(Bolivian big-eared mice;Auliscomys boliviensisand moun-
tain viscachas; Lagidium peruanum;Pearson 1948; dassie
rats and rock hyrax; George and Crowther 1981). In the
latter case, crevices were divided between the two species
according to height, the maximum crevice size used by the
smaller-bodied dassie rat equaling the skull height of the
rock hyrax. The highly complex nature of rocky substrate,
with its abundance of crevices and fissures, is clearly one
reason why so many rodents make use of rocky habitat.
Predator avoidance
Mammals also make use of rocky substrate because it can
enhance their ability to protect themselves against preda-
tors. Some species use rock promontories and ledges as
lookout posts for predator detection (Tyser 1980; Mares
and Lacher 1987). In gundis, this behavior appears to be
cooperative, with group members taking turns reposing on
ledges and watching out for intruders (Gouat and Gouat
1989). Other species prefer rocky habitat because rocks can
be used to hide a burrow or nest. For example, burrows
of both the golden-mantled ground squirrel (Spermophilus
lateralis) and the least chipmunk (Tamias minimus) are lo-
cated under large rocks more often than would be predicted
from the distribution of boulders in their habitat (Bihr and
Smith 1998). Species that live in more complex rocky en-
virons conceal themselves from predators by foraging be-
neath rocky overhangs, or evade capture by darting into
nearby cracks and crevices. For the common spiny mouse
(Acomys cahirinus), the pygmy rock mouse (Petromyscus
collinus), and collared pikas (Ochotona collaris), the choice
of foraging location depends greatly on the presence of such
nearby cover (Holmes 1991; Brown et al. 1998; Mandelik
et al. 2003). In all of these instances, rocks are used in an
attempt to minimize predation pressure.
Rock-dwelling rodents are also able to evade predation
through camouflage. In at least three rock-dwelling species,
the canyon mouse (Peromyscus crinitus), a spiny mouse
(Acomyssp.), and the rock pocket mouse (Chaetodipus in-
termedius), alternative color morphs exist for individuals
inhabiting different colored substrate (Johnson and Arm-
strong 1987; Sicard and Tranier 1996; Hoekstra and Nach-
man 2003). For theAcomyssp., three color morphs (brown,
red, and gray) can be found in Burkina Faso, each of which
appears to be adapted to the coloration of the soil in its
particular habitat (Sicard and Tranier 1996). Populations of
the rock pocket mouse contain either light or dark-colored
morphs, although this is a deceptively simple categoriza-
tion, since alternative genes are responsible for the adaptive
melanism in different dark-colored populations (Hoekstra
and Nachman 2003). Habitat-dependent selection main-
tains the positive association between coat color and habi-
tat color observed in rock pocket mice (Hoekstra et al.
2004).This ability of rodents to camouflage themselves on
rocky substrate to avoid predation may partially explain
why there are such a large number of rodents that associate
with rocks.Thermoregulation
Rock-dwelling rodents occur in almost every ecosystem,
ranging from extremely hot, dry deserts to very cold alpine
regions. The geographic ranges of several species, including
golden spiny mice (Acomys russatus), common gundis, and
cliff chipmunk (Tamias dorsalis) extend from one environ-
mental extreme to the other, making it necessary for these
species to be able to adapt to a wide range of climatic condi-
tions (Haim and Borut 1975; Gouat and Gouat 1984; Hart
1992). Rock-dwelling rodents are able to survive in such
extreme environments in part because their rocky habitat
can help them to properly thermoregulate.
Many rock-dwelling rodents are able to survive in harsh
environments because of the insulating ability of rocks
to moderate extreme fluctuations in ambient temperature.
Rocks cool down and heat up slowly, so temperatures in-
side crevices are generally cooler than ambient during the
heat of the day and warmer than ambient at night (Clough
1972; George 1986). Temperatures inside crevices are also
generally much more stable than ambient temperature (Sale
1966). For example, the temperature of bushy-tailed wood-
rat (Neotoma cinerea) burrows constructed in rock crevices
fluctuates by only a few degrees Celsius daily, whereas am-
bient temperature may fluctuate by up to 20 C (Brown 1968,
as cited in Smith 1997). This thermal stability of rocky en-
virons may be enhanced by snow cover insulation, per-
haps explaining why some alpine rodents such as the snow
vole (Chionomys nivalis) and the bank vole (Clethrionomys
glareolus) use rocks as their preferred over-wintering micro-
habitat (Karlsson 1988; Luque-Larena et al. 2002b). The
moderated temperatures found within rocky crevices help
to provide a warm home for species living in cold climates
and a cool refuge for inhabitants of warm environments.
Rodents also use the thermal properties of rock surfaces
to help with thermoregulation. In hot climates, rock surface
temperature is generally higher than that of ambient tem-
perature (George 1986; Fredericksen et al. 2003). Many
diurnal species make use of the higher temperatures on
rock surfaces to warm up on cold mornings. Social rock-
dwelling rodents, including mountain viscachas and gun-
dis, often huddle together in piles during this early morning
bask (Pearson 1948; Gouat 1991). The heat-retaining and
insulating properties of rock help many rock-dwelling spe-
cies to properly thermoregulate, thereby enabling them to
survive in extreme environments.Socioecology of Rock-Dwelling Rodents 419