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Wolf-Bison Interactions in Yellowstone National Park


Bison abundance and distribution.—Bison in YNP that historically have winter ranges restricted to Lamar (northern range), Mary Mountain (Hayden Valley-Firehole), and Pelican Valley (Meagher 1973; Fig. 1) have undergone major changes in numbers and distribution during the past 15 years (Meagher 1989; Meagher et al. 1997). Geographic designations no longer represent distinct wintering subpopulations because numbers occupying those locales change throughout winter. Lamar and Hayden valleys presently function as major summer range; summer use is limited on traditional winter ranges.

When wolves were reintroduced to YNP in March 1995, bison numbered about 4,000, with about 200 left in Pelican Valley in April. The herd decreased to about 3,500 in winter 1995-1996, with the majority in the Mary Mountain herd. A small number of bison again spent the entire winter in Pelican Valley. The population comprised about 3,400 bison in early winter 1996-1997. That winter was exceptionally severe, and with the ongoing changes in bison distribution (Meagher 1998), large numbers of bison moved out of YNP. Management actions outside YNP removed 1,084 bison, and about 400 died naturally within the park.

About 2,100 bison were counted in early winter 1997-98 (Table 1).Within the Mary Mountain segment, 523 bison lived in Hayden Valley, 472 in the Nez Perce-Firehole River area, and 352 along the Madison River or in the western boundary area. By late winter-early spring, when counts were less accurate because bison are scattered, 1,769 were counted on 30 April 1998. Of those, 333 were on the Northern Range, 80 were in Pelican Valley, and 1,356 were in the Mary Mountain area. In December 1998, 2,203 bison were counted in YNP (Table 1). A March survey yielded 1,683 bison, with 349 on the northern range, 192 in Pelican Valley, and 1,142 in the Mary Mountain herd.

  Figure 1 - Map of Yellowstone National Park with shaded areas indicating winter ranges of bison in 1995

Fig. 1.  Winter ranges of bison in Yellowstone National Park when wolves were reintroduced in 1995.

TABLE 1.  Distribution and abundance of bison and elk in Yellowstone National Park, December 1994-1999.
Winter season Bison Elk
Pelican Valley Mary Mountain Northern range Total Pelican Valley Mary Mountain Northern Range Total
1994-1995 1,123 2,156 694 3,973 0 651a 16,500 17,151
1995-1996 469 2,277 742 3,488 0 537a 16,000b 16,537
1996-1997 719a 1,874c 775c 3,367 0 438a 14,000b 14,438
1997-1998 365 1,347 393 2,105 0 594a 11,692 12,286
1998-1999 366 1,418 419 2,203 0 545a 11,742 12,287
a Eberhardt et al. (1998).
b No survey; number represents an estimate.
c Survey conducted October 1996.
d From R. A. Garrott and L. L. Eberhardt, pers. comm.

Wolf distribution.—All wolf packs in YNP (3-11 packs in 1995-1999) were exposed to bison, and all but 2 packs regularly encountered them. In Pelican Valley, where the Crystal Creek pack wintered, we saw only 9 elk while aerially tracking wolves over 4 winters (1995-1999; Table 1).

Five other wolf packs coexisted with bison, but all packs also had access to elk. In 1995-1999, 4 wolf packs occupied the Northern Range of YNP. The Crystal Creek, Leopold, Rose Creek, and Druid Peak packs all shared winter and summer ranges with bison but also shared range with the Northern Yellowstone elk herd, consisting of 11,692-16,500 animals (Lemke et al. 1998). In winter 1998-1999, the Nez Perce pack occupied the Madison-Firehole area of YNP, part of the overwintering area of the Mary Mountain bison herd, and an area occupied by 537-651 non-migratory elk and 1,347-2,277 bison (Table 1; Eberhardt et al. 1998).

Wolf-bison interactions.—From April 1995 through March 1999, field personnel observed 44 independent wolf-bison encounters, resulting in 57 total interactions (13 interactions involved the same bison and wolves), and saw 4 bison (7%) being killed; remains of 10 other wolf-killed bison were found (Table 2).

During that same period, we observed 372 separate wolf-elk interactions, during which wolves killed 77 elk (21%). Hence, wolves were more successful killing elk than killing bison when they encountered them (χ² = 5.18, d.f. = 1, P = 0.03) . We documented 589 other wolf-killed elk during that period. Although there were more elk in YNP than bison (Table 1; elk outnumbered bison 5.6:1), the ratio of elk:bison killed by wolves was much higher (47.6:1).

The 5 year average for the elk and bison population over the study period showed that elk comprised 83% (14,540) and bison 17% (3,027) of the available prey base. Based on our wolf-prey encounter rates of 372 (87%) for elk and 57 (13%) for bison, we found that wolves did not approach elk more often than they approached bison (χ² = 2.08, d.f. = 1, P = 0.15).

One to 15 wolves (4.1 0.5 SE) approached 1-55 bison (10.9 1.5). Encounters lasted from 24 s to 9.5 h (17.5 10.0 min, median = 3.0 min). Approach of wolves toward bison was direct with no attempt at concealment. At least 17 (30%) encounters involved pups and adults, and 5 (9%) encounters involved only pups. Forty-three (75%) bison approached by wolves stood their ground and did not flee; 12 (21%) of those animals were lone bison. Of the 45 encounters of multiple bison, 38 (84%) involved the bison grouping tightly, and in 32 (84%) of those cases, bison stood and faced the wolves.

When wolves attempted to attack bison they did so from the rear, when bison were running away (25%; n = 14). If bison did not run, wolves quickly lost interest. Hence, whether or not an attack occurred, duration of encounter, chase distance, and ultimate outcome of the encounter (failed attempt or successful kill) was determined by behavior of the bison. The conclusions of 2 attacks were observed, and wolves were attacking the neck of the bison, but we do not know if the wolves attacked the rear first.

Wolves killed primarily calves and cows. Ten (71%) of the 14 bison kills were made in March or April (Table 2). A calf and a bull with a broken leg were killed in December 1998, and lone calves were killed in January and February 1999. We detected only 2 kills in 1997, 3 in 1998, and 9 in 1999.

Six different wolf packs killed bison, but 10 of 14 kills were made by 2 packs. The Crystal Creek pack made 4 kills, the Nez Perce pack made 6, and the Druid Peak, Rose Creek, Montana pups, and a lone uncollared wolf made 1 kill each. These kills involved 58-60 different wolves. None of those wolves had experience with bison prior to reintroduction in YNP, except for possibly the Druid Peak pack, which contained 2 members that as pups may have been exposed to bison in British Columbia.

TABLE 2.  Sex, age, and condition and date of kill for wolf-killed bison in Yellowstone National Park, 1997-1999.
Date Killed Sex Age Conditiona Location Pack size Wolf pack
8 April 1997 Unknown Calf Poor Norris Junction 5 Sawtooth
12 April 1997 Female Adult Unknown Pelican Valley 2 Crystal
30 March 1998 Unknown Adult Unknown Pelican Valley 8 Crystal
10 December 1998 Male Adult Broken leg Lamar Valley 7 Druid
30 December 1998 Unknown Calf Unknown Firehole River 7 Nez Perce
8 January 1999 Unknown Calf Poor Firehole River 7 Nez Perce
13 February 1999 Female Calf Poor Firehole River 7 Nez Perce
10 March 1999 Unknown Calf Poor Firehole River 7 Nez Perce
12-13 March 1999 Female Adult Fair Slough Creek 18 Rose
17 March 1999 Female Adult Unknown Pelican Valley 14 Crystal
23 March 1999 Unknown Calf Unknown Nez Perce Creek 7 Nez Perce
24 March 1999 Unknown Calf Probably poor Gibbon Meadows 1 1 wolfb
28 March 1999 Unknown Yearling Probably poor Pelican Valley 14 Crystal
28 March 1999 Unknown Calf Probably poor Firehole River 7 Nez Perce
a Condition was determined either by inspection of carcass or by observation of individual before death.
bAttack on bison calf also involved 4 coyotes.

Accounts of wolf-killed bison.—Only 4 bison were observed being killed by wolves; the other bison were determined after the fact to have been killed by wolves (Table 2). Of those 4 bison, only 1 kill-sequence was observed from start to finish. On 17 March 1999, 14 wolves from the Crystal Creek pack, which resided in Pelican Valley, attacked a group of about 55 bison. The attack lasted 9.5 h Wolves chased bison from areas of no snow to deep (1-2 m) snow, attacking them while they were in the deep snow with all members of the pack. A maximum of 14 wolves were observed biting bison simultaneously. After testing and attacking bison like this all day, wolves killed an adult female.

Another yearling bison was observed being killed by the same wolves at the same location on 28 March 1999. The entire kill was not observed. Fourteen wolves had 2 bison, a cow and a yearling, separated from a herd of 60-70 animals. The bison cow stood while wolves attacked the yearling. Ten wolves attacked the yearling simultaneously with most of the attack focused on the neck. The yearling was killed while the cow remained motionless 5 m away, but she was never approached by the wolves.

The other 2 bison observed being killed by wolves were both calves in late winter. One kill involved 5 yearling wolves that attacked a malnourished lone bison calf on 8 April 1997 (Table 2). That group, referred to as the Sawtooth wolves, was not a pack but a temporary affiliation of young wolves released together. Another kill involved a lone wolf, with 4 coyotes (Canis latrans) simultaneously attacking a malnourished lone bison calf on 24 March 1999. The wolf attacked the neck of the bison and the coyotes attacked a hind leg. Bites to the neck eventually killed the calf. The wolf then chased the coyotes away, but they remained 50 m away while the wolf fed on the calf.

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