The Norway rat is the largest of the commensal (i.e., living in close association with humans) rodents. The head and body are seven to ten inches long and the tail is an additional six to eight inches. It has a stocky body and weighs seven to 18 ounces. The fur is coarse, shaggy and brown with some black hairs. The muzzle is blunt, eyes and ears are small, and the tail, which is bi-colored, is shorter than the head and body combined. Norway rat droppings are up to ¾-inch long with blunt ends.
Adults are sexually mature in two to five months. Females produce three to six litters per year, each averaging seven to eight young. Adults live from six to twelve months. They have poor sight but keen senses of smell, taste, hearing, and touch.
Rats are nocturnal. They are shy about new objects and very cautious when things change in their environment and along their established runs. Outdoors, Norway rats prefer to nest in burrows in the soil, e.g., under sidewalks and concrete pads, stream/river banks, railroad track beds, next to buildings, in low ground cover, etc. The burrows typically have one main entry hole and at least one escape hole. The rats easily enter buildings through ½-inch and larger gaps. In buildings they prefer to nest in the lower levels of the building, e.g., crawlspace, basement, loading dock and sewers. They prefer foods such as meat, fish, and cereals and require a separate nonfood water source. Their foraging range is 100 to 150 feet from their nest. Rats are associated with various diseases and occasionally bite. Plague is of little concern because it has not occurred in rats in the United States for many years. However, leptospirosis is vectored by rats, and, thus, is a disease of great concern. This disease is acquired by eating food and drinking water, which are contaminated with infected rat urine. Rats also cause significant structural damage and product destruction.