The Norway rat is the largest of the commensal rodents and the most common of the two commensal rat species. It damages materials by gnawing, eats and contaminates stored food, and also carries disease. It can be found worldwide and throughout the United States.
The Norway rat has a combined head and body length of 7" - 9 1/2". Its tail is an additional 6" - 8" long. It will average 7 - 18 ounces in weight. The Norway rat has a coarse, shaggy brown fur, with scattered black hairs. The underside is gray to yellowish white.
The muzzle of the Norway rat is blunt, with small eyes and small ears. It is a heavy bodied rat. The tail is bicolored, darker above than below, and is shorter than the rat's head and body combined. Droppings of the Norway rat are about 3/4" long with a capsule shape and blunt ends.
The Norway rat exhibits many signs of infestation:
- gnaw marks (new marks rough, old are smooth and greasy)
- droppings (fresh are soft and moist, old are dry and hard)
- tracks (front, 4 toes - hind, 5 toes, hind print usually longer)
- rub marks (fresh are soft, greasy - old are dry, flaky)
- burrows (earthen banks, under slabs and walls, opening hard packed)
- runways (consistently follow same path along walls, stacked products)
- damaged goods (prefer meat, fish, dog food and cereal)
Norway rats reach sexual maturity in 2-5 months. Pregnancy lasts about 22 days. Young are blind and naked, with hair appearing in about a week and eyes opening in about 2 weeks. They are weaned in about 3-4 weeks. They produce 3-6 litters per year with an average of 7-8 young per litter. Each female averages about 20 weaned offspring per year. Adults live an average of 6-12 months.
Norway rats have poor vision and are color blind. They have a keen sense of hearing, smelling, touching and tasting. They use their vibrissae (whiskers) to feel their way along runways. They are good runners, climbers, jumpers and swimmers.
The Norway rat requires about 3/4-1 ounce of food per day. It needs 1/2-1 ounce of water daily with the water coming from a nonfood source. It produces about 30-180 droppings each day. It also excretes 1/2 ounce of urine daily.
Rats are primarily nocturnal and are very cautious. Although they constantly explore their surroundings, they shy away from new objects and changes in their environments. Outdoors, they prefer to nest in burrows in the soil along embankments, rubbish piles, under concrete slabs, etc. Burrows have at least one entry and one bolt hole (emergency exit) that is usually hidden under grass, debris, etc. Rats are social animals and many burrows may be constructed in the same areas.
Norway rats require an opening of 1/2" to gain entry into buildings. Indoors, they usually nest in basements and lower portions of buildings in piles of debris, sewers, or stored items which are not disturbed. They may occasionally be found in attics, ceilings and other high places.
Although they will eat practically anything, Norway rats prefer meat, fish and cereal. If the food is disagreeable, they develop food shyness. If acceptable, they eat their fill at one sitting and will return time after time. They almost always require a separate water source. Norway rats will travel 100-150 feet from their harborage for food or water. They will gnaw through almost anything to get to food and water.
Once established, Norway rats tend to follow the same path constantly. Runways are usually along vertical surfaces and will be clean and worn. They may migrate when necessary due to overpopulation. Because they are shy and suspicious, they may avoid new objects, even food, for several days.
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