nance hierarchies, and for inducing dispersal of males
(Crowcroft 1955; Anderson and Hill 1965). In feral house
mice, male-male dominance likely is significant for access
to resources such as food or mates, and influences dispersal
via male-male contact. To our knowledge, these male-male
effects have not been observed directly in feral house mice,
but are inferred from trapping data, radio-tracking, and
spatial dispersion patterns.
As in rats, aggression by female house mice occurs at
lower frequencies and intensities than in males (Mackin-
tosh 1981), although it has been investigated in more detail.
The ecological functions of aggression by females involve
competition for resources, possible influences on dispersal,
and maternal aggression, exhibited by postpartum females
toward almost all intruders (Svare 1977, 1981). Both ecol-
ogy and evolution have likely influenced aggression in fe-
male house mice. In their ancestral habitat, with a relatively
even distribution of resources such as food, there would be
little need for females to compete with other females, but
they would need to protect their young through maternal
aggression if breeding space became limited (Wolff and Pe-
terson 1998). In the commensal system, with males hold-
ing territories and each territory potentially containing
more than one female, female-female tolerance is important
to maintain cohesive breeding within the deme, especially
among related females (Solomon and Keane, chap. 4 this
volume).
Social odors
As social nocturnal animals (Calhoun 1962a; Taylor et al.
1991; Berdoy 1994) it is not surprising that olfaction plays
an important role in rat societies (there are also suggestions
that rats use ultrasound for short range echolocation; Ros-
enzweig et al. 1955; Riley and Rosenzweig 1957; Kalt-
wasser and Schnitzler 1981; and for social communication;
Sales and Pye 1974; Barfield and Geyer 1975; Sales and
Smith 1978; White et al. 1990; Blanchard at al. 1991). Ol-
factory signals emanating from the animals’ body, breath,
urine, and feces convey an enormous level of information,
ranging from individual recognition, sexual status, and
dominance status to stress levels in both rats and mice as
well as rodents in general (see Roberts, chap. 22 this vol-
ume). Olfactory signals might be encoded through the pres-
ence of single components, but also through the ratio of sev-
eral chemicals or through the calibration of a fluctuating
signal with another stable, odorous one. The scent from se-
baceous glands, which are located on a rat’s hindquarters,
is often sniffed by individuals of both sexes, and has at least
22 volatile components that can vary between individuals
(Natynczuck and Macdonald 1994a, 1994b). Olfaction is
also useful in predator avoidance. Rats, including labora-
tory rats that have never been in contact with predators
for generations, remain innately aversive to predator odors
(Vernet-Maury et al. 1968, 1984; Berdoy and Macdonald
1991; Tanapat et al. 2001; see the following for how this
aversion can be manipulated by parasites). Social odors are
relevant to virtually all aspects of rats’ lives, including mat-
ing behavior, aggression, parental behavior, and food selec-
tion, and have been reviewed extensively for rats (Brown
and Macdonald 1985) and other rodents (Roberts, chap. 22
this volume). The finding that rats have an estimated 2,070
genes and pseudogenes involved with olfaction (37% more
than mice) suggests that we are a long way from unraveling
its complexity (Rat Genome Sequencing Project Consor-
tium 2004; table 32.1).
Although rats and mice of both sexes are known to scent
mark by depositing urine, this has been generally investi-
gated more in mice, where scent marking clearly plays a sig-
nificant role in social behavior (Bronson 1976; Hurst 1987,
1989, 1990a, 1990b, 1990c, and see extensive review for
rodents in general by Roberts, chap. 22 this volume). Dom-
inant males typically mark territorial boundaries with urine,
and intensely counter-mark both resident and nonresident
males. Urine marking can also be used to predict social
dominance in male house mice in that males destined to be-
come dominant leave more marks on a greater area than
those that will likely become subordinate (Drickamer 2001).
Female house mice respond to urine by investigation and
counter-marking toward urine from resident and neighbor-
ing females, and females mark at a higher frequency to in-
dicate higher social and breeding status. Between the sexes,
females are more attracted to marks from their resident
dominant males than other males. In addition to the im-
portance of urine cues for social behavior, mice of all ages
and sexes appear to use urine marks as a means of orienta-
tion (Hurst 1990b, 1990c, 1990d). For a complete discus-
sion of urine marking in mice, rats, and other rodents see
Roberts, chap. 22 this volume.Mate choice
Rats and mice clearly exert mate choice. Mating behavior
is more complex in rats, but the proximate mechanisms of
discrimination, such as the possible role of the Major His-
tocompatibililty Complex (MHC) and Major Urinary Pro-
teins (MUPs; Penn and Potts 1999; Hurst et al. 2001; Rob-
erts and Gosling 2003), as well as the fitness consequences
of free mate choice have been more investigated in mice.
The dynamics of mating in rats appears to be strongly
dependent upon density, but their mating patterns show
multimale-multifemale mating, with a potentially complex
(and largely not understood) pattern of intromissions (e.g.,
see McClintock, Anisko, and Adler 1982 for observations386 Chapter Thirty-Two