Mus musculus is the house mouse. This mouse is believed to be the second most populous mammalian species on Earth, after Homo sapiens. House mice almost always live in close proximity of humans. Laboratory mice are strains of house mice that form important model organisms in biology and medicine; they are the most commonly used laboratory mammal.
Physical description
House mice are light brown to black, with short hair and a light belly. Laboratory mice and pet mice are often white. The ears and tail have little hair. Adults weigh some 12 to 38 grams; their body (including tail) is about 15-19 centimeters long, with the tail usually accounting for a bit more than half of it. House mice, especially males, have a characteristic musky odor.
Young males and females are not easily distinguished; females have a significantly smaller distance between their anus and genital opening, it is also often hairless. When sexually mature, the most striking and obvious difference is the presence of testicles on the males. These are relatively large compared to the rest of the body, being approximately half the size of the mouses' skull and usually hairless. Females have 5 pair of mammary glands and nipples; males do not have nipples.
Behavior
House mice usually walk, run or stand on all fours, but when eating, fighting or orienting themselves, they stand only on the hind legs, supported by the tail. When running, the horizontal tail serves for balance; the end stands up vertically, unless the mouse is frightened. The mice are good jumpers, climbers and swimmers.
Mice are mostly active during dusk or night; they do not like bright lights. They live in a wide variety of hidden places that are near food sources and constructs nests from various soft materials. Mice are territorial and one dominant male usually lives together with several females and young. If two or more males are held together in a cage, they will often turn aggressive unless they are raised together from birth.
House mice primarily feed on plant matter, but they will also accept meat and dairy products. They will drink water but require little of it, relying mainly on the moisture present in their food. They will eat their feces to acquire nutrients produced by bacteria in their guts.
Life cycle and reproduction
Female house mice have an estrous cycle that is 4-6 days long, with estrus itself lasting less than a day. The gestation period is about 19-21 days, and they give birth to an average litter of 7-13 young. One female can have some 5-10 litters per year, so their population can increase very quickly. Breeding occurs throughout the year (however, animals living in the wild don't reproduce in the colder months). The newborn are blind and furless. Fur starts to grow some three days after birth and the eyes open one to two weeks after birth. Females reach sexual maturity at about 6 weeks and males at about 8 weeks, but both can breed as early as 35 days.
House mice live about 1-2 years on average. The Methuselah mouse contest is a competition to construct extremely long-lived laboratory mice. As of 2004, the record holder was a genetically engineered mouse that lived for 1819 days, nearly 5 years.
Senses and communication
As primarily nocturnal animals, house mice have little or no color vision. They have a sharp sense of hearing and can perceive ultrasound, possibly up to 100kHz. They communicate both in the human audible range with squeaks (for long-distance warnings), and in the ultrasound range (for short-distance communication). They can sense surfaces and air movements with their whiskers. They have a sharp sense of hearing.
They also rely on pheromones, which they sense with the Jacobson's organ at the bottom of their nose.
Mice and men
House mice usually live in close proximity of humans, in or around houses or fields. Originally native to Asia (probably Northern India), they spread to Europe and humans introduced them all over the world only fairly recently.
House mice can transmit diseases, and their droppings can spoil foods. They can also cause substantial damage when feeding on grain.
It is thought that house mice were the primary reason for the taming of the domestic cat. Various mousetraps have been developed to catch mice. Generally, rats are more harmful to humans than mice.
House mice have been bred as pets for a long time, producing numerous strains of "fancy mice " with unusual colors or behaviors. The laboratory strains are of prime importance in science.
Subspecies
A number of subspecies of Mus musculus have been described:
- Mus musculus bactrianus (southwestern Asian house mouse)
- Mus musculus castaneus (southeastern Asian house mouse)
- Mus musculus domesticus or Mus domesticus (western European house mouse)
- Mus musculus gentilulus
- Mus musculus homourus
- Mus musculus molossinus
- Mus musculus musculus (eastern European house mouse)
- Mus musculus praetextus
- Mus musculus wagneri
Laboratory mice
Mice are convenient in research because their physiology is similar to that of humans (though rats are a better models for certain diseases) and their short life cycle makes breeding easy. They are mainly used to model human diseases in order to develop new drugs, and to test the safety of proposed drugs.
The US Animal Welfare Act covers most mammals but specifically excludes laboratory mice and rats. Most academic research institutes seek voluntary accreditation which requires certain minimal standards of care for laboratory animals. This accredation is a prerequisite for federal funding.
Most laboratory mice are hybrids of different subspecies, most commonly of Mus musculus domesticus and Mus musculus musculus. Laboratory mice are often white, and some are albinos. Many (but not all) laboratory strands are inbred, so as to make them genetically almost identical. The different strands are identified with short letter-digit combinations; for instance, the strand whose genome was sequenced in 2002 (see below) is C57BL/6J.
The first such inbred strains were produced by Clarence Cook Little in 1909. Little was influential in promoting the mouse as a laboratory organism.
Genome
Sequencing of the mouse's genome was completed in late 2002. It is about 2.5 billion base pairs long and contains roughly 30,000 genes, about as many as the human genome.
Mutant and transgenic strands
Various mutant strands of mice have been created by a number of methods:
- Mice resulting from ordinary breeding
- Waltzing mice, showing a peculiar style of walking due to a mutation affecting their inner ear
- Nude mice , lacking hair and a thymus. The mice don't produce T lymphocytes and therefore don't mount most immune responses. They are used for research in immunology and transplantation.
- Transgenic mice , with foreign genes inserted into their genome
- Knockout mice, where a specific gene was made inoperable by a technique known as gene knockout; the aim is to study the function of the gene's product or to simulate a human disease.
Since 1998, it has been possible to clone mice from cells derived from adult animals.
See also
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