which direct fitness decreases with group size, individuals
are likely forced to live together due to the prevalence of
ecological or other extrinsic factors that limit dispersal and
independent breeding (Lacey 2004; Safran et al., in press).
In between are species in which the relationship between
group size and fitness is more complex and may be non-
linear (e.g., yellow-bellied marmots, Marmota flaviventris;
Armitage and Schwartz 2000). Although the inclusive fit-
ness consequences of group living may differ somewhat due
to the effects of indirect fitness gains, we believe that re-
lationships between group size and direct fitness provide
valuable information regarding the factors that promote
philopatry and, hence, sociality (see also Jennions and Mac-
donald 1994).
Data regarding the direct fitness consequences of social-
ity are available for only a few species of group-living, com-
munally nesting rodents. In several of these taxa, the per
capita number of offspring produced does not appear to
vary with group size (e.g., dormice, Glis glis;Pilastro et al.
1994; white-footed and deer mice; Wolff 1994b). In these
species, tenure in the natal group tends to be brief, and in-
dividuals may switch from living in a group to living alone
(or vice versa) between successive rounds of reproduction
(Armitage 1991; Marin and Pilastro 1994; Wolff 1994b).
These data suggest that the fitness consequences of solitary
versus group life do not differ greatly, with the result that
individuals are behaviorally flexible and can alter their so-
cial setting in response to small changes in environmental
conditions.
In two of the remaining species for which data are avail-
able, direct fitness is negatively correlated with group size.
In black-tailed prairie dogs (Cynomys ludovicianus), the
number of young reared to weaning declines with group
size (Hoogland 1995), indicating that females pay a direct
fitness cost to remain in their natal group (coterie). Simi-
larly, direct fitness is negatively correlated with group size
in the colonial tuco-tuco (Ctenomys sociabilis;Lacey 2004),
a communally nesting rodent from southern Argentina. In
both of these species, females frequently remain in their
natal group for life (Hoogland 1995; Lacey and Wieczorek
2004), implying that dispersal is sufficiently difficult and
dangerous that natal philopatry is favored even though it
entails an apparent direct fitness cost.
No communally nesting rodents are known to exhibit
a consistently positive relationship between group size and
per capita direct fitness. Although intrinsic benefits to group
living have been suggested for at least one plural-breeding
social rodent, the degu (Octodon degus;Ebensperger and
Wallem 2002; Ebensperger and Bozinovic 2000), data re-
garding the direct fitness consequences of sociality in this
species are not available. Among yellow-bellied marmots,
net reproductive rate increases with group size in smaller
groups, but this relationship becomes negative at larger
group sizes (Armitage and Schwartz 2000). Finally, in some
singular-breeding species such as naked mole-rats, the di-
rect fitness of reproductive individuals may increase with
group size, but the strong reproductive skew within groups
dictates that nonbreeding individuals experience a decrease
in direct fitness by remaining in their natal group (Lacey
and Sherman 1997). An important implication of these data
is that rodent groups form primarily due to the net effects
of extrinsic (ecological) constraints on natal dispersal.
While detailed field studies of multiple social species are re-
quired to verify this interpretation, the realization that for248 Chapter Twenty-One
Figure 21.1 Schematic representation of the relationships between ecological constraints and benefits of philopatry models to explain natal philopatry and group
living.