Although behavioral observations provided reliable evi-
dence that femaleD. spectabilismated with neighbors (Ran-
dall 1991a), DNA analysis revealed that many offspring
were fathered by nonneighboring males (Winters and Waser
2003). Because neighbors can be related, females may mate
with more distant males to avoid inbreeding. Males travel
considerable distances to visit the territories of estrous fe-
males, and females mate more than once during behavioral
estrus (Randall 1991a). In addition to promoting toler-
ance among neighbors, it seems that recognition of related
neighbors could facilitate mate choice by females to prevent
inbreeding.
Recognition of social status may be another means of
promoting social order in kangaroo rats (Shier and Yoerg
1999; Newmark and Jenkins 2000; Shier 2003). Shier
(2003) found that D. herrmanniformed a nearly linear
dominance hierarchy almost immediately upon meeting;
familiarization with dyad partners enhanced the relation-
ships. For D. heermanni,socialization (exposure to conspe-
cifics in general) rather than familiarization with particular
individuals seems to be the main factor mediating aggres-
sion (Yoerg 1999). Subordinate individuals avoid contact
with more aggressive individuals by retreating into the bur-
row and footdrumming (Shier and Yoerg 1999).
Not all species of kangaroo rat, however, are more toler-
ant of familiar neighbors than unfamiliar strangers. Dipod-
omys desertidid not direct less aggression toward neighbors
than nonneighbors in paired encounters in the field (Sulli-
van 2000). During playback experiments of footdrumming,
D. desertiapproached the speaker, presumably to chase
away the intruder, rather than stand and footdrum in re-
sponse to playbacks of conspecific footdrums, as seen in
D. spectabilisand D. ingens(Randall 1997).
Communication by scent and footdrumming
Kangaroo rats have evolved complex systems to commu-
nicate identity. Familiarity is communicated by scent at
sandbathing sites and through footdrumming exchanges
(table 31.2; Randall 1993; Randall 1997; Perri and Ran-
dall 1999; Murdock and Randall 2001). Territorial kanga-
roo rats, in which both sexes defend territories, have sexu-
ally monomorphic dorsal glands that are used for territorial
marking independent of age, sex, season, and gonadal hor-
mones (table 31.2; Randall 1993). Sebum from the gland is
deposited during sandbathing, and both males and females
discriminate familiar from unfamiliar scent at these sites.
Species with overlapping home ranges, however, have a sex-
ually dimorphic scent gland, larger in males than in females,
controlled by androgens. Scent from the gland is probably
important for identification of males by females during mat-
ing; females, but not males, discriminate between dorsal
gland scent of familiar and unfamiliar males (Randall 1993).
Kangaroo rats use both seismic and airborne signals gen-
erated by drumming their feet to communicate to conspe-
cifics (Randall and Lewis 1997; Randall 2001). Footdrum-
ming patterns are species-specific and range from single
thumps in D. desertito individually specific footdrumming
signatures in D. spectabilisand possibly D. ingens(Randall
1993, 1994, 1995, 1997, 2001). The footdrumming effec-
tively communicates ownership of a territory and usually is
sufficient to deter a visitor without chases or fights, except
in the case of D. deserti,mentioned earlier.
Footdrumming occurs in contexts other than territorial
advertisement. Male kangaroo rats footdrum during com-
petitive interactions when they compete for estrous females
(Randall 1987b; table 31.2). Footdrumming in D. heer-
manniis done inside the burrow by the kangaroo rat, avoid-
ing contact with a more dominant individual outside the
burrow (Shier and Yoerg 1999) and outside the burrow af-
ter mating (Shier, personal communication).
Kangaroo rats also footdrum to communicate to preda-
tors. Dipodomys spectabilis, D. ingens,and D. desertiac-
tively approach snakes, jump back, and footdrum to com-
municate directly to the snake that the kangaroo rat is alert,
is not easy prey, and to go away (Randall and Matocq 1997;
Randall and Boltas King 2001). Dipodomys ingensalso
footdrums on hearing the approach of a kit fox (Vulpes
macrotus), its major predator. In a field experiment, D. in-
gensdiscriminated playbacks of footfalls of an approaching
kit fox and the movement of snakes from controls of a rab-
bit hopping and tape noise (Busch 2003). Kangaroo rats
also discriminated the odor of snakes and kit foxes from
controls in laboratory experiments (Randall et al. 1995;
DeAngelo 2002).Social GerbilsIn contrast to the kangaroo rats that are strictly solitary,
gerbils show a social continuum ranging from solitary to
communal (Eisenberg 1967; Daly and Daly 1975a, 1975b;
Dempster and Perrin 1989a; Pavlinov et al. 1990; Randall
1994; Shenbrot et al. 1999; Gromov 2000; table 31.1).
Russian scientists provide the best information about a so-
cial continuum in the genus Meriones. From comparative
studies the following species have been classified from the
least to the most social: Meriones tamariscinus, M. meridi-
anus, M. libycus,and M. unguiculatus(Shilova et al. 1983;
Chabovsky et al 1990; Goltsman and Borisova 1993; Cha-
bovsky and Popov 1994; Goltsman et al. 1994; Gromov
et al. 1996). Social designation was based on the sharing of
a burrow by a male-female pair and the stability of male-
female relationships over time. Social behavior, however,
varied with density and season.
Two species of gerbils have been studied in enough de-376 Chapter Thirty-One