Discussion
Test of hypotheses
H
0
1: wolf predation represents additive mortality for prey
populations. We found evidence to support our hypothesis
that wolf predation represented additive mortality for both
moose and caribou. Wolf predation is usually additive when
prey are below the “nutrient-climate ceiling” (Theberge
1990; Gasaway et al. 1992). During our study, moose and
caribou remained at low to moderate densities. Wolves in
our study killed proportionally more calf, yearling, and old
moose and fewer prime-age animals. This age pattern was
similar to other Alaska and Yukon studies, where moose
were also below the nutrient-climate ceiling (Fig. 7) (see
Peterson et al. 1984; Ballard et al. 1987; Hayes et al. 1991;
Gasaway et al. 1992).
Gasaway et al. (1992) estimated additive and compensa-
tory mortality of moose on the basis of marrow-fat indices.
Using his values, we found that 21 of 27 adults (77%) were in
the largely additive mortality age-class (middle-aged). The
remaining six were very old adults (>12 years of age) that
we considered compensatory losses. Calves were also in the
additive mortality class, but they showed lower marrow-fat
indices than adults. These lower indices can be explained
by the higher energetic requirements of calves for growth
(Peterson et al. 1984). Both nutrition and age data are con-
sistent with the hypothesis that wolf predation on moose was
mainly additive. We had too few samples to estimate caribou
condition.
H
0
2: the kill rate by wolves is dependent on prey density;
H
a
2: the kill rate by wolves is independent of prey density
and related to wolf-pack size. We found evidence for reject-
ing H
0
2 and accepting H
a
2. The kill rate by wolves was in-
dependent of moose density (Table 2) and pack size was the
only variable of six tested that was related to kill rate. On
average, large packs killed moose more often than did small
packs, which is similar to the results of other studies
(Ballard et al. 1987; Hayes et al. 1991; Thurber and Peterson
1993; Dale et al. 1994). Nevertheless, many of our small
packs killed moose as often as larger packs did, which are
similar to the findings of Hayes et al. (1991) and Thurber
and Peterson (1993).
H
0
3: the kill rate by wolves on moose calves depends on
the proportion of calves in winter populations. We obtained
evidence for rejecting H
0
3. The kill rate on moose calves
was not related to the number of calves available in winter,
contrary to the findings of other studies (Peterson 1977;
© 2000 NRC Canada
54
Can. J. Zool. Vol. 78, 2000
Fig. 5. Two models of wolf predation rates on moose, based on
grand mean kill rates by wolves and pack-size kill rates during
each year of the study.
Fig. 6. Relations between moose and caribou calf survival rates
and wolf density in the FSA during each winter. The percentage
of moose calves was estimated from March counts and the per-
centage of caribou calves from October counts (R. Farnell,
unpublished data). The thick line shows the relation for caribou
calves and the thin line shows the relation for moose calves.
Fig. 7. Ages of moose (excluding calves) killed by wolves dur-
ing this study (FSA) and four other studies in Alaska and the
Yukon. Other sources of data were as follows: Kenai Peninsula,
Alaska, from Peterson et al. (1984); Nelchina, Alaska, from
Ballard et al. (1987); Coast Mountains, Yukon, from Hayes et al.
(1991); and Game Management Unit 20E, Alaska, from Gasaway
et al. (1992).
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Peterson et al. 1984). In most winters calves were abundant,
but many vulnerable yearling moose were also available to
wolves (Larsen and Ward 1995), apparently reducing calves’
importance in wolves’ diet.
H
0
4: the kill rate by wolves on moose is reduced when
caribou availability exceeds that of moose. We had evidence
for rejecting H
0
4. Wolves did not prey heavily on caribou
that temporarily migrated into their pack territories, unlike
wolves in Alaska (Dale et al. 1994, 1995). Wolves continued
to kill mainly moose, even though caribou outnumbered
moose and probably posed less of a risk to hunt (Haugen
1987). We believe that there was little benefit to preying on
caribou because many calf and yearling moose were avail-
able in most winters and were also highly profitable and
low-risk prey.
Snow depth, snowshoe hare availability, and search rate
by wolves
Snow depth did not influence the rate at which wolves
killed moose. Huggard (1993) and Mech et al. (1998) showed
that snowfall can add substantial prey-density-independent
variation to wolf predation rates. Low scavenging rates by
wolves in all winters of our study indicated that snow depth
probably did not reduce ungulate survival rates (Fuller 1991;
J
drzejewski et al. 1992; Huggard 1993). We conclude that
winters were not severe enough to affect any measurable
change in wolves’ kill rates.
Snowshoe hare abundance had no detectable influence on
the rate at which wolves killed moose. Snowshoe hares were
abundant during 1990 and 1991, when moose and caribou
were rapidly increasing, competition for ungulates was low-
est, and many vulnerable, young moose and caribou were
available. In this ecological context, we believe that there
were few incentives for wolves to hunt snowshoe hares. Al-
though wolves might survive on snowshoe hares during the
peak of the cycle, they might not maintain the behavior nec-
essary to enable them to defend large territories in winter.
Our data were consistent with those of Messier and Crête
(1985) and Dale et al. (1995), who found that wolves’ search
rates were independent of prey density. Differences in prey
density in our study might not have been sufficiently large to
be detectable by the methods we used to measure search
rates.
Consumption rate of wolves
Wolves’ consumption rate was 8.7 kg/wolf/day, which is
higher than rates estimated in previous studies (Thurber and
Peterson 1993 and references therein). The apparent con-
sumption rates for our study wolves were excessive. For ex-
ample, wolves in small packs would have had to consume an
average of 30% (12.7 kg) of their body mass each day of
winter if they consumed all edible portions. Adjusting for
biomass lost to ravens (RA) reduced our estimate of con-
sumption to between 4.1 and 6.4 kg/wolf/day in packs of all
sizes.
All packs handled moose carcasses in 2.6–3.3 days. Prom-
berger (1992) found that large groups of ravens removed
up to 37 kg of food/day from ungulate carcasses and he esti-
mated that ravens removed proportionally more edible prey
from small packs. Juvenile ravens form large cooperative
flocks in winter (Heinrich 1991). These subadult flocks
compete with small wolf packs because the small packs can-
not handle kills as quickly as larger packs can. Other studies
have shown that competition from scavengers can influence
the kill rates of other carnivores (Harrison 1990; Cooper
1991). We believe that where ravens are common, they can
have a significant impact on wolves’ kill and consumption
rates.
Optimal foraging-group size
The optimal foraging-group size was 2 wolves, which is
similar to the findings of other wolf studies (Hayes et al.
1991; Thurber and Peterson 1993). Advantages for group-
living carnivores include greater foraging efficiency (Bertram
1978; Nudds 1978), inclusive fitness (Bertram 1978; Rod-
man 1981), defense of young (Packer and Ruttan 1988), and
protection of kills (Packer et al. 1990; Cooper 1991). Rodman
(1981) argued that for larger wolf packs, the decline in for-
aging efficiency is offset by members improving their inclu-
sive fitness through the addition of close relatives to the
population (Rodman 1981). Schmidt and Mech (1997) argued
that wolves live in packs primarily in order to share their
kills with their young for kin-selection reasons, until youn-
ger wolves gain hunting and killing experience that improves
their fitness after dispersal.
Predation rate by wolves on moose
We estimated that wolves killed 7% of moose older than
calves in winter 1990 and 10–16% or more after 1991.
These rates are higher than the annual adult mortality rates
of 5–9% in stable or increasing moose populations in Alaska
and the Yukon (Gasaway et al. 1983; Ballard et al. 1987;
Larsen et al. 1989; Gasaway et al. 1992). In our study area,
Larsen and Ward (1995) estimated a 5% mortality rate until
the winter of 1992. Our predation-rate modeling predicted
that wolves would reduce adult moose survival rates to lev-
els that could not be sustained by recruitment.
Our results support the model of Walters et al. (1981),
who found that the number of wolf packs was the best deter-
minant of wolf predation rates. Higher kill rates by wolves
in small packs enable them to remove a larger than expected
proportion of moose from a population. The relatively high
wolf predation rate in the early years of our study was re-
lated to the organization of wolves into many small packs
whose kill rates were nearly equivalent to those of larger
packs. Our results show that in order to model wolf preda-
tion rates, researchers need to know the number and sizes of
wolf packs that are killing prey. Table 4 shows three hypo-
thetical models of predation rates by 100 wolves on moose
in winter, depending on different pack-size frequencies.
Model 1 has the highest proportion in pairs (34%), and
wolves removed 27% more moose than in model 3 which
has 10% pairs, and 16% more than model 2, which has 20%
pairs. In a stable wolf population, we could expect that pack
density will not change but mean pack size will grow to
about 10 wolves (Zimen 1976; Hayes and Harestad 2000a).
Thus, using the same model parameters, 200 wolves orga-
nized into 20 packs in the same hypothetical area should kill
about 920 moose during winter, only slightly more than in
model 1 with half the number of wolves.
Although we found no other ecological determinants of
kill rate beside wolf-pack size, kill rates could change if
© 2000 NRC Canada
Hayes et al.
55
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