Rodent Species Identification
Norway Rat: Identification and Behavior
Rattus norvegicus
A homeowner guide to identifying Norway rats, understanding their behavior, and planning removal and long-term prevention.
How to Identify Norway Rat
Norway rats are the larger of the two common commensal rats found in US homes. Adults grow to a total length of about 13 to 18 inches, including the tail. Adults weigh 7 to 18 ounces, compared with 5 to 10 ounces for a roof rat. The body is large and robust, with a blunt nose, small ears that do not reach the eyes when folded over, and a tail shorter than the head and body combined.
Fur is usually brown or red-gray above and white-gray on the belly. Young rats can be mistaken for house mice, but very young rats have large heads and feet in proportion to their bodies. Adult Norway rats are large and robust, where roof rats are sleek and agile.
Droppings are another clue. Norway rat droppings are about 1/2 inch long and can reach 3/4 inch, with blunt ends. Fresh droppings are black or nearly black and look wet; after a few weeks they turn gray and crumble easily. Droppings turn up along runways, in feeding areas and near shelter, usually clumped in small groups.
13 to 18 inches total length
- Compared to a roof rat:Slightly larger
- Compared to a house mouse:Adults weigh 7 to 18 ounces; a house mouse weighs about 1/2 ounce
Distinguishing From Similar Species
- vs Roof Rat: Norway rats are larger and stockier, with a shorter tail and smaller ears. They usually stay in basements and on ground floors, though infestations in ceiling voids are not uncommon.
- vs Pack Rat: Pack rats (woodrats) have furry tails. Norway rats have scaly, mostly hairless tails.
Where Norway Rats Live
Norway rats occur in all 50 states. The roof rat, by contrast, is more at home in warm climates.
Habitat selection centers on ground level and below-ground environments. They burrow along building foundations, under buildings and concrete slabs, and beneath trash piles or woodpiles. Indoors they tend to stay in basements or on the ground floor. They also live in and around cellars, warehouses, stores, docks and sewers.
Roof rats are the more agile climbers, though most Norway rats are good climbers too. UC IPM notes that Norway rat infestations in ceiling voids are not uncommon.
Outdoors, ICWDM lists stream banks, pond edges and dumps among the places Norway rats burrow.
Norway Rat Behavior
Norway rats are nocturnal and usually become active around dusk, when they look for food and water. Some are active by day when populations are high, or when they are disturbed or hungry. Nightly foraging ranges vary significantly with local conditions and may be anywhere from 100 to 500 feet from their nests (UC IPM), though ICWDM says they seldom travel more than 400 feet for food or water.
Norway rats generally live in groups dominated by a large male. Females produce 3 to 6 litters a year, with 6 to 12 young per litter.
Norway rats are omnivorous and prefer grains, meat, fish, nuts and some fruits. They avoid new objects and foods, often for several days. Leaving traps baited but unset until the bait has been taken several times helps overcome that caution.
Movement is edge-driven and habitual. Norway rats habitually use the same routes, traveling along walls, along fences and under bushes night after night. They are excellent swimmers.
Damage Norway Rats Cause in Homes
Norway rat incisors grow continuously at about 5 inches per year. They gnaw on all types of materials, including plastic, wood and soft metals such as copper and lead. Their tooth marks are larger than those of mice.
Gnawed wiring can create a fire risk.
Food contamination extends well beyond what rats consume. One rat probably contaminates about 10 times the food it eats with urine and droppings.
Health Risks From Norway Rats
Norway rats can carry pathogens that reach people through urine, droppings, saliva, bites, and contaminated food or surfaces. Diseases linked to Norway rats include murine typhus, leptospirosis, salmonellosis and rat-bite fever. Seoul virus, a hantavirus, is carried primarily by Norway rats and has been found in wild rats in the US. CDC warns against sweeping or vacuuming droppings, urine or nesting material, and rat-bite fever can also spread through contaminated food or drink. Anyone who develops illness after rodent exposure should contact a healthcare provider.
Vector-borne disease adds a second transmission pathway. Flea-borne typhus is one example: fleas become infected when they bite infected rats, cats or opossums. Plague is more commonly associated with roof rats than with Norway rats; the last US urban outbreak of rat-associated plague was in Los Angeles in 1924 and 1925.
Rats have been associated with medically significant allergic responses in people.
How Norway Rats Enter Homes
ICWDM advises sealing all holes and openings larger than 1/2 inch across to keep Norway rats out. A rat may gnaw a smaller gap to enlarge it.
UC IPM lists openings for water pipes, electric wires, sewer pipes, drain spouts and vents among the gaps to seal, and notes that rats can enter through toilets or broken drains.
Norway Rat Control Considerations
Norway rat control centers on ground-level and below-grade areas: burrow entries near the foundation, basement and crawl space perimeters, and utility penetrations. Traps go close to walls and on active runways, left baited but unset until the bait has been taken several times.
Exclusion for Norway rats targets the ground-level envelope, from foundation seams and cracks to utility penetrations, basement windows, garage thresholds, vent screens, and sewer caps.
For a Norway rat problem, Rat Control describes ground-level trap placement, a foundation inspection, and attention to burrows near the house.
Call to get a trusted local pro out for Norway rat control.
Find Service Near YouWhere Norway Rats Are Found
ICWDM records Norway rats in every one of the contiguous 48 states as well as Alaska and Hawaii. They are generally found at lower elevations but may occur wherever humans live.
In Texas, the Mammals of Texas account says Norway rats are not so common in the southern half of the state as the black rat, another name for the roof rat.
Signs You Have a Norway Rat Problem
Large blunt-ended droppings
Norway rat droppings are about 1/2 inch long, up to 3/4 inch, with blunt ends. Fresh ones are black or nearly black and look wet; after a few weeks they turn gray and crumble. Look along runways, in feeding areas and near shelter.
Smudge marks along walls
Norway rats habitually use the same routes, and smudge (rub) marks from oil and dirt appear on beams, rafters, pipes and walls along them.
Burrows in yard or near foundation
Burrows appear along foundations, under concrete slabs and beneath woodpiles or trash piles. Active burrow entries are about 2 to 4 inches in diameter.
Activity starting around dusk
Norway rat activity is usually in basements and on ground floors, though ceiling voids are not uncommon. They usually become active around dusk.
Gnaw marks on wood, wire, and containers
Rats gnaw wood, plastic and soft metals such as copper and lead. Rat tooth marks are wider than mouse marks.
Similar Species and Commonly Confused Rodents
Common Questions About Norway Rats
Size and body shape are the fastest ways. Norway rats are larger and stockier, with blunt snouts and small ears. Roof rats are slimmer with pointed snouts and larger ears. Tail length also differs: Norway rat tails are shorter than the head and body; roof rat tails are longer than the head and body. Habitat helps too: Norway rats tend to stay in basements and on ground floors, while roof rats favor attics and trees.
Sources and Further Reading
Sources for Norway rat facts on this page:
- Pest Notes: Rats (UC Statewide IPM Program (UC IPM))
- Norway Rats (Internet Center for Wildlife Damage Management (ICWDM))
- Norway Rat Biology (Internet Center for Wildlife Damage Management (ICWDM))
- Norway Rat Damage Identification (Internet Center for Wildlife Damage Management (ICWDM))
- Norway Rat Damage Prevention and Control Methods (Internet Center for Wildlife Damage Management (ICWDM))
- Rodent Pests (Southern Nevada Health District)
- Old World Rats (Washington Department of Fish and Wildlife (WDFW))
- Norway or Brown Rat (The Mammals of Texas, online edition) (Natural Science Research Laboratory, Texas Tech University)
- How to Clean Up After Rodents (Centers for Disease Control and Prevention (CDC))
- Veterinary Guidance for Testing for Seoul Virus in Pet Rats (Centers for Disease Control and Prevention (CDC))
- About Rat-bite Fever (Centers for Disease Control and Prevention (CDC))
- About Flea-borne Typhus (Centers for Disease Control and Prevention (CDC))
- How Plague Spreads (Centers for Disease Control and Prevention (CDC))
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