Dustin Penn

September, 1999

Department of Biology

University of Utah


A House Mouse Primer

House mice are useful animals for solving problems in evolutionary biology and behavioral ecology for several reasons. First, house mice are the model organism for biomedical sciences, and therefore they are one of the world's best-studied animals. Second, many well-characterized strains of laboratory mice have been developed through selective breeding and, more recently, genetic engineering. However, since the vast majority of studies on house mice has been conducted in the artificial conditions of the laboratory, there is still much to learn about their behavior and ecology in natural conditions. This primer provides basic information about what is known about the evolution, behavior, and ecology of house mice. You will find that the lives of these little rodents is interesting and surprisingly complicated.

Evolutionary History

House mice are rodents (order Rodentia) because they have two pairs of chisel-like, self-sharpening incisors. Rodents are an extremely diverse lineage with 3,000 species accounting for 40% of all mammalian species. House mice belong to the family Muridae (Old World mice) and the genus Mus. The commensal Mus musculus consists of four recognizable forms or morphotypes. These four morphotypes may be considered distinct species (M. musculus, M. domesticus, M. castaneus, and M.bactrianus.) or subspecies of Mus musculus (e.g., M. musculus domesticus). Crosses between these four types in captivity produce offspring, indicating that they are recently diverged "subspecies" (biological species concept). In the wild, however, their distributions are nearly non-overlapping and gene exchange is limited where the types come into contact. Because the forms maintain their integrity in the face of hybridization, they are distinct evolutionary lineages and can therefore be considered "species" (phylogenetic species concept) . House mice originally evolved in Europe and Asia and have recently spread throughout the world. M. domesticus and M. musculus are the most widely distributed of the four forms. Mus musculus evolved in eastern Europe and Asia. M. domesticus is originally from western Europe, and has colonized North and South America, Africa, and Australia. The wild house mice in the U.S. are Mus domesticus (our native mice are "deer mice" or Peromyscus).

Inbred laboratory strains of mice were originally obtained by geneticists from pet mouse breeders 100 years ago. People have been breeding mice in Egypt, Greece, and China for at least 4,000 years. They were kept them in temples or homes to predict the future or as lucky charms, and ancient Romans used them as medicine. The Japanese bred white and colored mice systematically 300 years ago. Subsequently, the laboratory mouse arose as a hybrid of various Mus lineages (M. musculus, M. domesticus, and and possibly M. bactrianus and M. castaneus). Geneticists have bred many different strains of laboratory mice. Some strains, such as Swiss Webster, are outbred, whereas others, such as Balb/c, DBA, and B6, are inbred (genetically homozygous), maintained by brother-sister matings. When two strains are genetically identical at all loci except at a particular locus, such as the MHC, they are called "congenic" strains (e.g., B6 carries the b MHC haplotype whereas B6.A carries the a haplotype).


House mice usually live near humans and are therefore called "human commensals". Rather than being commensals, house mice are usually kleptoparasites as they have been stealing our food stores since the agricultural revolution (indeed, the names "mouse" and "mus" come from the Sanskrit "mush" derived from a verb meaning "to steal"). Since house mice have been living in human structures for over six thousand years, our enclosures provide reasonably natural environments. Mice are crepuscular (active at dawn and dusk) and eat grains and fruits, but they also eat insects and other small invertebrates. Their natural predators include snakes and cats, and they carry a diversity of pathogens and parasites (though the only mouse pathogen that provides a significant risk to humans is the lymphocytic choriomeningitis(LCMV) virus.

Social Behavior

House mice tend to live in stable social groups consisting of a single dominant, territorial male and 4-12 other adults. This mating system is called "harem polygyny" because the females mate with the territorial male and generally refuse to mate with the subordinate males. However, females will also leave their territory and solicit matings from neighboring territorial males. A 7-year capture-release study found that most adult animals remained faithful to a locality for extended periods. Several studies have found genetic differentiation indicating that dispersal is low enough to create a potential inbreeding risk. Many studies in the laboratory indicate the inbreeding is detrimental, and we have recently found that the fitness costs of inbreeding is much worse under the stressful, competitive conditions.

Mating and Reproductive Behavior

Females reach reproductive maturity between 4-8 weeks and males at about 7 weeks. Females come into puberty earlier when exposed to male odors, whereas they delay it when exposed to the odor of females. Females breed year round and come into estrous every four days in the laboratory (estrous condition can be diagnosed by a vaginal smear), although estrous can be behaviorally and pheromonally inhibited in the wild. Mating occurs during the late proestrus/early estrus portion of the cycle, and takes place over a period of 15-60 min (unless males rape the female): males mount the female and withdraws repeatedly without ejaculation (intromission) from one to 100 times until ejaculation occurs (which you can recognize because males clinch the female and fall over on their side). The function of intromission behavior is unclear, but it may be a form of courtship behavior in which to assess potential mates . Upon ejaculation, males deposit a sperm plug that blocks the entrance to the vagina. The refractory period lasts for about an hour, and full sperm count is not built up again for two days, but a male may mate with up to three females in one night.

Once pregnant, gestation lasts between 18-22 days, but it is not uncommon for it to be extended (delayed implantation) to 25 d or more when a female is lactating . Females may abort the pregnancy (actually they block implantation) if they are stressed, or interestingly, if the stud male is replaced by another foreign male (the Bruce effect). Before giving birth, females construct nests for their young, ranging in complexity from no nest, a platform nest, a bowl-shaped nest, to an enclosed chamber . Which type of nest a female constructs depends on the ambient temperature indicating that nests function as a thermoregulatory device for young.

Females have 5-8 pups per litter in wild mice (6-10 in lab mice). Sex ratios of pups are usually equal, though food-deprived females produce fewer males , probably because runted males are unlikely to compete for territories and mates (differences in weaning weight persist into adulthood ). Also, pups from post-partum litters are heavier than first litters and are male biased . The position of a fetus in the uterus, relative to its siblings (intrauterine position), appears to affect its sexual development, i.e., males that develop between two females are feminized and females between two males are androgenized . After giving birth, females enter post-partum estrous and usually mate that night (6-24 hrs after giving birth).

Pups are weaned at around 14-15 days and pregnant dams withdraw milk earlier than non-pregnant dams . Mice are considered to be adults at 60 days. Adults are sexual dimorphic with males being larger than the females and having larger scent glands. Mice can live for nearly three years in captivity, but they rarely live for more than a year in the wild. A study in a population enclosure found that only 52% of the females become pregnant (either due to differential mating or abortion) and only 15% of their offspring reach weaning age . Females in the wild will only produce two surviving offspring in her lifetime if the population size is stable.

Male Behavior

Males establish dominance and territorial status by severe aggression . Indeed, losers may be killed when there is nowhere to escape (Crowcroft, 1966). Dominant males have a higher reproductive success than subordinates in the laboratory (, but see ). Our behavioral observations and genetic data both indicate that territorial males obtain nearly all matings, with subordinates only occasionally successfully mating. When fighting over territories, larger males have an advantage over smaller males , residents have an advantage over non-residents , and unparasitzed males have an advantage over parasitized males . Once males become territorial they patrol their territory performing tail-rattling displays and chasing away other males. They cover their territory with small scent marks of urine, whereas subordinate males deposit their urine in small puddles in the corner . Males fight over real-estate because females prefer males with high quality territories .

Female Behavior

Female mice prefer to nest in complex environments, with plenty of hiding places. Not surprisingly, they have strong preferences for males who occupy high quality territories . Females assess the scent marks of males and interestingly base their mating preferences on a male's phenotypic quality as well his territory quality. For example, females prefer males who can successfully defend their territories against rivals . They prefer the odor of dominant to subordinate males , and they can even discriminate the sons of dominant males from the sons of subordinate males by their odor . Females prefer the odor of uninfected males to parasitized males , and males having dissimilar genes at the major histocompatibility complex (MHC) . Females can recognize siblings, even unfamiliar ones, and prefer unrelated males to siblings .

Female house mice often nest communally and nurse each other's pups, like lions and a few other mammals. Cooperative breeding is a strategy to reduce pup loss from infanticide . Infanticide can be quite high (30% in communal nests and 60% in non-communal nests). Familiarity among females increases the reproductive success of communally nesting females, especially if the females are closely related . Females tend to nest with their sisters in the wild , and they tend to communally nest with MHC-similar females, supporting the idea that the MHC provides a kin recognition system . Not all female behavior is cooperative and much of the variance in reproductive success among females may be due to competition among females.

Ecological Genetics

Wild house mice populations often carry a gene called the "T-locus", closely linked to the MHC, that somehow sabotages sperm cells carrying an alternative alleles so that only sperm carrying the T allele fertilize eggs (this phenomenon is called "meiotic drive" or "segregation distortion"). The T allele is prevented from reaching fixation because males carrying T alleles have lower fertility than other males and females prefer not to mate with them (e.g., a polymorphism maintained by balancing selection) .


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