or conspecifics (e.g., Sherman 1980a, 1981b; Murie and
Michener 1984; Wolff 1993b), and alarm calling (e.g., Sher-
man 1977, 1980a; Hoogland 1995).
In field experiments with bank voles (Mappes et al.
1995) and Townsend’s voles (Lambin and Yoccoz 1998),
survival and recruitment of juveniles was enhanced when
females had kin as neighbors, whereas no fitness benefits
were found for gray-tailed voles (Microtus canicaudus,Dal-
ton 2000). In an experimentally manipulated population
of Richardson’s ground squirrels (Spermophilus richard-
sonii), females residing in kin clusters had significantly
greater reproductive success compared to females in nonkin
cluster groups (Davis 1984c). Having female kin as neigh-
bors increases female reproductive success in some other
species of ground-dwelling sciurids (e.g., Belding’s ground
squirrels, Sherman 1980a; black-tailed prairie dogs, Hoog-
land 1995; yellow-bellied marmots (Marmota flaviventriis,
Armitage and Schwartz 2000). However, the effect of pop-
ulation kin-structure on female reproductive success still
remains to be clarified for the vast majority of species in
which female kin clusters occur.
Cooperative breeding
Cooperative breeding — care of young by individuals other
than the genetic parents — is seen in a small percentage of
rodents, many of which display natal philopatry (40 spe-
cies from 9 of 30 families [Solomon and Getz 1997; Blum-
stein and Armitage 1999; Hayes 2000]). In these species,
females rear offspring in extended family groups where off-
spring remain after weaning, and participate in care of sub-
sequent litters born to their mothers (figure 4.2). In about
half of these cooperatively breeding species, more than one
female breeds within a social group and these females may
be kin but not necessarily mothers and young (Packer et al.
1992; Pusey and Packer 1994; Lewis and Pusey 1997; Eben-
sperger et al. 2004). Typically, all breeding females in the
group provide care for young, which are reared in a com-
munal nest (Sayler and Salmon 1971; Hayes 2000).
One of the adaptive hypotheses for alloparental care of
young is that breeders benefit directly by increasing their
lifetime reproductive success (Brown 1987; Solomon and
Hayes, in press). Alloparents (conspecifics that participate
in offspring care) may decrease the workload of breed-
ers, which could result in more reproductive attempts per
breeding season, or increase the success of individual breed-
ing attempts by increasing the quantity or quality of pups.
However, in laboratory studies, neither the presence of al-
loparents nor greater numbers of alloparents affected litter
size at weaning in prairie voles, pine voles (Microtus pine-
torum), or Mongolian gerbils (Meriones unguiculatus,Fried
1987; Solomon 1991; French 1994; Hayes and Solomon
2004). There were also no effects of living in a cooperatively
breeding group (as opposed to living solitarily) on num-
bers of weaned young in field studies of deer mice (Wolff
1994b), white-footed mice (Wolff 1994b), fat dormice [Glis
glis,Pilastro et al. 1996] or yellow-bellied marmots (Van
Vuren and Armitage 1994a). However, a negative relation-
ship between number of offspring weaned and group size
was found in black-tailed prairie dogs (Hoogland 1995)
and social tuco-tucos (Ctenomys sociabilis,Lacey 2004),
indicating that the presence of additional conspecifics can be
detrimental rather than beneficial. In contrast, alpine mar-
mots (Marmota marmota) and possibly golden marmots
(M. caudata aurea) warm related juveniles in the hiber-
naculum during winter, which results in increased over-
winter survival of pups (Arnold 1993; Blumstein and Ar-
nold 1998).
Another way that alloparents may benefit the breeders is
by increasing the quality of offspring produced. One mea-
sure of offspring quality is body size at weaning. Allopar-
ental care may result in increased offspring size relative to
offspring reared by a single female. Large body size at wean-
ing may result in numerous potential benefits. Offspring that
are heavier at weaning survive better than offspring that are
lighter at weaning (Solomon 1991; Huber et al. 2001). Off-
spring that are heavier at weaning also tend to be heavier as
adults. These individuals are preferred as social, and pre-
sumably mating, partners (Solomon 1993a but see Getz
et al. 2004) and are likely to be able to out-compete male
conspecifics for mates (Sheridan and Tamarin 1988). In
some species, such as house mice, heavier offspring become
sexually mature sooner than lighter individuals (Fuchs
1982). Finally, weaning weight of females affects the growth
of her pups (Solomon 1994). Females that were larger at
weaning had pups that grew faster prior to weaning. In-
creased size of weanlings was reported when prairie voles
were raised with alloparents under environmentally chal-
lenging conditions (e.g., temperatures or limited food; Sol-
omon 1991; Hayes and Solomon 2004), suggesting that
cooperative breeding can increase the quality of the off-
spring produced by a female.Conclusions and Future Directions for ResearchThe types of studies reviewed in this chapter have produced
many exciting new ideas showing that female fitness plays
a larger role in sexual selection and evolution of mating sys-
tems than previously thought. To make a stronger argument
that a particular mechanism, whether precopulatory or
postcopulatory, results in sexual selection by female mate
choice, we need to show that (1) females respond differently
to some males compared to other conspecific males, and
that this discrimination occurs in nature, (2) females’ dis-Reproductive Strategies in Female Rodents 55