Fear and Foraging
In deserts, it appears that most, if not all, small rodents end
their days at the hands, claws, jaws, or talons of a predator
(Brown and Harney 1993). In one study, Merriam’s kanga-
roo rats (Dipodomys merriami) suffered one predation loss
per 134 nights of activity (Behrends et al. 1986; Daly et al.
1992). Mortality came from snakes, owls, shrikes, and coy-
otes. The risks faced by rodents vary temporally and spa-
tially. At small spatial scales, proximity to shrub cover seems
critical to predation risk. All else being equal, most desert
rodents favor the shrub microhabitat over the open (Brown
and Lieberman 1973; Price 1978; Thompson 1982; Brown
1989; Hughes and Ward 1993; Brown et al.). Even the In-
dian crested porcupine (Hystrix indica), the largest species
of desert rodent, prefers areas with high vegetation cover
(Brown and Alkon 1990).
Voles appear to prefer areas of taller and denser vegeta-
tion to areas with less dense vegetation (Jacob and Brown
2000; Pusenius and Schmidt 2002). Similarly, perception of
predation risk increases as thirteen-lined ground squirrels
(Spermophilus tridecemlineatus) venture farther from their
burrow (Thorson et al. 1998), as tree squirrels (Sciurus car-
olinensisand S. niger) forage farther from trees (Lima et al.
1985; Dill and Houtman 1989; Bowers et al. 1993; Brown
and Morgan 1995), and as rodents such as white-footed
mice (Peromyscus leucopus) and some voles (Clethrionomys
sp.) inhabit margins of woods relative to forest interiors
(Morris and Davidson 2000; Wolf and Batzli 2004). Aus-
tralian house mice (Mus musculus) inhabiting an open agri-
cultural habitat consisting of grain fields, pastures, and non-
managed fence lines, avoided pastures and preferred dense
crop fields until harvest. Following the grain harvest and the
removal of cover from the cropland, the mice switched pref-
erences and activity to the fence-line habitat, the only re-
maining habitat with protective cover (Ylönen et al. 2002).
Regardless of scale, rodents respond to spatial heterogene-
ity in predation risk by biasing activity toward safer areas,
by demanding a higher harvest rate from risky areas, and
by using food patches less thoroughly in risky relative to
safe habitats.
Presence of predators: Do rodents learn fear
from prior experience?
Temporal patterns of risk influence foraging behavior.
Abramsky et al. (2002) flew trained barn owls for short
durations over rodent enclosures at a sand-dune habitat
in the Negev Desert, Israel. In response, gerbils (Gerbillus
sp.) greatly reduced foraging activity during the period of
flights, but by the end of the night total gerbil activity was
the same on grids with and without trained-owl flights. In
response to owl flights, the gerbils simply rearranged their
nightly activity to avoid periods when owls were active.
Aviary experiments have shown how desert rodents can re-
spond nightly to the presence or absence of predators such
as owls (Brown et al. 1988; Kotler et al. 1991; Longland
and Price 1991). One species of gerbil (Gerbillus pyra-
midum) returns to baseline levels of activity within one
night of the removal of owls from the aviary, whereas a
smaller, more vulnerable species (G. andersoni) requires up
to three days to return to baseline activity levels following
the “trauma” of a night with owls (Kotler 1997).Circadian rhythms and predation risk
Besides the irregular arrival and departure of predators
from an area, rodents experience three important scales of
predation risk: day versus night, changes in moon phase,
and seasonal changes in predator abundance. It is interest-
ing that most rodents opt for either complete nocturnality
or diurnality (Bartness and Albers 2000). Given that their
food resources do not seem to vary strongly between night
and day, it seems most likely that rodents are avoiding cli-
matic or predation stresses by adopting diurnal or noctur-
nal activity. Nocturnal rodents tend to seek cover, avoid be-
ing detected by predators, lack complex social structures
such as colonies, and do not seem to use alarm calls. In con-
trast, diurnal rodents (especially sciurids) rely heavily on vi-
sion to detect predators, prefer open sight lines, and pro-
vide examples of sociality. In colonies, alarm calls and other
social interactions assist with detecting and avoiding pred-
ators (Blumstein, chap. 27 this volume).
Voles and spiny mice (Elvert et al. 1999) provide two
prominent examples of rodents that break the “rule” by
having 24-hour patterns of foraging activity. Despite con-
siderable variation, voles appear to be “polyphasic,” with
a 2- to 3-hour cycle of rest and foraging (Madison 1985;
Ylönen 1988; Halle 2000b). Polyphasic activity may be an
adaptation to herbivory and a relatively low-quality diet.
Such activity patterns are common for large ungulates (e.g.,
black rhinoceros, Diceros bicornis;Kiwia 1989).
Predation risk sculpts the polyphasic activity pattern in
a manner that makes boreal vole activity more like that of
nocturnal desert rodents (Halle 2000a). Voles become more
nocturnal and less synchronous in their activity times in
fragmented habitats with microhabitats of cover and open.
Furthermore, their use of the open microhabitat becomes
almost entirely nocturnal. Open microhabitats may pre-
sent rodents with intense exposure to predators. Voles in-
tegrate habitat structure, predation risk, and the activity of
other voles into their own foraging decisions. While this is
merely speculation, a polyphasic activity pattern may influ-
ence their predator’s hunting success by overdispersing the330 Chapter Twenty-Eight