for geneticists, whereas rats have been used as a model for
a wide range of biomedical endeavors, from psychology
to biochemistry. Moreover, since 2004, R. norvegicushas
joined Mus domesticusand Homo sapiensas the first three
mammals to have their genomes sequenced (International
Human Genome Sequencing Consortium 2001; Mouse Ge-
nome Sequencing Consortium 2002; Rat Genome Sequenc-
ing Project Consortium 2004). While the motivation behind
these large-scale sequencing efforts is obviously medical,
some of the current and most probably future results stem-
ming from such genomic comparisons should be heeded by
whole animal and field biologists.
Although rats and mice are two mammal species that
have had profound effects on humans, both positive and
negative, no attempt has been made to evaluate and /or com-
pare various aspects of their social evolution or life history
that may have preadapted these species for this outcome.
Therefore, the objective of this chapter is to present a brief
summary of what is known about the social organization
and life history of two of the most commonly studied ro-
dents, rats (genusRattus) and house mice (genusMus).
Given the overall paucity of information on the various spe-
cies within each genus, our focus here will be onRattus nor-
vegicusand Mus musculus. In this chapter we present the
basic social and behavioral ecology of these two species.
Evolutionary Origins
Molecular data suggest that humans and rodents shared
a common ancestor some 80 million years ago (mya), and
that rats and mice diverged about 12 to 24 mya (Adkins
et al. 2001; Springer et al.2003; Rat Genome Sequencing
Project Consortium 2004). With between fifty to fifty-six
species, the genus Rattusis, in terms of number of spe-
cies, among the largest of mammalian genera (Wilson and
Reeder 1993; Nowak 1999). The genus is sorted into five
groups: the xanthurus group (species occurring in Sulawesi
and nearby islands), the native New Guinea species, the na-
tive Australian species, the rattusgroup, and the norvegicus
group (Wilson and Reeder 1993).
The Norway rat (Rattus norvegicusBerkenhout 1769),
also called Brown Norway, is not always brown (Watson
1944; Smith 1958; Figala 1963), does not come from Nor-
way and was not always called a rat. The Norway rat was
initially called Mus,later replaced by Epimysbefore its
formal Linnean nomenclatural description by Berkenhout
(1769), who mistakenly believed that the species came from
Norway (Barnett 2002; see also Yosida 1980). Rather, fos-
sil evidence suggests that the rat (perhaps suitably the first
sign of the Chinese zodiac) likely originated from the vast
Asian plains of northern China and Mongolia (Meng et al.
1994). Although it is unclear when the rat became com-
mensal, its later spread to other parts of the world is directly
attributed to its relationship with humans (Southern 1964;
Robinson 1965). While the black rat (R. rattus),the species
responsible for the bubonic plague (but see also Ruiz 2001
for R. norvegicus) via the flea vector (Xenopsylla cheopsis)
is thought to have been part of the European landscape
at least since the middle ages, reliable records of R. norveg-
icusin European countries date back only to the eighteenth
century. Rats may have colonized Europe after crossing the
Volga River following an earthquake in 1727 (Grzimek
1968; and see Barnett 2002), and later came to America
on the ships of the new settlers in the latter part of the eigh-
teenth century (Lantz 1909; Grzimek 1968; see also Twigg
1975).
In contrast, archeological evidence suggests that house
mice likely exhibited social and behavioral flexibility en-
abling them to coexist with humans at least 10,000 years
before the present (Auffray et al. 1988). Fossil and molecu-
lar data suggest that the early representative of the mouse
genus probably first appeared in the Miocene, about 5 mya,
and underwent several radiations before the appearance of
M. musculusby the early Pleistocene, about 0.5 mya (Broth-
well 1981; Boursot et al. 1993; Galtier et al. 2004). Fossil
and subfossil finds indicate that these species occurred at
various locations in Asia, Europe, and Africa (Dixon 1972;
Corbet 1974; Tchernov 1975; Storch and Uerpmann 1976).
One recent classification of species of the genus Mus,us-
ing morphometric and biochemical techniques, divides the
group into as many as thirty-seven distinct species (Wilson
and Reeder 1993). There are disagreements as to whether
M. domesticusis a distinct species or is, instead, a sub-
species of M. musculus(Boursot et al. 1993), although we
are not concerned with that taxonomic discussion in this
chapter. Mus musculus(Linnaeus 1758) is recognized as
the taxon that has become worldwide in its distribution.
The work that we summarize was all done with this species.
Mus musculusis also the taxon from which most of the lab-
oratory strains of house mice are derived.
Our primary focus in this review is on field studies of
rats and mice, which, by their very nature, are both feral
and commensal, and thus it is often not possible to com-
pletely distinguish confined conditions from truly feral or
wild conditions.Social OrganizationRadio-tracking and trapping studies highlight the flexibility
of rat population dynamics and social organization. Data
from poor environments with sparsely distributed food,
such as rural environment and field settings, suggest that
feral rat societies are characterized by low population den-
sity, slow sexual maturity, and single male territoriality,Comparative Social Organization and Life History of Rattusand Mus 381