male offspring spent more time than female offspring suck-
ing from the highest-yielding teats, although this pattern
appeared to result from the behavior of young and not from
the mother’s active promotion or discouragement of par-
ticular offspring from sucking from specific teat locations.
Clark et al. (1990) found that female gerbils rearing all-male
litters were much more likely than females rearing all-female
litters to be in the nest with young and to have pups attached
to their nipples. Norway rat mothers and house mouse
mothers spend more time licking the anogenital region of
male than female pups, and also show enhanced nursing and
nest building when rearing all-male litters (Moore and Mo-
relli 1979; Richmond and Sachs 1984; Alleva et al. 1989).
Rat mothers appear to use olfactory cues to discriminate
the gender of their offspring (Moore 1981) and the spe-
cific chemosignal comes from the preputial glands of pups
(Moore and Samonte 1986).
Other studies of differential postnatal parental invest-
ment compared mothers with unrestricted access to food to
mothers whose food was restricted during lactation; greater
investment by food-restricted mothers in female offspring
was predicted. Food-restricted eastern woodrats (Neotoma
floridana) invested more in female offspring, as evidenced
by higher mortality and reduced growth of male offspring
(McClure 1981). Female-biased investment also is reported
for food-restricted golden hamster mothers (Labov et al.
1986). In contrast, Sikes (1995, 1996b) found no evidence
of sex-biased maternal investment in food-restricted eastern
woodrats and northern grasshopper mice (Onychomys leu-
cogaster;also see Sikes chap. 11 this volume). Finally, recent
evidence indicates that male-biased mortality in polygynous
species may occur independently of parental discrimination.
Moses et al. (1998), working with bushy-tailed woodrats
(Neotoma cinerea), suggested that male-biased mortality in
offspring of food-restricted mothers might reflect the greater
energetic demands of male offspring, resulting from sexual
selection for faster growth and greater body size. Compar-
ative data for rodents on differential postnatal maternal
investment in male and female offspring remain equivocal
and the topic requires further study.
Concurrent pregnancy
Postpartum mating in some groups of rodents results in con-
current pregnancy and lactation (Gilbert 1984). Although
less costly than lactation, pregnancy imposes energetic
costs (e.g., bank voles, Clethrionomys glareolus;Kaczmar-
ski 1966). Levels of maternal care by pregnant females are
generally lower than in nonpregnant females; such differ-
ences arise late in lactation as birth of the new litter ap-
proaches (wild house mouse, Mus domesticus;König and
Markl 1987; Norway rats; Rowland 1981; Wuensch and
Cooper, 1981; but see McGuire 1997 for red-backed voles,
Clethrionomys gapperi,and Krackow and Hoeck 1989 for
house mice). Lower levels of maternal care by pregnant fe-
males could result from the increased energetic demands
faced by such females. Other options for pregnant females
include diverting energy from young in utero or from them-
selves (Oswald and McClure 1987). All studies noted were
conducted in laboratory conditions with abundant food
nearby, and no temperature stresses.Social environment
Many laboratory studies have examined effects of social ex-
perience on rodent parental behavior (McGuire 1988; Leh-
mann and Feldon 2000; and reviews by Dewsbury 1985;
Brown 1993; Kinsley 1994). Here, we discuss how presence
of other males or mating opportunities influences paternal
care, and review studies that examine how the composition
of a social group influences parental behavior.
Males often disproportionately increase reproductive
success by seeking additional matings rather than by pro-
viding paternal care (Trivers 1972; Clutton-Brock 1989b).
Thus paternal care in rodents should decrease as mating op-
portunities increase, and this has been found in the field for
two normally monogamous species, hoary marmots (Ba-
rash 1975a) and muskrats (Marinelli and Messier 1995). In
the laboratory, parent-offspring interactions in polygynous
meadow voles were studied in a 2.4 by 1.2 m enclosure
(Storey et al. 1994). Introduction of an estrous female did
not significantly reduce the time fathers spent in the nest
with their pups, even though many fathers mated with the
introduced females. The enclosure’s size may have made
it easy for males to mate with estrous females without sig-
nificantly reducing their time in the natal nest (Storey et al.
1994). Difficulties observing paternal care in the field and
the need to provide extensive space in the laboratory make
it challenging to study the relationship between paternal
care and mating opportunities in rodents.
Field and laboratory studies show that the presence of
one parent can influence care shown by the other parent. Pa-
ternal presence correlates with decreased maternal behavior
in species such as rock cavies (Kerodon rupestris,studied
in laboratory cages; Tasse 1986), Norway rats (studied in an
outdoor pen; Calhoun 1962a), gerbils (studied in labora-
tory cages; Elwood and Broom 1978), red-backed voles
(studied in seminatural laboratory environment; McGuire
1997), and muskrats (studied in the field; Marinelli and
Messier 1995). The muskrat example is interesting because
free-living polygynous males only provided care to young of
their first mate, and these primary females displayed lower
levels of maternal behavior than did secondary females, who
compensated for the lack of male assistance by increasing236 Chapter Twenty