House Mouse

Mus musculus is the common 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 and physiology
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 40 grams; their body (including tail) is about 15-19 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. 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 mouse's skull and usually hairless. They can be retracted into the body. Females have 5 pairs of mammary glands and nipples; males have no nipples.

House mice have Harderian glands near their eyes which produce a reddish-brown discharge when the animals are stressed.

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 windpipe, as was reported in (Terszowski 2006).

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. 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 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.

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
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

Special populations
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 up to 1 m 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. 

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, 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 instance, the strain 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.

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 more simplistic.

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 strains
Various mutant strains of mice have been created by a number of methods:
 * Mice resulting from ordinary breeding
 * Mice with unusual regenerative capacities.
 * 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 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
 * Large mice, with an inserted rat growth hormone gene
 * Oncomice, with an activated oncogene, so as to significantly increase the incidence of cancer
 * Doogie mice, with enhanced NMDA receptor function, resulting in improved memory and learning
 * 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.
 * Strong mice, with a disabled myostatin gene.
 * NOD mice develop type 1 diabetes.

Since 1998, it has been possible to clone mice from cells derived from adult animals.

External links and sources

 * Comprehensive house mouse information, including pictures, by the University of Michigan Museum of Zoology
 * The mouse life cycle
 * 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
 * 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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