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Nordic Society Oikos
Foraging under Predation Risk in the Wild Guinea Pig Cavia aperea
Author(s): M. H. Cassini
Vol. 62, No. 1 (Oct., 1991), pp. 20-24
Published by: Blackwell Publishing on behalf of Nordic Society Oikos
Stable URL: http://www.jstor.org/stable/3545441
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62: 20-24. Copenhagen
risk in the wild guinea
M. H. Cassini
Cassini, M. H. 1991. Foraging
risk in the wild guinea pig Cavia
aperea. - Oikos 62: 20-24
about the foraging-antipredation
rous rodents Cavia
were tested: (a) shorter residence
times and (b) greater
detection, (c) shorter
(d) greater scanning
are alone than
they are in
groups. Over a total of 123 complete foraging
at less than four
m from the cover.
and (d), suggesting
that (1) cavies improved
efficiency by joining a
group, which allowed them to produce longer bouts and to invest more time in
bouts, and (2) foraging
formation had a role in antipredation
defense. (b) was also supported,
but in the case of (a), the observed
to the one expected,
that is residence
increased. Observed behaviour
cavies reduce risk at greater
the cover by increasing
rates and by progressively
ones, and returning
to the cover in a hurry.
M. H. Cassini,
Laboratorio de Fisiologia
de Biologia y
2490, (1428) Buenos Aires, Argentina.
Dept of Zoology, Univ. of Oxford,
OXI 3PS, UK
Classical optimal foraging models (e.g., the marginal
value theorem, Charnov 1976) consider an animal ex-
ploiting a food patch as exclusively engaged in foraging
activity. That is, they assume an optimal strategy simply
maximizes the net rate of energy intake or other cur-
rency related to foraging. However, many animals must
attend to other activities which compete with foraging,
such as antipredator vigilance or territorial defence. In
recent years, the trade-off between foraging and preda-
tion avoidance has become an important new field in
behavioural ecology. Insects, fishes, birds, and mam-
mals have been studied with respect to the effect of
predation risk over foraging decision rules such as (1)
how much time to spend foraging (Caraco 1979a, b,
Barnard 1980, Caraco et al. 1980, Gluck 1987a), (2)
which foraging strategy to use (Milinski and Heller
1978, Lima 1985, Lima et al. 1985, Metcalfe et al.
1987b, Pitcher et al. 1988), (3) where to eat (Sih 1980,
Lima et al. 1987, Newman and Caraco 1987, Holbrook
and Schmitt 1988, Werner and Hall 1988), (4) which
types of prey to consume (Metcalfe et al. 1987a, Mor-
gan 1988), and (5) when to leave a patch (Newman et al.
The aim of this paper is to analyse the effects of
predation risk on the patch exploitation by wild guinea
pigs Cavia aperea, in natural conditions.
Cavias' patch use rules have been studied in the lab-
oratory. Domestic guinea pigs were trained to forage
under different environmental conditions, which in-
cluded manipulations of travel time between patches,
patch gain function, and patch quality between and
within environments (Cassini 1989, Cassini et al. 1990).
Accepted 17 May 1991
OIKOS 62:1 (1991)
The results of these experiments
ocally all predictions
of discrete rate maximizing
risk is equalized, cavies leave patches in a way that
seems to approach
of their foraging
effect of predation
risk over patch use behaviour.
in the field because:
conditions are an excellent
model for the study
(2) although this conflict has been studied in the lab
Pitcher et al. 1988),
arose (because the effect of
to the predator
may last over several
when a hierarchically
law decision level (as how
long to stay in a patch) was studied
Wild guinea pigs C. aperea
has a cover zone with high
and dense vegetation,
use as a protection
(Rood 1972), and a foraging
would depend, among
tor detection has been studied in other
species, e.g., see
Ferguson 1987, Metcalfe 1984). In short vegetation,
each one consist-
ing of a relatively
to the foraging
zone close to
the cover. While
over the line projected
by their backs. This scanning
or alert behaviour
to have an anti-predator
to cover and foraging
size have been
as two factors
which affect foraging
risk (for a review, see Pulliam
Caraco 1984). In animals
bouts like the
cavies', it has been found that perceived
is greater as the animal
away from the
cover, because the probability
detected attack is lower (Davis 1973,
Lima 1985, New-
man and Caraco 1987). Evidence that foraging in a
detection of predators
alone has been reported
In this work, the effects of distance
to cover and of
socialization in cavies' foraging behaviour
lyzed by testing the following
Caraco 1984): (1) shorter
if the behaviour
of cavies has no relation
to the distance to cover, that is, the animals
times, and (4) greater
ning rates are expected when cavies forage alone than
when in a group.
Cavies C. aperea
of the Para-
na Delta (Buenos Aires Province,
served. A open garden
used as study area. There was a well defined limit be-
tween this open ground
and the surrounding
forest vegetation characteristic
to eat in the open garden, where they were easy to
because the grass
was cut weekly.
were made in two garden
uous to cover, one 4 m x 21 m and the other 4 m x 12
m, both of which
into cells of 1 m x 3 m
so as to determine
0-1 m, 1-2 m, 2-3 m, 3-4 m and >4 m. Cavies
used these sites daily, from 0600 to 1100 h, and from
1700 to 2100
The two study
completely covered by vegetation of homogeneous
it could be assumed
that there were
differences in vegetable
relation to distance to cover. Small ferrets (Galictis
cuja) and domestic
dogs and cats have been observed
attacking cavies in the area. Polyborus plancus, Milvago
chimango, Asio flammeus and Buteo magnirostris are
(Rood 1972, Dalby 1975).
ber 1989, between 0600 and 2030 h. Animals
by eye or with
a 3 m high
vation site. An IEpson
PX-8 Geneva computer
on a focal animal
mann 1974) computed
(1) total duration,
distance to cover, (3) maximum
as the number
circle of 3 m around
the focal animal), (4) residence
times at each distance category zone (visits without
behaviour or with agressive
at different distances
from cover, (6)
numbers and rates of alert positions when the focal
was alone and when it foraged
in a group.
were not individually
but at least
cavies visited the two study sites, since
was observed simultaneously.
A total of 123 complete
bouts was registered.
Bouts lasted from 4 s to 1452 s, with an average of
s. The mean
rate was 0.049 s-1 that is, about three
OIKOS 62:1 (1991) 21
60 man test statistic = 11.48, p = 0.004), (2) no significant
.*A ^^^differences between the 1-2 m and 2-3 m (Wilcoxon
50- test, p > 0.05), and (3) significant differences between
these and 0-1 m (Wilcoxon test, p < 0.02).
40- When foraging in a group, individual cavies made
longer bouts than when they foraged alone (Mann-
30- Whitney test statistic = 293, p = 0.007) (Fig. 3A). On
the other hand, the scanning rates were marginally grea-
20- ter when cavies foraged alone during the whole foraging
bout, than when they did it with other conspecifics at
10- any moment of the bout (Mann-Whitney test statistic =
9.66, p < 0.07)'(Fig. 3B).
0 Cavies usually did not visit distant cover zones by
0- 1 1 - 2 2 - 3 3 - 4 travelling to them directly. Instead, they used to stop
Maximal distance (meters) several times to forage in nearer zones. However, they
commonly returned from distant zones quickly and
600 without stopping. This observation was supported by
R the result that residence times in the first metre to cover
0-1 1 -2 2-3 3-4
Maximal distance (meters)
Fig. 1. Absolute frequencies of foraging bouts (A), and mean
duration (+ standard deviation) of foraging bout (B), in rela-
tion to maximal distance to cover. N = 123.
0-1 1-2 2-3
Distance to cover (meters)
reached per foraging bout are represented in Fig. 1A.
The number of ocurrences were inversely related to
maximal distances (r = -0.99, t = -28.67, n = 4, p =
0.001). On the other hand, Fig. 1B shows that complete
foraging bouts were longer as maximal distances in-
creased (r = 0.54, t = 7.12, n = 123, p < 0.001). That is,
bouts in which cavies travel long distances became more
rare but resulted in longer total durations.
Mean residence times related to distance from cover
are shown in Fig. 2A. A Friedman test shows that there
were significant differences between residence times
(Friedman test statistic = 11.48, p = 0.003). Multiple
comparisons between means pointed out that residence
time within the first metre was significantly lower than
at greater distances (Wilcoxon tests, ps < 0.001), while
there were no statistical differences between the means
of the second and the third distance categories (p >
The same comparisons were made for scanning rates,
showing the same tendencies (Fig. 2B): (1) over-all
significant differences among the three means (Fried-
0-1 1-2 2-3
Distance to cover (meters)
Fig. 2. Mean patch residence time (A) and mean scanning rate
(B) in relation to distance to cover. Asterisks show statistical
differences. (*: p<0.02) N = 28 for each distance category.
Only samples are included in which cavies foraged at the three
distance categories in the same bout are included.
OIKOS 62:1 (1991)
Alone In groups
0.00 Alone In groups
Fig. 3. Mean foraging bout duration, and mean scanning rate
when cavies foraging alone (n=70) or in a group (n=15).
Asterisks show statistical differences (*: p <0.05).
were significantly greater when cavies went out (mean
= 57.43 s, SE = 10.00 s) than when they returned
(mean = 15.16 s, SE = 6.23 s) to cover (Wilcoxon test,
p < 0.0001).
The aim of this study was to analyse how cavies ex-
ploited their environment when they were under preda-
tion risk. The effects of distance to cover and foraging in
a group on visit duration and scanning rate were stud-
The first tested prediction was that patch residence
times should be shorter as distance increases, in order to
reduce exposure time to predators, because the prob-
ability of escaping a detected attack would be lower at
greater distances to protective cover. Most of the stud-
ies on the effect of predation risk on patch exploitation
examined patch selection rather than patch residence
time (Holmes 1984, Brown et al. 1988, Holbrook and
Schmitt 1988). Only one work (Newman et al. 1988),
dealing with the effect of distance from protective cover
on patch departure rule was found. In this work, gray
squirrels, Sciurus carolinensis, showed shorter residence
times at greater distance to cover, as was expected.
However, the results obtained with cavies showed the
opposite trend to this prediction: animals stayed for
shorter periods of time at the nearest areas. Lima et al.
(1987) found that three species of passerine birds (Junco
hyemalis, Melospiza melodia and Pipilo erithrophtal-
mus) rarely foraged as close as possible to cover. The
authors suggested that these birds perceive cover as a
protective place but also as an attack source, and pro-
posed that the use of space in this case reflects a trade-
off between the perceived risk of foraging too near to
the cover, and foraging too far from it. This interpreta-
tion could explain why cavies showed shorter residence
times in the nearest zones. The prediction that patch
residence times should be shorter as distance increases
is based on the assumption that food quality and quan-
tity is the same in the whole foraging area. An alterna-
tive explanation can be reached if this assumption is not
fulfilled. Under these conditions, cavies should stay
longer at greater distances where they can find better
patches. Independent measures of food types used by
cavies in natural conditions would be necessary to dis-
tinguish this hypothesis from that which postulate that
the cover per se is avoided.
The second prediction tested was that scanning rates
should increase at greater distances to cover because of
the increased predation risk. This prediction found sup-
port in the present results: cavies scanned less fre-
quently at shorter distances. However, results in other
species have not always agreed with this prediction. For
example, Lima (1987) observed just the opposite trend
in the house sparrow, Passer domesticus. In this species,
the antipredation strategy might be to minimize the
exposure time to predators by eating more quickly and
scanning less frequently, which results in shorter visits
to more distant places. An extreme case of this beha-
viour is that of the black-capped chikadee Parus atrica-
pillus or the squirrel S. carolinensis, both of which carry
the food to cover; in this case, the decision to consume
or to carry the food depends, apart from the distance to
cover, on the handling time and the carrying cost of
different food types (Lima 1985, Lima et al. 1985).
In summary, wild guinea pigs reduce predation risk at
grater distances to cover by increasing scanning rates
and by reaching more distant zones progressively forag-
ing first in near zones and returning to the cover in a
hurry. This strategy differs from those of other prey
species, which diminish their exposure time at longer
distances by eating faster and scanning less, or by carry-
ing the food to cover.
The last two predictions tested are related to the
advantages of group foraging. As predicted, patch resi-
OIKOS 62:1 (1991) 23
dence times were greater and scanning rates lower when
cavies were foraging among group members than when
they foraged alone. These results suggest that individual
wild guinea pigs improved their foraging efficiency by
joining a group, which allowed them to (1) feed longer,
and (2) invest more time in grazing during bouts. Fur-
thermore, if vigilance was shared among group mem-
bers, then a cavie would gain protection against preda-
tion by joining a group. Evidence supporting such
group-derived benefits has been obtained in several spe-
cies, e.g., fishes Pimphales notatus (Morgan 1988), os-
triches Struthio camelus (Bertram 1980), swallows Hi-
rundo pyrrhonota (Brown and Brown 1987), juncos
Junco phaeonotus (Caraco 1979b), goldfinches Cardue-
lis carduelis (Gluck 1987b), herons Ardea herodias
(Krebs 1974), and prairie dogs Cynomys spp. (Hoo-
In summary, foraging in a group seems to reduce
predation risk and/or increase foraging rates, and cavies
grouping tendencies can be included in this general
- I thank
L. Vila, A. Kacelnik,
P. Lemoine and M. Gabelli
comments on the manu-
and 0. Suarez for lending
me her house in The Delta.
This research was partly funded by a research
to E. T. Segura.
pleted while I was visiting the AFRC Unit of Ecology and
Evolution (Director: J. R. Krebs FRS), Dept of Zoology,
Univ. of Oxford,
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