pothesis applies to m-driven rather than the N-driven sys-
tem of voles.
Predation Risk Affecting Social Interactions
Rodents exhibit a broad range of social complexity levels,
ranging from primarily solitary taxa to the highly integrated
colonies of naked mole-rats (Randall 1994). The family Sci-
uridae has a Social Complexity Index of 0.27 for the wood-
chuck (Marmota monax) up to 1.12 for the black-tailed
prairie dog (Cynomys ludovicianus) (as defined and cal-
culated by Blumstein and Armitage 1997). Such a score
for the woodchuck indicates a low level of social interac-
tion and structure, one that is relatively typical for many
rodent species. In this system, solitary adult females main-
tain small home ranges; males maintain larger home ranges
that often include the ranges of several females; males and
females live apart; breeding is promiscuous, with some
antagonistic behavior of males; and the young may reside
with the mother or within the mother’s home range into
or close to adulthood. Prairie dogs represent an example
of a colonial lifestyle without strict division of labor and
with only inconspicuous dominance hierarchies. Coteries
of often related individuals maintain a shared system of
burrows, share sentinel duties, and cultivate social bonds
through ritualized behaviors (Hoogland 1995). Levels of
sociality likely influence and have been influenced by
numerous life history and environmental factors. For in-
stance, Devillard et al. (2004) showed that among ground-
dwelling sciurids male-biased dispersal increases with so-
cial complexity. To what extent has predation risk sculpted
and in turn been sculpted by social interactions among
rodents?
A striking feature of sociality in rodents is that most
colonial rodents are diurnal. Diurnal and colonial rodent
species exist among species of Cynomys, Marmota, Degus,
Xerus, Rhombomys, Parotomys, Petromys, Spermophilus,
Rhabdomys,and others. The naked mole-rat and Damara
mole-rat (Cryptomys damarensis) provide conspicuous ex-
ceptions to coloniality and diurnality, but these are fossor-
ial rodents. The nature of predation risk likely plays a role.
The veil of darkness conceals both prey and predator from
each other’s view, rendering the night less propitious for the
benefits of group wariness and group foraging.
Alarm calls are rare among nocturnal rodents. Sight
lines and vision may be requisites for either the recognition
of the need for, or the effectiveness of, group vigilance and
group defense. Not surprisingly, the complexity of alarm
calls increases with the size and complexity of rodent colo-
nies (Blumstein and Armitage 1997b, 1998a). Furthermore,
most colonial rodents occupy relatively open landscapes.
Prairie dogs will actually destroy vegetation and shrubs
that obstruct lines of sight. The black-tailed prairie dog
is notable but not unique for a repertoire of alarm calls
that varies and adjusts for different species of predators
and different approaches of predators (Hoogland 1995; see
Boero 1992 for discussion of European marmot, Marmota
marmota).
The great gerbil (Rhombomys opimus) of the Kyzylkum
desert, Uzbekhistan, is diurnal and colonial (Rogovin et al.
2004). It exhibits a social structure also found in the cape
ground squirrel (Xerus inauris) of the deserts of southern
Africa (Waterman, chap. 3 this volume). Both species tend
to have colonies of up to thirty individuals built around sev-
eral adult females and their offspring. A single adult male
may continuously or periodically share the colony. Most
males live alone or in pairs, sometimes moving in and out
of colonies (Waterman 1995, 1997). In the great gerbil
a rhythmic alarm warns of a distant predator, whereas
an intense whistle warns of imminent threat from a preda-
tor nearby (Randall and Rogovin 2002). Foot drumming
also warns of dangers when emitted outside (in response to
snakes) or inside (in response to a dog) of the burrow. Fur-
thermore, solitary gerbils do not emit alarm calls, and adult
gerbils call more frequently in the presence of young gerbils
(Randall et al. 2000).
Predation risk clearly plays a role in the colonial life and
alarm calls of diurnal rodents (Blumstein, chap. 27 this vol-
ume). Yet a critical research question remains. Did some
other benefit to social complexity drive the evolution of
coloniality in these rodents? If this is the case, sentinels,
alarm calls, and group wariness may have evolved as adap-
tations useful to coloniality. Or did the antipredator advan-
tages of sociality drive the evolution of coloniality, as beau-
tifully demonstrated by Hoogland (1981) for prairie dogs?
We think the latter hypothesis is more likely and believe
that in the absence of predation risk most forms of colo-
niality would not have evolved in rodents.
While lacking coloniality, nocturnal desert rodents do
exhibit rudimentary forms of sociality and calls in response
to predators. Foot drumming is widespread across diverse
families of desert rodents (Randall 2001). Randall (2001)
noted that most species foot drum in response to preda-
tion risk. Solitary, territorial rodents such as the bannertail
kangaroo rat have complex patterns of airborne and seis-
mically transmitted foot drums that communicate terri-
toriality and the presence of predators. When it occurs in
response to predators, foot drumming in the bannertail
kangaroo rat serves as parental care (warning to a female’s
current litter) or individual defense (indicates to the pred-
ator that it has been detected; Randall and Matocq 1997).Fear and the Foraging, Breeding, and Sociality of Rodents 337