Common house mouse biology
The wild house mouse (Mus domesticus) is
common in North America, and was introduced from western Europe. It occurs
in commensal form when found living in association with man-made structures,
or as a feral form when living predominantly independent of human dwellings
such as in native or introduced plant communities, in wastelands and agricultural
fields. For lack of reports it seems to be absent from woodland habitats
(Sage R.D.,1981). Mice of laboratory inbred strains have average life span
of 17-27 months, however maximum life span of free-living individuals
is less than 18 months. In attic M. musculus population Petrusewicz
and Andrzejewski (1962) found mean residency time to be less than 2.5 months
(c.f. Sage R.D.,1981).
Mice are very social animals forming tightly knit
social groups - demes. The size of established social groups
is very variable. Mice are expert colonizers. On one hand they strongly
resist immigrants into their existing groups, on the other hand their extensive
exploratory behaviour displayed by both sexes encourages them to locate
and colonize empty spatial and social niches elsewhere. The rate of colonization
mainly depends on the complexity of the environment. All weanling mice
seem to go through a dispersal phase. If the resulting exploration fails
to locate a new territory the individual returns to the social group of
its birth. Males however tend to be more persistent in their emigrating
efforts (Lidicker W.Z. Jr.,1976)
The home ranges of M. domesticus are rather small, usually less than 15 metres. Many studies indicate that the size of home range largely reflects the scarcity of food resources. Aside from their frequent short-term movements about their home range, long distance dispersals of 1000 metres or even as long as 2400 metres were reported in literature. However, such moves are uncommon and dispersal of house mice is rather limited. In studies of indoor populations only 5-8% of mouse population moved from the attic to out-doors or to lower floors (Petrusewicz and Andrzejewski,1962; Adamczyk and Walkowa,1971). Only 12 of 500 M. domesticus living in basement were trapped outside (Young et al.,1950)(c.f. Sage R.D., 1981). Strecker R.L. (1954) demonstrated that rate of dispersal in M. musculus was very low when food supply was in excess, but increased dramatically when food supply became limited. The dispersing animals were healthy, and of both sexes. They included visibly pregnant females as well as juveniles larger 10.1 grams.
Several studies suggest that mouse populations
sometimes consist of two separate segments which differ in their movement
and social structure. They are residents and transients. The residents
have developed a rigid social arrangement whereas the transients seemingly
have not. In agricultural landscape, Shchipanov N.A. and Shutova M.I. (1989)
refer to transients as "mobile reserves". They form temporary colonies
which are source of migratory animals. Mobile reserves apparently play
a crucial role in rapid regeneration of disrupted resident colonies.
Reproduction in Mus domesticus and its
closely related species
Mus musculus in temperate regions is reported
to take place from Spring to Autumn, in some studies reporting on indoor
mice, reproduction occurs all year round. The size of females reported
to be engaged in reproduction ranged from 14.4 gm to more than 22.0 gm.
After 19-20 days of gestation period pups are born in a litter averaging
5-8 pups. In laboratory, larger litters such as 11 are not uncommon. An
interesting observation however is concerning the reported sex ratios.
Generally, low density populations have more males than females, but reverse
is found to be true at higher levels. This imbalance was noted at the level
of litters, and therefore appears to be controlled by the females (Sage
Mouse Life Cycle
As temperatures warm up in the spring and food becomes abundant, breeding of the winter surviving individuals starts and continues in an increasing rate. The rate of reproduction is likely limited only by food availability at this stage. As the level of population reaches density where the density dependent factors such as socially mediated repression begin to predominate, a decreasing number of females reproduce. Ultimately the reproduction in all females ceases as the food resources become reduced. Which of the two density suppressing factors begin to operate sooner primarily depends on the availability of food in any particular situation. If food is available in abundance, population ceases to grow due to socially mediated repression of reproduction and becomes stable. Any reduction in population at this point occurs only through predation, emigration and natural, or induced dieoffs. Sage R.D. (1981) believes that such high density populations where almost no female is reproductively active represents the extreme condition in wild populations. Most feral populations are reported to start declining without reaching plague densities either through food limitation or climate-induced mortality due to approaching winter. Winter season takes severe toll on feral M. domesticus.
Commensal populations on the other hand, are spared of the climate-induced fluctuations, and seem to be exposed to somewhat different regulatory pressures. Sage R.D. (1981) cited two studies in which indoor living mice "voluntarily" declined to extinction (Evans F.C. 1949, Petrusewicz and Andrzejewski 1962). Based on known mouse biology he speculated that it was the founding females producing most of the colony offspring, and repressing their daughters from reproduction (it is a well established fact that onset of reproductive age in young, virgin females is delayed by exposure to urine of adult females). The adult females survive well in the most favourable home ranges. With advancing age, only their reproductive capabilities are impaired, not their social standing. Under these conditions they might survive long enough to repress their own offspring long enough so they too, lose their capability to repopulate the area. Thus the chain of recruitment of young into aging population would be broken.
This scenario would only be possible in isolated populations without immigration from extraneous sources, which, in case of most human settlements is highly unlikely to occur.