Seward Peninsula, Alaska
On 12 May 2015, I drove along the Nome-Teller highway with my supervisor, David. We slowed down to watch some musk oxen (Ovibos moschatus) that had strayed close to the road. An Alaska DOT truck driver stopped and waved us over. “There’s a bear chasing the musk ox,” he told us. We dragged out our scope and watched events unfold about a kilometer off the road.
The adult musk oxen were scattered over a far ridge, a mixture of yearlings and grown adults. The brown bear (Ursus arctos horribilis) galloped toward a small group of oxen, chasing them at top speed straight up the ridge. The oxen kept ahead by a dozen meters, running over snow and tundra. The bear cut them off each time they angled back to join up with the larger herd. When the bear slowed, the musk oxen slowed to a walk. When he started galloping, they galloped out of reach.
The small group of oxen reached the top of the ridge and rejoined the larger herd of about 20 animals. While the bear caught up, the oxen formed a defensive circle, where the adults stood shoulder-to-shoulder facing outward. I knew that they formed this ring to defend their young, and I thought it was over–the bear had to give up.
The bear stopped several meters from the circle. Bear and oxen faced each other, motionless for several seconds. Then, the bear charged. The defensive circle buckled as the bear’s target pulled away before contact. Oxen ran in every direction. The bear resumed pursuit of two large bulls. He chased them down the ridge and over a bank of snow. 
The chase had now lasted about 30 minutes. The bear fell behind and eventually collapsed onto the snow. It lay still, sides heaving, before it began digging itself into the snowbank, and then did not move. After ten minutes, we left the scene.
The event seemed unusual to me. This group did not seem to have any young calves, which would be easier prey. The bear may have been young and inexperienced, or recently emerged from its winter den with a desperate need for food. Alaska Department of Fish and Game has recorded an increase in musk ox kills by grizzly bears in recent decades, suggesting that bears are learning how to prey on even the dangerous adults.
The bear was unsuccessful in killing a musk ox, but it did manage to break up the defensive circle. If their defense is more a bluff than true threat, it may not work against an adaptable bear.