Growth and reproductive performance are two key indicators of dietary adequacy. Other than the fact that it lacks a gallbladder, the rat's digestive tract resembles that of other omnivorous rodents in that the stomach contains both nonglandular and glandular regions, the small intestine is of moderate length, and the cecum is relatively well developed (Bivin et al., 1979 Vorontsov, 1979). The Norway rat is omnivorous, eating a wide variety of seeds, grains, and other plant matter as well as invertebrates and small vertebrates (Nowak, 1991). Females produce 1 to 12 litters per year, and those in a colony nurture their young collectively. In the wild, the Norway rat exhibits both territorial and colonial behavior and typically occupies underground burrows (Calhoun, 1963). The origin and historical development of the major strains of laboratory rats have been reviewed by Lindsey (1979). ![]() The Norway rat is not indigenous to Europe however, it is believed to have originated in Asia and to have taken advantage of human movement in expanding its range worldwide (Nowak, 1991). The albinos of the Norway rat were first domesticated in Europe in the early 19th century and came into use as experimental animals shortly thereafter (Lindsey, 1979). The laboratory rat is a domesticated Norway rat, which in nature is one of the most widespread and abundant of the more than 70 species of the genus Rattus (family Muridae).
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