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Mus musculus

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Mus musculus

Conservation status
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Rodentia
Family: Muridae
Subfamily: Murinae
Genus: Mus
Subgenus: Mus
Species: M. musculus
Binomial name
Mus musculus
Linnaeus, 1758
Subspecies
  • Mus musculus bactrianus
  • Mus musculus castaneus
  • Mus musculus domesticus
  • Mus musculus gentilulus
  • Mus musculus musculus
House mouse range

The house mouse (Mus musculus) is a small rodent, a mouse. It is one of the most numerous species of the genus Mus. Although a wild animal, the house mouse mainly lives associated with humans, causing damage to crops and stored food.

The house mouse has been domesticated as the pet or fancy mouse, and as the laboratory mouse, which is one of the most important model organisms in biology and medicine. It is by far the animal most commonly genetically altered for scientific research.[2]

Characteristics

House mice have an adult body length (nose to base of tail) of 7.5–10 cm (3.0–3.9 in) and a tail length of 5–10 cm (2.0–3.9 in). The weight is typically 10–25 g (0.4–0.9 oz). They vary in color from white to grey to light brown to black. They have short hair and a light belly. The ears and tail have little hair. The hind feet are short compared to Apodemus mice, only 15–19 mm (0.59–0.75 in) long; the normal gait is a run with a stride of about 4.5 cm (1.8 in), though they can jump vertically up to 45 cm (18 in).[3] The voice is a high-pitched squeak.[4][5] House mice thrive under a variety of conditions: they are found in and around homes and commercial structures, as well as in open fields and agricultural lands.


Young males and females are not easily distinguished: females have a significantly smaller distance between their anus and genital opening. Females have five pairs of mammary glands and nipples; males have no nipples.[6] When sexually mature, the most striking and obvious difference is the presence of testicles on the males. These are 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.[7]

Subspecies

The three widely accepted subspecies are increasingly treated as distinct species:[8][9]

  • Mus musculus castaneus (southern and southeastern Asia)
  • Mus musculus domesticus (western Europe, southwestern Asia, Americas, Africa, and Oceania)
  • Mus musculus musculus (eastern Europe and northern Asia)

Two additional subspecies have been recognized more recently:[9]

  • Mus musculus bactrianus (central Asia)
  • Mus musculus gentilulus (Arabian Peninsula; Madagascar)[10]

Many more names have been given to house mice, but are now regarded as synonyms of other subspecies. Some populations are hybrids of different subspecies, including the Japanese house mouse (M. m. molossinus).[9][11]

Behavior

House mice usually run, walk, or stand on all fours, but when eating, fighting, or orienting themselves, they rear up on their hind legs with additional support from the tail. When they run, 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 crepuscular or nocturnal; they are averse to bright lights. The average sleep time of a captive house mouse is reported to be 12.5 hours per day.[12] They live in a wide variety of hidden places 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 housed together in a cage, they will often become aggressive unless they have been raised together from birth.

House mice primarily feed on plant matter, but are omnivorous. They will eat their own faeces to acquire nutrients produced by bacteria in their intestines. House mice, like most other rodents, do not vomit.

Mice are generally afraid of rats which often kill and eat mice, a behavior known as muricide. Despite this, free-living populations of rats and mice do exist together in forest areas in New Zealand, North America and elsewhere. House mice are generally poor competitors and in most areas cannot survive away from human settlements in areas where other small mammals, such as wood mice, are present.[13] However, in some areas (such as Australia), mice are able to coexist with other small rodent species.[14]

Social behavior

The social behavior of the house mouse is not rigidly fixed into species-specific patterns but is instead adaptable to the environmental conditions, such as the availability of food and space.[15][16] This adaptability allows house mice to inhabit diverse areas ranging from sandy dunes to apartment buildings.[15]

House mice have two forms of social behavior, the expression of which depends on the environmental context. House mice in buildings and other urbanized areas with close proximity to humans are known as commensal.[15] Commensal mice populations often have an excessive food source resulting in high population densities and small home ranges. This causes a switch from territorial behavior to a hierarchy of individuals.[15][17] When populations have an excess of food, there is less female-female aggression, which usually occurs to gain access to food or to prevent infanticide.[15] Male-male aggression occurs in commensal populations, mainly to defend female mates and protect a small territory.[15][16] The high level of male-male aggression, with a low female-female aggression level is common in polygamous populations.[18] The social unit of commensal house mouse populations generally consists of one male and two or more females, usually related.[18][19] These groups breed cooperatively, with the females communally nursing. This cooperative breeding and rearing by related females helps increase reproductive success. When no related females are present, breeding groups can form from non-related females.[19]

In open areas such as shrubs and fields, the house mouse population is known as noncommensal. These populations are often limited by water or food supply and have large territories.[16] Female-female aggression in the noncommensal house mouse populations is much higher, reaching a level generally attributed to free-ranging species. Male aggression is also higher in noncommensal populations. In commensal populations, males come into contact with other males quite frequently due to high population densities and aggression must be mediated or the risk of injury becomes too great.[15]

Both commensal and noncommensal house mouse males aggressively defend their territory and act to exclude all intruders. Males mark their territory by scent marking with urine. In marked territories, intruders showed significantly lower aggression than the territory residents.[16] House mice show a male-biased dispersal; males generally leave their birth sites and migrate to form new territories whereas females generally stay and are opportunistic breeders rather than seasonal.[20]

Senses and communication

Vision

As primarily nocturnal animals, house mice have little or no color vision. Their visual apparatus is basically similar to humans, but differs markedly in at least one respect. The ventral area of the mouse retina has a much greater density of ultraviolet-sensitive cones than other areas of the retina, although the biological significance of this structure is unknown.[21][22][23]

Olfaction

House mice also rely on pheromones for social communication, some of which are produced by the preputial glands of both sexes. The tear fluid and urine of male mice also contains pheromones, such as major urinary proteins.[24][25] 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. At least 10 different compounds, such as alkanes, alcohols, etc., are detectable in the urine. Among them, 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.[26]

Odours from adult males or from pregnant or lactating females can speed up or retard sexual maturation in juvenile females and synchronise reproductive cycles in mature females (i.e. the Whitten effect). Odours of unfamiliar male mice may terminate pregnancies, i.e. the Bruce effect.

Touch

Mice can sense surfaces and air movements with their whiskers which are also used during thigmotaxis. If mice are blind from birth, super-normal growth of the vibrissae occurs presumably as a compensatory response,[27] or if the vibrissae are absent, the use of vision is intensified.[28]

Life cycle and reproduction


Female house mice have an estrous cycle about four to six 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 come into estrus after 72 hours.

Male house mice court females by emitting characteristic ultrasonic calls in the 30 kHz–110 kHz 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 behaviour. Males can be induced to emit these calls by female pheromones. The vocalizations appear to differ between individuals and have been compared to bird songs because of their complexity.[29] While females have the capability to produce ultrasonic calls, they typically do not do so during mating behaviour.

Following copulation, female mice will normally develop a copulation 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 six to eight). One female can have 5 to 10 litters per year, so the mice population can increase very quickly. Breeding occurs throughout the year. (However, animals living in the wild do not reproduce in the colder months, even though they do not hibernate.) The newborn are blind and without fur. Fur begins growing about three days after birth, and the eyes open one to two weeks after birth. Females reach sexual maturity at about six weeks of age and males at about eight weeks, but both can copulate as early as five weeks.

Life expectancy

House mice usually live under a year in the wild, due to 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 1,819 days ().[30] Another record holder that was kept in an enriched environment but did not receive any genetic, pharmacological, or dietary treatment lived for 1,551 days ().[31][32]

Mice and humans

House mice usually live in proximity to humans, in or around houses or fields. Originally native to Asia (probably northern India),[33] they spread to the Mediterranean Basin about 8000 BC, only spreading into the rest of Europe around 1000 BC.[34] This time lag is thought to be because the mice require agrarian human settlements above a certain size.[34] 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, one study suggests the possibility of a previously unsuspected early link between Northern Europe and Madeira on the basis of the origin of Madeiran mice.[35]


House mice can transmit diseases, and can damage food and food packaging. Some of the diseases the house mouse carries can be deadly: for example, leptospirosis, murine typhus, rickettsialpox, tularemia, lymphocytic choriomeningitis [36] and potentially bubonic plague. House mice can also cause substantial damage when feeding on grain. House mice were thought to be the primary reason for the taming of the domestic cat. Various mousetraps have been developed to catch 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 BC version.[37] Human domestication led to numerous strains of "fancy" or hobby mice with a variety of colours and a docile temperament.[38] Domestic varieties of the house mouse, called "feeder" mice, are also used as food for some carnivorous pet reptiles, birds, arthropods, and fish. Mice bred for this purpose are genetically identical to other domestic mice, and they can be kept as pets themselves.[38]

Invasive species

Mice have become an invasive species on islands to where they have spread during the period of European exploration and colonisation.

New Zealand had no land mammals prior to human occupation and the house mouse is one of many species that have been introduced. Mice are responsible for the reduction in native bird species since they eat some of the same foods as birds. They are also known to kill lizards and have a huge effect on native insects.[39]

Gough Island in the South Atlantic is used by 20 species of seabird for breeding, including almost all of the world's Tristan albatross (Diomedea dabbenena) and Atlantic petrel (Pterodroma incerta). 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 1 m tall, but are largely immobile, by working in groups and gnawing on them until they bleed to death.[40]

Diseases (zoonoses) and allergies

In 1999, a study completed at the Dickson Centre for Sleep Studies discovered that the protein FgFF, present in mouse droppings and urine, was indicated as a possible cause of late onset asthma, hairloss and rhinobalonitis ("balloon nose") in insomnia sufferers. The findings suggested that mice and their offspring were present on the pillows and bedsheets of the subjects of the study.

Laboratory mice


Mice are the most commonly used mammalian research model with hundreds of established inbred, outbred, and transgenic strains. They are common experimental animals in biology and psychology, primarily because they are mammals, are relatively easy to maintain and handle, reproduce quickly, and share a high degree of homology with humans. The mouse genome has been sequenced, and many mouse genes have human homologues. In addition to being small, relatively inexpensive, and easily maintained, several generations of mice can be observed in a relatively short period of time as mice reproduce very quickly.[41]

Most laboratory mice are hybrids of different subspecies, most commonly of Mus musculus domesticus and Mus musculus musculus. Laboratory mice can have a variety of coat colours, including agouti, black and albino. Many (but not all) laboratory strains are inbred, so as to make them genetically almost identical. The different strains are identified with specific 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.[42]


Legislation of use in science

United Kingdom

In the UK, as with all other vertebrates and some invertebrates, any scientific procedure which is likely to cause lasting distress or suffering is regulated by the Home Office under the Animals (Scientific Procedures) Act 1986. Detailed data on the use of mice and other species in research in the UK are published each year.[43] In the UK in 2012, there was a total of 3,058,800 regulated procedures on mice in research, which represents 74% of all scientific procedures on animals in the UK in that year.

United States

In the United States, laboratory mice are not regulated under the Animal Welfare Act administered by the USDA APHIS. However, the Public Health Service Act (PHS) as administered by the National Institutes of Health does offer a standard for their care and use. Compliance with PHS is required to receive federal funding. PHS policy is administered by the Office of Laboratory Animal Welfare. Many academic research institutes seek accreditation voluntarily, often through Association for Assessment and Accreditation of Laboratory Animal Care, which maintains the standards of care found within The Guide for the Care and Use of Laboratory Animals and the PHS policy. This accreditation is voluntary, not a prerequisite, for federal funding.[44]

Genome

Sequencing of the mouse genome was completed in late 2002. The haploid genome is about three billion base pairs long (3000 Mb distributed over 20 chromosomes), therefore equal to the size of the human genome.[45] 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. The current count of primary coding genes is 23,139.[46] For comparison, humans have an estimated 20,774.[47]

T-haplotype

The t-haplotype is a selfish element that works to disable the function of the wild house mouse, Mus musculus, sperm to ensure the fertilization of the female egg with their own sperm. This gamete killer designed and structured to suppress recombination of genes, is a single unit, of linked genes, located near the centromere of chromosome 17 and is approximately 30-40 Mb long.[48] One part of the t-haplotype is the responder-insensitive allele Tcr. Tcr gives protection from the distorting drivers, since it shows haploid-specific expression, which means only sperm that carry the haploid are rescued from being killed.[49] While advantageous, the frequency of this selfish element is reported as low.[50]

The low t–haplotype frequency in the wild house mouse population is a paradox. Although +/t male mice carry equal ratios of both gamete types (+ and t) and the wild type chromosome becomes functionally inactivated as well as 90% of the offspring inherit the t chromosomes, wild house mouse populations have remained polymorphic.[50] It would be hypothesized that the high transmission distortion ratio of t-haplotypes would become fixed in a natural population, if 90% of the offspring were inheriting the t chromosome, but due to several factors, this is not the case. Investigations of different subspecies mice populations in different locations have found there were low t frequencies in enclosure populations, as well as in different subspecies. Huang et al. (2001),[51] in Taiwan, observed a low frequency in the subspecies, Mus castaneus, which has also been observed by both Ardlie and Silver (1998)[50] and Carroll et al. (2004).[52] Based on these findings, the general mechanisms of these low t frequencies in mouse populations are similar across subspecies and geographical location, making the unraveling of this paradox beneficial to not only the Mus domesticus species of mouse, but also for other species of mouse, such as Mus castaneus.

Explanations for the low frequency of t-haplotypes include factors such as population size, inbreeding, heterozygosity, and polyandry in the wild house mouse population.

Mutant and transgenic strains


Various mutant strains of mice have been created by a number of methods. A small selection from the many available strains:

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

Injection procedures

Routes of administration of injections in laboratory mice are mainly subcutaneous, intraperitoneal and intravenous. Intramuscular administration is not recommended due to small muscle mass.[54] Intracerebral administration is also possible. Each procedure has recommended injection site, approximate needle gauge and recommended maximal injected volume at a single time at one site, as given in table:

Route Recommended site[54] Needle gauge[54] Maximal volume[55]
subcutaneous dorsum, between scapula 25-26 ga 2-3 ml
intraperitoneal left lower quadrant 25-27 ga 2-3 ml
intravenous lateral tail vein 27-28 ga 0.2 ml
intramuscular hindlimb, caudal thigh 26-27 ga 0.05 ml
intracerebral cranium 27 ga

To facilitate intravenous injection into the tail, laboratory mice can be carefully warmed under heat lamps to vasodilate the vessels.[54]

Anesthesia

A common regimen for general anesthesia for the house mouse is ketamine (in the dose of 100 mg per kg body weight) plus xylazine (in the dose of 5–10 mg per kg), injected by the intraperitoneal route.[56] It has a duration of effect of about 30 minutes.[56]

Euthanasia

Approved procedures for euthanasia of laboratory mice include compressed CO2 gas, injectable barbiturate anesthetics, inhalable anesthetics, such as Halothane, and physical methods, such as cervical dislocation and decapitation.[57]

Pathogen susceptibility

A recent study detected a murine astrovirus in laboratory mice held at more than half of the US and Japanese institutes investigated.[58] Murine astrovirus was found in nine mice strains, including NSG, NOD-SCID, NSG-3GS, C57BL6-Timp-3-/-, uPA-NOG, B6J, ICR, Bash2, and BALB/C, with various degree of prevalence. The pathogenicity of the murine astrovirus was not known.

References

Further reading

External links

Taxonomy

  • http://www.findmice.org/

Genetics

  • National Center for Biotechnology Information (NCBI)
  • Ensembl Project
  • Vega Mus musculus genome browser, includes NOD mouse sequence and annotation

Media

  • , from www.digimorph.org
  • 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
  • Arkive Photographs.Short text.

Further reading

  • Biology of the Mouse, from the Louisiana Veterinary Medical Association
  • Nature Mouse Special 2002
  • Biology of Laboratory Rodents by David G. Besselsen
  • Comprehensive house mouse information, including pictures, by the University of Michigan Museum of Zoology
  • 'Fancy Mice', includes much behavioral and physiological information
  • Some information on muricide
  • Vocalizations during copulation
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