predation-rate study periods we found 179 of the moose kills
and 25 caribou kills (Appendix, Table A2).
Wolves preyed on moose calves more often than on other
age-classes. Calves accounted for 31% (n = 88) of killed
moose (Table 1). We found no consistent relation between
the proportion of calves in the wolves’ diet and the propor-
tion of calves available each winter (Table 1).
The age of 27 killed adult moose that were aged was 8.9 ±
0.9 (mean ± SE) years, ranging from 2 to 15 years (Fig. 1).
Wolves killed 28 female and 18 male moose (>1 year old).
Mean age did not differ between the sexes. We found 30 car-
ibou carcasses but we could not distinguish sex or age from
aircraft. Large wolf packs completely consumed caribou in a
few hours, leaving few remains for identification.
Nearly all killed prey were apparently not in starving con-
dition at the time of death. Starvation levels are <10% mar-
row fat for moose calves and <20% for adults (Peterson et
al. 1984). Marrow fat content of wolf-killed calves (n = 23)
was 34 ± 4% (mean ± SE; range 11–78%) and that of adults
(n = 26) was 77 ± 3% (mean ± SE; range 52–95%) (Fig. 2).
No moose were within starvation range, but 35% of calves
were close. Seven adult caribou had 66 ± 14% (mean ± SE)
marrow fat (range 8–95%).
Kill and consumption rates by wolves
We studied kill rates in 21 different wolf packs during 4
winters (Appendix, Table A2). Traveling pack size ranged
from 2 to 20 wolves. The predation period was 20 ± 1.3
(mean ± SE) days, ranging from 6 to 39 days (Appendix,
Table A1). We measured kill rates of small packs (2 or 3
wolves) during 18 predation periods, medium packs (4–
9 wolves) during 13 periods, and large packs (
≥
10 wolves)
during 14 periods. In total, we sampled kill rates of 283
wolves during 6153 wolf-days (982 pack-days). We ob-
served packs for 71 ± 0.9% (mean ± SE) of all days during
predation periods (Appendix, Table A2).
Moose composed 94% (57 764 kg) of the biomass of
ungulates killed. The kill rates were 0.045 ± 0.004 (mean ±
SE; range 0.013–0.123) moose/day by each wolf and 0.193 ±
0.085 moose/day by each pack. Other studies showed that
pack size strongly affected kill rates (Hayes et al. 1991;
Thurber and Peterson 1993; Dale et al. 1994). The log-
transformed model y = log
10
of pack size minimized hetero-
scedasticity for both kilograms (mass) of prey killed per wolf
each day (KGWD) and the number of moose killed per wolf
each day (MWD). Log
10
y = pack size was the best linear
model for the period between moose kills (days per moose
kill, DMK). Thurber and Peterson (1993) used the same log-
transformed models in a similar analysis of wolf kill rates.
Because we measured kill rates of some packs more than
once, we examined the data for dependence problems. We
examined a regression equation for KGWD and log
10
pack
size using data from the last (or only) predation period for
the 21 different packs studied (y = –17.4 – 5.35 log
10
pack
size). Parameters differed little from the equation for the
pooled predation data (y = –16.8 – 5.4 log
10
pack size). Thus,
we used the pooled rates in our regression analyses. We also
tested for any relation between kill rate and intervals be-
tween relocations, expressed as the percentage of days on
© 2000 NRC Canada
Hayes et al.
51
Wolf-killed moose
Moose in March
population
H
0
: P
k
= P
p
Year
No.
Proportion of
calves (P
k
)
No.
Proportion of
calves (P
p
)
χ
2
P
1990
55
0.55
156
0.36
13.8
<0.01
1991
16
0.25
265
0.37
1.0
<0.01
1992
135
0.26
215
0.26
1.2
0.28
1993
33
0.12
101
0.22
0.6
0.44
1994
47
0.32
332
0.11
14.3
<0.01
Note: The
χ
2
values show the differences between the proportion of
calves in the kill sample (observed) and the proportion of live calves in
winter (expected). Yates’ corrected
χ
2
was used for 1991 and 1993
because of small sample sizes of calves in the kill sample.
Table 1. Proportions of moose calves killed by wolves and in
late winter composition counts.
Fig. 1. Frequencies of moose in age-classes older than calves
that were killed by wolves during winter in the study area.
Fig. 2. Marrow-fat indices (%) for adult and calf moose killed
by wolves during winter. “SA” is starvation level for adult
moose and “SC” is starvation level for calf moose (Gasaway
et al. 1992).
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which wolves were seen. We found no correlation (Table 2),
indicating that we sampled daily activities often enough to
find most kills.
Kill rate was significantly correlated only with wolf-pack
size (Table 2). It was not related to (i) daily area (km
2
) in
which wolf packs traveled, (
ii) percentage of days on which
wolves were followed, (iii) annual ratio of wolf numbers to
moose numbers, (iv) number of wolf packs, (v) snow depth,
or (vi) moose density (Table 2). The kill rate by wolves on
moose calves (log
10
days/calf kill) was not related to any
variable, including the proportion of calves alive in winter
(Table 2).
Both KGWD (r
2
= 0.40, df = 44, P < 0.001) and MWD
(Fig. 3;
r
2
= 0.57, df = 43, P < 0.001) were inversely related
to log
10
pack size. Log
10
DMK was inversely related to
wolf-pack size (Fig. 4; r
2
= 0.37, df = 43. P < 0.001). We
excluded small packs to test whether kill rates remained sig-
nificantly correlated with the sizes of larger packs (4–20
wolves). KGWD remained inversely related to log
10
pack
size (
r
2
= 0.37, df = 26, P = 0.001). Excluding small packs
did not improve the relation between log
10
DMK and moose
density (
r
2
= 0.007, df = 25, P = 0.69). Excluding wolf pairs
did improve the relation between log
10
days/calf kill and the
ratio of numbers of live calves to numbers of adult moose
(r
2
= 0.11, df = 24, P = 0.11), but it was not significant.
© 2000 NRC Canada
52
Can. J. Zool. Vol. 78, 2000
Dependent variable
Independent variable
r
2
df
P
kg/wolf/day
km
2
/day
0.01
44
0.49
Moose density
0.03
44
0.28
Moose/wolf
0.002
44
0.78
Number of packs
0.001
44
0.97
Percentage of days seen
0.03
44
0.28
log
10
pack size
0.40
44
<0.001
Moose/wolf/day
log
10
pack size
0.57
43
<0.001
log
10
days/kill
km
2
/day
0.001
43
0.87
Moose density
0.02
43
0.93
Moose/wolf
0.006
43
0.98
Percentage of days seen
0.10
43
0.52
Snow depth
0.003
41
0.75
Pack size
0.37
43
<0.001
log
10
days/calf kill
Moose density
0.001
31
0.90
Percentage of moose calves
alive in late winter
0.001
31
0.84
Pack size
0.004
44
0.74
Snow depth
0.008
41
0.58
Note: Values in boldface type indicate that the independent variable is significantly related.
Table 2. Results of linear regression analysis of kill rates by wolves on ungulates (kg/wolf/day),
moose (moose/wolf/day), and killing intervals on moose (log
10
days/moose kill) and moose
calves (log
10
days/calf kill) with various independent variables.
Fig. 3. Kill rates during winter by wolf packs of different sizes
in the FSA (log
10
y = 0.93 – 0.03x).
Fig. 4. Intervals between moose kills during winter for wolf
packs of different sizes in the FSA (log
10
y = 0.93 – 0.03x).
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