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The Behaviour ofthe House Mouse

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Barbara König

367 Institute of Evolutionary Biology and Environmental Studies,

University of Zurich, Switzerland

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IntroductionThe house mouse (Mus musculus) is the mostsuccessful and most widely distributed invasivemammal except for humans ([1], cited in [2]).For thousands of years, house mice have beencommensal with and have been transported byhumans throughout the world [3]. Archaeologicaland paleontological evidence shows that the asso-ciation of humans and house mice took placeimmediately after the first postglacial tempera-ture rebound during the Bølling/Allerød(12 700–10 700 BC). The subsequent house mouseexpansion was initiated when new agriculturaland husbandry practices were established, suchas large-scale grain storage, during the Neolithicrevolution (reviewed in [2]). Worldwide, housemice are found in a variety of habitats: intemperate and tropical zones, as well as insubarctic regions; on farmland or coral reefislands; in grain storage facilities, rice fields and

The Laboratory Mouse� 2012 Elsevier Ltd. All rights reserved.ISBN 978-0-12-382008-2

in coal mines; in deserts and on tropical islands;from sea level to up to altitudes of several thou-sand metres; house mice have been evenobserved to reproduce successfully in frozencarcasses in cold stores in the port of London[4]. To allow such flexibility in habitat use andin distribution, flexibility in behaviour, especiallyin maternal and social strategies, is required.

Taxonomy andbiogeography of thehouse mouseMice of the genus Mus evolved on the Indiansubcontinent, fromwhere they radiated in severaldirections [5]. All commensal house mice andlaboratory mice belong to the species Mus muscu-lus, which consists of the four well-describedsubspecies Mus musculus domesticus, M. m. musculus,

DOI: 10.1016/B978-0-12-382008-2.00016-7

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M. m. bactrianus and M. m. castaneus (for recentreviews on the taxonomy, systematics and bioge-ography of house mice see [6], [7] and Chapter1.1). These subspecies have non-overlappingranges under natural conditions, and aremorpho-logically and molecularly distinct. Nevertheless,they can reproduce and produced fertileoffspring in the laboratory [8].

The western house mouse (M. m. domesticus)followed a Mediterranean route out of its NearEastern place of origin, and colonized westernEurope, commensal with humans, 2000 years ago[2]. Nowadays it occurs in western Europe, NorthAfrica and the Middle East to south-west Iran.The eastern house mouse (M. m. musculus), on theother hand, followed a route north of the BlackSea and along the Danube [9], and today is foundall over northern Asia as well as in eastern, centraland Scandinavian Europe.M. m. castaneus is foundall over South East Asia, and M. m. bactrianus inAfghanistan and Pakistan (with some evidencethat this group of ‘central mice’ might representan ancestral population [7]).

Long-distance colonization of the rest of theworld is a recent phenomenon. M. m. domesticusis the subspecies with the widest distribution,and expanded its range through passive trans-port with humans to the Americas, Australia,sub-Saharan Africa and Oceanian islands [2, 7].The ecological success of house mice as an inva-sive species thus is linked to their commensalinteraction with humans, which resulted in thespecies reaching and thriving in places outsideits natural regions of origin.

The house mousein researchThe house mouse has been a model and a tool formedicine and biology for many centuries, and hascontributed enormously to our knowledge ofgenetics and physiology of mammals (for a histor-ical overview and summary see [10]). The mousewasused for anatomical studies in the 17thcentury;it helped to document Mendelian segregation andlinkage of genes in the early 20th century, andcontributed to the neo-darwinian synthesis alongwith other classic organisms such as Drosophila; it

was the second mammaldafter humansdforwhich the genome has been sequenced; it servedand still serves as a model in evo-developmentalstudies, and is an important study species forbiomedical research and pathology.

The origin and history of laboratory mice aredescribed in detail in Chapter 1.1. Nowadays over300 different inbred strains, and a variety ofadditional outbred strains, are known. Inheritedvariation is the basis of the differences betweenstrains, but it has to be kept in mind that thecommonly used strains only carry a small partof the variation found in wild mice. Behaviouralanalyses have been done for many inbred strains,with the discovery of genetic variability inbehaviour within and between strains [11–13].Differences have been documented forvarious traits, for example social behaviour [14],maternal behaviour [15–17], lactation perfor-mance [18, 19], activity and aggression [20], repro-ductive output and growth of pups afterweaning [21], and length of an oestrous cycle oroestrous duration [22]; for a review of geneticdifferences described for naturally occurringpopulations see [23].

Genetically, all inbred strains analysed so fardid not originate in only one house mouse subspe-cies, but are a combination of different subspecies.Nevertheless, M. m. domesticus is predominant [8].Since most studies on free-living house mice, aswell as on wild or laboratory mice kept in labora-tory animal facilities or in semi-natural enclosures,have been done on M. m. domesticus, the followingreview of house mouse behaviour is primarilybased on observations on or experiments withthis subspecies. Latham and Mason [23] recentlycompiled a review on the behavioural biology offree-living house mice, M. musculus.

Behaviouralflexibility in thewestern housemouseThe western house mouse (M. m. domesticus) isconsidered a prime example of adaptability to

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very diverse habitat and climatic conditions[3,24]. Behavioural flexibility is a remarkablefeature of the subspecies and has been consid-ered as a predisposition for its ecological success[25–27]. This is highlighted by the mouse’scomplex history of repeated successful coloniza-tion and the concomitant adaptations to newenvironments. Its association with human activi-ties and dwellings, and the fact that humansmay transport it over long distances, thus havea profound impact on the population dynamicsand social structure of the species. As a result,house mice occur in patchily distributed smallto large populations, and gene flow between pop-ulations varies in intensity both in space and intime. Most house mouse populations can thus beconsidered as genetically almost unique [24, 28].

In his classic review on the reproductiveecology of the house mouse, Bronson [3] empha-sized ecological opportunism and colonizing abil-ities of the species. Nevertheless, he suggestedclassifying mouse populations according towhether they exist commensally with humans(commensal populations), or independently ofhuman activity (feral populations). In Europecommensal populations occur in anthropogenichabitats, such as farm buildings and grain stores.Feral populations, on the other hand, are foundin grasslands and cultivated areas, and arerestricted to islands in Europe [29].

House mice have a high reproductive poten-tial. Under favourable environmental conditionsand in laboratory animal facilities, mice can sexu-ally mature at 6–8weeks, and females can givebirth to a litter every month. Such an enormousreproductive output has been interpreted as anadaptation to a colonizing life strategy, which hasto cope with variable environmental conditionsandhighmortality [30–32]. The average life expec-tancy of free-living housemice is only a fewweeks(100–190 days), which ismainly due to high juvenilemortality [33–35]. Some individuals can neverthe-less survive formore than twowinters, and labora-tory mice live up to 3 years or even longer.

Feeding and foragingbehaviourIn terms of food, house mice are omnivorous.They feed predominantly on seeds, nuts, fruits

and roots, but also eat meat and prey on livinginsects [4, 23]. Foraging takes place during regularpatrolling of the territory [36]. Mice eat up to 20%of their body weight daily [37, 38], with lactatingfemales more than doubling their caloric intakeper day [19]. Typically, a mouse consumes about200 small meals in a 24h period, repeatedlyvisiting approximately 20–30 food sites [23].Generally, mice are not reluctant to try newfood. As pups they learn about food from theirmother even before they are weaned. Subadultsand adults assess by smell during grooming ofgroup members what food the others have eaten,and can establish socially learned food prefer-ences [39]. Such allo-grooming not only assiststransfer of information about food, but alsomaintains social relationships. Self-grooming, onthe other hand, is important for hygiene andinsulation [23].

During foraging or exploration mice avoidunsheltered or exposed areas, and sites wherethere is a high risk of predation. They find carni-vore faeces and related scents aversive, and avoidthe urine and faeces of recently frightenedconspecifics (reviewed in [23]). For females thelocal presence of a male, and status of his associ-ated females, may also be important, as thebreeding success of female mice is criticallydependent on their ability to establish a nest sitewithin a male territory [40, 41].

When mice explore a new area, they do soslowly and carefully, following physical struc-tures such as barriers and walls. They frequentlypause during excursions, and rear up or makelong stretches forward, sniffing at new objects.Gradually they make longer excursions. Inbetween such short-range explorative excur-sions, they often return to a familiar or safearea, following visual landmarks and the olfac-tory marks produced by the plantar glands ontheir feet [23, 36, 42].

Mice are very sensitive to movement andchanges in light intensity, and also use visualcues to demarcate territorial boundaries [43], tonavigate or to move to cover [44, 45]. They havelittle colour perception and are insensitive tored wavelengths (they lack a long-wavelengthphoto pigment). On the other hand, they aresensitive to ultraviolet wavelengths, which maybe an adaptation to crepuscular activity [46],and may be used in navigation and in foraging

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(many fruits, seeds and even larvae reflect inultraviolet [23]). Hearing is well developed inmice and they are able to hear noises from10 kHz to ultrasound over 100 kHz [47, 48].

Activity and territorialityHouse mice are most active from dusk to dawn,and thus are considered as crepuscular ornocturnal. Still, commensal mice can also beactive during the day in the absence of predation[36]. Little is known about how much time free-living mice are active. Laboratory mice spendless than 50% of the entire day active [23].

Commensal mice are territorial, and althoughthe dominant male is usually the most aggressive,adults of both sexes contribute to territorialdefence [23, 36, 49]. Territorial boundaries oftenoverlap with physical structures in the environ-ment. All group members mark territorial bound-aries as well as conspicuous objects, or thesurroundings of feeding and nesting sites, byurinary odour cues. In particular, dominant terri-torialmales frequently deposit such scentmarks atboundaries, since refreshing their own marks isa signal of competitive ability, providing informa-tion about territorial and sexual status [3, 50–53],and females prefer to mate with dominant males[54, 55]. Territorial males also try to over-marktheir competitor’s urine marks in neighbouringterritories. Urinemarks of all mice living in a terri-tory can be deposited so frequently that they form‘pillars’ several millimetres high [56, 57]. Oncedeposited, urine marks can last for up to 2 days,due to non-volatile major urinary proteins(MUP), which contain information about individ-uality, sex, dominance status and reproductivecondition, and stimulate aggression among malesand oestrous in females (for recent reviews onolfactory mediated information through MUPssee [53, 58–60]).

Besides such urinary cues, mice also depositolfactory marks produced by plantar glands onthe feet [23, 36], resulting in well-worn runways.Individuals travel their entire territory daily,covering and marking the same routes repeat-edly. Commensal mice therefore have beendescribed as creatures of habit [23]. Throughthese routines, mice acquire highly habitualresponses (e.g. dashes to safety), which they canperform extremely rapidly and with minimal

sensory input. Predictable movement about theterritory therefore is not only an essential partof territorial defence, but also allows the animalto build up a detailed, continually updatedpicture of its domain [36, 61].

Male intraspecific behaviour is characterizedby aggressiveness and dominance, and functionsto defend a territory and access to reproductivelyactive females (still, intraspecific aggressionvaries between laboratory strains, betweensubspecies and also between different Musspecies) [54, 62–65]. Unfamiliar intruders aregenerally aggressively driven out of the territory;if they are unable to escape, they are likely to facesevere injury or even death [36, 61, 66].

Social structureThe social structure of the western house mousevaries with habitat, resource partitioning anddensity [3, 27, 67, 68]. Most typically, house micelive in small social groups that consist of a domi-nant male, one or several adult females withtheir litters and up to several subordinate miceof both sexes [3, 24, 49, 69–71]. The social systemof house mice has been classified as polygynous,but there is increasing evidence that femalehouse mice are actively polyandrous [35, 72–74].The genetic mating system thus is better charac-terized as polygynandrous. Why females matewith several males during one oestrous cycleis not yet understood. It has been shown inexperimental studies that polyandry increasesoffspring postnatal survival [75], facilitatesinbreeding avoidance [76], selects for increasedsperm numbers and motility [77] or may beunderstood as a female counterstrategy to miti-gate the negative effects of a selfish geneticelement, the t haplotype [35].

House mice are plural breeders, with severalbreeding females per group. Within groupsadult females cooperate in some kinds ofcommunal care, such as babysitting, social ther-moregulation or defence of pups. The moststriking example of cooperation, however, isnon-offspring nursing, also prominent amonglaboratory mice (for a review see [78]). Suchnon-offspring nursing occurs when two, or some-times more, females pool their litters ina communal nest and indiscriminately nursetheir own young and non-offspring [79–82].

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The causes and consequences of communalnursing will be discussed below.

Dispersal behaviourIn house mice, as in most other mammals,dispersal behaviour is one of the most importantlife-history traits [66, 83, 84]. A detailed study withwild house mice under semi-natural conditions[85, 86] revealed a dispersal pattern very muchin agreement with knowledge from stablecommensal populations [70, 84]. Nearly all maleoffspring disperse from the natal group unlessan individual has successfully supplanted thecurrent territorial male, which might happen tobe the father of that male. The decision ofwhen to disperse seems to be governed mainlyby processes intrinsic to the dispersing animal,independent of sexual maturation. Young malesoften stay resident for some time after reachingsexual maturity, exhibiting no intrasexualaggression, and disperse late; others becomeaggressive early and disperse if not successful ingaining territory ownership [66, 86]. Females, onthe other hand, stay in the natal group as longas there is a chance of starting to breed withinthat territory. Otherwise, at high local populationdensities, females disperse at about the same ageas males, but without an observable increase inovert aggression [85, 86]. Since females can stayand successfully reproduce in their natal group,such groups often are extended family groups,comprising several related females.

Although male house mice are well knownfor their high aggressiveness, exceptions havebeen identified recently. In at least one popula-tion of wild house mice, adult males have beenfound to be reluctant to engage in aggressiveinteractions [87]. This confirms a finding ofBenus and co-workers [88–90] who studied wild-stock-derived mice selected for long and shortattack latency. These mice differ in a whole arrayof behavioural traits, including readiness foraggressive interactions for dominance status,anxiety in novel situations, and explorativeactivity. The authors argue that these two pheno-types reflect two dispersal strategies. One strategywould lead to socially competent males that areable to stay in the natal group for a longer period;the other one would produce males of a moreactive phenotype that is characterized by active

exploration of opportunities for dominanceand early dispersal. Hence, it is tempting tosuppose that some ecological alterations haveselected for more socially competent and lessaggressive male mice under some circumstances,differing strongly from the aggressive and early-dispersing male phenotype encountered in mosthouse mouse populations. Such selection mighthave taken place in house mice on the Isle ofMan, where males are rather non-aggressive [87].

Laboratory mice are generally less aggressiveand more socially tolerant towards conspecificsthan wild house mice, presumably as the resultof selection during domestication.

Flexibilityin maternalreproductivestrategiesAs emphasized earlier, house mice have a veryhigh reproductive potential. This is due to thefact that females give birth to 4–12 young aftera gestation period of 19–21 days (average littersizes differ for different inbred strains and pop-ulations [8]), have a postpartum oestrous 12–18 hafter birth of a litter, and thus can produce thenext litter after just 1 month. Implantationof the litter conceived during the postpartumoestrous is delayed by a few days [37](reviewed in [91]).

Maternal careMaternal behaviour in mammals is characterizedby lactation. Via the milk, offspring are providedwith nutrients, calories, vitamins, minerals, andpassive immune protection (lymphocytes andantibodies), for growth and for metabolism [31,92]. As a consequence, a long lactation period isbeneficial for the pups. For the mother, on theother hand, the energetic costs of lactation mayinfluence her survival and future reproduction.The metabolic demands of lactation are enor-mous, especially for small animals, and in housemice they are more than four times higher than

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the energetic costs of gestation [93]. Daily energyoutput in milk per unit body weight is approxi-mately 16 times higher than in an animal thesize of a cow [94]. Such high and sustained milkproduction in small mammals is only possiblebecause of a high metabolic rate that also relatesto a decrease in lifespan [95, 96]. In addition, thelonger the lactation period, or the more milkproduced, the more delayed is the birth of thenext litter [33, 34]. As a consequence, lactatingfemales have to make a trade-off betweencurrent and future reproduction, and we expectflexible maternal strategies to compromisebetween offspring benefits and maternal costsduring lactation under different environmentalconditions. The potential for rather flexiblematernal strategies is already illustrated by theobservation that reproductive performancevaries among different inbred strains [91, 97].

Maternal behaviour in house mice consists ofnursing, licking and grooming pups (licking theanogenital region stimulates defecation inpups), nest-building behaviour, huddling overpups to keep them warm (under conditions oflow temperature) and retrieving pups to thenest, either when females move the nest toanother place, or when a nursing mother hasleft the nest, and some pups were dragged alongbecause they were still attached to a teat [31]. It isinteresting to note that male house mice, whenkept in a monogamous pair with a female, showthe same parental behaviours as females towardstheir offspring, except for nursing [31].

House mice as well as laboratory mice buildnests in which they sleep or rest. Such nests areoften relatively small and open, but are moreclosely built during periods of cold environmentalconditions [91]. Pregnant and lactating females, onthe other hand, build maternal nests (fromapproximately 4 days after mating onwards)that are two to three times the size of a sleepingnest, with one or two entrances and completelyenclosed. Maternal nests are an important compo-nent of maternal behaviour. When given thechoice between different bedding materials,including an option with no bedding materialpresent, female laboratory mice never gave birthin cages without bedding [98]. Under natural orsemi-natural conditions, access to a safe and pro-tected nesting site seems to be a prerequisite forsuccessful reproduction in females, because such

sites improve protection from disturbances byconspecifics [41, 99–101]. It is therefore highly rec-ommended to provide mice with nest-buildingmaterial in laboratory animal facilities.

Maternal aggression refers to aggressivebehaviour of a lactating female when defendingher litter [91]. Females become intensely aggres-sive towards other individuals during the finaldays of pregnancy and during lactation, andwill vigorously protect their nest, biting intruders’heads and bellies [102, 103]. Such increasedaggression may function to allow the female todefend her nest and pups against infanticidalconspecifics (both male and female house micehave been observed to kill pups when encoun-tered in a foreign nest), or assess the dominancestatus of any intruding males [104–106].

Pup development andweaningThe development of house mouse pups can besummarized as follows [19, 31]. At birth (day ofbirth of a litter refers to day 1 of lactation),pups weigh 1.2–1.4 g, with individual pups fromsmall litters being heavier that those from largelitters. They are fully dependent on their motherfor nutrition, thermoregulation and protection.By day 9 they are covered by dense fur (thinhair will have grown a few days earlier); theyopen their eyes at day 14; and they begin toactively leave the nest when 16 days old. Evenduring the first 2 weeks of life, mouse pupsproduce a variety of sounds, mostly ultrasounds,which function to elicit maternal care in a situa-tion of distress, danger or hunger [107–109]. Therate of these ultrasound calls reaches amaximumat around day 8, and decreases after thatuntil these calls disappear after 14–16 days[109–111].

Weaning of pups is a flexible maternalstrategy in female house mice. Pups feed exclu-sively on milk until they are 16 days old; at17 days they begin to eat solid food, but neverthe-less are still nursed by their mother [31]. A drop innursing activity to less than 1% defines weaning.Weaning age differs according to litter size,with small litters (litter size �6) being weaned atday 21, and larger litters at day 23. Furthermore,weaning is not completed before day 23 when

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a female is simultaneously lactating and preg-nant. After weaning nursing is replaced byresting with body contact to the mother, withoutnursing attempts by the pups. During days 17–22pups regularly try to initiate nursing, and femalesoften rest without body contact with the pups, forexample in a separate corner of the cage, whenthey have finished nursing. If space is ratherlimited, pups often succeed in approaching themother and initiate sucking. When the micewere kept in at least two small Macrolon cages(type 2) linked by tubes, however, females typi-cally slept or rested alone in a cage other thanthe one with the nest, thereby avoiding suchsucking attempts by their pups [31]. Resting aloneand remaining far from the litter indicate thefemale’s active role in avoiding the offspring’sincreasing demands during the period ofweaning (days 17–22). Nevertheless, there is nomaternal aggression towards the young duringthe weaning period. After weaning the relation-ship between mother and young appears freeof conflict. Nursing is replaced by resting withbody contact, and the offspring no longer try tosuck [31].

Lactation performanceThe main energy source in the milk of housemice is fat, which allows for rapid postnatalgrowth of a relatively large number of young.A detailed analysis of milk production over thewhole lactation period in laboratory micerevealed that lipids provide on average morethan 80% of the energy available for the sucklingyoung [19]. Daily milk production, maternal bodyweight and food consumption increase after thefirst days of lactation to a maximum duringdays 9–16, and decrease again afterwards. Femalesdo not store body fat during pregnancy to be usedfor milk production but have to increase theirdaily food consumption during lactation [93,112]. As in other small rodents, a reversibleenlargement of the digestive tract tissue mayresult in the same assimilation efficiencyof lactating females as in non-reproducingindividuals [113–116].

Females meet the energy demand ofa growing litter not only by increasing theamount of milk produced, but also by improvingthe quality until day 16 of lactation [19]. During

peak lactation both lipid concentrations and totalsolids reach maximal values. The concentrationof calcium in the milk, which is essential for thebone synthesis of the young, also increases withage and growth of the litter until the onset ofweaning. Iron, which is essential for synthesis ofred blood cells, is maximal during the first week.

Although the energy needs for the pups’metabolism have only been measured throughchanges in pup body weight, metabolic use seemsto increase steeply after day 16. Until then thesum of all energy losses of an individual young(growth, metabolism, urine, faeces) were approx-imately the same as the amount of energyabsorbed via milk. After that, however, theenergy demands of metabolism and growthwere much higher than the energy supplied bythe mother. This might suggest that young housemice have to increase their energy intake byconsumption of solid food at the age of 17 daysto allow for a positive energy budgetda necessityfor growth [19]. This corresponds to the earliestage at which young mice were first seen to eatsolid food [31].

With increasing litter size, females adjust theamount of milk produced according to thehigher demands of many offspring, but regula-tion is imperfect [19]. Weaning weight of indi-vidual pups from large litters (litter size �8) ismore than one gram lower than that of pupsraised in small litters (litter size 5–6). Femalehouse mice produce approximately 100 g milkof an energy equivalent of 1100–1200 kJ to reara litter. With such an investment, a female caneither wean a litter of six young with a bodyweight of 10.6 g each, or a litter of 7.3 youngweighing 9.4 g each [19].

When analysed under different environ-mental and reproductive conditions, femalehouse mice with litter sizes of 6–10 young adjusttheir maternal behaviour according to the devel-opmental state of the litter and do not wean theirpups below a minimal threshold weight of, onaverage, 9 g [31]. A weaning weight below thisthreshold, as found in young of very large litters(7.1 g per young in a litter of 12), would result ininitial, but reversible, weight loss of pups. Asa consequence of such low weaning weight,daughters will experience delayed maturity anddelayed onset of reproduction [33]. A femalerearing a very large litter therefore experiences

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relatively low reproductive success in terms ofgranddaughters produced, compared to its rela-tively high energy expenditure in milk. A motherwill gain higher reproductive success by dividingthe energy available per lactation to the largestnumber of young she can raise to an averageweaning weight of 9 g, instead of producinga smaller number of larger young or a largernumber of smaller young [19].

In laboratory animal facilities weaning is typi-cally imposed at 21 days, corresponding to thetime when the next litter will be born if thefemale was mated postpartum [91]. However,weaning is only rarely finished at 21 days natu-rally and in laboratory mice, as previously illus-trated. In addition, newborn pups do not sufferfrom delayed growth in the presence of an olderlitter [32]. The data presented on maternal strate-gies therefore recommend not weaning mousepups before they are 23 days old.

Communal nursing of littersAs mentioned before, another maternal strategyin female house mice is communal nursing ofpups. Non-offspring nursing is an integral partof the reproductive behaviour of house mice inmultifemale groups, although females do notalways communally nurse when given the oppor-tunity to do so under free-living conditions (ownobservations). Communal or non-offspringnursing has been described for approximately70 mammalian species (in 12 orders) and forreproducing and non-reproducing females;nevertheless, in only 10% of such species werenon-offspring nursed as much as own young, asis the case in house mice [117–122].

When direct descendants of wild-caughthouse mice were kept under standardized condi-tions in the laboratory or in semi-natural enclo-sures, female lifetime reproductive successdiffered significantly as a function of the socialenvironment. A female’s lifetime reproductivesuccess was defined as the number of offspringweaned within an experimental lifespan of6months, standardized as 120 days after firstintroduction of an unfamiliar, genetically unre-lated adult male. Under natural conditions, sucha life expectancy seems to be realistic for femalehouse mice that survived at least until maturity[67, 34]. A female sharing a nest with a familiar

sister (i.e. females that grew up with each otherand had never been separated) weaned signifi-cantly more offspring within an experimentallifespan than a female living monogamouslyand rearing litters alone. The lifetime reproduc-tive success of a female living with a previouslyunfamiliar, unrelated partner lay somewhere inbetween [32, 80, 123]. Thus, communal rearingof young is beneficial due to improved individuallifetime reproductive success, as long as femaleschoose a familiar sister. These laboratory dataare supported by observations of free-livingmice and of wild mice in semi-natural enclosures.First, the degree of relatedness among commu-nally nesting females was higher than expectedby chance, which has been interpreted as ‘geneticassortment’ among females [124]. Second, innatural populations and in semi-natural enclo-sures female house mice spatially associateand communally nest with kin [40, 41, 124–126].Third, under semi-natural conditions femalespreferred for communal nursing a same-sexpartner that had a similar major histocompati-bility locus (MHC); since similarity in the MHCcorrelates with genetic relatedness, such assorta-tive social partner choice has been interpretedas kin preference [127].

During mate choice house mice of both sexesalso use MHC genetic cues; here, however, toavoid mating with unfamiliar related conspecificsand to mate preferentially with unrelated individ-uals (for recent reviews see [128, 129]). Proteinsproduced by the MHC are excreted through theurine and used as olfactory recognition cues.The recognition mechanism is probably pheno-type matching, meaning that an individual withan odour similar to one’s own is treated as kin.Such a mechanism may potentially also allowfemales to discriminate between a related andan unrelated potential social partner.

The effect of improved lifetime reproductivesuccess was due to improved probability to repro-duce and to improved offspring survival offemales sharing a nest with a sister [130]. Famil-iarity during juvenile development and notgenetic relatedness per se proved to be of para-mount importance for this effect, despite theexistence of a geneticmechanism to identify relat-edness [131]. This may suggest that a physiologicalmechanism is involved which requires someperiod of adaptation to or some synchronization

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with a partner. On the other hand, for communalnursing to bemost productive it may be importantthat females have information about each other’sbehaviour, which is best guaranteed amongfamiliar partners. Nevertheless, due to the basicfamily structure of housemouse breeding groups,there is a high probability that a female that choo-ses a familiar partner for communal care ofyoung will direct her cooperation and care ofnon-offspring towards kin.

Despite such cooperation, females neverthe-less compete over the opportunity to reproduce.Within breeding groups females establish eitheregalitarian reproductive relationships (bothfemales wean young within their experimentallifespan) or despotic ones (only one female repro-duces successfully). Lifetime reproductive successin egalitarian relationships is similar for bothfemales, irrespective of the degree of relatednessto or familiarity with the female partner.However, egalitarian reproduction is significantlymore common among familiar sisters than amongunfamiliar and unrelated females, with sistersvery rarely establishing despotic relationships.Future research has yet to show whether the pres-ence of an adult, non-reproducing daughterimproves a mother’s reproductive success, as nohelping effect has been documented for housemice so far.

Competition among females further increaseswith increasing group size. Individual lifetimereproductive success in groups of three females(either three sisters or three unfamiliar, unrelatedfemales) is significantly lower than that ofmonog-amous females. These data suggest that forsuccessful cooperation it might be important forfemales to be able to establish stable pairs [78].

Nevertheless, irrespective of relatedness,communal care in pairs with egalitarian repro-duction involves direct and mutualistic fitnessbenefits for both partners. The phenotypicallyaltruistic behaviour of non-offspring nursing inhouse mice proves to be genetically ‘selfish’ bymaximizing a female’s lifetime reproductivesuccess. Nevertheless, the mutualistic benefitsare influenced by relatedness and juvenile famil-iarity among the females, because of improvedprobability for egalitarian reproduction amongsisters that have grown up together.

During communal nursing, female housemice do not discriminate between own young

and non-offspring, because of constraints inrecognition abilities [80, 130]. As a consequence,cooperative females may run the risk of beingexploited by a female partner with a larger littersize [78]. Because nursing is indiscriminate,lactating females adjust milk production notaccording to their own litter size but to the sizeof the communal nest. Energetic investment isthen shared equally among the members ofa communal nest (König and Neuhäusser-Wespy,in preparation). Such equalized investment there-fore might be a prerequisite for cooperation, andsuggests the importance of social partner choicefor a female’s reproductive success.

Recent research has shown that female housemice display non-random preferences for groupmembers, and that social partner choice yieldssignificant fitness benefits. Females were experi-mentally allowed to establish non-random associ-ations to another female when kept in groups ofsix in seminatural enclosures. Afterwards,females were kept with either a previouslypreferred female partner or with a partner ofrandom association, and mated with an adult,genetically unrelated male over an experimentallifespan of 6months. Females kept in pairs witha preferred partner had a significantly higherprobability of giving birth and establishing anegalitarian, cooperative relationship, resulting inhigher reproductive success than females innon-preferred pairs [132].

Within groups, social relationships amongfemales thus appear to be structured by coopera-tion and by the existence and resolution ofconflicts. A highly flexible social and especiallymaternal behavioural repertoire neverthelessallows female house mice to reproduce undervariable environmental conditions and adjusttheir parental investment even to changing situa-tions. Such flexibility can be considered as animportant component to understand the species’ecological success as an invasive species that isable to live in a large variety of habitats, and itsalmost worldwide distribution.

The Nobel Prize in Medicine in 2011 wasawarded to Bruce A. Beutler, Jules A. Hoffmannand Ralph M. Steinman. The Nobel Prize wasawarded to Bruce A. Beutler, for the positionalcloning of the TLR4 gene in mice, togetherwith Jules A. Hoffmann, who described thefunction of the TLR receptor in fruit flies.

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Ralph M. Steinman received the Nobel Prize forthe discovery of dendritic cells, first reported inmice [133, 134].

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