Norway rats are presently cosmopolitan in distribution, having been spread throughout the world by cargo ships. They are typically ground dwellers as opposed to black rats, Rattus rattus, which tend to climb more. Norway rats build underground burrows that contain long branching tunnels, multiple exits, and chambers for food storage and nests (Whitaker, 1980; Nowack, 1991). When inhabiting buildings they usually are found in the lower levels, occupying cellars and basements, and thriving in sewers (Nowak, 1991). They are common in cities and suburban areas throughout the Gulf states. They also occur in crop fields, well away from human habitations (Lowery, 1974). The typical home range for this species is between 25 and 150 m in diameter (Nowak, 1991). In the wild, they may live up to three years, but few live beyond two years of age, with most surviving only one year (Davis, 1948a; Whitaker, 1980). Davis (1951e) estimated an overall mortality rate of 95% year, and a 97.5% mortality rate for weaned rats. Usually movements and migration are relatively rare (Davis, 1951e). However, when conditions of crowding occur, Norway rats may carry out mass migrations (Whitaker, 1980).
Reproduction and Fecundity: Norway rats reproduce throughout the year with peaks in spring (March-June) and fall (September-October) (Davis, 1951b). Females may bear as many as 12 litters per year of 2-22 young each. However, typically they bear 5 litters of 7-11 young (Lowery, 1974; Whitaker, 1980). Davis (1951e) reported an average of 8.7 young per litter, and 4.3 pregnancies per year based on pregnancy data of several hundred thousand Norway rats from cities of the United States and India. Because of high suckling mortality rates, this author estimated an average of 10 young weaned per female, per year. There is a postpartum estrus within 18 hours of birth (Nowak, 1991). Young are born naked and blind. They open their eyes in two weeks and are weaned at 3-4 weeks (Lowery, 1974; Whitaker, 1980). Females may begin to breed at three months of age (Whitaker, 1980).
Trophic Interactions: Norway rats are omnivorous. They feed on meat, insects, wild plants, seeds, stored grains, soap, hides, paper, etc. (Whitaker, 1980; Nowak, 1991). Norway rats have been reported to prefer animal matter (Whitaker, 1980; Nowak, 1991). They will kill poultry, feed on eggs, and are excellent at catching fish (Whitaker, 1980; Nowak, 1991). Mice and newly born farm animals such as lambs and pigs have also been reported as food items (Nowak, 1991).
Major predators include snakes, owls, hawks, skunks, weasels, minks, cats, and dogs (Whitaker, 1980). Mortality due to intraspecific conflicts and cannibalism is high (Lowery, 1974).