with exclusive male ranges overlapping those of several fe-
males. In rich environments, where food is abundant and
clumped (such as urban areas, grain stores, and rubbish
tips) commensal rat societies are generally characterized
by high densities, small home ranges, rapid sexual matu-
rity, and polygynandry (Calhoun 1962a; Fenn 1989; Mac-
donald and Fenn 1995; also in Macdonald et al.1999). In
stable, rich environments, rat colonies are likely to be orga-
nized in multimale /multifemale “clan” territories (see also
Calhoun 1962a), within which males compete for estrous
females. Within home ranges, rats retreat to secure sites
(burrow systems in feral environments) that can act as “in-
formation centers” where rats can passively transmit infor-
mation about the environment to each other.
House mice also have a flexible and variable social sys-
tem and spatial dispersion pattern, depending on where
they live. Male territoriality and a polygynous mating sys-
tem characterize most house mice that are truly commensal
with humans. Feral house mice living in field settings and
away from human habitation exhibit a flexible pattern of
overlapping home ranges, both within each sex and across
sexes (Brown 1962; Berry 1968, 1981; Bronson 1979;
Mikesic and Drickamer 1992), and their mating system,
though not fully understood, involves both constrained
promiscuity and multiple paternity. Multiple paternity oc-
curs in about 35% of the litters in a field setting (Drick-
amer, unpublished data). Home range overlap within and
between sexes constrains promiscuity by limiting mating
with those individuals that share breeding space (Dricka-
mer and Robinson, unpublished).
Density and home ranges
Plasticity in rats and mice social organization is matched by
flexibility in home ranges, and consequently the ability to
live in highly variable densities. Estimates of rat abundance
in urban areas show 1,000-fold differences in rat density,
from 1 rat per 100 people to 1,000 rats per 100 people. Be-
cause rat densities and movements are heavily influenced
by the spatial and temporal distribution of food resources
such estimates, even when reliable (e.g., see Davis and Fales
1949, 1950; Davis 1950), are of specific rather than gen-
eral value. The reported killing of 2,650 rats in a single
night and 16,000 in one month in a horse slaughterhouse
in France in 1840 provides a relative estimate of rat abun-
dance in a small area (Shipley 1908 in Twigg 1975). While
current control programs would prevent the buildup of
such densities, these accounts demonstrate the flexibility of
the rat population dynamics. Rat colonies have typically
been maintained at a density of several rats per m^2 in out-
door enclosures and, while social processes eventually limit
population size (Davis and Christian 1956, 1958), rats show
little constraint by self-regulation when food is unlimited
(Calhoun 1962). In laboratory cages, current guidelines
recommend a maximum density of adult rats equivalent to
about 25 rats per m^2 (or 250,000/ha), while mice can be
stocked in densities equivalent to about 100 adults per m^2
(equivalent to 1,000,000/ha, see the following).
Even in relatively rugged environments, rat populations
can be viable away from human habitations (e.g., South
Georgia Island; Pye and Bonner 1980). Kozlov (1979) re-
ported that rat populations could be found 10 km away
from villages in northern Kazakhstan, and within a year
had moved up to 70 km away (in Pye and Bonner 1980).
Following Elton and Chitty’s pioneer work (Chitty 1954a —
see also Errington 1935; Aisenstadt 1945; Davis et al. 1948;
Davis 1949; Barnett 1955), most studies of rats in the wild
have been carried out under the framework of their role as
agricultural pests. In farmland, up to three-quarters of rat
populations may occupy stable home ranges (Hartley and
Bishop 1979), while the rest are transient animals. Studies
of farm rats in temperate regions (Czechoslovakia; Zapletal
1964; Moravia; Homolka 1983; United Kingdom; Brodie
1981; Hartley and Bishop 1979; Huson and Rennison 1981;
Middleton 1954; Cowan et al. 2003; United States; Erring-
ton 1935; USSR; Aisenstadt 1945) suggest that hedges and
fields constitute only temporary habitats, although they can
provide an important reservoir for disease (see the follow-
ing) as well as a source for the reinfestation of buildings.
Sizes of home ranges are equally flexible and dependent
upon food availability, including seasonal variation in har-
vest. Linear home range sizes vary from less than 100 m
in habitat where food is plentiful (e.g., urban areas, grain
stores) to almost a kilometer in poorer rural environments
(Davis et al.1948; Taylor 1978; Taylor and Quy 1978;
Hardy and Taylor 1979; Hartley and Bishop 1979; Stroud
1982; Fenn et al.1987; Macdonald and Fenn 1995). Stud-
ies based on radio-tracking typically show greater move-
ments than those based on capture-mark-recapture and, in
addition, rats can also travel long distances on foraging sor-
ties. The longest radio-tracked movement in a single night
was a 3.3 km round trip made by a male rat (Taylor and
Quy 1978; Hardy and Taylor 1979). Steiniger also reported
that rats would travel an estimated 6 km on an overnight
round trip (including swimming across a 50-m river) to
take eels from the nets at ebb tide in North Germany (in
Twigg 1975).
House mice living in commensal situations in and
around human-made structures can attain densities of more
than 14 mice /m^2 or 10,000/ha (Laurie 1946; Rowe et al.
1963), although the area involved is often only a fraction
of this size, as the mice are highly concentrated. When out-
breaks occur (fig. 32.2), densities of commensal house mice
can exceed the equivalent of 100,000/ha (Hall 1927), but382 Chapter Thirty-Two