Although there are many types of mouse and
rat, the words most often are used to refer to
the house mouse, Mus musculus, and the Norway
rat, Rattus norvegicus. These two species belong to
a family of rodents known as the Muridae, the Old
World mice and rats, and there are over 450 species
within this group. Both species underwent
their early evolution in the wilds of Asia. As humans
appeared and began to settle in farms and
villages, mice and rats became associated with
them. As humans migrated to other parts of the
world and as commercial exchanges took place
around the world, mice and rats went along for
the ride and became established in the OldWorld
and the New World, and today may be found almost
any place on earth where humans are found.
At birth, mice and rats are unimposing animals,
as the pups are born in an altricial state and
are naked, sightless, and helpless. They develop
rapidly, however, and by three weeks of age are
weaned from their mother. By 1.5 to 2 months of
age for the mouse, and 2 to 3 months of age for the
rat, they are sexually mature. Their powers of reproduction
are phenomenal and many litters,
some containing more than a dozen young, can be
produced in a single year. Asingle female mouse
mayproduce over one hundred young in one year.
As adults, mice and rats still are not imposing.
They have a long, scaly, scantily haired tail, and
are a grayish brown color, which is somewhat
paler in the belly. Although they have poor sight,
their senses of smell, hearing, and taste are all excellent.
The Good and Bad of Mice and Rats
All of the other mammalian species combined, as well as all other animals, do not cause as much damage and destruction to humans as do mice and rats. The two species have been able to adjust to living with and near humans and their habitations. Their diets are omnivorous and include all types of foods, grains, and grain products. They can climb, burrow, and swim, and can invade nearly all buildings, houses, barns, warehouses, and other structures. As a result, they cause billions of damage each year around the world. Human food is eaten or destroyed by contamination with urine and feces. The urinemaycontain bacteria, causing diseases such as leptospeosis. Food poisoningmaybe caused by salmonellosis in their feces. Many children and some adults are bitten each year by rats, especially in rundown urban areas. Even those mice and rats that live under semiwild conditions may affect humans in their effect on other wildlife. Ground-nesting birds and the young of other animals are susceptible to predation by mice and rats. The costs of these pests to humans is tremendous. Some consolation for having to deal with the extensive bad side of mice and rats may be found in their beneficial use as laboratory animals. Both species are used widely in medical and genetic research. The mouse has been used to elucidate many basic principles of genetics, and today, its many genetic variants serve as models for many human disorders, including genetic diseases and cancer. As studies define the human genome, similar studies are defining the mouse and rat genomes. The laboratory rat, although not as genetically well known as the mouse, has been used extensively in physiological and psychological research. It is unlikely that humans will ever be able to eliminate completely mice and rats and control their destruction, but it also is unlikely that their importance to both basic and applied science will ever be diminished, at least in the foreseeable future.
Family: Muridae, with fifteen subfamilies, 241 genera, and 1,082 species
Geographical location: Worldwide, through introduction by humans
Habitat: Occurs in highly varied habitats, usually in association with humans
Gestational period: Nineteen to twenty-two days, although this may be lengthened in nursing females due to delayed implantation
Life span: Around one year in the wild; laboratory rats and mice may live three to four years or longer
Special anatomy: The scantily haired tail helps to distinguish house mice and Norway rats from most other types of mice and rats
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