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Conservation status: Least concern (LR/lc)
The common House Mouse (Mus musculus) is one of the most numerous species of the genus Mus equivalent to the common term mouse. It is a small mammal and a rodent. It is probably the second most populous mammalian species on Earth (after humans). House mice almost always live in close proximity to humans. Laboratory mice belong to strains of house mice and are some of the most important model organisms in biology and medicine; they are by far the most commonly used laboratory mammal.
Body and genome
House mice are light brown to black, with short hair and a light belly. The ears and tail have little hair. Adults weigh some 0.4 to 1.4 ounces (12 to 40 grams); their body (including tail) is about 6 to 7.5 inches (15 to 19 centimeters) long, with the tail usually accounting for a bit more than half of it.
Young males and females are not easily distinguished; females have a significantly smaller distance between their anus and genital opening. Females have 5 pairs of mammary glands and nipples; males have no nipples. 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 and can be retracted into the body.
In addition to the regular pea-size thymus organ in the chest, house mice have a second functional pinhead-size thymus organ in the neck next to the trachea, as was reported in (Terszowski 2006).
Sequencing of the mouse genome was completed in late 2002. The haploid genome is about 3 billion bases long (3000 Mb) and therefore equal to the size of the human genome. Estimating the number of genes contained in the mouse genome is difficult, in part because the definition of a gene is still being debated and extended. A common estimate is 30,000 to 50,000 genes, about as many as in the human genome.
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. Mice are good jumpers, climbers and swimmers.
Mice are mostly active during dusk or night (nocturnal); they do not like bright lights. They live in a wide variety of hidden places that are near food sources and construct nests from various soft materials. Mice are territorial and one dominant male usually lives together with several females and young. Dominant males respect each other's territory and normally enter another's territory only if it is vacant. If two or more males are held together in a cage, they will often turn aggressive unless they have been 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. House mice, like other rodents, do not vomit.
Mice are afraid of rats, which often kill and (partially) eat them. This rat behavior is known as muricide. Despite this behaviour free-living populations of rats and mice do exist together, such as in New Zealand forests. House mice are poor competitors, and in most areas cannot survive away from human settlements in areas where other small mammals, such as wood mice, are present (Tattersall, Smith and Nowell 1997). In other areas (such as Australia) mice are able to co-exist with other small rodent species (Moro and Morris 2000).
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).
House mice also rely on pheromones. Most of these are produced by the preputial glands of both sexes and are excreted with urine. The tear fluid of male mice also contains pheromones (Kimoto 2005). Mice detect pheromones mainly with the vomeronasal organ (Jacobson's organ), located at the bottom of the nose.
The urine of house mice, especially that of males, has a characteristic strong odor. In (Achiraman 2002), ten different compounds such as alkanes, alcohols, etc. were detected in the urine. Among the ten, five compounds are specific to males, namely 3-cyclohexene-1-methanol, Aminotriazole (3-amino-s-triazole), 4-ethyl phenol, 3-ethyl-2,7-dimethyl octane and 1-iodoundecane.
The mice can sense surfaces and air movements with their whiskers.
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. If several females are held together under crowded conditions, they will often not have an estrus at all; if they are then exposed to male urine, they will become estrous after 72 hours.
Male house mice court females by emitting characteristic ultrasonic calls in the 30kHz - 110kHz range. The calls are most frequent during courtship when the male is sniffing and following the female. However, the calls continue after mating has begun at which time the calls are coincident with mounting behavior. Males can be induced to emit these calls by female pheromones. The vocalizations appear to be different in different individuals and have been compared to birdsongs because of their complexity. (Holy 2005) While females have the capability to produce ultrasonic calls, they typically do not do so during mating behavior.
Following copulation, female mice will normally develop a vaginal plug which prevents further copulation. This plug stays in place for some 24 hours. The gestation period is about 19-21 days, and they give birth to a litter of 3-14 young (average 6-8). 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, even though they don't hibernate). 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 five weeks.
House mice usually live under a year in the wild, because of a high level of predation and exposure to harsh environments. In protected environments, however, they often live two to three years. The Methuselah Mouse Prize is a competition to breed or engineer extremely long-lived laboratory mice. As of 2005, the record holder was a genetically engineered mouse that lived for 1819 days, nearly 5 years. Another record holder that was kept in a stimulating environment but did not receive any genetic, pharmacological or dietary treatment lived for 1551 days, over 4 years.
Mice and humans
- See also: Fancy mouse
House mice usually live in proximity to humans, in or around houses or fields. Originally native to Asia (probably Northern India) (Boursot et al 1996), they spread to the Mediterranean Basin about 8000 BC, only spreading into the rest of Europe around 1000 BC (Cucci, Vigne and Auffrey 2005). This time lag is thought to be because the mice require agrarian human settlements above a certain size (Cucci, Vigne and Auffrey 2005). They have since been spread to all parts of the globe by humans.
Many studies have been done on mouse phylogenies to reconstruct early human movements; for example, Gunduz et al 2001, showed a previously unsuspected early link between Denmark and Madeira on the basis of the origin of the Madeiran mice.
House mice can transmit diseases, and can damage food and food packaging. 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.
The first written reference to mice kept as pets occurs in the Erya, the oldest extant Chinese dictionary, from a mention in an 1100 B.C. version. Human domestication led to numerous strains of "fancy" or hobby mice with a variety of colors and a docile temperament. Domestic varieties of the house mouse called "feeder" mice are also used as food for some carnivorous pet reptiles, arthropods and fish. Mice bred for this purpose are genetically identical to other domestic mice, and can be kept as pets themselves.
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, to test the safety of proposed drugs, and in basic research.
The United States 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 accreditation 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 strains are inbred, so as to make them genetically almost identical. The different strains are identified with short letter-digit combinations; for example C57BL/6 and BALB/c.
The first such inbred strains were produced by Clarence Cook Little in 1909. Little was influential in promoting the mouse as a laboratory organism.
The behavioral patterns of laboratory mice are significantly different from those of most common house mice due to years of lab breeding. These behaviors are much simpler.
Mutant and transgenic strains
Various mutant strains of mice have been created by a number of methods:
- Mice resulting from ordinary breeding.
- NOD mice, which develop type 1 diabetes.
- Mice with unusual regenerative capacities.  
- "Waltzing" mice, which walk in a circular pattern due to a mutation adversely affecting their inner ear.
- Immunodeficient nude mice, lacking hair and a thymus. The mice don't produce T lymphocytes and therefore don't mount cellular immune responses. They are used for research in immunology and transplantation.
- Severe Combined Immunodeficient or SCID mice, with an almost completely defective immune system
- 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 purpose is to study the function of the gene's product or to simulate a human disease.
- Fat mice, prone to obesity due to a carboxypeptidase E-deficiency.
- Strong mice, with a disabled myostatin gene.
Mice threatening bird species
Gough Island in the South Atlantic is used by numerous seabirds for breeding, including almost all of the world's Tristan Albatross and Atlantic petrel. Until house mice arrived on the island in the 19th century with seamen, the birds did not have any mammalian predators. The mice have since grown unusually large and have learned to attack albatross chicks, which can be nearly one metre tall but are largely immobile, by working in groups and gnawing on them until they bleed to death. The estimated 700,000 mice on the island kill a total of over 1 million bird chicks per year. 
There are 3 well recognized Mus musculus sub-species:
- Mus musculus musculus (eastern European house mouse)
- Mus musculus castaneus (southeastern Asian house mouse)
- Mus musculus domesticus (western European house mouse)
An additional sub-species was described by Prager, Orrego and Sage (1998) from the Arabian Peninsula:
- Mus musculus gentilulus
The following were previously identified as sub-species, but have since been found to belong to the sub-species above:
- Mus musculus homourus
- Mus musculus molossinus (the Japanese house mouse; actually a hybrid of musculus, castaneus and domesticus; Bonhomme 1989)
- Mus musculus bactrianus (southwestern Asian house mouse)
- Mus musculus praetextus
- Mus musculus wagneri
- the National Centre for Replacement, Refinement, and Reduction of Animals in Research
- Waterston, R.H., Lindblad-Toh, K., Birney, E., Rogers, J., Abril, J.F., Agarwal, P., Agarwala, R., Ainscough, R., Alexandersson, M., An, P., et al. 2002. Initial sequencing and comparative analysis of the mouse genome. Nature 420, 520-562 (5 December 2002); section "Mouse genes" gives an estimate of 30,000-40,000. Note that out of these, only about 23,000 are currently "known" (source: "known genes" statistic, Ensembl Genebuild: Ensembl, Apr 2006, from assembly: NCBI m36, Dec 2005, found at http://www.ensembl.org/Mus_musculus/index.html)
- American Fancy Rat and Mouse Association
- the Rat and Mouse Club of America
- the Rat and Mouse Club of America
- Amori (1996). Mus musculus. 2006 IUCN Red List of Threatened Species. IUCN 2006. Retrieved on 12 May 2006.
- Bonhomme F., Miyashita N., Boursot., Catalan J. and Moriwaki K (1989) Genetic variation and polyphyletic origin in Japanese Mus musculus. Heredity, 63, 299- 308.
- Boursot P., Din W., Anand R., Darviche D., Dod B., Von Deimling F., Talwar G. P. and Bonhomme F. (1996) Origin and radiation of the house mouse: mitochondrial DNA phylogeny. Journal of Evolutionary Biology, 9, 391-415.
- Cucchi T., Vigne J-D. and Auffray J-C. (2005) First occurrence of the house mouse (Mus musculus domesticus Schwarz & Schwarz, 1943) in the Western Mediterranean: a zooarchaeological revision of subfossil occurrences. Biological Journal of the Linnean Society, 84, 429-445.
- Gündüz I., Auffray J.-C., Britton-Davidian J., Catalan J., Ganem G., Ramalhinho M.G., Mathias M.L. and Searle J.B. (2001) Molecular studies on the colonization of the Madeiran archipelago by house mice. Molecular Ecology, 10, 2023-2029.
- Kimoto H., Haga S., Sato K., Touhara K. (2005) Sex-specific peptides from exocrine glands stimulate mouse vomeronasal sensory neurons. Nature, 437. 898 - 901. Abstract
- Holy TE, Guo Z (2005) Ultrasonic Songs of Male Mice. PLoS Biol 3(12): e386. Full Text
- Moro, D. and Morris, K. (2000) Movements and refugia of Lakeland Downs short-tailed mice, Leggadina lakedownensis, and house mice, Mus domesticus, on Thevenard Island, Western Australia. Wildlife Research 27, 11-20.
- Prager E. M., Orrego C. and Sage R. D. (1998) Genetic variation and phylogeography of Central Asian and other house mice, including a major new mitochondrial lineage in Yemen. Genetics 150, 835-861.
- Achiraman S, Archunan G. (2002) Characterization of urinary volatiles in Swiss male mice (Mus musculus): bioassay of identified compounds. J Biosci. 2002 Dec;27(7):679-86. PMID 12571373
- Terszowski G et al. (2006) Evidence for a Functional Second Thymus in Mice. Science. 2006 Mar 2. PMID 16513945
- Tattersall F. H., Smith, R. H. & Nowell, F. (1997). Experimental colonization of contrasting habitats by house mice. Zeitschrift für Säugetierkunde. 62: 350-358.
- Comprehensive house mouse information, including pictures, by the University of Michigan Museum of Zoology
- High-Resolution Brain Maps and Brain Atlases of Mus musculus
- Biology of the Mouse, from the Louisiana Veterinary Medical Association
- Biology of Laboratory Rodents by David G. Besselsen
- Mus musculus, by the Museum of Texas Tech University
- Nature Mouse Special 2002
- Ensembl Mus musculus genome browser, from the Ensembl Project
- Vega Mus musculus genome browser, includes NOD mouse sequence and annotation
- Taxonomy entry from NCBI, with comprehensive links to database information about Mus musculus
- Pictures, movies and applets showing the anatomy of Mus musculus, from www.digimorph.org
- Monica Lawlor: "A Home For A Mouse", Humane Innovations and Alternatives (Vol 8, 1994). Description of behavior and senses.
- 'Fancy Mice', includes much behavioral and physiological information
- Some information on muricide
- Vincenzo Covelli: "Guide to the Necropsy of the Mouse", pictures and explanation of a mouse autopsy
- Vocalizations during copulation
- "Songs" of male mice
- Michael Purdy: "Researchers add mice to list of creatures that sing in the presence of mates"-Study of male mouse "song" with mouse song recording (MP3), by Washington University Medical School
- "Surprise organ discovered in mice; Mice are shown to have two thymus organs, not just one" March 2, 2006, Nature online
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