Columbian ground squirrels (S. columbianus;King 1989a,
1989b). Similarly, kin-differential behavior and the spatial
aggregation of female kin are common to both marmots
(Armitage, chap. 30 this volume) and prairie dogs (Hoog-
land, chap. 37 this volume).
Given the potential importance of kin selection to ground
squirrel sociality, the mechanisms allowing beneficent be-
havior to be directed toward kin have also been the sub-
ject of comparative analysis (Holmes and Sherman 1982;
Holmes 1984b; Schwagmeyer 1988a; Holmes and Mateo,
chap. 19 this volume). Holmes (1984b) noted preliminarily
that an increasing reliance on more direct mechanisms of
kin recognition (i.e., those in which phenotype matching
or perhaps recognition alleles precluded a reliance upon so-
cial familiarization) paralleled increasing social complex-
ity in contrasting thirteen-lined and Belding’s ground squir-
rel kin recognition abilities. Consistent with that notion,
Schwagmeyer (1988a) reported that greater spatial cluster-
ing and more complex social behaviors, such as frequent
allogrooming and even joint hibernation, coincided with
the appearance of kin recognition mechanisms that were
not dependent upon familiarity in her review of the so-
cial behavior and kin recognition mechanisms of thirteen-
lined, Richardson’s, Belding’s, and Arctic ground squirrels.
Schwagmeyer (1988a) ultimately concluded that more re-
fined kin recognition abilities evolve where opportunities
to express nepotism exist. The subsequent findings of Hare
and Murie (1996) that social discrimination in highly social
Columbian ground squirrels is dependent entirely upon so-
cial familiarization, and the more recent demonstration by
Mateo (2002) that relatively asocial golden-mantled ground
squirrels (Spermophilus lateralis) make exacting discrimi-
nations on the basis of kinship in the absence of familiarity,
suggest, however, that no overall correlation exists between
social recognition mechanisms and sociality. Exacting kin
discrimination among solitary species may result from se-
lection pressure relating to mate choice (Mateo 2002), and
may serve to promote an optimal balance between inbreed-
ing and outbreeding (Bateson 1983). Hence, the nature of
recognition mechanisms is unlikely to prove useful in as-
sessing the contribution of kin selection to the evolution
of sociality.
Regardless of the mechanism promoting kin-biased be-
havior, it is undeniable that kin selection can play a sub-
stantial role in maintaining and defining the limits of soci-
ality in certain ground squirrel species. What is not clear,
however, is the extent to which kin selection enjoys primacy
in terms of promoting the evolutionary appearance of so-
ciality, or to what extent kin selection is necessary to ac-
count for the wide range of social constructs evident among
ground squirrels at present. Comparative analyses of mar-
mot sociality both among (Armitage 1987b) and within spe-
cies (yellow-bellied marmots; Armitage 1988) call into ques-
tion the extent to which evolutionary explanations of soci-
ality need to invoke kin selection. Armitage (1987b) agrees
with Michener (1983a) that matrilines are the fundamen-
tal unit of ground squirrel societies. Unlike Michener, how-
ever, Armitage argues that the formation of matrilines is
an expression of parental investment (a component en-
hancing direct fitness), and only incidentally allows bene-
fits to accrue via kin selection (promoting indirect fitness).
In support of his contention, Armitage (1987b) notes that
yellow-bellied marmot matrilines begin as mother/daughter
associations, and that sister/sister associations result only
from the joint recruitment of daughters to the natal area
and the subsequent death of their mother. He also reports
that there is no evidence that females associating in a ma-
triline trade off direct for indirect fitness benefits, as matri-
line size has no effect on the per capita production of de-
scendants. Finally, he notes that the small size of matrilines,
owing in part to mortality, but more tellingly through fis-
sioning of larger matrilines, is inconsistent with the opera-
tion of a mechanism promoting the ongoing association of
kin, as would be expected if sociality were predicated upon
kin selection. For yellow-bellied marmots, the average re-
latedness of members of a matriline was typically 0.5, and
when relatedness decreased, the matriline subdivided (Ar-
mitage 1988). The fact that matrilines split when burrow
systems are available (Armitage 1988), however, again sug-
gests that both relatedness and resources contribute to the
spatial aggregation of female kin.
In yellow-bellied marmots, the recruitment of daughters
was positively correlated with female reproductive success,
and thus cooperation among females would prove selec-
tively advantageous if that cooperation facilitated the ex-
clusion of immigrants from access to the resources of the
natal area (Armitage 1988). From the subordinate’s per-
spective, any loss of reproductive opportunities early in life
would be more than compensated for by the reproductive
payoffs of later becoming the dominant female within a ma-
triline. Further, where individuals have a limited probabil-
ity of successfully dispersing and establishing themselves as
immigrants (Armitage 1999), natal philopatry may simply
make the best of a bad situation and allow some fitness ben-
efit to accrue via kin selection (Emlen 1982; Woolfenden
and Fitzpatrick 1990). The adult female’s selective recruit-
ment of daughters over other potential allies reveals that
parental investment also contributes to matriline formation
(Armitage 1988). To illustrate the broader applicability of
this principle, Armitage (1988) notes that the delayed dis-
persal of black-tailed relative to while-tailed prairie dog
young reported by Hoogland (1981) was most readily in-
terpreted as a product of parental investment increasing off-
spring survival, and enhancing coterie membership in such
a way that facilitates the defense of the resources of the na-
tal area. That one’s own reproductive success remains the350 Chapter Twenty-Nine