are more often at densities approaching 1,000/ha (Pearson
1963). Outbreaks of field populations of mice are common
in the main cereal-growing regions of Australia (Singleton
et al. 2005) and in the beech forests of New Zealand fol-
lowing mast years (Ruscoe et al. 2001). House mice living
under feral conditions are generally found at densities rang-
ing from 10 –100 mice /ha (Elton 1942), even when living
in field enclosures (Drickamer and Gillie 1998). Changes in
densities of mouse populations across different habitats
in an agricultural landscape and the processes likely to be
driving these dynamics have been detailed in the Austra-
lian wheatlands (Newsome 1969; Singleton 1989; Single-
ton and Redhead 1989).
When resources are scarce or widely dispersed, as in nat-
ural or field settings, house mice are often not territorial,
but exhibit overlapping home ranges whose size can be
highly variable (e.g., 0.0002 – 8 ha; Chambers et al. 2000).
Studies in the agricultural landscape of southern Queens-
land (Australia) reported that most individuals were site at-
tached during the breeding season, with an extensive over-
lap of home range of both sexes and with breeding males
occupying home ranges about twice as large as breeding fe-
males. After breeding, home ranges increased by about ten-
fold as most mice became nomadic. Similarly, Chambers
et al. (2000) reported from studies in an agricultural site of
northwestern Victoria (Australia) that nonbreeding mice
seemed to be nomadic when densities were low, as well evi-
dence for exclusive home range by females (at all densities),
low to moderate home range overlap for males when densi-
ties were low, and a switch to a more gregarious phase af-
ter breeding and when densities were high. The flexible so-
cial system and related changes in breeding structure and
care of young have likely contributed to this species success
throughout its worldwide distribution (Berry 1981).
Although there is considerable variation among method-
ologies, results, and interpretations of the numerous studies
of rats and mice, we can make several generalizations. Mice
appear to have a more flexible social system than rats, and
both species are opportunistic with respect to reproduction
and rapid population growth when food availability is high.Dispersal
Dispersal has not been studied directly in the wild, but
laboratory studies suggest that it is socially as well as food
driven. Calhoun (1962a), in his pioneering work on rats
housed in a large outdoor enclosure, observed that subor-
dinate males were excluded from the more favorable sites.
Farmland data suggest that colonists are usually young
males approaching sexual maturity (Zapletal 1964; Telle
1966; Bishop and Hartley 1976; Farhang-Azad and South-
wick 1979), with females more likely to stay in stable breed-
ing environments (Leslie et al.1952; Kendal 1984; see also
Calhoun 1962a for results in large outdoor enclosures). It
appears thus that dispersal is male biased, and it is likely
that around reliable food supplies resident males exclude
transient immigrants (see Nunes, chap. 13 this volume for
examples and theory of dispersal).
A similar pattern of dispersal appears to occur in mice.
Young females have a greater tendency to remain philo-
patric than males; however, both sexes do disperse to some
extent. Males emigrate from their maternal site as early as
40 days of age and females at about 70 days of age (Brown
1953; Berry and Bronson 1992; Drickamer unpublished
data). As in most species, males may disperse farther than
females, but no clear sex differences have been noted in the
tendency to disperse (Rowe et al. 1963; Drickamer unpub-
lished data). Aggressive behavior by adult males toward ju-
venile males has been reported as a precipitating factor
in juvenile dispersal (Bronson 1979); however, this has not
been well documented for mice or other species of mam-
mals (Wolff 1993a, 1994a).Foraging and feeding
Rats and mice are mostly nocturnal, presumably to avoid
predation, although both are able to modify activity pat-
terns. In a long-term monitoring of feeding activity, Ber-
doy (1994) reported how wild rats in an outdoor enclosure
(fig. 32.3) adjusted feeding intensity to maintain nocturnal-
ity. As night length decreased during the summer months,
rats compensated by increasing feeding activity, mostly in
the last quarter of the night. This nonuniform compensa-
tion may point to time-budget conflicts between feeding and
social interactions, which took place in the first part of the
night. It may also highlight the fact that rats feed in a wayComparative Social Organization and Life History of Rattusand Mus 383Figure 32.2 Mice seem to have been associated with humans for over
10,000 years, and are also well known for their impressive population out-
breaks. Photo by G. Singleton.