they endangered?

Norway rats are widespread and abundant and are not considered threatened. 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.

University of Michigan Museum of ZoologyNational Science Foundation

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. "Rattus norvegicus" (On-line), Animal Diversity Web. Accessed May 25, 2024 at

BioKIDS is sponsored in part by the Interagency Education Research Initiative. It is a partnership of the University of Michigan School of Education, University of Michigan Museum of Zoology, and the Detroit Public Schools. This material is based upon work supported by the National Science Foundation under Grant DRL-0628151.
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