Rattus norvegicus

Brown rats like to live in the vicinity of people, since there is always a place to hide and something to eat. They eat just about everything: seeds, grains, beets, insects, meat, eggs, worms, young birds, small rodents, snails, amphibians and garbage. Brown rats are active at night. They are hunted by owls and predators (including domestic cats), and fiercely pursued by people. They are so intelligent that they aren't readily caught in traps.

Considered one of the most serious mammalian pests ever known, the brown rat is a true omnivore, eating a huge range of food including invertebrates, frogs, small mammals, birds' eggs, scavenged meat and bones, cereals and seeds (4), fruit, carrion, and any food discarded by humans (1). They have also been known to catch and eat fish (2). They are mainly nocturnal, but like many mammals they become increasingly active in the day where they are undisturbed by humans (4). They generally move around on the ground, but are also expert jumpers, climbers and swimmers (1), holding their tail aloft for balance when swimming (2). Brown rats live in 'packs' in which there is a dominant male who gains priority access to food, water and resting sites, and defends a harem of females, preventing subordinate males from mating (1). This rat is one of the most prolific of all mammals (1). Females become sexually mature at just 8-12 weeks, gestation is between 21 and 23 days, and females are able to conceive whilst suckling a previous litter, often mating within 18 hours of giving birth (1). They can breed throughout the year if the weather is mild and there is plenty of food; 13 litters are possible each year, each one consisting of 7-9 young (4). Maximum-known longevity is 3 years, but in the wild, life span is probably less than 18 months (4). The greatest cause of mortality is poisoning by rodenticides, but predators such as cats, foxes, dogs, mink, stoats and owls also take their toll (4).

The brown or Norway rat is a supreme generalist; its opportunistic lifestyle, agility and prolific breeding potential have helped it to colonise practically every part of the world (2). The fur of this rat varies in colour but is generally brown or greyish-brown with some black hairs, becoming paler on the underside (2). The tail is long and largely hairless, the naked ears are prominent (1) and the eyes are relatively small (2).

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.

Biogeographic Regions: nearctic (Introduced ); palearctic (Introduced , Native ); oriental (Introduced ); ethiopian (Introduced ); neotropical (Introduced ); australian (Introduced ); oceanic islands (Introduced )

Other Geographic Terms: cosmopolitan

Widespread (Nile Valley, Delta, coastal areas).

This species was originally native to south-east Siberia, north-east China and parts of Japan, but it occurs worldwide as an introduced species.

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.

Biogeographic Regions: nearctic (Introduced ); palearctic (Native ); oriental (Introduced ); ethiopian (Introduced ); neotropical (Introduced ); australian (Introduced ); oceanic islands (Introduced )

Other Geographic Terms: cosmopolitan

occurs (regularly, as a native taxon) in multiple nations

Canada

Origin: Exotic

Regularity: Regularly occurring

Currently: Present

Confidence: Confident

Type of Residency: Year-round

United States

Origin: Exotic

Regularity: Regularly occurring

Currently: Present

Confidence: Confident

Type of Residency: Year-round

Global Range: Assumed native in eastern Asia; introduced worldwide but most common in colder climates of high latitudes; in warmer regions, restricted to habitats highly modified by humans (Musser and carleton, in Wilson and Reeder 2005).

The brown rat is believed to have originated from China. It reached Europe at some point in the early 18th century and was first seen in England in 1720 (4). At present it has a very wide distribution in Britain in both urban and rural areas, but is absent from a number of smaller islands. It is widespread and common in urban areas across the globe, with the exception of some tropical and subtropical areas (4).

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.

Range mass: 200 to 500 g.

Average length: 399 mm.

Other Physical Features: endothermic ; homoiothermic; bilateral symmetry

Sexual Dimorphism: male larger

Average basal metabolic rate: 1.404 W.

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.

Range mass: 140 to 500 g.

Average mass: 400 g.

Average length: 399 mm.

Other Physical Features: endothermic ; homoiothermic; bilateral symmetry

Sexual Dimorphism: male larger

Average basal metabolic rate: 1.404 W.

Length: 46 cm

Weight: 485 grams

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.

Habitat Regions: temperate ; terrestrial

Terrestrial Biomes: savanna or grassland ; chaparral ; forest ; scrub forest

Other Habitat Features: urban ; suburban ; agricultural ; riparian ; estuarine

Habitat and Ecology

Systems

• Terrestrial

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.

Habitat Regions: temperate ; tropical ; terrestrial

Terrestrial Biomes: taiga ; desert or dune ; savanna or grassland ; chaparral ; forest ; rainforest ; scrub forest ; mountains

Wetlands: marsh ; swamp ; bog

Other Habitat Features: urban ; suburban ; agricultural ; riparian

Comments: Buildings and other structures in cities, villages, and farm country, also dumps and open areas near abundant food. Most common in colder climates of high latitudes; in warmer regions restricted to habitats highly modified by humans (Musser and Carleton, in Wilson and Reeder 1993). In Hawaii: locally common in some lowland cane fields, rare or absent in extensive grassland and most native forests (Tomich 1986). In Oklahoma, moved from fields to buildings in fall (Caire et al. 1989). Young are born in nests in buildings, under debris, or underground.

This species is associated with farm buildings in rural areas. Refuse tips, sewer systems, hedges and field margins are also suitable habitats (4).

Non-Migrant: Yes. At least some populations of this species do not make significant seasonal migrations. Juvenile dispersal is not considered a migration.

Locally Migrant: No. No populations of this species make local extended movements (generally less than 200 km) at particular times of the year (e.g., to breeding or wintering grounds, to hibernation sites).

Locally Migrant: No. No populations of this species make annual migrations of over 200 km.

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 Actinopterygii and small rodents, such as Mus musculus, and they readily eat dead animals. In cities, Norway rats enjoy human food that is uneaten or spoiled.

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.

Animal Foods: birds; mammals; amphibians; reptiles; fish; eggs; carrion ; insects; terrestrial non-insect arthropods; mollusks; terrestrial worms; aquatic crustaceans; echinoderms; other marine invertebrates; zooplankton

Plant Foods: leaves; roots and tubers; wood, bark, or stems; seeds, grains, and nuts; fruit; nectar; flowers; sap or other plant fluids

Other Foods: fungus; detritus

Foraging Behavior: stores or caches food

Primary Diet: omnivore

Comments: Very opportunistic feeder; diet of vegetable and animal matter, garbage, and carrion.

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.

Mutualist Species:

• humans

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.

Known Predators:

• Felis silvestris
• Vulpes vulpes
• Falconiformes
• Mustela
• other small predators

Animal / dung saprobe
solitary or confluent gymnothecium of Arachniotus ruber is saprobic in/on dung or excretions of dung of Rattus norvegicus

Animal / pathogen
Foot and Mouth virus (FMD) infects Rattus norvegicus
Other: minor host/prey

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.

Ecosystem Impact: disperses seeds; soil aeration

Mutualist Species:

• humans (Homo sapiens)

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.

Known Predators:

• canids (Canidae)
• owls (Strigiformes)
• cats (Felidae)
• lizards and snakes (Squamata)
• humans (Homo sapiens)

Rattus norvegicus is prey of:
Mustela
Felis silvestris
Vulpes vulpes
Falconiformes

This list may not be complete but is based on published studies.

Rattus norvegicus preys on:
Podilymbus podiceps
Zenaida asiatica
Progne dominicensis
Phoebastria nigripes

This list may not be complete but is based on published studies.

Home range is usually less than an acre. Can become locally abundant. Lives in highly organized colonies. Throughout range in different habitats, median survival time relatively consistent at about 3 months, annual survival rate at about 5%.

"Nocturnal, fossorial"

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.

Communication Channels: visual ; tactile ; acoustic ; chemical

Other Communication Modes: scent marks ; vibrations

Perception Channels: visual ; acoustic

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.

Communication Channels: visual ; tactile ; acoustic ; chemical

Other Communication Modes: pheromones ; scent marks

Perception Channels: visual ; acoustic ; vibrations

Comments: Mostly nocturnal.

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.

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.

Range lifespan
Status: captivity: 4 (high) years.

Average lifespan
Status: wild: 2 years.

Typical lifespan
Status: captivity: 2 to 3 years.

Maximum longevity: 5 years (captivity) Observations: This common laboratory animal is one of the fastest ageing mammals. Even in captivity, most animals do not normally live more than 3-4 years. Maximum longevity has been reported to be 5-6 years (Caleb Finch 1990), though anecdotal reports of animals living more than 5 years have not been verified. Caloric restriction extends lifespan and appears to delay ageing (Yu et al. 1982).

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

Mating System: polygynous ; cooperative breeder

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.

Breeding interval: 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: 7.0.

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 (male): 3.0 months.

Key Reproductive Features: iteroparous ; year-round breeding ; gonochoric/gonochoristic/dioecious (sexes separate); fertilization ; viviparous

Average birth mass: 5.81 g.

Average number of offspring: 9.9.

Average age at sexual or reproductive maturity (male)
Sex: male: 70 days.

Average age at sexual or reproductive maturity (female)
Sex: female: 90 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.

Parental Investment: altricial ; female parental care

The mating system of R. norvegicus is best described as polygynandrous. Social animals, Norway rats tend to breed in large groups. Once a female enters her six-hour estrus period, she may mate as many as five-hundred times with competing males.

Mating System: polygynandrous (promiscuous) ; cooperative breeder

Although not technically a seasonal breeder, a mating increase occurs in the warmer months of the year. An average female is capable of giving birth approximately seven times per year. Around 18 hours after giving birth, females experience postpartum estrus, and mate again. This reproductive function is responsible for the huge birthrates of Norway rats, which can reach 60 young each year per female. After a short gestation period of 22 to 24 days, the litter of approximately 8 pups is born. The young are very small and underdeveloped. It takes 14 to 17 days for the young's eyes to open. Newborns weigh an average of 5 grams and are milk-fed until weaning occurs at 3 to 4 weeks, and the young then leave the nest.

Often, the litters of numerous females will occupy the same nest, and all the young are cared for by the adults, regardless of who the true mothers are. This communal care makes the species something of a cooperative breeder.

Males usually reach sexual maturity at 3 months and females at 4. However, it is usually the female who mates first because competition for mates among males prevents the smaller, less-dominant individuals from succeeding immediately. Rattus norvegicus is capable of mating for up to two years.

Breeding interval: Norway rats may breed up to 7 times per year

Breeding season: Breeding occurs year round, but is less pronounced during colder months.

Range number of offspring: 2 to 14.

Range gestation period: 22 to 24 days.

Range weaning age: 3 to 4 weeks.

Range time to independence: 4 to 5 weeks.

Range age at sexual or reproductive maturity (female): 3 to 4 months.

Range age at sexual or reproductive maturity (male): 3 to 4 months.

Key Reproductive Features: iteroparous ; year-round breeding ; gonochoric/gonochoristic/dioecious (sexes separate); fertilization ; viviparous ; post-partum estrous

Average birth mass: 5.81 g.

Average number of offspring: 9.9.

Parental care is provided by females. Because these animals often nest communally, the litters of several different females often occupy the same nest. In nesting groups of more than one female, if a mother is killed, the other females will take over nursing the newborns. Males do not particupate in parental care.

Parental Investment: no parental involvement; altricial ; female parental care ; pre-fertilization (Protecting: Female); pre-hatching/birth (Provisioning: Female, Protecting: Female); pre-weaning/fledging (Provisioning: Female, Protecting: Female); pre-independence (Protecting: Female)

Breeds throughout the year, particularly from spring to fall. Gestation lasts 21-24 days or up to a week longer in lactating females. Litter size is 2-14 (average about 9). Averages 6 litters per year (range 3-12). Sexually mature in 80-85 days.

Barcode of Life Data Systems (BOLDS) Stats
Public Records: 60
Specimens with Barcodes: 88
Species With Barcodes: 1

The following is a representative barcode sequence, the centroid of all available sequences for this species.

There are 41 barcode sequences available from BOLD and GenBank.  Below is a sequence of the barcode region Cytochrome oxidase subunit 1 (COI or COX1) from a member of the species.  See the BOLD taxonomy browser for more complete information about this specimen and other sequences.

Download FASTA File

Norway rats are widespread and abundant and are not considered threatened. Humans often try to control the numbers of rats by trapping and poisoning.

IUCN Red List of Threatened Species: least concern

US Federal List: no special status

CITES: no special status

Year Assessed

Assessor/s

Reviewer/s

Contributor/s

Justification

History

• 1996
  Lower Risk/least concern

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

US Federal List: no special status

CITES: no special status

State of Michigan List: no special status

IUCN Red List of Threatened Species: least concern

Non-native, resident.

Canada

Rounded National Status Rank: NNA - Not Applicable

United States

Rounded National Status Rank: NNA - Not Applicable

Rounded Global Status Rank: G5 - Secure

Not legally protected in the UK. No conservation designations (3).

Population

Population Trend

Major Threats

Brown rats are controlled as they are serious pests. They carry dangerous diseases, and cause serious damage to property and crops (1). Brown rats have developed resistance to rodenticides, and many newer poisons are unfortunately highly toxic to many other vertebrates (4).

Conservation Actions

Not relevant.

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.

Negative Impacts: injures humans (bites or stings, carries human disease); crop pest; causes or carries domestic animal disease ; household pest

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.

Positive Impacts: pet trade ; research and education

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.

Negative Impacts: injures humans (bites or stings, causes disease in humans , carries human disease); crop pest; causes or carries domestic animal disease ; household pest

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.

Positive Impacts: pet trade ; research and education

Comments: Often a severe pest; eats stored grain and small farm animals, public health menace, etc.

Species Impact: Implicated as a contributor in the decline of the ancient murrelet and other burrow-nesting seabirds (see Bertram and Nagorsen 1995).

In New York, Draud et al. (2004) documented a high rate of predation on hatchling and juvenile diamondback terrapins by Norway rats.