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Wolf-Bison Interactions in Yellowstone National Park


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 have been in captivity since 5-months of age, killed a dying bison calf within 3 weeks of their release. Other packs killed adult bison within ≤25 months of release where no other prey were available (4 of 14 kills), where bison predominated (9 of 14 kills), or when bison were especially vulnerable (3 of the 14; 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 (Mech 1970; Mech et al. 1998). The 1 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. For example, the wolf population in Minnesota expanded from its wilderness reservoir where deer and moose were the primary prey into semiagricultural land and at first rarely attacked livestock (Fritts and Mech 1981). Eventually, the population spread to agricultural land, and wolves learned to kill livestock, which became included regularly in their diet (Fritts et al. 1992). A similar situation occurred with wolves in Montana (Diamond 1994) after they had been closely exposed to livestock for a year (E. E. Bangs, pers. comm.). However, because livestock are often kept near human habitations in open areas away from cover and are only seasonally available, it may take wolves longer to learn to include them regularly as prey. Our study suggests that wolves can adapt quickly to killing novel prey if the need arises or if an individual is physically vulnerable.

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