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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 ą 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 ą 0.4 (SE)% (Table 1).

During the 1998 study, wolves made at least 37 attempts to kill an elk and succeeded in 5 (15 ą 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 ą 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
Pack	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
Variable	Bulls	Cows^c	Bulls	Cows^c
n	14^c	19	17	6
mean^d	6.1	15.2	4.8	13.0
range^d	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)
Pack	1997	1998	1997	1998	1997	1998	1997	1998
Druid	5	8	11^a	16	3,324	3,676	22.2	15.3
Rose	8	14	14^b	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	2^c	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	-	4^d	-	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
Pack	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 ą SE			23 ą 5			89 ą 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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Menlo Park, CA [caww54]

Winter Severity and Wolf Predation on a Formerly Wolf-free Elk Herd

Results

Winter Severity and Wolf Predation on
a Formerly Wolf-free Elk Herd