investigating their unfamiliar genetic father than an un-
familiar, unrelated adult male, demonstrating daughter-
father recognition despite the absence of any prior asso-
ciation between daughters and their fathers (Kruczek and
Golas 2003). Before concluding that voles cannot recognize
their unfamiliar relatives, recall that the lack of differential
treatment does not necessarily indicate the absence of rec-
ognition abilities (Gamboa et al. 1991), and we suggest that
recognition abilities are more likely to be detected with
direct- rather than indirect-inference studies. Until a num-
ber of direct-inference studies have been conducted on vole
kin recognition, it will be unclear whether the apparent lack
of kin-biased behaviors via phenotype matching in Micro-
tusis due to an inability to discriminate among kin or be-
cause Hamilton’s (1964) rule is not satisfied, resulting in
no preferential treatment of kin that, under some circum-
stances, can be recognized. Indeed, the results of some re-
cent cross-fostering studies suggest the value of a direct
inference study and the potential for phenotype matching
to facilitate recognition of unfamiliar kin. For example,
mandarin voles (Microtus mandarinus) display social pref-
erences for their unfamiliar siblings over unfamiliar nonsi-
blings that approach statistical significance (Fadao et al.
2000), and demonstrate mate preferences for unfamiliar
nonsiblings over unfamiliar siblings (Fadao et al. 2002), a
mate preference that also seems to exist in Brandt’s voles
(M. brandtii;Yu et al. 2004).
Summary
Behavioral ecologists have been very active in pursuing the-
oretical and empirical work on kin recognition, largely in
response to Hamilton’s (1964) development of inclusive
fitness theory, and various species of rodents have fig-
ured prominently in the growth of this body of knowledge.
In this chapter, we have addressed some of the concep-
tual and methodological issues surrounding kin recogni-
tion, reviewed part of the model-system work that has been
done on ground squirrel (Spermophilus) kin recognition,
and identified important questions for future research. We
believe that there are good reasons for separating differen-
tial treatment of kin, which can reveal discrimination abili-
ties, from preferential treatment of kin, which may or may
not follow from kin recognition, depending on the costs
and benefits of kin favoritism and the recognition context.
This distinction is important because a lack of differential
treatment of kin (e.g., nepotism, agonism, avoidance of in-
breeding) does not necessarily indicate a lack of a recog-
nition mechanism. Selection will operate separately on the
proximate bases of recognition (e.g., recognition cues, per-
ceptual abilities) as well as the ultimate functions of recog-
nition (e.g., nepotism, mate choice). We also suggest that in
empirical studies of discrimination abilities, fitness-neutral
bioassays like differences in olfactory investigation are more
appropriate for studying kin recognition than are bioassays
that have clear fitness consequences like food sharing, co-
operative care, or differences in agonistic behavior (e.g., kin
favoritism).
Early work on ground-squirrel kin recognition revealed
that two proximate mechanisms, prior association and phe-
notype matching, explained many instances of kin recogni-
tion, and that these mechanisms also operate in many other
taxa, including several species of rodents. Prior association
mediates recognition of previously encountered kin, such as
siblings that develop together in a nest or burrow isolated
from other conspecifics. In contrast, phenotypic match-
ing mediates recognition of never-before-encountered kin,
such as paternal half-siblings meeting for the first time, or
unequally related individuals that shared a common early
rearing environment. A species’ ecological niche and social
system will influence both the evolution of these recogni-
tion mechanisms as well as their use across various social
contexts.
In future studies, we encourage investigators to examine
kin recognition in cooperative and communal breeders, be-
cause when multiple individuals share a common rearing
environment, intriguing questions arise as to whether and
how different categories of kin might be recognized by var-
ious individuals.228 Chapter Nineteen