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Wolf-Bison Interactions in Yellowstone National Park

Introduction

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 white-tailed deer (Odocoileus virginianus) routinely without taking domestic livestock in or near their territories (Fritts and Mech 1981). Reintroduced wolves in Montana denned in a livestock pasture (Diamond 1994) for about a year before they killed the surrounding cattle (E. E. Bangs, pers. comm.). Some wolf packs in northeastern Minnesota kill deer but rarely take moose (Alces alces—Mech 1966, 1977; Mech and Frenzel 1971). Thus, the predatory behavior of wolves reintroduced to Yellowstone National Park (YNP—Fritts et al. 1997), 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, would kill bison (Bison bison), their largest and most formidable prey (Carbyn et al. 1993).

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 (Lopez 1978). Wood Buffalo National Park and the adjacent Slave River Lowlands in Canada is 1 of the few areas where a wolf-bison system has been preserved and where wolves regularly prey on bison (Carbyn et al. 1993; Van Camp 1987). 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 (Larter et al. 1994).

We expected wolves reintroduced to YNP to avoid or rarely kill bison, instead preferring the familiar, more abundant, more easily killed elk (Cervus elaphus—Carbyn 1983; Mech 1970). We predicted that wolves would have to learn to kill bison, and time to 1st bison kill would be longer than time to 1st 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.

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