Conclusions
Water scarcity, extreme temperatures, and unpredictable
and limited productivity of food place broad constraints on
the behavior of semifossorial desert rodents. As a result, the
majority are “solitary specialists.” The rodents included in
this review are primarily solitary and nocturnal. Feeding
adaptations vary and include granivores, folivores, omni-
vores, and insectivores. The social semifossorial rodents, on
the other hand, are diurnal folivores (table 31.1).
High productivity of green vegetation is an important
factor necessary to support group living in social rodents.
Under most desert conditions food may be too dispersed,
low in quality, and temporally unpredictable to support
more than a single animal and a solitary social structure.
When social groups do form, group sizes tend to be un-
stable and expand and contract with conditions of food
abundance; group living may increase social stress enough
to influence survival (Rogovin, Randall et al. 2003). For
these reasons, although social desert rodents should be
subjected to the same rules predicated for the evolution
of sociality in other animals (Alexander 1974; Pulliam and
Caraco 1984; Wrangham and Rubenstein 1986), the link
between sociality and traditional costs and benefits of soci-
ality are not as compelling for the desert rodents examined
in this paper as they are in other rodent groups (see reviews
by Ebensperger [2001a] and Lacey and Sherman, chap. 21
this volume). Both group formation and reproductive strate-
gies seem to be opportunistic responses to harsh environ-
ments with highly variable and limited food productivity.
Although semifossorial desert rodents occupy open hab-
itats in which group vigilance and detection of predators
would be predicted, a complex system of alarm signals has
not evolved in them, with the exception of the great ger-
bil. Constraints on group predator defenses may occur be-
cause of nocturnal activity that has evolved to facilitate wa-
ter conservation. Visual detection of approaching predators
and subsequent alarm calls to communicate predation risk
would not be effective at night. Instead, some desert ro-
dents have evolved acute low-frequency hearing to detect
approaching predators, effective bipedal locomotion to es-
cape from predators in open habitats, and footdrumming
behavior for individual defense.
Social tolerance is important for desert rodents. Besides
saving time and energy, avoidance of aggressive interactions
minimizes the effects of physiological stress and water loss.
Social tolerance is promoted by philopatry in both social
and solitary species. In social species, philopatry is impor-
tant for group formation; delayed dispersal leads to coop-
erative groups of related females in R. opimus. In solitary
species, philopatry might lead to recognition of related
neighbors to increase tolerance and decrease inbreeding.
Kangaroo rats provide a model system for understand-
ing the social behavior of solitary rodents. Behavioral differ-
ences among species are related to body size, spacing, food
storage, social tolerance, communication, and response to
predators. Social tolerance varies on a social continuum
from the most socially tolerant species, D. merriami,to
the least tolerant, D. deserti. If future research shows that
solitary kangaroo rats are more socially tolerant of related
neighbors than unrelated neighbors, a behavioral link in
the evolution of mammalian sociality will be made. In the
future, there would be a much better understanding of so-
cial structures in general if solitary mammals were not
considered asocial and dismissed as having no clear social
relationships.
Social flexibility may be more common in desert rodents
than previously assumed. Rodents seen together in high
numbers may reflect high population densities rather than
a stable social structure (Randall 1994). Whether group
living is an evolved strategy to increase inclusive fitness, a
consequence of local food abundance, or delayed emigra-
tion resulting from habitat saturation has not been dem-
onstrated for most species of desert rodents (Lacey 2000).
Further research that experimentally tests the relative in-
fluence of food abundance, habitat saturation, and density-
dependent factors leading to delayed dispersal and shared
space by female kin will help to elucidate the proximate
causation and ultimate benefits of group living in deserts
(Wolff 1993b, 1994b). However, because even the most so-
cial desert rodents (e.g., the Mongolian gerbil) seem to ex-
hibit flexible social behavior, researchers should exercise
caution when generalizing about the social structure of des-
ert rodents.SummaryDesert rodents must often survive and reproduce in harsh
conditions of scarce water, extreme temperatures, and lim-
ited plant growth. These environmental constraints, espe-
cially the limited abundance and patchy distribution of
food, impose limitations on the formation of social groups
in semifossorial desert rodents. Both social and solitary des-
ert rodents exhibit behavioral flexibility, probably to fa-
cilitate responses to the changing and unpredictable condi-
tions of arid environments. Social tolerance is important
to save time and energy and to minimize the effects of ag-
gression on physiological stress and water loss. When social
groups do form, group sizes tend to be unstable; they ex-
pand and contract with conditions of food abundance. Ger-
bils are semifossorial desert rodents that range from soli-
tary to communal. The great gerbil (Rhombomys opimus)
is one of a few social gerbils in a group of primarily solitary378 Chapter Thirty-One