Norway rats reach sexual maturity at 3 months of age. Female Norway rats may come into heat every 4 or 5 days and they can mate within 1 or 2 days after a litter is born. Females produce 3 to 6 litters per year. The peak time of breeding often is in spring and fall, with activity declining during the heat of summer and often stopping in winter, depending on habitat. Trends of seasonal reproduction are most pronounced in areas of extreme climates.
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 eat solid food at 2½ to 3 weeks, become independent at about 3 to 4 weeks.
Norway rats usually construct nests in burrows below ground or at ground level (Figure 6). Nests may be lined with shredded paper, cloth, or other fibrous material. Each female has her own nest chamber, but Norway rats may share a burrow and may raise their young together. In northern climates, rats prefer to locate nests near sources of heat.
Norway rats are nocturnal. They usually become active around dusk, when they seek food and water. Some individuals may be active during the day when populations of rats are high or to avoid humans at night.
Norway rats rely on their excellent senses of hearing, smell, taste, and touch. They have poor eyesight and are considered color-blind. Therefore, bait can be dyed distinctive colors without causing avoidance by rats, as long as the dye does not have an objectionable taste or odor. They use their sense of smell to locate food and to recognize other rats. They can detect contaminants in food at levels as low as 0.5 parts per million.
Norway rats gain entry to structures by gnawing, climbing, jumping, and swimming. A rat travels an area about 100 to 150 feet in diameter daily. Rats seldom travel more than 400 feet from their burrows for food or water. Rats explore and learn about their environment, memorizing the locations of pathways, obstacles, food and water, shelter, and other elements in their domain. They quickly detect, and tend to avoid, new objects in their environment. Objects such as traps and bait stations often are avoided for several days. Rats may investigate baited, unset traps after a short period of avoidance, which will aid in overcoming their fear. Expanded-trigger traps set directly on travel routes may catch rats immediately.
Rats initially avoid novel food items in their environment. They may eat very small amounts, and subsequent feeding will depend on the flavor of the food and its physiological effect. If the food contains a toxicant or some other substance that soon produces ill effects, but not death, the food may be associated with the illness. “Bait-shyness” is a major problem when single-dose, acute toxicants are the main rodenticides in use.
Norway rats live in close association with humans. In urban and suburban areas, they live in and around residences, cellars, warehouses, stores, slaughterhouses, docks, and sewers. On farms they may inhabit barns, granaries, buildings for livestock, silos, and kennels. They may burrow to make nests under buildings and other structures, beneath slabs of concrete, along banks of streams, around ponds, in dumps, and at other locations where suitable food, water, and shelter are present. Although most are good climbers, Norway rats tend to inhabit the lower floors of buildings.
A single Norway rat needs about 110 calories per day. Rats eat nearly any type of food, but when given a choice they select a nutritionally balanced diet and fresh items over stale or contaminated foods. They prefer grains, meat, fish, nuts, some types of fruit, and insects such as cockroaches. Rats require ½ to 1 ounce of water per day when feeding on dry foods, but need less when moist foods are available. Food that is discarded in household garbage offers a well-balanced diet and satisfies moisture needs of Norway rats.