cial dominance and aggression (Oakeshott 1974), and
chance (Berry and Jakobson 1974). Thus numerous exper-
imental studies have demonstrated that various proximate
mechanisms, which have fitness consequences, are used by
mice in assessing mates. Evidence from field and laboratory
studies indicates fitness consequences for males and females
choosing a preferred mating partner (Drickamer et al.
2000; Gowaty et al. 2003). The progeny of females mated
with a preferred male showed greater pre- and post-weaning
survival and more reproduction in field enclosures than
the progeny from females mated with a nonpreferred male.
Males mated with nonpreferred females produced fewer
offspring with lower viability than males mated with pre-
ferred females. Progeny of both sexes when males mated
with preferred females built better nests, and males were
more aggressive than progeny from matings between males
and nonpreferred females.Reproduction and maturation
Rats and mice are well known for their high reproductive
rate. Both species show postpartum estrus and are theoret-
ically capable of producing a new litter every month or so
(table 32.1). Reproductive output is, however, strongly in-
fluenced by environmental factors, and life-history param-
eters in the wild are better documented in mice.
Rats can breed throughout the year (Barnett 2002); re-
production is affected by a seasonal fluctuation in food
availability. Pregnant females have been captured from early
spring until late summer (Davis and Hall 1951; Farhang-
Azad and Southwick 1979; Fenn 1989). Cold winters were
associated with no reproduction (Leslie et al. 1952; An-
drews et al.1972; Lattanzio and Chapman 1980; Pye and
Bonner 1980), but Fenn (1989) found that winter breeding
in the UK was stimulated by provision of additional food in388 Chapter Thirty-Two
Figure 32.5 Relative mating success of the top four males (to control for a
dilution effect) in four colonies of wild rats. When colony size was small, the
dominant male exhibited the greater mating success. But, in larger colony sizes,
its mating success was more variable and sometimes lower than the next two
highest ranking males (Berdoy and Macdonald, unpublished data; see also Mac-
donald et al. 1999).
Figure 32.6 A series of eight 0.1-ha enclosures were used for extensive studies of the social system, reproduction, effects
of olfactory cues, and mate choice in feral house mice. Photo by L. Drickamer.