Photo Credit: National Park Service

SCIENTIFIC NAME: Rattus norvegicus
OTHER NAMES: Norway rat, Norwegian rat, Hanover rat, sewer rat, and wharf rat.
STATUS: Exotic. Breeder. A commensal rodent brought to the United States by early European colonists, albeit considerable later (ca. 1775) than the black rat and house mouse. Requires food, water, and harborage provided by humans. Often targeted for eradication because of potential economic damage and health concerns. 
DESCRIPTION: The brown rat is one of the largest rat species. Its body can reach 10 inches in length, with a slightly shorter tail for a total length ranging from 12 to 18 inches. Males can weight up to 18 oz., but average 12 oz. Female are somewhat smaller weighting 5 to 11 oz. In wild populations their coarse fur is brown to dark grey in color, while various strains of captive bred rats may be white, black, brown, or a spotty mixture of these colors. Their undersides are usually lighter or even white in color. Their tail and ears are bald.  As with all rodents their incisors are erupting (growing) continuously.  They have acute hearing and a keen sense of smell, which they use to find food and even to distinguish one another. They are vocal and also use body posture to communicate.
DISTRIBUTION: Brown rats were originally found only in China, Asia, and Mongolia, but now have become one of the most common rats on all continents except Antarctica. They are not found in the Artic region. It is believed the spread of the brown rat occurred during the Middle Ages.  They followed the expansion of human populations and trade routes to new areas and now live almost everywhere humans do; particularly urban/man-made environments. Although untrue, it was believed that the Norway rat migrated to Europe on Norwegian ships, thus receiving its name the brown rat.
HABITAT: Originally the brown rat inhabited forest and brushy damp areas next to river banks. However, they have adapted to all types of human-associated habitats. Wherever food and shelter is found, a substantial population of brown rats can usually be found as well. Brown rats inhabit open fields, woodlands, barns, sewer lines, garbage dumps and many other habitats near human settlements. They can even live within heavily populated cities. It has been stated that in some impoverished areas with old buildings and facilities with high moisture, there can be as many or more rats than humans living there. These rats have become somewhat dependant on humans for food and shelter. The home range of a brown rat depends on the density of their population and availability of food. They normally travel in linear “runways” only 65 to 170 feet from their nest.  They often dig under buildings and other structures creating extensive burrow systems.  The brown rat is not known to be an agile climber, yet they have been seen traveling along wires from one building to another.  They are excellent swimmers, both on and below the surface.
FEEDING HABITS: Brown rats are omnivores, consuming a wide variety of things based on food availability. They will consume insects, eggs, fish (i.e. live fingerlings), nestling bird chicks and small sparrows, mollusks, lizards, grains, fruits, green vegetation, and garbage.  They have been known to attack human babies and kill young pigs and lambs. Brown rats will consume approximately a third of their body weight within a 24 hour period.  They are mostly nocturnal (active during the night).
LIFE HISTORY AND ECOLOGY: The brown rat is found in large hierarchical colonies or groups. The size of these groups depends largely on availability of food. Die offs may occur starting with rats in the lower social order until food sources increase. If conditions, such as food, weather, and shelter are suitable, females can produce up to twelve litters a year.  The gestation period is only 21 days and females can breed again within eighteen hours of giving birth.  Nests are constructed of twigs, leaves, paper, rags, and other debris. Litters can range from five to twenty young, but usually average seven. Nests may contain litters from several females. All the young are cared for by all mothers even if one mother were to be killed. Males do not participate in parental care. Brown rats are born blind and naked and completely dependant on the females. They are weaned at approximately three weeks of age. Males can begin to reproduce at three months of age whereas females begin breeding at four months of age. The life span of a brown rat varies considerably from an average of one year to a maximum of three years. Brown rats can be aggressive in driving out other native rats and mice. They are commonly used as laboratory rats for medical (diseases and drugs) and biological (genetics) research and also sold as pet rats or food for snakes.
Field Guide to Mammals, North America. 1996. National Audubon Society. Chanticleer Press, Inc. 937 pp.
Bowers, N., R. Bowen, and K. Kaufman. 2004. Mammals of North America. Houghton Mifflin Co., New York, NY. 351 pp.
Burt, W.H. and R. P. Grossehnheider. 1980. Peterson Field Guide, Mammals. Houghton Mifflin Co., New York, NY. 271 pp.
Author: Daniel Toole, Wildlife Biologist, Division of Wildlife and Freshwater Fisheries