Mice and rats are well-known rodents people usually associate with cities. Known as commensal, some species of mice and rats have a long history of living close to people. Fossil records place evidence of House Mice in a Neolithic Turkish community more than 8,000 years ago. But there are also wild species of mice and rats scampering around the forests and plains of North America that steer clear of humans.
There are over 70 species of mice and rats widely distributed across North America. Washington is home to 12 of these species—nine native species: Great Basin Pocket Mouse, Kangaroo Rat, Western and Pacific Jumping Mice, Bushy-tailed Woodrat, Northern Grasshopper Mouse, Deer Mouse, Keen’s Mouse and the Western Harvest Mouse and three non-native or introduced species: House Mouse, Norway Rat and Black Rat.
The most common mice and rats and the species that cause the most damage are the non-native House Mouse (Mus musculus), Norway Rat (Rattus norvegicus) and Black or Roof Rat (Rattus rattus). All three of these species are in the family Muridae, which is the largest family of rodents and mammals in the world. It contains over 700 species naturally found in Eurasia, Africa, and Australia.
The spread of these "old world" mice and rats is directly linked to human development and movement. Their dispersal coincides with the construction of houses and barns, the development of agriculture, and the beginning of ship travel.
Did you know? Rats and mice are the most abundant mammals on the planet.
The House Mouse ranges in color from light brown to black, usually with lighter underparts. Domestic strains of House Mice are predominantly white with black or gray markings.
The Norway or Brown Rat, as its name suggests, is brown with lighter underparts and a shorter tail while Black Rats are darker in color all over with slender bodies, pointed muzzles and tails that are longer than their bodies.
Wild mouse and rat species vary in color from pale yellow to dark brown; they have large ears and eyes and long tails. Some species like the Kangaroo Rat and jumping mice have specialized hind legs for hopping.
Did you know? Norway rats aren’t actually from Norway and are thought to have originated in central Asia.
The natural habitat of wild mice and rats includes forests and grasslands, and their range extends from sea level to high mountain elevations.
Norway Rats, Roof Rats, and House Mice, however, are found in cities and suburbs where food and shelter are plentiful. Norway Rats are ground dwellers and often live in cellars, basements and lower floors of buildings. Because they swim well, they can also survive in sewers.
House Mice build loose nests of shredded paper and fabric behind rafters, in woodpiles, in storage areas or other hidden locations where there is a nearby source of food.
Black Rats build nests in attics, trees and overgrown shrubbery, and Norway Rats prefer to nest underground.
Development and Family Structure
Old world mice and rats breed continuously and, depending on the species, have between one and 12 litters per year. Litters average from five to six young who are born blind and without fur. Fully furred after 10 days and weaned at about three weeks, House Mice reach sexual maturity between five and six weeks. Rats reach sexual maturity at three to four months old.
Did you know? Rats prefer to nest where water is easily available.
Wild mice and rats are omnivores, eating a variety of vegetable matter, insects and meat. Commensal rodents feed on any human food available, as well as birdseed and pet food.
Mice and rats, in general, are nocturnal and will even become inactive is the moon is too bright. They are good climbers and jumpers and some species of rats are also good swimmers.
Norway Rats are capable of producing ultrasonic vocalizations to communicate with their mothers, as babies, and to alert others to predators or other dangers.
Rats and mice are very social animals, they are very intelligent and curious, and despite popular opinion they are actually quite clean; they groom themselves several times a day.
Did you know? Rats and mice are less likely than dogs or cats to catch and transmit parasites and viruses due to their cleanliness.
Living with Mice and Rats
Because they are gnawers, mice and rats can cause damage to buildings, especially when they chew through insulation and wiring. Rodents are attracted to stored food, especially grain and seed, and they can contaminate it with their urine and feces.
Property owners can discourage and exclude rodents by creating an unfriendly habitat.
Keep rodents away from food
Follow these steps to protect food:
- Store food in secure containers.
- Dispose of garbage in metal cans with tight-fitting lids.
- Do not put food scraps on open compost piles.
- Bury food waste in an underground composter or in a lidded worm box
- If you feed birds, store seed in secure metal containers. Put only as much seed in feeders as will be eaten in a day, and clean up spilled seed and hulls immediately.
Keep rodents out of your home
Rodents can enter buildings through holes as small as one-half inch in diameter and they can usually climb interior walls. You need to seal your house and other buildings thoroughly.
Use hardware cloth with squares one-quarter inch or smaller to patch larger holes, screen vent openings, and skirt building foundations at least 12 inches underground.
- Stuff steel wool into cracks and around drainpipes and other small openings.
- Seal the stuffing with expanding spray foam, caulking or cement.
- Seal all holes and cracks in walls, eaves, and roofs.
- Inspect your home regularly and repair any potential entry points.
Make your yard unattractive
Rodents also nest outdoors, especially where overgrown plants offer cover. You can eliminate their shelter sites by:
- Mowing long grass.
- Removing weeds and brush.
- Trimming shrubs at ground level and large ground cover plants such as ivy.
- Removing debris and woodpiles close to buildings.
Rodents are also drawn to pet food. It is best not to feed cats and dogs outside, but if you have no other choice, pick up bowls, spilled food and leftovers promptly.
Bowers N. Bowers R & Kaufman K. (2004). Kaufman Field Guide to Mammals of North America. New York, NY: HIllstar Editions L.C.
Burke Museum. February 1, 2016. Mammals of Washington: Deer Mouse.
Washington Department of Fish and Wildlife. February 1, 2016. Living with Wildlife: Rats.