The House Mouse (Mus musculus) is one of the most numerous species of the genus Mus commonly termed a mouse. It is a small mammal and a rodent. In most parts of the world, they 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.
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 (Terszowski 2006).
An additional subspecies was described by Prager, Orrego and Sage (1998) from the Arabian Peninsula:
The following were previously identified as subspecies, but have since been found to belong to the subspecies above:
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 droppings 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 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 (Tattersall, Smith and Nowell 1997). However in some areas (such as Australia) mice are able to co-exist with other small rodent species (Moro and Morris 2000).
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.
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 - 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 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. (Holy 2005) 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 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.
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 colours 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 the most commonly used animal research model with hundreds of established inbred, outbred, and transgenic strains. In the United States, they are generally 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 mandate their humane treatment. 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.
Mice are common experimental animals in biology and psychology primarily because they are mammals, and thus share a high degree of homology with humans. 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. Mice are a primary mammalian model organism, as are rats. There are many additional benefits of mice 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. Some mice can become docile if raised from birth and given sufficient human contact. However, certain strains have been known to be quite temperamental.
Most laboratory mice are hybrids of different subspecies, most commonly of Mus musculus domesticus and Mus musculus musculus. Laboratory mice come in 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.
The behavioural patterns of laboratory mice are significantly different from those of most common house mice due to years of lab breeding. These behaviours are much simpler.
Since 1998, it has been possible to clone mice from cells derived from adult animals.
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