Grizzly Bear, Ursus arctos, usurps Bison calf, Bison bison, captured by Wolves, Canis lupus, in Yellowstone National Park, Wyoming

We describe an adult Grizzly Bear (Ursies arctos) usurping a Bison (Bison bison) calf from a pack of five Wolves (Canis lupus) attempting to kill the Bison in Yellowstone National Park during early spring. Five Wolves grabbed the hind end and neck of the calf while it was trailing behind two adult male Bison. In 3 minutes a Grizzly Bear arrived and displaced the two Wolves attacking the hind end. For 1 minute the Grizzly Bear attacked the rear of the Bison while three Wolves attacked the front end. The Grizzly Bear subsequently pulled the struggling calf from the Wolves and made the kill. The Wolves were unable to displace the Grizzly Bear from the carcass. Our observation demonstrates the capacity for Grizzly Bears to exploit tire predatory abilities of Gray Wolves restored to Yellowstone National Park. Kleptoparasitism by Grizzly Bears on Wolf-captured ungulates may be a selective pressure promoting group living in Wolves, and could provide an important new food resource to threatened Grizzly Bears in the Greater Yellowstone Ecosystem.