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Patterns of Prey Selection by Wolves
in Denali National Park, Alaska

Results


One hundred and seven wolves were captured and radio-tagged in 25 packs distributed throughout Denali during 1986 to 1991. Some packs included up to four wolves wearing active collars concurrently because radio-collars usually lasted for several years. During each year, five to 19 packs included radio-tagged members.

Remains of 294 moose, 225 caribou, and 63 sheep were found by tracking wolves. Of these, 245 moose, 221 caribou, and 60 sheep were considered wolf kills or probable wolf kills (hereafter pooled as "kills"). This is our basic sample from which various subsamples were examined for different analyses. Some 167 moose, 165 caribou, and 49 sheep were examined from the ground to determine species, age, sex, condition, abnormalities, and cause of death.

Our tallies of kills found with collared packs probably were not a representative sample of the kills made by these packs for several reasons: 1) flying efforts were not distributed evenly over the year; 2) wolves spend more time at the kills of larger animals, and 3) larger kills such as moose are more visible and identifiable from the air.

Overall, moose represented 47% of the kills found, caribou 42%, and sheep 11% (Table 2). Because of the biases (previously discussed), this sample probably exaggerates the relative numbers of moose taken and minimizes the proportion of sheep. However, the proportions may more accurately represent the relative biomass of the three prey consumed. About 15% of all moose eaten by wolves (about 40% of the bulls and 9% of cows and calves) were thought to have been scavenged. They were not included in the kill sample. Haber (1977) also found that Denali wolves scavenged considerably on bull moose.

Table 2.  Composition of prey (unadjusted for weight) killed by wolves, or probably killed by wolves, in Denali National Park, Alaska, October-April 1986-1992.
Species Winter
1985-86 1986-87 1987-88 1988-89 1989-90 1990-91 1991-92 TOTAL
n % n % n % n % n % n % n % n %
Moose
bulls 1 14 9 24 4 10 14 18 3 5 -- -- 1 6 32 11
cows 3 43 14 37 12 30 14 18 3 5 6 14 5 28 57 20
calves 2 29 6 16 10 25 14 18 6 11 9 21 4 22 51 18
sum 6 86 29 77 26 65 42 54 12 21 15 35 10 56 140 49
Caribou
bulls 1 14 4 11 4 10 18 23 10 18 9 21 5 28 51 18
cows -- -- 2 5 3 8 5 6 16 28 16 37 1 6 43 15
calves -- -- -- -- -- -- -- -- 10 18 2 5 1 6 13 5
sum 1 14 6 16 7 18 23 29 36 64 27 63 7 40 107 38
Sheep
rams -- -- -- -- 5 13 6 8 6 11 -- -- -- -- 17 6
ewes -- -- 3 8 1 3 6 8 3 5 1 2 -- -- 14 5
lambs -- -- -- -- 1 3 1 1 -- -- -- -- 1 6 3 1
sum -- -- 3 8 7 19 13 17 9 16 1 2 1 6 34 12
TOTAL 7 100 38 100 40 100 78 100 57 100 43 100 18 100 281 100

Figure 1.
Fig. 1  Annual proportions of each species, sex, and age of ungulates killed by wolves, or probably killed by wolves, in Denali National Park, Alaska, October through April 1986-1992.

Annual and Monthly Variation in Prey Composition

The composition of wolf kills varied by both year and month. Moose formed the largest percentage of the sample during the first 4 years of the study, but caribou predominated in 1989-90 and 1990-91 (Table 2, Fig. 1). As caribou kills increased, both moose and sheep kills decreased. This trend held true both for the observed proportions of kills in our sample and for the proportions derived after adjustment for weight.

On a monthly basis, the total sample of kills suggested that wolves tended to kill moose calves year around, caribou calves in May, June, March, and April (1990-1992 only), caribou bulls year around, but primarily in July through November, bull moose year around, but primarily in November and December, caribou cows primarily February through June, cow moose October through May, and sheep primarily September through April and especially December (Table 3, Fig. 2). Some of these trends may have resulted from sampling error or bias. Chi square analysis indicated that significantly more moose were taken November through February, more caribou cows and calves in March, and more sheep in December. Although on a monthly basis the preponderance of caribou bulls taken August to October was not significantly different from its proportion of prey in other months (Table 3), bull caribou formed the single most predominant prey type for any single period of the year (Fig. 2).

Table 3.  Frequency of wolf kills or probable kills of moose, caribou, and Dall sheep in Denali National Park, Alaska, 1986-1992.1
Species Month
J F M A M J J A S O N D Total wtd. Mean of X%
Adults
Moose 10 23 16 11 6 3 0 1 1 10 14 5 100 36
Caribou 1 14 34 14 11 2 2 8 10 20 9 2 127 53
Sheep 1 10 7 2 0 1 0 0 1 3 4 4 33 11
p2 0.01 0.02 0.60 0.82 0.33 0.64 0.42 0.11 0.12 0.75 0.19 0.02  
Adults and Calves1
Moose 14 34 30 19 - - - - - 16 20 7 140 55
Caribou 1 16 41 17 - - - - - 21 9 2 107 32
Sheep 1 11 8 3 - - - - - 3 4 4 34 13
p2 0.05 0.59 0.03 0.37 - - - - - 0.07 0.85 0.18  
1 data are not given for May through September for the adults and calf part of table because of the strong bias against finding kills of young animals during that period

2 probability of no significant difference in proportion of prey species in a given month compared with year-round weighted mean proportion.

Fig. 2  Monthly proportions of each species, sex, and age of ungulates killed by wolves, or probably killed by wolves, in Denali National Park, Alaska, October through April 1986-1992.

Age and Sex Structure of the Kill

Denali wolves killed primarily calves and old moose and caribou of both sexes (Figs. 3 and 4), except during multiple caribou kills (see below). Although our sample of Dall Sheep was small, apparently individuals aged ≥5 years were taken disproportionately (Fig. 5). These results parallel those of Murie (1944), Burles and Hoefs (1984), Sumanik (1987), and Hayes et al. (1991) for sheep; Mech (1966a), Peterson (1977), Haber (1977), Fuller and Keith (1980), Peterson et al. (1984), Ballard et al. (1987), Bjorge and Gunson (1989), Hayes et al. (1991), and Gasaway et al. (1992) for moose; and Parker and Luttich (1986) for caribou. Our oldest kills of each species were estimated at ≤20 years old, ≤22 years old, and ≤13 years old for moose, caribou, and sheep, respectively.

There was no significant difference between the sex ratio of wolf-killed moose and the sex ratio of the herd (Meier et al. 1991), but wolves killed significantly more male caribou (119:100; χ² = 13.7, 1 df, P < 0.001) than the proportion in the population (Adams et al. 1989a).

This differs from the even sex ratio found by Parker and Luttich (1986) for wolf-killed caribou in Labrador. Samples of the males of each species killed by wolves were younger than those of the females (Figs. 3-5), although only the male moose and sheep were significantly younger (P < 0.05). Ballard et al. (1987) found no significant difference between the sex ratio of wolf-killed caribou and that of caribou surveyed by air in south-central Alaska.

During late winter and spring 1990 and 1991, record amounts of snow fell in the study area (Table 1), and several multiple wolf kills of caribou were found. For both males and females, the age distributions of caribou killed during these bouts of mass predation (Fig. 4) were significantly younger than those killed individually and at other times of the year (P < .01). Contrary to reports from other areas (Kelsall 1957, Eide and Ballard 1982, Miller et al. 1985, 1988b), Denali wolves returned repeatedly to feed on the frozen carcasses of the multiple kills.

Figure 3.
Fig. 3  Age structure of moose killed by wolves, or probably killed by wolves, in Denali National Park, Alaska, October through April 1986-1992. (Calves are ≥0.5-years old).


Figure 4.
Fig. 4  Age structure of caribou killed by wolves, or probably killed by wolves, in Denali National Park, Alaska, October through April 1986-1992.


Figure 5.
Fig. 5  Age structure of Dall sheep killed by wolves, or probably killed by wolves, in Denali National Park, Alaska, October through April 1986-1992.

Snow and Patterns of Prey Selection

The most striking change in the composition of wolf kills during our study was the major increase in the proportion and composition of caribou killed beginning the second consecutive winter (1989-1990) with above-average snowfall (Table 1, Fig. 1). The proportion of adult moose we found killed in that winter greatly decreased (Fig. 1). Although winter 1988-89 was the first year of above-average snowfall during the study, the increased proportion of wolf-killed caribou cows was not found until 1989-90 and 1990-91 (Table 2), both of which also had above-average snowfall (Table 1). This may be evidence for a cumulative snow effect (Mech et al. 1987) on caribou condition. The proportion of caribou in observed wolf kills each year, including those we could not sex or age, increased with the cumulative snowfall (Table 1) for that year (R2 = 0.70; P = 0.02). When a sub-sample of caribou kills (Table 2) that could be sexed and aged was examined relative to snowfall, it was the proportion of caribou cows in the wolf kills that varied most with snowfall (R2 = 0.54; P = 0.06; bulls, R2 = 0.33; P = 0.17). There was no substantial change in proportion of time spent locating various wolf packs, or in the geographic area of coverage, that might account for the switch to caribou during the study.

The occurrence of caribou calves in the sample of winter wolf kills was related to snowfall (Table 1) during the winter they were in utero. Caribou calves were killed by wolves during winters that followed winters of above-average snowfall, whereas after winters of below-average snowfall, no wolf-killed caribou calves were found the next winter (Table 2). This parallels the results of neonate caribou survival in the same herd. Following two winters of below-average snowfall, mortality of calves ≤30 days old averaged 39%, whereas following three winters of above-average snowfall, the average was 67% (Adams et al. 1993).

No corresponding relationship was found between winter snowfall and proportion of moose calves killed by wolves. Where moose are the primary prey of wolves, calf vulnerability to wolf predation is related to snowfall (Peterson 1977). In Denali, increased moose calf vulnerability based on snowfall may have been masked by the increased kill of caribou.

Figure 6.
Fig. 6  Distributions of percent marrow fat in prey killed by wolves, or probably killed by wolves, in Denali National Park, Alaska, October through April 1986-1992.

Nutritional Condition of Wolf-Killed Prey

The mean marrow-fat content of moose, caribou and sheep killed by wolves was low during each of the winters (October-April) of the study. Means for moose were lowest in 1990-91 (Table 4), when snow was deepest (Table 1). Marrow fat was less in moose and caribou calves than in adults, less in male caribou during October through May (no January data) than in June through September, and less in bull moose than in cows (Table 5). Percent marrow fat in 29 caribou killed in multiples during winters of deep snow averaged 66±4% compared with 47±5% for 44 caribou killed individually during all winters. The distribution of marrow fat for each species contained individuals with <20%->90% (Fig. 6).

Table 4.  Percent femur marrow fat content of prey killed by wolves, or probably killed by wolves, in Denali National Park, Alaska, October-May 1986-1991.
SPECIES WINTER
1985-1986 1986-1987 1987-1988 1988-1989 1989-1990 1990-1991
Mean of X ±SE n Mean of X ±SE n Mean of X ±SE n Mean of X ±SE n Mean of X ±SE n Mean of X ±SE n
Moose 58 10 4 63 5 30 64 5 24 71 4 22 56 12 8 28 7 6
Caribou 79 - 1 48 13 5 40 12 6 42 7 21 59 5 11 55 5 20
Sheep - - - 93 - 2 58 13 5 78 12 6 88 - 2 - - -

Table 5.  Percent femur marrow fat content of prey killed by wolves, or probably killed by wolves, in Denali National Park, Alaska, October-May 1986-1992.
SPECIES January February March April May June
Mean of X ±SE n Mean of X ±SE n Mean of X ±SE n Mean of X ±SE n Mean of X ±SE n Mean of X ±SE n
Moose
bulls 58 - 1 63 12 6 60 27 5 62 - 1 73 - 2 84 - 1
cows 84 2 7 74 6 13 62 24 7 68 9 6 58 - 2 74 - 1
calves 54 9 3 67 5 7 32 8 10 37 - 2 10 - 1 38 16 3
Caribou
bulls - - - 33 12 3 57 9 9 68 10 6 52 - 1 14 - 1
cows - - - 70 7 10 62 6 22 86 - 1 62 13 5 - - -
calves - - - 31 - 2 45 7 9 38 2 3 21 2 9 - - -
Sheep
rams - - - 65 - 2 - - - 12 - 1 - - - - - -
ewes - - - - - - 85 - 2 56 - 2 - - - - - -
lambs - - - - - - - - - - - - - - - - - -
 
SPECIES July August September October November December
Mean of X ąSE n Mean of X ąSE n Mean of X ąSE n Mean of X ąSE n Mean of X ąSE n Mean of X ąSE n
Moose
bulls - - - - - - - - - 76 - 2 53 - 2 41 15 3
cows - - - 87 - 1 - - - 69 6 3 80 7 4 87 - 1
calves - - - - - - 45 - - 44 10 3 52 - 2 18 - 2
Caribou
bulls 71 - 1 84 5 3 85 2 4 39 8 12 24 9 7 - - -
cows - - - 92 - 1 85 1 3 10 - 1 - - - 78 - 1
calves - - - - - - - - - - - - - - - - - -
Sheep
rams - - - - - - 92 - 1 - - - 91 - 2 64 - 1
ewes - - - - - - - - - 90 - 2 88 - 1 93 - 2

Skeletal Abnormalities of Prey Killed by Wolves

More than a third of the wolf-killed moose examined showed mandibular necrosis, and a third or more had arthritis in their lumbosacral or coxofemoral joints (Table 6). Similarly, jaw necrosis and arthritis was found in wolf-killed moose on Isle Royale (Peterson 1977), on Kenai Peninsula, Alaska (Peterson et al. 1984), and earlier in Denali (Haber 1977). Arthritis afflicted male moose as young as five years of age, whereas no sign of it was found in cows ≤14 years old (Table 6). This contrasts with reports that arthritis afflicted moose cows as young as eight years on Isle Royale (Peterson 1977) and six years of age on Kenai Peninsula (Peterson et al. 1984). On Isle Royale, as well as in our study, the incidence of arthritis was higher in male moose (Peterson 1977).

More wolf-killed sheep than moose showed necrosis (Table 6). Murie (1944) also reported a high rate of necrosis in Denali sheep. We found no arthritis in our small sheep sample, except for a severely arthritic mandibular ramus on a 10-year-old ram.

The incidence of mandibular necrosis and arthritis in wolf-killed caribou was lower than in sheep or moose (Table 6), but the incidence of necrosis in our sample was higher than the incidences reported by Doerr and Dietrich (1979) and several authors whose work they summarized. The incidence was higher in male caribou than in females, but not significantly so (P = 0.14). Doerr and Dietrich (1979) found significantly more males with necrosis than females.

Table 6.  Incidence of skeletal abnormalities in prey killed by wolves, or probably killed by wolves in Denali National Park, Alaska, October-April 1986-1992.
SPECIES Mandibular Necrosis Arthritis
n affected (%) n affected (%)
Dall Sheep 
   Female 18 8(44) 6 0(0)
   Male 24 13(54) 5 0(0)
Caribou 
   Female 60 0(0) 43 2(5)
   Male 66 3(5) 52 7(13)
Moose 
   Female 81 29(36) 41 15(37)
     Age 1-13 - - 23 0(0)
     Age 14-20 - - 18 15(83)1
   Male 80 28(35) 52 25(48)
     Age 1-4 - - 10 0(0)
     Age 5-15 - - 42 25(60)1
1 Different at P = 0.07


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