tion is that mothers of large litters may need to spend more
time outside the nest foraging to meet nutritional demands
associated with providing milk to a large number of young
(Priestnall 1972; Mendl 1988). Another explanation is that
mothers of small litters must help pups maintain their body
temperature, whereas pups in larger litters may not require
such help because they can huddle with more littermates
(Priestnall 1972; Mendl 1988). Other evidence suggests,
however, that littermates are relatively ineffective at main-
taining a warm nesting environment (Webb et al. 1990). Fi-
nally, mothers of large litters may experience problems with
hyperthermia, and so spend more time away from pups, dis-
sipating heat (see Jans and Leon 1983 regarding maternal
hyperthermia). Given the diversity of rodents, the absence
of detailed behavioral studies, and the possibility that more
than one of these explanations may apply to a case, we can-
not establish a single cause for the observed pattern.
A second general pattern to emerge is that mothers wean
large litters later than small litters (see table 20.2, column
Age at weaning). Mothers may wean offspring when they
reach a certain minimum weight, and this weight is achieved
earlier in small than in large litters (Cameron 1973; König
and Markl 1987).
No clear patterns emerge concerning litter size and the
time that mothers spend nursing, grooming young, or nest
building (see table 20.2, columns Nurse, Groom pup, Nest
build). This inconsistency is chiefly because there are few
comparable studies and criteria for behavioral categories
differ across studies. Two reports indicate that maternal ag-
gression toward unfamiliar male conspecifics increases with
litter size (see table 20.2, column Attack). This result sup-
ports predictions that risks to mothers of defending young
should not change with litter size, but that benefits of such
defense should increase with number of offspring (Maestri-
pieri and Alleva 1990; Jonsson et al. 2002).
Much less is known about litter size variation and pa-
ternal care, but some observations are available. For ex-
ample, male gerbils (Meriones unguiculatus) exhibited more
frequent pup grooming and body contact when litters were
large but more nest building when litters were small (El-
wood and Broom 1978). Paternal care in social voles is
essentially independent of litter size (Libhaber and Eilam
2004).
Artificially manipulating litter size creates complications
that make interpretation of results difficult. Mothers in na-
ture may adaptively adjust their number of offspring ac-
cording to their own condition, abilities, and prevailing en-
vironmental conditions, so measures of parental behavior
may not vary with litter size. In contrast, females that nat-
urally give birth to small litters and then have their litter size
experimentally increased may be greatly challenged. Fur-
ther, litter augmentation would be a very unusual situation
under natural conditions. Although reductions in litter size
would not be unusual in field populations, experimental re-
duction to very small litter sizes (e.g., one or two pups) may
be extreme and may disrupt milk production or thermo-
regulation in the nest. Finally, pups can display fidelity to
certain nipples (e.g., in prairie and pine voles; McGuire
1998; McGuire and Sullivan 2001) and transferring pups
between litters, even on day 1 postpartum, can disrupt
development of such preferences. Some researchers have
teased apart the adjustments that nursing mothers make us-
ing methods to change offspring food demand (by manipu-
lating their access to solid food) without changing litter
size, but this approach only works for precocial species
such as guinea pigs (Laurien-Kehnen and Trillmich 2003).
Additional studies of unmanipulated litters of different sizes
are needed.
An intriguing cross-species example of litter size varia-
tion is reported in naked mole-rats (Sherman et al. 1999).
While most rodents have mean litter sizes equal to about
one-half the number of mammae (Gilbert 1986), naked
mole-rat queens raise litters on average equal to the number
of mammae (about 12). Extremely large litters —up to 28
and 27 offspring — are reported in field and laboratory col-
onies, respectively. Sherman et al. (1999) state that such
large litters are possible because offspring take turns nurs-
ing at the same nipple and colony members feed and pro-
tect the queen.Gender of offspring
Differential investment by mothers in male and female off-
spring has been examined in the context of parental invest-
ment theory, especially as it relates to mating systems (Triv-
ers 1972; Trivers and Willard 1973; Sikes, chap. 11 this
volume). In polygynous species (where variance in repro-
ductive success is typically greater for males than females)
mothers in good condition are predicted to bias their invest-
ment toward sons, while mothers in poor condition should
invest in daughters. Sex-biased parental investment may be
reflected in the sex ratio of offspring produced or in dif-
ferent amounts of care shown to sons and daughters dur-
ing the postnatal period (for reviews see Clutton-Brock
et al. 1981; Clutton-Brock and Iason 1986; Cockburn et al.
2002). There is some evidence that female rodents adap-
tively manipulate the sex ratios of their litters during the
prenatal period (e.g., coypus; Gosling 1986; golden ham-
sters, Mesocricetus auratus;Labov et al. 1986; house mice,
Mus musculus;Krackow and Hoeck 1989; Krackow 1997).
Here, we focus on differential parental investment during
the postnatal period.
Some studies examined postnatal maternal investment in
male and female offspring under ad libitum food conditions.
For example, Gosling et al. (1984) found differential invest-
ment in male and female offspring in the polygynous coypu;Parental Care 235