House Mouse, Mus musculus

Even in the wild, this rodent is associated with humans by destroying crops and stored food. The house mouse is also known as the fancy mouse, a common pet. It is also a widely used laboratory animal, important for testing in genetics, biology, and medicine. There are three recognized subspecies of the house mouse. These mice thrive in a number of locations including fields, houses, and commercial structures.

An adult male house mouse can have a body length of up to 3.9 in, and tail length reaching 3.9 inches. The average weight of a house mouse is between 0.4 ounces and 0.9 ounces. The color of the house mouse’s short fur varies greatly, including brown, white, black, and grey, with little hair appearing on the tail and ears. Although they can jump vertically reaching a height of up to eighteen inches, their normal stride is a run with a gait of up to 1.8 inches. It is difficult to differentiate between male and female house mice when they are young; however, when they have reached adult hood males will have large testicles, and females will have five pairs of mammary glands.

House mice will walk, run, or stand on four feet, but will spend a lot of their time standing on their hind legs as well. These moments include eating, fighting, and or adjusting themselves, using their long tail to balance properly. They will also use their tail for balance when running by sticking it upwards. Mice are very good at many activities including swimming, climbing, and jumping. House mice are typically active during dusk and at night, choosing habitats that have an abundant source of food. They are omnivores, but eat mostly plant materials. They have been known to eat their own droppings in order to obtain nutrients produced from their intestines. Like other rodents, house mice do not vomit.

Male house mice are very dominant, and usually only one will live with a group of females and young. Although the house mouse is territorial, it is not likely to enter another male’s territory unless it is vacant. It will, if not raised together, attack other males if they are placed together. House mice are dependent upon humans, and in most cases cannot live where there are other rodents. However, in North America, New Zealand, and Australia there have been reports of mice living near other rodents such as rats and wood mice.

House mice use their acute senses to obtain information about their surroundings. They can see well in the dark, but have little use for colored vision. Their eye structure is very similar to that of humans, but differs in that the middle part of the retina has enhanced ultraviolet sensitive cones. The significance of their eye structure is not yet known. House mice can hear a wide range of sounds including the ultrasound range (eighty Hz to one hundred kHz), but most are sensitive to the range between fifteen to twenty kHz. Humans can hear mice squeaks used for long distance communication, while ultrasound is used for short-distance communication and cannot be heard by humans. Whiskers are used for sensing shifting air currents and surfaces. These senses may be enhanced if a mouse is blind from birth.

Olfaction, or the use of scenting pheromones for communication, is common in house mice. They use an organ known as Jacobson’s organ, which is located under the nose, to sense pheromones that are sometimes left by the preputial glands on both males and females and by urine. House mice urine, particularly male urine, has a very strong odor. It can contain up to ten types of chemical combinations, including alcohols and alkanes. Five of these compounds are found only in male urine. In juvenile females, odors from adult males or pregnant or lactating females can slow their reproductive growth. This is known as the Whitten effect . Odors from unknown males are known to terminate pregnancies in female mice, and this is known as the Bruce effect .

Female house mice are able to mate for a short span of four to six days. If there are a large number of female mice in one group, they may not be able to mate. If exposed to male urine, however, they will be able to mate within seventy-two hours. The courtship process begins with the male using ultrasonic calls at a range of thirty kHz to one hundred and ten kHz range, and these calls will continue through mating. These calls have been compared to birdsong and are individual to each male mouse. Females are able to make the same calls, but do not do so during mating. Pregnancy in female house mice can last between nineteen to twenty-one days, after which a litter of up to fourteen young can be born. The typical litter size is between six and eight young. Five to ten litters can be produced in one year, causing mice populations to increase rapidly. There is no mating season for house mice, although mice living in the wild do not mate during colder months. Babies are born hairless and blind, but can see at one to two weeks. Captive house mice can sleep an average of 12.5 hours a day and they do not hibernate.

Due to a high level of predation, house mice in the wild do not usually live more than a year. In captivity or safe environments, they can live to be three years old. In order to win the Methuselah Mouse Prize , a competition where mice are bred to live longer lives, the most recent winner (2005) genetically engineered a mouse that lived for nearly five years. A close contender that required no special genetic, dietary or pharmalogical enhancements lived for over four years.

House mice are native to Asia, most likely India, and spread to the Mediterranean Basin around 8000 BC. They only reached Europe at around 1000 BC. This lapse in range growth is thought to be due to the mice’s natural attachment to human agrarian settlements. Humans spread house mice around the world after that time. Studies have been done on the evolution of the house mouse to identify early human movements, and a link between Madeira and Denmark was found through these studies of the origin of Madeiran mice. The earliest record of pet mice can be found in the Erya, the oldest Chinese dictionary in existence, mentioned in an 1100 BC version. Fancy mice, or “hobby” mice, were domesticated to have a docile temperament and a variety of colors. These mice have also been used as a food source for fish, birds, meat-eating pet reptiles, and arthropods.

House mice are commonly known as pests, not only for humans but also for other animals. On Gough Island in the South Atlantic, there are twenty species of seabirds that use the island for breeding; these include the Atlantic Petrel and the Tristan Albatross. During the nineteenth century, humans introduced mice onto the island, and now there is an estimated mouse population of 700,000. These mice learned to overtake baby albatross birds, which previously did not have any mammalian predators. The mice will group together and bite the baby bird until it dies of bleeding. Over one million chicks can be killed in a year by the house mice. Diseases carried by house mice include Leptospirosis, Rickettsialpox, lymphocytic choriomeningitis, and possibly the Bubonic Plague. They can damage crops through excessive feeding.  It is thought that the cat was domesticated in order to control the mouse population.

House mice are the most commonly used mammal for laboratory testing. They are so popular because of their ability to reproduce quickly, are handled easily, and because they have a similar degree of homology with humans. These mice have a genome (set of genes) that can be altered, and many even have human homologues. House mice are inexpensive and several generations can be observed in a laboratory setting. With a haploid genome equal in size to that of humans, approximately 3 billion bases long, it is hard to estimate the exact number of genes in a mouse, due to the fact the definition of a gene is still being debated. The estimation of 23,786 genes is the current guess.

Mice are bred to be as genetically similar as possible, so inbreeding does occur. There are also outbred and transgenic types of laboratory mice. The two most typically bred mice are Mus musculus domesticus and Mus musculus musculus, and these two subspecies form hybrids, often inbred. The laboratory house mouse can be a variety of colors, including black, agouti, or albino. Scientists can identify between different strains of mice by using a code of numbers and letters, like these; C57BL/6 and BALB/c. Mice hybrid such as these were first created in 1909 by Clarence Cook Little , and was influential in making the mice a laboratory animal.

There is a paradox among wild house mice involving the reproductive ability of males, and this is caused by something known as the t-haplotype. This element targets the male’s sperm, making It unable to reach the eggs in female mice. The confusion lies in the fact that this element attacks selfishly, yet the sperm with t-haplotypes survive. There is more confusion involved because there is such a low frequency of the t-haplotype occurring, even with male mice carrying the element. Mice populations still remain polymorphic, and many factors influence this trend including inbreeding, population size, and heterozygosity.

Many mutant strains of mice have been created in laboratories, including NOD mice which develop Type 1 diabetes, muscular mice known as “mighty mice”, mice that have been injected with rat genes that grow abnormally large, and even mice with regenerative capabilities known as MRL mice.

House mice are not regulated in laboratories under the Animal Welfare Act in the United Sates. There is a standard of care that has been enacted by National Institutes of Health with the Public Health Service Act (PHS). These standards must be met in order to receive funding from the federal government. A number of academic research institutes will o seek accreditation by the Association for Assessment and Accreditation of Laboratory Animal Care, but this is not mandatory. When in the UK, if a procedure would bring long lasting harm to a vertebrate or invertebrate, then it is controlled by the Home Office through the Animals (Scientific Procedures) Act 1986.

Image Caption: Common house mouse (Mus musculus), wild type. Credit: Wikipedia