The Norway Rat is one of the most well-known and common rats, and also one of the largest. Thought to have originated in northern China, this rodent has now spread to all continents and is the dominant rat in Europe and much of North America.
The Norway rat (Rattus norvegicus) lives in close association with people. In urban or suburban areas they live in and around residences, in cellars, warehouses, stores, slaughterhouses, docks, and in sewers. On farms, they may inhabit barns, granaries, livestock buildings, silos, and kennels. They may burrow to make nests under buildings and other structures, beneath concrete slabs, along stream banks, around ponds, in garbage dumps and at other locations where suitable food, water, and shelter are present. Also called the brown rat, house rat, barn rat sewer rat, gray rat or wharf rat, it is a slightly larger animal than the roof rat. Adult Norway rats weigh an average of 1 pound. Their fur is coarse and usually brownish or reddish-gray above and whitish gray on the belly. Blackish individuals occur in some locations. Although they can climb, Norway rats tend to inhabit the lower floors of multi-story buildings.
Norway rats are primarily nocturnal. They usually become active about dusk, when they begin to seek food and water. Some individuals may be active during daylight hours when rat populations are high.
Rats have poor eyesight, relying more on their hearing and their excellent senses of smell, taste, and touch. They are considered color-blind. Therefore, for safety reasons, baits can be dyed distinctive colors without causing avoidance by rats, as long as the dye does not have an objectionable taste or odor.
Rats use their keen sense of smell to locate food items and to recognize other rats. Their sense of taste is excellent, and they can detect some contaminants in their food at levels as low as 0.5 parts per million.
Norway rats usually construct nests in below-ground burrows or at ground level. Nests may be lined with shredded paper, cloth, or other fibrous material. Litters of 6 to 12 young are born 21 to 23 days after conception. Newborn rats are hairless and their eyes are closed, but they grow rapidly. They can eat solid food at 2 1/2 to 3 weeks. They become completely independent at about 3 to 4 weeks and reach reproductive maturity at 3 months of age.
Females may come into heat every 4 or 5 days, and they may mate within a day or two after a litter is born. Breeding often peaks in spring and fall, with reproductive activity declining during the heat of summer and often stopping completely in winter, depending on habitat. These seasonal trends are most pronounced in more severe climates. The average female rat has 4 to 6 litters per year and may successfully wean 20 or more offspring annually.
Norway rats have physical capabilities that enable them to gain entry to structures by gnawing, climbing, jumping, swimming, and other tactics.
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