winter. Rats show delayed onset of sexual maturity and fac-
ultative cessation of spermatogenesis when food is scarce
(Fenn 1989; Macdonald et al.1999).
In mice, breeding intensity and the proportion of sexu-
ally mature females that are pregnant or lactating at any
one time also varies with climate, resource availability, and
whether the mice are commensal or feral (Pelikan 1981). In
feral populations, the average number of females breeding
at any one time is around 40%, but this can peak at more
than 80% during summer months (Pelikan 1981; Drick-
amer unpublished data; Singleton et al. 2001). In commen-
sal locations, it is possible that some females may have eight
to ten litters or more in a lifetime, though this would be ex-
ceptional. In feral situations, most females can be expected
to have two to four litters during a breeding season (Pelikan
1981; Drickamer, unpublished data).
Litter size exhibits wide variation; in Europe, it varies
seasonally and with habitat. Average litter size for commen-
sal mice in winter ranges from 5.0 –5.5 pups, rising to 6.5 –
6.7 pups in summer months (Pelikan 1981). Feral mice in
Europe have summer litter sizes of about 8.0 pups. In labo-
ratory colonies of wild house mice and field populations of
feral house mice in seminatural enclosures in the United
States, summer litter sizes average 5.4 –5.6 pups (Dricka-
mer 1977b, unpublished data). Feral mice in agricultural
areas in Australia show average litter sizes ranging from five
to nine pups, depending on seasons and years (Singleton
et al. 2001).
Females attain sexual maturity at 50 – 60 days of age, as
evidenced by first vaginal estrus (Drickamer 1979); its tim-
ing can be influenced by a combination of factors. Factors
that stimulate early estrus include high nutrition followed
by rapid weight gain and heavier body mass (Monteiro and
Falconer 1966; Vandenbergh et al. 1972), and social fac-
tors, particularly urinary chemosignals from adult males
(Vandenbergh and Coppola 1986; Drickamer 1986a, 1999;
see also Drickamer, chap. 9 this volume).
House mice can breed throughout the year and generally
do so in commensal situations if sufficient food resources
are available (Southern 1964; Berry 1970; Pelikan 1981).
In natural or feral settings house mice exhibit seasonal
breeding, which involves either cessation of reproduction
for winter months, or in some instances, a summer seasonal
peak with diminished reproduction in winter (Berry 1970;
Bronson 1979; Singleton et al. 2001). Bronson and Perrigo
(1987) argue that virtually all of the variance in reproduc-
tion for house mice can be explained by energetics, and that
readily available food is the key to variation in reproduc-
tion in different habitats and climatic conditions. From a
dataset on 18 years of breeding, Singleton et al. (2001) sug-
gest that reproductive changes may be driven by food qual-
ity, particularly protein.
Perhaps the only life table published for house mice is
that of Berry and Jakobson (1974) for Skokholm Island (see
also Krebs et al. 1995). Life expectancy at birth varies sea-
sonally with time of birth, from a high of over 15 weeks for
mice born in late spring to a low of 11 weeks for mice born
in fall. Longevity was up to 18 months for a very few indi-
viduals born in late spring and was somewhat less for mice
born later in the breeding season. As would be expected for
an opportunistic species, about 50% of all young die before
attaining puberty. Data for feral house mice living in field
enclosures indicate that only 22.3% of young survive to re-
cruitment (Rossinni et al. unpublished). Analyses of these
same data revealed that several factors influence survival in
wild house mice, including genetics, weather, density, body
mass, and season of birth. The only published study with a
life table indicated that these factors influence survival for
house mice; consequently, additional experimental data are
needed to confirm the relevance of ecological factors to life-
history parameters.Parental care and infanticide
As altricial species, both rats and mice need prolonged and
intense parental care, which is provided almost entirely by
the female (Lonstein and De Vries 2000c), although labo-
ratory evidence hints at a possible influence of males in mice
(Wright and Brown 2000).
While the proximate mechanisms of parental behavior
(Rosenblatt et al. 1979; Brown 1993; Lonstein and De
Vries 2000b) and infanticide (Brown 1986) have been in-
vestigated extensively in laboratory rats, virtually nothing is
known about parental care and infanticide in wild rats and
it is unclear to what extent some of the laboratory findings
on these behaviors (e.g., communal nesting; Mennella et al.
1990; Schultz and Lore 1993) occur in wild rats. Parental
care is provided by the female although, perhaps surpris-
ingly, laboratory evidence indicates that, among prewean-
ing juveniles, males show more parental responsivenessto-
ward pups than do females. These gender differences are
reversed after weaning (see reviews in Stern 1996; Lonstein
and De Vries 2000c). Infanticide in wild rats is likely to
be common, if not frequent (Calhoun 1962a; Robitaille
and Bovet 1976; Mennella and Moltz 1988). M. Berdoy (un-
published data) observed infanticidal events by both males
and females in an enclosed colony of wild rats (see also film
footage in Berdoy 2002 for lab rats). In one instance, the
dominant male in the colony was observed to wait outside
the burrow of a female at dusk and kill a pup about every
30 minutes whenever she was not there. However, by the
following day, about half of the litter remained unharmed
and subsequently survived to sexual maturity (M. Berdoy,
unpublished data). Given the evidence of olfaction-basedComparative Social Organization and Life History of Rattusand Mus 389