The house mouse (Mus musculus) is a small rodent, a mouse, one of the most numerous species of the genus Mus.

As 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 most commonly used genetically altered laboratory mammal.^[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 colour from white to grey, and 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.

Reconstructed image of a House Mouse skull.

Young males and females are not easily distinguished: like people, 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.^[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

There are three widely accepted subspecies, 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 more recently recognized:^[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 ("molossinus").^[9][11]

Behaviour

Eating

Video of a Japanese pet mouse eating at Ueno Zoo.

House mice usually run, walk, or stand on all fours, but when eating, fighting, or orienting themselves, they stand only on the hind legs, supported by 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 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 are omnivorous. They will eat their droppings to acquire nutrients produced by bacteria in their intestines. House mice, like most other rodents, do not vomit.

Mice are afraid of rats, which often kill and (partially) eat mice. This rat behaviour is known as muricide. Despite this behaviour 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.^[12] However, in some areas (such as Australia), mice are able to co-exist with other small rodent species.^[13]

Senses and communication

Vision

As primarily nocturnal animals, house mice have little or no colour 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 (UV) sensitive cones than other areas of the retina, although the biological significance of this structure is unknown.^[14][15][16]

Hearing

Mice can hear over a broad spectrum of frequencies. They can detect frequencies from 80 Hz to 100 kHz (i.e. into the ultrasound range), but are most sensitive in the 15–20 kHz range and around 50 kHz. They communicate both in the human audible range with squeaks (for long-distance warnings), and in the ultrasound range (for short-distance communication).^{[citation needed]}

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.^[17][18] 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 ten different compounds such as alkanes, alcohols, etc. are detectable 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.^[19]

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 there is super-normal growth of the vibrissae presumably as a compensatory response,^[20] or if the vibrissae are absent, the use of vision is intensified.^[21]

Life cycle and reproduction

A 7-day-old mouse suckling on an anaesthetized mother.

A baby mouse, 4 days old.

2 weeks old, just about to open its eyes.

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 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 be different in different individuals and have been compared to birdsongs because of their complexity.^[22] 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 6–8). One female can have some 5–10 litters per year, so the mice 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 without fur. Fur starts to grow some three days after birth and the eyes open one to two weeks after birth. Males reach sexual maturity at about 6 weeks and females at about 8 weeks, but both can breed as early as five weeks.

Sleep

This section requires expansion.

The average sleep time of a captive House mouse is said^{[by whom?]} to be 12½ hours per day.^[23]

Life expectancy

House mice usually live under a year in the wild. This is 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^[update], the record holder was a genetically engineered mouse that lived for 1819 days (nearly five 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 four years).

Mice and humans

House mice usually live in proximity to humans, in or around houses or fields. Originally native to Asia (probably northern India),^[24] they spread to the Mediterranean Basin about 8000 BC, only spreading into the rest of Europe around 1000 BC.^[25] This time lag is thought to be because the mice require agrarian human settlements above a certain size.^[25] 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 showed a previously unsuspected early link between Denmark and Madeira on the basis of the origin of the Madeiran mice.^[26]

An individually ventilated and sealed cage for laboratory mice

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 ^[27] and potentially Bubonic plague.^{[citation needed]} House mice 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.

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.^[28] Human domestication led to numerous strains of "fancy" or hobby mice with a variety of colours and a docile temperament.^[29] 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.^[29]

Mice as an invasive species

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 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 one million bird chicks per year.^[30]

Laboratory mice

Laboratory mouse (note the ear tag)

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, there are further benefits to the use of mice in laboratory research. Because mice can reproduce quickly, several generations of mice can be observed in a relatively short period of time.^[31]

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.^[32]

Albino laboratory mice

Legislation of use in science

In the United States, laboratory mice are not regulated under the Animal Welfare Act (AWA) administered by the USDA APHIS. However, the Public Health Service Act (PHS) as administered by the National Institutes of Health (NIH) does offer a standard for their care and use. Compliance with PHS is required in order to receive federal funding. PHS Policy is administered by the Office of Laboratory Animal Welfare (OLAW). Many academic research institutes seek accreditation voluntarily, often through Association for Assessment and Accreditation of Laboratory Animal Care (AAALAC), 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.^[33] 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.

Genome

mice genetically engineered to express green fluorescent protein under UV light

Sequencing of the mouse genome was completed in late 2002. The haploid genome is about three billion bases long (3000 Mb distributed over 20 chromosomes) and therefore equal to the size of the human genome.^[34] 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 estimated gene count is 23,786.^[35] This estimate takes into account knowledge of molecular biology as well as comparative genomic data. For comparison, humans are estimated to have 23,686 genes.^[36]

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.^[37] 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 that only sperm that carry the haploid are rescued from being killed.^[38] While advantageous, the frequency of this selfish element is reported as low.^[39]

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.^[39] It would be hypothesized that the high TRD, 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. There have been investigations of different subspecies mice populations in different locations. Studies have found that there were low t frequencies in enclosure populations as well as in different subspecies. Huang et al. (2001),^[40] in Taiwan, observed a low frequency in the subspecies, Mus castaneus, which has also been observed by both Ardlie and Silver (1998)^[39] and Carroll et al. (2004).^[41] Based on these findings, it can be assumed that 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 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

A knockout mouse (left) that is a model of obesity, compared with a normal mouse.

Various mutant strains of mice have been created by a number of methods:

• Mice resulting from ordinary breeding.
  • NOD mice, which develop diabetes mellitus type 1.
  • MRL mice, mice with unusual regenerative capacities.^[42]
  • "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.
  • Abnormally 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 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 muscular mice, with a disabled myostatin gene, nicknamed "mighty mice."

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.^[43] 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:

To facilitate intravenous injection into the tail, laboratory mice can be carefully warmed under heat lamps to vasodilate the vessels.^[43]

Anesthesia

A common regimen for general anesthesia for the house mouse is ketamine (in the dose of 100 mg per kilogram body weight) plus xylazine (in the dose of 5–10 mg/kg), injected by the intraperitoneal route.^[45] It has a duration of effect of about 30 minutes.^[45]

Notes

References

A mouse (plural: mice) is a small mammal belonging to the order of rodents, characteristically having a pointed snout, small rounded ears, and a long naked or almost hairless tail. The best known mouse species is the common house mouse (Mus musculus). It is also a popular pet. In some places, certain kinds of field mice are also common. This rodent is eaten by large birds such as hawks and eagles. They are known to invade homes for food and occasionally shelter.

The American White-footed Mouse (Peromyscus leucopus) and the deer mouse (Peromyscus maniculatus), as well as other common species of mouse-like rodents around the world, also sometimes live in houses. These, however, are in other genera.

Cats, wild dogs, foxes, birds of prey, snakes and even certain kinds of arthropods have been known to prey heavily upon mice. Nevertheless, because of its remarkable adaptability to almost any environment, the mouse is one of the most successful mammalian genera living on Earth today.

Mice can at times be harmful rodents, damaging and eating crops,^[1] causing structural damage and spreading diseases through their parasites and feces.^[2] In North America, breathing dust that has come in contact with mouse excrement has been linked to hantavirus, which may lead to Hantavirus Pulmonary Syndrome (HPS).

Primarily nocturnal animals, mice compensate for their poor eyesight with a keen sense of hearing, and rely especially on their sense of smell to locate food and avoid predators.^[3]

Mice and rats are the most commonly used animals for laboratory tests.^[4]

Reproduction

1 day old pups

Breeding onset is at about 50 days of age in both females and males, although females may have their first estrus at 25–40 days. Mice are polyestrous and breed year round; ovulation is spontaneous. The duration of the estrous cycle is 4–5 days and estrus itself lasts about 12 hours, occurring in the evening. Vaginal smears are useful in timed matings to determine the stage of the estrous cycle. Mating is usually nocturnal and may be confirmed by the presence of a copulatory plug in the vagina up to 24 hours post-copulation. The presence of sperm on a vaginal smear is also a reliable indicator of mating.^[5]

Female mice housed together tend to go into anestrus and do not cycle. If exposed to a male mouse or the pheromones of a male mouse, most of the females will go into estrus in about 72 hours. This synchronization of the estrous cycle is known as the Whitten effect. The exposure of a recently bred mouse to the pheromones of a strange male mouse may prevent implantation (or pseudopregnancy), a phenomenon known as the Bruce effect.^[5]

The average gestation period is 20 days. A fertile postpartum estrus occurs 14–24 hours following parturition, and simultaneous lactation and gestation prolongs gestation 3–10 days owing to delayed implantation. The average litter size is 10–12 during optimum production, but is highly strain-dependent. As a general rule, inbred mice tend to have longer gestation periods and smaller litters than outbred and hybrid mice. The young are called pups and weigh 0.5–1.5 g (0.018–0.053 oz) at birth, are hairless, and have closed eyelids and ears. Cannibalism is uncommon, but females should not be disturbed during parturition and for at least 2 days postpartum. Pups are weaned at 3 weeks of age; weaning weight is 10–12 g (0.35–0.42 oz). If the postpartum estrus is not utilized, the female resumes cycling 2–5 days post-weaning.^[5]

Newborn male mice are distinguished from newborn females by noting the greater anogenital distance and larger genital papilla in the male. This is best accomplished by lifting the tails of littermates and comparing perineums.^[5]

Laboratory mice

Knockout mice

Mice are common experimental animals in biology and psychology primarily because they are mammals, and also because they share a high degree of homology with humans. They are the most commonly used mammalian model organism, more common than rats. The mouse genome has been sequenced, and virtually all mouse genes have human homologs. They can also be manipulated in ways that would be considered unethical to do with humans (note Animal Rights). A knockout mouse is a genetically engineered mouse that has had one or more of its genes made inoperable through a gene knockout.

There are other reasons why mice are used in laboratory research. Mice are small, inexpensive, easily maintained, and can reproduce quickly. Several generations of mice can be observed in a relatively short period of time. Mice are generally very docile if raised from birth and given sufficient human contact. However, certain strains have been known to be quite temperamental. Mice and rats have the same organs in the same places, just different proportions.

Subgenera

Field mouse of the subgenus Mus.

All members of the Mus genus are referred to as mice. However, the term mouse can also be applied to species outside of this genus. Mouse often refers to any small muroid rodent, while rat refers to larger muroid rodents. Therefore these terms are not taxonomically specific. For simplicity, only the rodent subgenera belonging to the Mus genus are listed here.

Genus Mus - Typical mice

• Subgenus Coelomys (East Asia)
• Subgenus Mus (Eurasia to North Africa, except for the House mouse which is worldwide.)
• Subgenus Nannomys (Sub-Saharan Africa)
• Subgenus Pyromys (East Asia)
• Subgenus and species Mus lepidoides

As pets

Pet mice

This unreferenced section requires citations to ensure verifiability.

Many people buy mice as companion pets. They can be playful, loving and can grow used to being handled. Like pet rats, pet mice should not be left unsupervised outside as they have many natural predators, including (but not limited to) birds, lizards, cats, and dogs. Male mice tend to have a stronger odor than the females. However, mice are careful groomers and as pets they never need bathing. Well looked after mice can make ideal pets. Some common mouse care products are:

• Cage – Usually a hamster or gerbil cage, but special mouse cages are now available.
• Food – Special pelleted and seed-based food is available. Mice can generally eat most rodent food (for rats, mice, hamsters, gerbils, etc.)
• Bedding – Usually made of hardwood pulp, such as aspen, sometimes from shredded, uninked paper or recycled virgin wood pulp. Using corn husk bedding is avoided because it promotes Aspergillis fungus, and can grow mold once it gets wet, which is rough on their feet.

Nutrition

In nature, mice are largely herbivores, consuming any kind of fruit or grain from plants.^[6] Consequently, mice adapt well to urban areas and are known for eating almost all types of food scraps. In captivity, mice are commonly fed commercial pelleted mouse diet. These diets are nutritionally complete, but they still need a large variety of vegetables. Food intake is approximately 15 g (0.53 oz) per 100 g (3.5 oz) of body weight per day; water intake is approximately 15 ml (0.53 imp fl oz; 0.51 US fl oz) per 100 g of body weight per day.^[5]

As food

"Pinkie" mice for sale as reptile food

Mice are a staple in the diet of many small carnivores. Humans have eaten mice since prehistoric times and still eat them as a delicacy throughout eastern Zambia and northern Malawi,^[7], where they are a seasonal source of protein. Mice are no longer routinely consumed by humans elsewhere.

In various countries mice are used as food^[8] for pets such as snakes, lizards, frogs, tarantulas and birds of prey, and many pet stores carry mice for this purpose. Some countries, such as Germany and the United Kingdom, have banned the practice of feeding live mice, citing ethical concerns regarding both predator and prey.^[9]

Common terms used to refer to different ages/sizes of mice when sold for pet food are "pinkies", "fuzzies", "crawlers", "hoppers", and "adults".^[10] Pinkies are newborn mice that have not yet grown fur; fuzzies have some fur but are not very mobile; hoppers have a full coat of hair and are fully mobile but are smaller than adult mice. Mice without fur are easier for the animal to consume; however, mice with fur may be more convincing as animal feed. These terms are also used to refer to the various growth stages of rats (see Fancy rat).

Use for sense of smell

Israeli scientists have tested mice as a new form of airport security detector. It consists of three concealed cartridges, each containing eight specially trained mice. If they sense traces of explosives or drugs, they will trigger the alarm. According to the New Scientist, the mice work four-hour shifts and are more accurate than using dogs or x-ray machines.^[11]

See also

Look up mouse in Wiktionary, the free dictionary.

Wikiquote has a collection of quotations related to: Mice

• Genetically modified mouse
• House mouse
• List of fictional mice and rats
• Mouse trap
• Mouse (computing)
• Musophobia (fear of mice)
• Rat
• Vacanti mouse
• Mouse brain development timeline

Notes

Wikimedia Commons has media related to: Mus