We studied interactions of reintroduced wolves (Canis lupus) with bison (Bison bison) in Yellowstone National Park. Only 2 of 41 wolves in this study had been exposed to bison before their translocation.
The question of how readily wolves (Canis lupus) learn to kill species of prey new to them has generated considerable discussion but limited testing. Certain wolf packs in northwestern Minnesota killed whitetailed deer (Odocoileus virginianus) routinely without taking domestic livestock in or near their territories. Reintroduced wolves in Montana denned in a livestock pasture for about a year before they killed the surrounding cattle. Some wolf packs in northeastern Minnesota kill deer but rarely take moose (Alces alces). Thus, the predatory behavior of wolves reintroduced to Yellowstone National Park, where a wide variety of prey exists, will shed light on wolf adaptation to novel prey. Of particular interest is the question of how soon after reintroduction YNP wolves, including inexperienced individuals, will kill bison (Bison bison), their largest and most formidable prey.
Historically, wolves and bison coexisted over vast areas of North America, but populations of both were drastically reduced because of predator control and market hunting. Wood Buffalo National Park and the adjacent Slave River Lowlands in Canada is one of the few areas where a wolf-bison system has been preserved and where wolves regularly prey on bison. Bison there are the main prey of wolves, so questions of learning and selectivity are not pertinent. In the Mackenzie Bison Sanctuary in Canada, wolves avoid bison and kill moose, even though bison are more abundant.
We expected wolves reintroduced to YNP to avoid or rarely kill bison, instead preferring the familiar, more abundant, and more easily killed elk. We predicted that wolves would have to learn to kill bison, and time to first bison kill would be longer than time to first elk kill. We also expected that when wolves did kill bison, they would take the most vulnerable animals, e.g., calves or animals weakened by harsh winters. Our wolf-bison study is ongoing, but we report here on initial circumstances of wolves interacting with and learning to kill their most formidable prey when prey that would be easier to kill were available.
MATERIALS AND METHODS
Yellowstone National Park...and the surrounding area (Greater Yellowstone area) support an estimated 120,000 elk, 87,000 mule deer (Odocoileus hemionus), an unknown but low number of whitetailed deer, 5,800 moose, 3,900 bighorn sheep (Ovis canadensis), 2,000-4,000 bison, 800-1,000 mountain goats (Oreamnus americanus), and 400 pronghorn (Antilocapra americanus)....
In March 1995, 14 wolves in three packs and in April 1996, 17 wolves in four packs were reintroduced into YNP. Wolves released in 1995 were captured just east of Jasper National Park, Alberta, Canada, where elk, deer, and moose were their main prey. The nearest bison were [more than] 600 km away. Because wolves sometimes disperse distances of [more than] 886 km some donor wolves might have had some experience killing bison. Wolves reintroduced in 1996 came from an area where they preyed on bison. In addition to the 31 Canadian wolves, 10 bison-naive wolves were brought to YNP in September 1996 from northwestern Montana....
When wolves were sighted aerially, we noted time of day, location, habitat type, group size, age, activity, and sex (for collared wolves). We circled for several minutes if wolves were involved in an activity such as attacking prey; otherwise, we left the area. Any interactions with prey observed from the ground were monitored with spotting scopes and recorded onto data forms in the field or into a dictaphone and later transferred to data forms. Ground observers recorded the same data as did aerial observers but also recorded sequence of events: which species approached (wolf or prey), numbers of wolves or prey involved, response of wolves and prey (stand, walk away, flee, or attack), duration of encounter, outcome, and distance over which the encounter took place. Observations reported here are limited to wolf-bison interactions. Bison numbers and distribution were recorded during aerial surveys as part of long-term research (Meagher 1998).
Wolf-killed prey were defined as prey observed being killed or prey determined as killed by wolves by observing wolves feeding on a carcass and then finding other sign that indicated that the prey had been killed by wolves. For example, we looked for blood in the snow along with signs of a struggle or searched for tracks indicative of a chase. Condition of wolf-killed bison was assessed either by marrow fat from a leg bone or observation of the bison before it was killed by wolves....
Bison abundance and distribution....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 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 1997-1998. 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 are less accurate because bison are scattered, 1,769 bison 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. 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.
Wolf distribution. All wolf packs in YNP (three to 11 packs in 1995-1999) were exposed to bison, and all but two packs regularly encountered them....
Five...wolf packs coexisted with bison, but all packs also had access to elk. In 1995-1999, four 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. 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 nonmigratory elk and 1,347-2,277 bison.
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 four bison (7%) being killed; remains of 10 other wolf-killed bison were found. 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....
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....Approach of wolves toward bison was direct, with no attempt at concealment....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. 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) were determined by behavior of the bison. The conclusions of two 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. 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 two kills in 1997, three in 1998, and nine in 1999.
Six different wolf packs killed bison, but 10 of 14 kills were made by two packs. The Crystal Creek pack made four kills, the Nez Perce pack made six, and the Druid Peak pack, Rose Creek pack, Montana pups, and a lone uncollared wolf made one kill each. Those 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 two members that as pups may have been exposed to bison in British Columbia.
Accounts of wolf-killed bison. Only four bison were observed being killed by wolves; the other bison were determined ofter the fact to have been killed by wolves. Of those four bison, only one 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 two 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 two bison observed being killed by wolves were both calves in late winter. One kill involved five yearling wolves that attacked a malnourished lone bison calf on 8 April 1997. That group, referred to as the Sawtooth wolves, was not a pack but was a temporary affiliation of young wolves released together. Another kill involved a lone wolf and four 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.
Our observations indicate that inexperienced wolves can learn to kill even the largest animals that the species preys on given the opportunity to do so. Yearling wolves that had been in captivity since five months of age killed a dying bison calf within three weeks of their release. Other packs killed adult bison within 25 [or less] months of release where no other prey were available (four of 14 kills), where bison predominated (nine of 14 kills), or when bison were especially vulnerable (three of these 14 kills; categories are not mutually exclusive).
These observations are contrary to our prediction that it would take several years for wolves to learn to kill bison, but they support our prediction that any bison that wolves did kill would be especially vulnerable ones. As with other large prey, wolves killed primarily calves and older adults in poor condition. The one bull they killed had a broken leg. We also found that bison kills increased from 1997 through 1999, indicating that with experience wolves were more successful killing bison.
Wolves succeeded more often with attacks on elk than attacks on bison. The combination of elk being more numerous and easier to kill explains why elk are the primary prey of wolves in YNP. Selection for elk by wolves when moose and deer were available was documented in Riding Mountain National Park, Manitoba (Carbyn 1983), suggesting that elk are the preferred prey of wolves when multiple prey are available. Nevertheless, some reintroduced wolves settled where bison were their only possible prey in winter. Clearly, under some conditions, wolves will prey on whatever animals are available, not just the easiest or safest prey species, thereby avoiding the need to migrate to follow prey (elk migrate from Pelican Valley in winter where the Crystal Creek pack resides all year). This finding is not consistent with observations that wolves in Alaska do not prey on bison (Miquelle 1985; S. DuBois and R. Toby, pers. comm.). Perhaps those situations have not been studied closely enough to detect such kills, or the wolf population has been artificially reduced below the level where they need to prey on bison. In YNP, we predict that bison will become a regular prey item for some wolves, at least in spring.
Our findings lend some insight into wolf predation on livestock....Our study suggests that wolves can adapt quickly to killing novel prey if the need arises or if an individual is physically vulnerable.
Figure 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.
Table 2: Sex, age, and condition and date of kill forwolf-killed bison in Yellowstone National Park, 1997-1999.