Factors Influencing Social Behavior of Desert Rodents
Physiological constraints
High temperatures and limited water are perhaps the most
unrelenting environmental pressures on the characterist-
ics of desert rodents. The major adaptations to these con-
ditions include low metabolic rates, efficient kidneys for
water conservation, and water produced through oxidative
metabolism (Schmidt-Nielsen 1975; French 1993). There
are also behavioral adaptations that have physiological ben-
efits. A majority of semifossorial rodents confine their activ-
ity to the cooler temperatures of night time (Randall 1994).
The rodents also avoid temperature extremes by retreating
into subterranean burrows, where cool temperatures and
high humidity provide a favorable microclimate for thermo-
regulation and water conservation, as well as protection
against predators and a place to store food (Kinlaw 1999).
Because burrows are so important to survival and repro-
duction, communal digging of burrows has been proposed
as a cause for the evolution of social groups of bathyergid
and hystricognath rodents (Faulkes et al. 1997; Ebensper-
ger and Cofré 2001; Ebensperger and Bozinovic 2000a).
Cooperative digging of burrows to locate food is especially
adaptive for subterranean mole-rats in arid habitats (Faulkes
et al. 1997). The role of communal digging of burrows is
less clear in other desert rodents (Kinlaw 1999). Under arid
conditions, burrow systems used for shelter can remain in-
tact for years or decades, and are passed from one genera-
tion to the next. In addition, solitary species seem perfectly
capable of digging sufficient burrow space for survival and
reproduction under most conditions; some semifossorial
desert rodents use existing rocky crevices and niches as
shelters rather than digging burrows (Shenbrot et al. 1999).
Stress is another physiological factor that could impact
the survival and reproduction of desert rodents. Although
semifossorial desert rodents are usually solitary, under some
conditions densities become high, forcing individuals into
group situations. These high-density conditions could in-
crease stress, leading to group instability, dispersal, and in-
creased concentrations of glucocorticoids (Sapolsky 1983).
The effects of group living on stress in a social desert rodent
across dramatic changes in food availability and population
densities have been tested in breeding males of the great ger-
bil, Rhombomys opimus(Rogovin, Randall et al. 2003).
Concentrations of fecal testosterone were lower and corti-
costerone higher during periods of high population densi-
ties when social groups were large compared with higher
testosterone and lower corticosterone concentrations dur-
ing lower densities when social groups were small (Rogo-
vin, Randall et al. 2003; Rogovin, Moshkin et al. 2003, and
unpublished data). Regression analysis of the relative role
of social factors showed a positive and significant relation-
ship between concentrations of male fecal corticosterone
and the number of adult females in a social group. These
findings suggest that social stress adversely influences sur-
vival of male gerbils and may be greater in higher popula-
tion densities because of increased social contact with mul-
tiple females in larger groups (Sapolsky 1983; Wingfield
et al. 1990).
Social interaction may also be stressful to solitary ro-
dents. Ganem and Nevo (1996) proposed that social toler-
ance is an adaptation to conserve energy and water as well
as to minimize stress. Ganen and Nevo compared geneti-
cally distinct populations of solitary mole-rats (Spalax eh-
renbergi) from different environments and found that the
arid-adapted rodents were more tolerant of conspecfics than
more aggressive individuals from a mesic habitat. The des-
ert rodents had lower concentrations of corticosterone and
higher urine osmolarity than the more aggressive mesic ro-
dents. (See Nevo chap. 25 for more information on behav-
ior of fossorial S. ehrenbergi.)Constraints of food abundance and distribution
The costs and benefits of obtaining and defending food
can be significant factors influencing the formation of social
groups in arid habitats (Jarvis et al. 1994). Arid-adapted
plants range from those that produce underground stor-
age structures in roots and tubers to permanent succulent
plants and widely distributed shrubs and grasses of low
quality and quantity. Because of the range in type, produc-
tivity, and distribution of plants, desert rodents vary in their
adaptations for obtaining food (Shenbrot et al. 1999). Be-
sides feeding on underground storage organs, desert ro-
dents feed on seeds (granivores), plant leaves and stems (fo-
livores), insects (insectivores), and a combination of food
types (omnivores; table 31.1).
The dispersed, patchy food and unpredictable rainfall in
arid habitats have been proposed as primary causes of group
formation in the subterranean bathyergid mole-rats (Jarvis
et al. 1994). According to the “aridity-food-distribution hy-
pothesis,” eusociality evolved in Heterocephalus glaberand
Cryptomys damarensisbecause cooperation was necessary
to dig burrows during short, unpredictable periods of rain-
fall to maximize the harvest of the patchily distributed roots
and tubers. Social mole-rats gain from cooperation because
the high quality and quantity of the resource feed many an-
imals once the food is found, which presumably compen-
sates for the high costs of burrowing (Jarvis et al. 1994;
Faulkes et al. 1997; Faulkes and Bennett chap. 36 this vol-
ume). Individual mole-rats in habitats with higher rainfallEnvironmental Constraints and the Evolution of Sociality in Semifossorial Desert Rodents 369