Commensal Rodents
Norway rat, Rattus norvegicus Hollister;
roof rat, Rattus rattus Linnaeus;
house mouse, Mus musculus Linnaeus


The Norway rat is the most common and largest of the commensal rodents. This species has a thick, heavy body. They grow up to 460 mm long including the tail which is always shorter than the body. The eyes and ears are small and the nose blunt. The upper body is usually reddish- or grayish-brown to black, and the underside is gray or yellowish-white. The droppings of this rodent are capsule shaped and 13 to 20 mm long. The Norway rat may also be known as the brown rate, house rat, barn rat, sewer rat, black rat, or wharf rat.

Smaller than the Norway rat, the roof rat has a light, slender body; a whip-like tail that is longer than its total body; large, naked ears; large eyes, and a pointed nose. It may grow up to 45.5 cm long. The droppings of roof rats are elongate, pointed and slightly smaller than those of the Norway rat.

House mice grow to between 13 and 20 cm from head to tail. It is light brown to black with soft, harsh or spiny fur; fur on the underside of the body is white. The house mouse does not have distinctive, white markings on its feet like the field mouse. This species leaves smooth droppings up to 6 mm long. The Norway rat, roof rat, and house mouse can be distinguished from other rodents by their scaly, almost hairless tails.


Distribution -- Commensal rodents are not native to the United States. These rodents were imported with European settlers. The Norway rat and house mouse now occur throughout the country. The roof rat, on the other hand, is found along the Pacific Coast and in the southeastern states from Texas through Virginia.

The Norway rat occurs in urban or rural areas where it constructs burrows in stream banks and canals and adjacent to redidences. The roof rat is a good climber and is frequently found living and nesting above ground level in trees or the upper stores of buildings. Because of the house mouse's ability to gain access to structures through small openings and to adapt to a wide variety of situations, it is much more common in buildings than either species of rat.

Feeding Habits -- Rats consume a wide range of substances including garbage, meat, fish and cereals. The house mouse feeds primarily on vegetable material such as seeds, fleshy roots, leaves and stems. Occasionally, however, it will eat meat or insects.

Damage -- Rodents destroy far more than they actually eat. Mice damage household articles by their gnawing activities, and they contaminate food. Besides damaging crops and grain reserves, rats often gnaw insulation from wires and have occasionally been known to cut through lead pipe and concrete dams. The tunneling activity of Norway rats causes structural damage to building foundations and platforms. Rats cause billions of dollars worth of damage throughout the world each year.

Life History -- Norway rats, roof rats, and house mice are the rodents most commonly found living in close association with man. They are called commensal rodents, referring to the fact that they live at man's expense without giving anything in return. Rats have very high reproductive rates. A pair of rats and their offspring could produce 1,500 more rats in only one year if they all survived. Rats breed at 3 to 5 months of age, carry their young for 21 to 33 days and have 6 to 12 young per litter, and 4 to 7 litters per year. The actual figures within these ranges depend on the availability of food, the density of rats in an area and the age of the female rat.

Although one female rat may produce up to 84 young in a year, on the average about 20 young per female per year survive to maturity. Female rats can breed again 1 to 2 days after giving birth. Breeding occurs all year; however, in most populations breeding peaks in the spring and fall. Generally, rat populations are lowest in the winter and, on the whole, control will be most effective at this time.

Rats usually construct their nests near food and water. Nests are generally hidden in quiet, secluded places and are made of soft materials such as paper, cloth, feathers, grass, etc. They are bowl-shaped and approximately 20 cm in diameter. Newborn rats are completely helpless and are born naked with their eyes and ears closed. They develop rapidly, however, and by 9 to 14 days are covered with hair and beginning to explore their environment. They nurse until they are about 4 to 5 weeks old and by 3 to 5 months are independent and sexually mature. The average life span of a rat is one year.

Mice mature rapidly and by 1.5 to 2 months of age are old enough to reproduce. They carry their young for 19 to 21 days and have 5 or 6 young per litter and 5 to 8 litters per year. A female mouse can breed again 2 to 4 days after giving birth. House mice breed throughout the year and do not show any seasonal breeding peaks.

Mice build nests of soft materials such as grass, paper, or cloth in hidden secluded areas. Nests are usually 13 cm in diameter with a small entrance hole on the side. Mice are born naked with their eyes and ears closed. They develop rapidly, and in 2 weeks they are covered with hair, their eyes and ears are open and they are no longer totally helpless. In another week they begin exploring the nest area and eating solid foods. At 5 to 8 weeks of age the mice are independent and ready to breed. House mice live 1 to 2 years.

A female mouse could produce 40 young in one year; however, this rarely if ever happens. The actual number of births and young that survive to maturity depends on the availability and quality of food and the degree to which the mice are crowded.


Successful rodent control must be a community effort and involves three different aspects: (1) sanitation, (2) rodent-proofing, and (3) rodent killing. Though the first two methods are most important, they also require the most effort and therefore are frequently neglected.

Sanitary practices such as the use of tight lids on garbage cans, destruction of refuse, removal of junk and proper storage of food and construction materials limit food supply and nesting sites for rodents. Packaged foodstuffs, boxes, lumber, firewood, etc., should be stored 30 to 45 cm (12 to 18 inches) off the floor or ground and away from walls. Storage in this manner provides access for inspection and allows control measures such as poisons or traps to be placed under stacked materials and next to walls where rats and mice travel. It also removes rodent hiding and nesting places. Livestock feed and other food removed from commercial packaging should be stored in metal containers to exclude rodents. Spilled food provides an easy meal for rats and mice and should be swept up regularly. Well-swept floors make detection of signs of rodent activity possible.

Rodent-proofing involves modifying structural details to keep rats and mice out of buildings. Thorough inspection is the first step and should be carried out semiannually. A list of possible problem sites and suggested ways to eliminate them follows.

  1. All openings where water pipes, drain spouts and vents enter a building should be tightly sealed with sheet metal patches or filled with concrete.

  2. Doors, windows and screens should close tightly to ensure that rodents cannot enter through them.

  3. Installing 24- to 26-gauge galvanized sheet metal flashing around wooden door jambs and metal kickplates on the outside of doors will prevent rats from gnawing entrances under or around doors.

  4. Floor drains and fan openings should be tightly covered with 19-gauge, 6 mm (1/4 inch) mesh, galvanized, hardware cloth.

  5. Removing materials stacked against the outside of an otherwise rodent-proof building will stop rats and mice from gaining entry into upper stories. In situations where it is not possible to stack materials elsewhere, rodent-proofing should be extended to a height of 91 cm (36 inches) above the highest probable level of piled materials.

  6. Nonconcrete basement floors and shallow foundations should be protected from burrowing rats with a cement curtain wall around the outer edge extending vertically 91 cm (36 inches) into the ground or in an "L" shape 61 cm (24 inches) into the ground with a 30 cm (12 inch) lip extending outward.

  7. Any other possible entrances (openings larger than 6 mm or 0.25 inches) should be blocked off so that rodents cannot get in.

Rodent-killing as a single control measure is expensive and effective for only brief periods of time. It is most effective if carried out during the winter when reproduction is at its lowest level. Also, as mentioned earlier, it is best to sanitize and rodent-proof prior to killing rodents when possible. If rodent infestations are severe, it may be better to kill the rodents first, because removal of harborage may cause them to migrate to nearby buildings.

Trapping is a common method of rat killing. Traps may or may not be baited but should always be placed in areas of rodent activity. Bacon, peanut butter, bread, and nutmeats make suitable baits. Mouse traps should be placed at intervals of about 1 meter (3 to 4 feet); rat traps should be set 4.5 to 9 m (15 to 30 feet) apart. To be most effective, they should be placed with the bait toward and the spring away from the wall. Traps are an alternative to rodenticides, especially where chemicals cannot be used; however, the use of traps is more labor intensive than chemicals.