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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)


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


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.

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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