Winter Severity and Wolf Predation on
a Formerly Wolf-free Elk Herd
Results
Observational Effort
We located and/or observed the 3 study packs on an average of 28 days and 29 days in 1997 and 1998, respectively. We were able to fly on 19 days in 1997 and 17 days in 1998. On many of these flights, we also observed the 4 other wolf packs that could not be located from the ground. We observed 24 wolves in 1997 and 57 wolves in 1998 at the remains of 55 and 62 kills or probable kills, respectively.
Wolf Hunting Success
During the 1997 study, the 3 main study packs made at least 65 attempts to
kill elk. They succeeded in 16 (26 ± 5[SE]%) attempts. Most elk chased by
wolves were in herds of up to 150, totaling 1,052 animals. Of those 1,052
elk, the wolves killed 16, or 3.0 ± 0.4 (SE)% (Table 1).
During the 1998 study, wolves made at least 37 attempts to kill an elk and succeeded in 5 (15 ± 6[SE]%) attempts. Most of the elk that were chased were in herds of up to 147, totaling 480 animals. Of those 480 elk, the wolves killed 5, or 1 ± 0.5(SE)% (Table 1).
| Table 1. Success rates of Yellowstone National Park wolves hunting elk, 17 March to 15 April 1997 and 2 to 31 March 1998 | ||||||||||
| Pack | Hunting attempts observed | Prey encountered | Prey killed | Prey killed/hunting attempts | Prey killed/total prey encountered | |||||
| 1997 | 1998 | 1997 | 1998 | 1997 | 1998 | 1997 | 1998 | 1997 | 1998 | |
| Leopold | 35 | 15 | 620 | 71 | 7 | 1 | 0.20 | 0.07 | 0.01 | 0.01 |
| Rose Creek | 8 | 8 | 32 | 322 | 2 | 2 | 0.25 | 0.25 | 0.06 | 0.01 |
| Druid | 22 | 14 | 400 | 87 | 7 | 2 | 0.32 | 0.14 | 0.02 | 0.02 |
| Total | 65 | 37 | 1,052 | 480 | 16 | 5 | ||||
| Weighted mean (SE) |
0.26 (0.05) |
0.15 (0.06) |
0.03 (0.004) |
0.01 (0.005) |
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Composition of Kills
Elk constituted 45 (87%) of the kills in 1997 and 61 (98%) in 1998. Other
kills included 6 moose and 1 mule deer in 1997 and 1 bison in 1998. A greater
proportion of male elk were killed in 1998 than in 1997 (Table 2).
| Table 2. Sexes and ages (yr) of adult elk killed by 3 wolf packs on the northern range of Yellowstone National Park from 17 March to 15 April 1997 and from 2 to 31 march 1998. (Elk of unknown sex not included.) | ||||
| Variable | 7 March to 15 April 1997 a | 2 March to 31 March 1998 b | ||
| Bulls | Cowsc | Bulls | Cowsc | |
| n | 14c | 19 | 17 | 6 |
| meand | 6.1 | 15.2 | 4.8 | 13.0 |
| ranged | 2.0-16.0 | 9.0-21.0 | 1.0-8.0 | 2.0-19.0 |
| a Plus 2
calves and 1 yearling b Plus 17 calves c Male:female versus expected 50:50—not significant d Based on 10 bulls and 18 cows in 1997 and 17 bulls and 4 cows in 1998 whose ages could be determined by tooth sections |
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Kill Rate
The mean 1997 kill rate of 17.1 kg of prey/wolf/day was higher than the 1998
rate of 6.1, a difference that was consistent among packs (Table 3). In addition
to killing more prey on a biomass/wolf basis in 1997, the wolves also killed
more individual animals (1.9/wolf in 1997 vs. 1.1/wolf in 1998).
| Table 3. Yellowstone National Park wolf mean minimum kill rates from 17 march to 15 April 1997 and from 2 to 31 March 1998. (Only packs from which kill rate data were collected are included.) | ||||||||
| Pack | No. wolves | No. elk kills | Biomass (kg) killed | Mean prey (kg/wolf/day) | ||||
| 1997 | 1998 | 1997 | 1998 | 1997 | 1998 | 1997 | 1998 | |
| Druid | 5 | 8 | 11a | 16 | 3,324 | 3,676 | 22.2 | 15.3 |
| Rose | 8 | 14 | 14b | 19 | 3,645 | 2,522 | 15.2 | 6.0 |
| Leopold | 5 | 8 | 12 | 9 | 2,558 | 1,988 | 17.1 | 8.3 |
| Thorofare | 2 | 5 | 2c | 3 | 1,204 | 500 | 20.0 | 3.3 |
| Soda Butte | 4 | 8 | 6 | 4 | 1,342 | 547 | 11.2 | 2.3 |
| Chief Joseph | - | 6 | - | 6 | - | 615 | - | 3.4 |
| Crystal | - | 8 | - | 4d | - | 1,042 | - | 4.3 |
| Total | 24 | 57 | 45 | 61 | 17.1 | 6.1 | ||
| a Plus 2
moose. b Plus 1 moose and 1 mule deer. c Plus 3 moose. d Plus 1 bison. |
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During our 1997 study, the Druid Peak pack, Rose Creek pack, Leopold pack, and scavengers consumed an average of 7% of the available food from their kills on the day they made the kill (n = 5), and 23% after 1 more day (n = 15; (Table 4); 5 kills were untouched to about 5% eaten. During 1998, wolves and scavengers consumed an average of 86% of the available food from their kills the day they made the kill (n = 14), and 89% after 1 more day. All kills were fed upon (n = 23).
| Table 4. Percent of Yellowstone National Park wolf-killed elk eaten, from 17 March to 15 April 1997 and from 2 to 31 March 1998, based on carcasses that could be examined within 36 hr after being killed in 1997 and 1998. (AM = adult male; AF = adult female; C = calf.) | ||||||
| Pack | 1997 kills examined | 1998 kills examined | ||||
| No. elk | Age-sex | % eaten | No. elk | Age-sex | % eaten | |
| Druid | 8 | 5AM, 3AF | 19 | 8 | 5AM, 3AF | 84 |
| Leopold | 4 | 1AM, 3AF | 23 | 2 | 2AM | 88 |
| Rose Creek | 3 | 2AM, 1AF | 32 | 13 | 1AF, 12C | 92 |
| Weighted mean ± SE | 23 ± 5 | 89 ± 5 | ||||
| Total | 15 | 8AM, 7AF | 23 | 7AM, 4AF, 12C | ||
Condition of Kills
Mean marrow fat content for 22 adult elk killed was 27% in 1997 and 70% for
20 adult elk in 1998. In 1998, marrow fat of the 12 calf and yearling kills
averaged 22% (range from 7 to 50%).
Ages of Kills
In 1997, all but 2 of the elk kills during our study were adults (including
1 yearling), whereas in 1998, almost half the elk killed in March were calves.
Of the adult elk killed, most were old females and males of various ages (Table
2).
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