Predatory Behavior of Grizzly Bears Feeding on Elk Calves in Yellowstone National Park, 1986-88

Abstract

Grizzly bears (Ursus arctos horribilis) were observed preying on elk calves (Cervus elaphus) on 60 occasions in Yellowstone National Park, with 29 confirmed kills. Some bears were deliberate predators and effectively preyed on elk calves for short periods e,ach spring, killing up to 1 calf daily. Primary hunting techniques were searching and chasing although some bears used a variety of techniques during a single hunt. They hunted both day and night and preyed on calves in the open and in the woods. Excess killing occurred when circumstances permitted. One bear caught 5 calves in a 15-minute interval. Elk used a variety of antipredator defenses and occasionally attacked predacious bears. The current level of this feeding behavior appears to be greater than previously reported. This is probably related to the increased availability of calves providing a greater opportunity for learning, and the adaptation of a more predatory behavior by some grizzly bears in Yellowstone. Int. Conf. Bear Res. and Manage. 8:335-341 Earlier investigators mentioned that grizzly bears in the Yellowstone area preyed on newborn elk calves but reported only briefly on this behavior. Murie (1944) stated that during the calving period bears would occa? sionally seize a calf while it was bedded. Johnson (1951) noted that grizzlies were assoeiated with elk calving grounds but did not report any episodes of predation by them. Craighead and Sumner (1982) reported that during the 1960's some grizzlies followed elk to their calving grounds or returned to these areas each spring. They noted bears appeared to locate calves by scent but gave no further details. Cole (1972) reported that grizzlies preyed on some newborn elk calves but noted his study did not adequately sample the calving period. During a study of grizzly bear behavior that began in the Yellowstone ecosystem in 1983, several episodes of elk calf predation were observed. It appeared to be more common and complex than previously reported, so an investigation of this predatory activity was added to the ongoing behavior study. This paper discusses the preda? tory behavior of bears feeding on newborn elk calves based upon field observations in Yellowstone National Park from 1986-1988. Thanks are extended to J.D. Varley and R.R. Knight for providing logistical support, equipment, and encouragement during this project. Thanks also go to park employees, volunteers, and other bear researchers for their assistance in the field work; and B. Blanchard and R. Knight for reviewing this manuscript. This project was funded by the Yellowstone Grizzly Foundation and per? formed under a research agreement with Yellowstone National Park and in cooperation with the Interagency Grizzly Bear Study Team.