Behavioral Ecology Vol. 7 No. 1: 30-34
Microhabitat use and behavior of voles under
weasel and raptor predation risk: predator
facilitation?
Erkki Korpimaki, Vesa Koivunen, and Hani Hakkarainen
Laboratory of Ecological Zoology, Department of Biology, University of Turku, FIN-20500 Turku,
Finland
An example of predator facilitation is that a microhabitat shift in a prey species induced by one predator increases the probability
of the prey falling victim to other predators. Least weasels (Mustela nivalis) hunt in dense plant cover, whereas kestrels (Falco
tinnunculus) hunt in habitats with sparse plant cover. Field voles (Microtus agrestis), the main food of weasels and kestrels,
prefer open country with a high grass layer. We simulated a multipredator environment in an aviary (3.0 X 4.8 X 2.2 m) to
find out whether predator facilitation plays a role in the interactions between voles, small mustelids, and raptors. In each
replicate, we placed a field vole in a pen including sides of high and low grass layers (cover and open). In a predator-free
situation, voles preferred cover but shifted to open when a weasel was introduced to cover. In the presence of a kestrel, voles
occupied cover and decreased their mobility. In the presence of a weasel plus a kestrel, voles behaved as under the kestrel risk
alone. Therefore, in these aviary circumstances, voles perceived the kestrel risk as greater than the weasel risk. Predator facilitation in the assemblage of predators subsisting on rodent prey may contribute to the crash of the four-year vole cycle: microhabitat shift due to an avoidance of weasel jaws may drive voles to raptor talons. Key mords: antipredatory behavior, field vole,
four-year vole cycle, kestrel, least weasel, microhabitat shift, predator facilitation. [Behav Ecol 7:30-34 (1996)]
I
tection against their own enemies, like raptors (Korpimaki
n multipredator environments, animals encounter the risk
and Norrdahl, 1989; Powell, 1973). Least weasels actively
of being killed by an assemblage of predators covering the
search for their prey and catch it after a pursuit but can also
spectrum from extremely hazardous to rather harmless (e.g.,
attack prey in ambush (Erlinge et al., 1974). Small mustelids
Polis and Holt, 1992; Polis etal., 1989; Taylor, 1984). Antipredscent-mark their home ranges (King, 1989), which reveals
atory behaviors reducing mortality from one predator type
their presence to small rodents that can recognize their odor
may not be efficient against another type and may even inand behave differently than when exposed to the odors of
crease the risk of being killed by other predators. Predator
larger carnivores (Jedrzejewski et al., 1993). The presence of
facilitation means behaviors (e.g., habitat or microhabitat
small mustelids or their scent induces a fast shift in temporal
shift) of prey animals that decrease mortality from one predactivity patterns of microtine rodents (Jedrzejewska and Jeator but which expose them to a second predator (Charnov
drzejewski, 1990).
et al., 1976). For example, in an aviary, barn owls (Tyto alba)
caught gerbils (GcrbUlus allenbyi and G. pyramidum) more
Diurnal raptors mainly use eyesight and nocturnal owls
from open microhabitats, and gerbils responded by shifting
hearing to discover prey. They hunt from a perch or on wing
to forage in the bush microhabitat (Kotler et al., 1991) where
locating vole prey even 100-200 m away [e.g., hovering Eurthey encountered increased risk of snake predation (Kotler et
asian kestrels (Falco tinnunculus), hereafter kestrel, Village,
al., 1992). Other examples of predator facilitation mainly
1990]. Because dense vegetative cover forms a shelter against
come from studies in aquatic ecosystems (e.g., Lima, 1992;
avian predators, hunting raptors prefer habitats with low vegPower, 1984; Walls et al., 1990 and references therein).
etative cover (e.g., Janes, 1985; Korpimaki, 1986; Preston,
1990). Also in an aviary, prey discovery and strike success of
Small rodents are the main food of a diverse assemblage of
great-horned owls (Bubo virginianus) were higher in the open
predators consisting of many different-sized carnivorous mamthan in the cover (Longland and Price, 1991).
mals, diurnal raptors, owls, and snakes, especially, when voles
are abundant in the peak phase of the four-year population
Field voles (Microtus agrestis) are herbivorous rodents that
cycle (e.g., Erlinge et al., 1983; Goszczynski, 1977; Korpimaki
occupy open country with high vegetative cover (Hansson,
and Norrdahl, 1991b; Korpimaki et al., 1991). Small mustelids
1987; Myllymaki, 1977). In our study area in western Finland,
(the stoat Mustela erminea and the least weasel M. nivalis nifield voles prefer uncultivated farmland areas and pastures
valis) and raptors create highly different risks for microtine
(Norrdahl and Korpimaki, 1993). In this area, small mustelids,
rodents, and this may induce predator facilitation. Small musdie kestrel, and two owl species (Asio otus and A. flammeus)
telids mainly use olfaction to discover prey (King, 1989) and
are the main open-country predators of voles (Korpimaki and
can dius locate their prey at a distance of several meters. BeNorrdahl, 1991a,b; Korpimaki et al., 1991).
cause of their small body size, least weasels are able to hunt
When a small mustelid enters a microhabitat (i.e., a patch
in the runnels of small rodents (Erlinge et al., 1974; Korpiwithin the home range of a vole) preferred by field voles,
maki et al., 1991). Their smallness makes them effective huntvoles can stay there and reduce their activity to decrease musers in a habitat with high plant cover, which also offers a protelid predation risk, or they can shift to an open patch with
high avian predation risk. We conducted an aviary experiment
to find out how voles actually respond in multipredator situations and whether predator facilitation plays a role in the
Received 30 November 1994; first revision 22 January 1995; second
interactions between voles, small mustelids, and raptors. We
revision 3 March 1995; accepted 8 March 1995.
asked four questions: (1) Do field voles prefer a microhabitat
1045-2249/96/S5.00 C 1996 International Society for Behavioral Ecology
Korpimaki et al. • Predator facilitation in small rodents
with high grass layer (cover) in a predator-free situation? (2)
If so, do field voles alter their microhabitat use and behavior
when a least weasel is introduced to cover? (3) Do field voles
change their microhabitat use and behavior when a kestrel is
present in the aviary? (4) What do field voles do in the presence of both a weasel and a kestrel?
MATERIAL AND METHODS
We conducted experiments in August 1993 in an aviary (bottom 3.0 X 4.8 m, height 2.2 m) situated at the Satakunta
Environmental Research Center of Turku University in Pori,
western Finland (61°30' N, 21°30' E). The field voles (n =
70) were caught in July and August 1993 in the vicinity of Pori
and Turku (60°30' N, 22°30' E) using Swedish Ugglan livetraps. The two male least weasels were captured in early August 1993 in the Kauhava region, western Finland (63° N, 23°
E) with Ugglan live-traps. The kestrels [7 adult (age +l-yr)
females and 6 adult males] were trapped in the same area in
late June 1993 by using a bal-chatri (Berger and Mueller,
1959). Both least weasels and kestrels were trapped and held
in captivity with the permission of the Finnish Ministry of the
Environment. After the experiment, we released them in the
same area where they were trapped.
We used two identical uncovered pens (bottom 2.3 X 1.6
m, height 0.5 m) located symmetrically around a perch
(height 1.4 m) for the kestrel. Both pens were partitioned off
in two equal parts, and these parts were connected with each
other by two plastic tubes (diameter 5 cm, length 10 cm) so
that voles were free to move between two parts of a pen. We
simulated an agricultural environment by placing a 3—4 cm
deep hay layer to one part of both pens (a good microhabitat,
hereafter referred to as cover) and a 0.5—1 cm deep hay layer
to a second part of both pens (a poor microhabitat, referred
to as open). Each part of the two pens also contained an open
feeding place for voles (pellets for laboratory mice and water).
We recorded the behavior of voles from a hide located on the
wall (3.5 m above the bottom) of the aviary.
Before each replicate, we measured the body mass and
checked the reproductive condition of voles (females: vagina
open or closed; males: testis developed or not). In each replicate, we used two similar (in sex and body size) field voles.
Each trial lasted for 45 min. We introduced each vole randomly to one part of the pen (one vole per pen). In the first
15 min., voles were allowed to become familiar with the pen.
In the next 15 min., we recorded the behavior of voles to
confirm that they behaved normally. In the control treatment,
hereafter CT, vole behavior was recorded in the last 15 min.
of each trial without further experimental intervention. In the
three experimental treatments, the first two 15-min. periods
were the same as in the CT. In the kestrel treatment (KT), we
released a kestrel in the aviary and observed the behavior of
field voles for 15 min. After a release, the kestrel usually flew
around the aviary for 1-2 min. and thereafter scanned from
the perch at a distance of 2-4 m from the pens. We used fullfed (satiated) falcons to remove the effects of strikes on vole
behavior. In the weasel treatment (WT), we introduced a least
weasel to cover of both pens (one weasel per pen) in a small
cage (10 X 40 X 10 cm). The weasel was placed on the cover
side of the pens only because in agricultural fields least weasels prefer to hunt in protective cover (Korpimaki E and Norrdahl K, unpublished radio-tracking data). Thereafter, we recorded vole behavior for 15 min. We used full-fed (satiated)
weasels to reduce the effects of movements on vole behavior.
In the kestrel plus weasel treatment (KWT), we released a
kestrel in the aviary and placed a least weasel in cover of both
pens, and recorded vole behavior for 15 min. In the last two
treatments, the bottom of the pen and hay of the cover treat-
31
ment were replaced by a similar bottom and hay to exclude
the possibility diat the weasel odor remained on the floor and
grass layer. Both kestrels and weasels were well accustomed to
the test cages before the experiment so that they did not frantically try to escape. We carried out a total of 11 replicates. In
the experiment, we used some voles more than once because
the total number of voles needed was 88 (2 pens X 4 treatments X 11 replicates), but we had only 70 voles. However,
we introduced each vole only once to one pen and treatment,
and the order in which we subjected voles to the different
treatments was random. This promotes the independence of
replicates. Each kestrel was introduced to only one replicate.
Between trials, we kept voles individually in cages (bottom 1
X 1 m, height 0.5 m), where they had unlimited access to
food and water.
The use of microhabitats (cover/open) and behavior of
voles in two trial pens was recorded at 1-min. intervals during
die 15-min. treatment. We classified vole behavior as follows:
(1) moving when a vole was mobile in the pen, (2) staying
when a vole was immobile in the pen, (3) eating when a vole
was feeding on pellets and drinking water, and (4) cleaning
when a vole was grooming its coat,
RESULTS
Field voles mostly inhabited cover when no predators were in
the aviary (mean proportion of time inhabiting cover during
the 15-min. trial: pen 1, 93% and pen 2, 83%). The experimental treatment significantly affected the microhabitat occupancy of voles, whereas the sex of voles and trial pen did
not have obvious effects on microhabitat use (Figure 1 and
Table 1). We found no significant interactions between treatment, sex, and trial pen. The between-treatment difference
mainly resulted from voles avoiding cover in WT, whereas in
KT and in KWT voles stayed in cover [pen 1 (Figure 1, above):
Tukey test for the difference in the arcsine-transformed proportion of time of voles inhabiting cover between CT and WT,
two-tailed p < .001; between KT and WT, p < .001; between
WT and KWT, p = .002] [pen 2 (Figure 1, below): p < .001,
p < .001 and p < .001, respectively].
Also, the experimental treatment strongly altered the behavior of field voles, whereas the sex of voles, trial pen, and
interaction between the three independent variables had no
obvious influence on vole activity (Figure 2 and Table 2).
Voles decreased their mobility as a response to KT (Tukey test
for die difference in die arcsine-transformed proportion of
time moving between CT and KT: pen 1, two-tailed p = .06;
pen 2, p = .13; pooled data from two pens p — .005), whereas
WT did not result in die similar response (p = .99, p = .89,
and p = .94, respectively). In KWT, voles reduced their mobility compared with CT (pen 1, p = .20; pen 2, p = .04;
pooled data from two pens p = .007). Field voles mosdy
moved or stayed in the trial pens and, therefore, showed no
obvious between-treatment difference in the time of eating
and grooming (Figure 2).
We also examined whether body mass and reproductive
condition of voles influenced dieir microhabitat use and behavior in trial pens. The only effect diat approached statistical
significance was that males with developed testis tended to
occupy cover less dian those with nondeveloped testis (F =
4.00,p= .052).
DISCUSSION
Four main findings emerged in our experiment First, field
voles preferred cover in a predator-free situation. Second,
field voles shifted to open when a least weasel entered cover.
Third, field voles preferred cover under the kestrel predation
32
Behavioral Ecology Vol. 7 No. 1
P«n 1
Table 1
ANOVA-table for the proportion of time of field voles inhabiting
cover on the experimental treatment (control, kestrel present,
weasel present, both kestrel and weasel present), sex of volei, and
trial pen (1 or 2) in 11 replicates (15 min. each)
Source of
variation
df
Mean
square
Treatment (T)
Sex(S)
Pen (P)
TXS
TX P
SX P
TXSX P
Error
3
1
1
3
3
1
3
71
5.76
0.20
0.05
0.03
0.15
0.01
0.17
0.22
F
P
26.50
0.90
0.24
0.12
0.68
0.03
0.79
<.001
.35
.63
.95
.57
.86
.50
The proportion of time was arcsine-transformed because of nonnormal distribution.
0.0
P«n 2
weasel predation risk. An explanation for this rather unexpected result may be an aviary artifact For example, kestrels
might be noisy whereas least weasels are silent, but kestrels in
our experiment were also silent. In addition, voles might discover that a caged weasel is unable to pursue and attack
whereas a free-flying kestrel can do so. However, this explanation seems unlikely because satiated kestrels in our trials
did not strike voles, and the presence of a caged weasel in
cover induced a significant avoidance of this pen. The microhabitat shift in the presence of a captive weasel was probably
attributable to weasel scent, which even humans can smell.
Also, laboratory experiments indicate that microtine rodents
may assess weasel odor as a real direat (Heikkila et al., 1993;
Ylonenetal., 1992).
Least weasel densities usually peak in the crash phase of the
four-year vole cycle (Korpimaki et al., 1991; Oksanen and OkTable 2
ANOVA-table for the proportion of time of field voles moving (A)
and staying (B) in the trial pens on the experimental treatment
(control, kestrel present, weasel present, both kestrel and weasel
present), sex of voles, and trial pen (1 or 2) in 11 replicates (15
min. each)
ao
Figure 1
(Top) The mean (±SD) proportion of time per replicate (n = 11,
15 min. each) of field voles occupying cover and open
microhabitats in four experimental treatments (CT = control, KT
= kestrel present, WT = least weasel present, and KWT = both
kestrel and weasel present) in pen 1. (Bottom) The same but for
pen 2.
risk and reduced dieir mobility. In accordance with our results, in a large enclosure (144 m!, each pen 4 X 12 m), bank
voles (Clethrionomys giarsoku) also avoided the pen visited by
the common weasel (Mustela nivalis vulgaris) (Jedrzejewski
and Jedrzejewska, 1990). Common voles (Microtus arvalis),
exposed to a kestrel model, also reduced dieir mobility (Gerkema and Verhulst, 1990).
Finally, simultaneous presence of a kestrel and a weasel induced a response much like the response to die kestrel alone,
suggesting that in diese aviary circumstances field voles perceived the kestrel predation risk as more hazardous than the
Source of
variation
df
Mean
square
F
P
(A) Dependent variable: time of moving
Treatment (T)
0.72
3
1
Sex(S)
1.00
Pen (P)
1
0.00
0.13
TXS
3
TX P
3
0.02
SX P
1
0.11
T XSx P
3
0.03
71
Error
0.08
9.07
1.23
0.06
1.62
0.28
1.34
0.33
<.001
.27
.81
.19
.84
.25
.80
(B) Dependent variable: time of staying
Treatment (T)
3
2.42
1
Sex(S)
1.00
Pen (P)
1
0.08
TXS
3
0.22
TX P
3
0.09
SX P
1
0.09
T XSXP
3
0.02
Error
71
0.21
11.58
0.46
0.38
1.04
0.42
0.45
0.08
<.001
.50
34
.38
.74
.51
.97
The proportion of time was arcsine-transformed because of nonnormal distribution.
Korpimaki et al. • Predator facilitation in small rodents
33
P«n 1
Pen 2
1969; Henttonen et al., 1977; Myllymaki, 1977; Norrdahl and
Korpimaki, 1993). However, the importance of predator facilitation to these habitat shifts remains undocumented because
our experiment was made in the scale of a vole home range.
Many factors also can induce these shifts (intra- and interspecific competition for space, lack of high quality food, etc.).
Future studies on radio-collared voles and least weasels could
reveal the importance of predator facilitation in die field.
Natural spatial scale of experiments is important when
studying prey behavior in the presence of predators (e.g., Korpimaki et al., 1994; Lima and Dill, 1990). We are well aware
that the abnormal spatial scale of our enclosure experiment
may restrict the application of the results to field circumstances. Despite diis, our results suggest that predator facilitation
may be potentially important in the assemblage of mammalian
and avian predators subsisting on microtine rodent prey. The
existence of multipredator environments has been largely neglected in studies on antipredatory behavior of microtine rodents. So far, the behavior of voles has been studied under
die risk of only one predator, like small mustelids (e.g., Heikkila et al., 1993; Jedrzejewska and Jedrzejewski, 1990; Ylonen
et al., 1992) or raptors (e.g., Gerkema and Verhulst, 1990;
Hakkarainen et al., 1992).
•
•
•
D
Eating
Ctoanlng
Staying
Moving
1.0
I
I
O8
We thank the staff of Satakunta Environmental Research Center, especially Jukka Jussila and Mikko Ojanen, for providing good working
facilities, and Voitto Haukisalmi, Bogumila Jedrzejewska, Wlodzimierz
Jedrzejewski, Kai Norrdahl, Christoph Rohner, and anonymous referees for comments on the manuscript. The study was supported by
the Academy of Finland.
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Figure 2
(Top) The mean (±SD) proportion of time per replicate (n = 11,
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