Foraging under Predation Risk in the Wild Guinea Pig Cavia aperea

Abstract

Four hypotheses about the foraging-antipredation behavioural conflict using herbivorous rodents Cavia aperea were tested: (a) shorter residence times and (b) greater scanning rates, are expected in foraging areas progressively more distant from cover, because foraging at greater distances from cover would increment predation risk; as group foraging would facilitate predator detection, (c) shorter residence times and (d) greater scanning rates, are expected when cavies are alone than when they are in foraging groups. Over a total of 123 complete foraging bout observations, cavies always foraged at less than four m from the cover. The results support predictions (c) and (d), suggesting that (1) cavies improved their foraging efficiency by joining a group, which allowed them to produce longer bouts and to invest more time in grazing during bouts, and (2) foraging group formation had a role in antipredation defense. (b) was also supported, but in the case of (a), the observed relation was the opposite to the one expected, that is residence times were longer as distance to cover increased. Observed behaviour suggests that cavies reduce risk at greater distance to the cover by increasing their scanning rates and by progressively reaching more distant zones, foraging first in near ones, and returning to the cover in a hurry.

Full text

Nordic Society Oikos
Foraging under Predation Risk in the Wild Guinea Pig Cavia aperea
Author(s): M. H. Cassini
Source:
Oikos,
Vol. 62, No. 1 (Oct., 1991), pp. 20-24
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OIKOS
62: 20-24. Copenhagen
1991
Foraging
under predation
risk in the wild guinea
pig
Cavia aperea
M. H. Cassini
Cassini, M. H. 1991. Foraging
under predation
risk in the wild guinea pig Cavia
aperea. - Oikos 62: 20-24
Four
hypotheses
about the foraging-antipredation
behavioural
conflict
using
herbivo-
rous rodents Cavia
aperea
were tested: (a) shorter residence
times and (b) greater
scanning
rates,
are expected
in foraging
areas
progressively
more
distant from
cover,
because
foraging
at greater
distances
from
cover would
increment
predation
risk;
as
group foraging
would facilitate
predator
detection, (c) shorter
residence
times and
(d) greater scanning
rates,
are expected
when cavies
are alone than
when
they are in
foraging
groups. Over a total of 123 complete foraging
bout observations,
cavies
always
foraged
at less than four
m from the cover.
The results
support
predictions (c)
and (d), suggesting
that (1) cavies improved
their foraging
efficiency by joining a
group, which allowed them to produce longer bouts and to invest more time in
grazing during
bouts, and (2) foraging
group
formation had a role in antipredation
defense. (b) was also supported,
but in the case of (a), the observed
relation
was the
opposite
to the one expected,
that is residence
times were
longer
as distance
to cover
increased. Observed behaviour
suggests
that
cavies reduce risk at greater
distance to
the cover by increasing
their scanning
rates and by progressively
reaching
more
distant
zones, foraging
first
in near
ones, and returning
to the cover in a hurry.
M. H. Cassini,
Laboratorio de Fisiologia
del Comportamiento,
Instituto
de Biologia y
Medicina
Experimental,
Obligado
2490, (1428) Buenos Aires, Argentina.
Present
address:
Dept of Zoology, Univ. of Oxford,
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.
1988).
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
OIKOS 62:1 (1991)
20

The results of these experiments
confirmed
unequiv-
ocally all predictions
of discrete rate maximizing
(and
delay minimizing)
models. Therefore,
when predation
risk is equalized, cavies leave patches in a way that
seems to approach
the optimization
of their foraging
efficiency.
These results
encouraged
the analysis
of the
effect of predation
risk over patch use behaviour.
This
study
was
performed
in the field because:
(1) wild
cavies
foraging
in natural
conditions are an excellent
biological
model for the study
of predation
feeding-conflicts,
and
(2) although this conflict has been studied in the lab
(Gluck
1987a,
Pitcher et al. 1988),
problems
in working
with simulated
predators
arose (because the effect of
exposure
to the predator
may last over several
trials),
mainly
when a hierarchically
law decision level (as how
long to stay in a patch) was studied
(Krebs
1980).
Wild guinea pigs C. aperea
are neotropical
herbivo-
rous
rodents
(Rood 1972).
Typically,
their
environment
has a cover zone with high
and dense vegetation,
which
they evidently
use as a protection
from
predator
attacks
(Rood 1972), and a foraging
zone where
predation
risk
would depend, among
other environmental
factors,
on
vegetation
height
(the importance
of visibility
in preda-
tor detection has been studied in other
species, e.g., see
Ferguson 1987, Metcalfe 1984). In short vegetation,
cavies
make
a series
of foraging
bouts,
each one consist-
ing of a relatively
brief
visit
to the foraging
zone close to
the cover. While
foraging,
cavies
periodically
stop graz-
ing and
elevate their
heads
from
over the line projected
by their backs. This scanning
or alert behaviour
seems
to have an anti-predator
function
(Rood 1972).
Distance
to cover and foraging
group
size have been
postulated
as two factors
which affect foraging
strate-
gies under
predator
risk (for a review, see Pulliam
and
Caraco 1984). In animals
with foraging
bouts like the
cavies', it has been found that perceived
predation
risk
is greater as the animal
moves further
away from the
protective
cover, because the probability
of escaping
a
detected attack is lower (Davis 1973,
Lima 1985, New-
man and Caraco 1987). Evidence that foraging in a
group
increments
detection of predators
compared
with
foraging
alone has been reported
in several
species
(see
Pulliam
and Caraco
1984).
In this work, the effects of distance
to cover and of
socialization in cavies' foraging behaviour
were ana-
lyzed by testing the following
predictions
(Pulliam
and
Caraco 1984): (1) shorter
patch residence
times (than
those expected
if the behaviour
of cavies has no relation
to the distance to cover, that is, the animals
walk with
constant
probability
of stopping),
and
(2) more
frequent
scanning
behaviour
are expected
at greater
distances to
cover;
(3) shorter
residence
times, and (4) greater
scan-
ning rates are expected when cavies forage alone than
when in a group.
Methods
Cavies C. aperea
from
a natural
population
of the Para-
na Delta (Buenos Aires Province,
Argentina)
were ob-
served. A open garden
of approximately
4000 m2
was
used as study area. There was a well defined limit be-
tween this open ground
and the surrounding
high and
compact secondary
forest vegetation characteristic
of
this
region.
Cavies
inhabit
this "cover
zone" and
leave it
to eat in the open garden, where they were easy to
observe
because the grass
was cut weekly.
Observations
were made in two garden
sites contig-
uous to cover, one 4 m x 21 m and the other 4 m x 12
m, both of which
were divided
into cells of 1 m x 3 m
with
stakes
so as to determine
five categories
of distance
to cover:
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
h, approximately.
The two study
sites
were
completely covered by vegetation of homogeneous
height. Therefore,
it could be assumed
that there were
no significant
differences in vegetable
biomass
supply
in
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
potential
aerial
predators
(Rood 1972, Dalby 1975).
Observations
were made
during
October
and
Novem-
ber 1989, between 0600 and 2030 h. Animals
were ob-
served
by eye or with
binoculars
from
a 3 m high
obser-
vation site. An IEpson
PX-8 Geneva computer
with a
program
based
on a focal animal
sampling
method
(Alt-
mann 1974) computed
the following
variables
of com-
plete foraging
bout observations:
(1) total duration,
(2)
maximum
distance to cover, (3) maximum
group size
(measured
as the number
of conspecifics
observed
in a
circle of 3 m around
the focal animal), (4) residence
times at each distance category zone (visits without
grazing
behaviour or with agressive
interactions
were
not included
in later
analyses),
(5) numbers
and
rates
of
alert behaviour
at different distances
from cover, (6)
numbers and rates of alert positions when the focal
animal
was alone and when it foraged
in a group.
The cavies
were not individually
marked,
but at least
eight different
cavies visited the two study sites, since
this number
of animals
was observed simultaneously.
Results
were statistically
analyzed
using
the Stat/Trans-
fer program,
and non-parametric
analyses.
Results
A total of 123 complete
foraging
bouts was registered.
Bouts lasted from 4 s to 1452 s, with an average of
211.49
s and
a standard
deviation
of 247.51
s. The mean
alert behaviour
rate was 0.049 s-1 that is, about three
alert positions
per minute.
Total numbers
of observations
of maximal
distances
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
B
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.
c
0
0
0)
O
a)
0)
0
0)
la
0
a(
OL
200 A
100-
0-
T
*
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 >
0.05).
The same comparisons were made for scanning rates,
showing the same tendencies (Fig. 2B): (1) over-all
significant differences among the three means (Fried-
s'
C
0
._
0
C
co
c(
0.06
0.05 -
0.04 -
0.03 -
0.02 -
0.01 -
0.00 -
B I
u-
*
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)
c
0
co
0
cU
m
0
0
0
._
co
0
0
22

0
a
o
c
0
0
0
._
0
LL
400
300-
200-
100-
0-
A
*
Alone In groups
Sociality
o
0
0
0)
c
._
0
0
0)
0.06
0.05
0.04
0.03
0.02
0.01
0.00 Alone In groups
Sociality
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).
Discussion
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-
ied.
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-
gland 1981).
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
phenomenon.
Acknowledgements
- I thank
L. Vila, A. Kacelnik,
G. Flores,
P. Lemoine and M. Gabelli
for their
comments on the manu-
script,
and 0. Suarez for lending
me her house in The Delta.
This research was partly funded by a research
grant from
CONICET
(Argentina)
to E. T. Segura.
The paper
was com-
pleted while I was visiting the AFRC Unit of Ecology and
Evolution (Director: J. R. Krebs FRS), Dept of Zoology,
Univ. of Oxford,
supported
by a Royal Society
scholarship.
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