when he engages in nasogenital circling and smells the vagi-
nal opening or when he is allowed to mount. These behav-
iors may give females more control over their choice of
mates and minimize harassment by males. Future studies
will have to determine whether this flexibility in receptivity
provides females with more opportunity for mate choice
and timing of pregnancies or whether the behavior is a re-
sult of some other factor, such as a reluctance to allow con-
tact in solitary animals that compete for resources.
Dispersal and philopatry
Delayed dispersal is proposed as a key to understanding
the evolution of sociality in many birds and mammals (Em-
len 1997). Long-lasting, stable societies of related individu-
als allow for the development of complex social systems,
and philopatry (the retention of young in the natal area)
is usually a prerequisite for the formation of social groups
(Greenwood 1980; Armitage 1981; Solomon and Getz
1997). Offspring often remain in the natal area in response
to ecological constraints of limited territories and other
resources (Emlen 1997; Hatchwell and Komedeur 2000).
Young that remain in the natal area in cooperative breeding
species become helpers and forgo their own reproduction
but gain inclusive fitness benefits (Emlen 1991).
Although philopatry is commonly associated with highly
social animals, there are many solitary species in which
dispersal is delayed and relatives continue to share space.
Young benefit by gaining access to territories important for
survival and reproduction, and the dangers of leaving the
natal area are eliminated. Adults may lose little in space or
food, and may gain in inclusive fitness because of the higher
fitness of their offspring (Waser and Jones 1983; Waser
1988; Wolff 1994b).
Philopatry has been observed in both social and soli-
tary desert rodents. There is direct evidence for philopatry
in two species of kangaroo rat, D. spectabilisand D. mer-
riami,and indirect evidence in several other species (Fitch
1948b; Rogovin 1981; Jones 1984, 1989; Randall 1993,
1994; Winters and Waser 2003). Despite their solitary life-
style as adults, juvenile male and female D. spectabilismay
remain in the maternal territory for several months after
weaning. When juveniles do disperse, they settle within
20 meters of their natal site, which can result in dispersed
kin clusters (Jones et al. 1988; Winters and Waser 2003).
Once established in a burrow, the majority of males and fe-
males remain in that same territory throughout their lives
(Jones 1993; Randall 1993, 1994). Philopatry occurs be-
cause mothers share territories with offspring, donate a sec-
ondary territory, or abdicate their territory and move to a
new burrow nearby as young mature (Waser 1988).
Philopatry leads to the continued association of kin,
which facilitates cooperation and social interactions in so-
cial gerbils in many of the same ways as in other social
mammals (Emlen 1997). Females of the social gerbil, R.
opimus,are philopatric, and are related to each other (Ran-
dall et al. 2005). This relationship may facilitate the high
degree of cooperation in these female groups.
The advantage of philopatry might not seem as obvious
in solitary rodents that compete with neighbors for food
and space. Recognition of neighbors as kin, however, could
minimize agonistic interactions between solitary animals. A
recent study using microsatellite genotypes over five succes-
sive breeding seasons confirmed that D. spectabilisneigh-
bors in adjacent territories are often close relatives (Winter
and Waser 2003). Because kin recognition is a prerequisite
for the formation of more gregarious groups (Sherman et al.
1997), future research should examine whether kangaroo
rats differentiate kin from nonkin and are more tolerant of
related neighbors than unrelated neighbors.Kangaroo Rats as a Model of Social Evolution
in Solitary Species of Desert RodentHow social are kangaroo rats?
A comparison of closely related species that share similar
ecological conditions is a useful way to establish evolution-
ary relationships (Sherman et al. 1995). In rodents, social
continua have been successfully constructed in terrestrial
sciurids (Armitage 1981, 1999a; Michener 1983a; Blum-
stein and Armitage 1997b), fossorial mole-rats (Jarvis and
Bennett 1991), and voles (Microtus;Wolff 1985a). Com-
parisons of evolutionary relationships within these groups
have been especially revealing because of a wide range of
social structures and clear correlations between sociality,
environmental variables, and life-history traits (Armitage
1981; Michener 1983a).
Why, then, compare sociality in a taxonomic group of
desert rodents that is considered intolerant and asocial, such
as kangaroo rats? As expressed in the title of a recent study
by Yoerg (1999, p. 317), “solitary is not asocial.” Sexes
meet for mating, juveniles associate with their mothers, and
adults interact with neighboring conspecifics with the gen-
eral objective of maintaining individual spacing (Eisenberg
1966; Randall 1993, 1994; Lacey 2000; Shier 2003).
Kangaroo rats have evident, if rudimentary, social struc-
tures (table 31.2). They are philopatric (Jones 1984, 1989,
1993), show a range of social tolerance, and have well de-
veloped means of communication (Randall 1993). Exten-
sion and development of any of these aspects of socialityEnvironmental Constraints and the Evolution of Sociality in Semifossorial Desert Rodents 373