Biggest and baddest
Bison are the largest land-dwelling mammal in North America so wolves and grizzly bears are the only large predators of adult bison. Bison have become inceasingly common in Yellowstone National Park but most wolves steer clear of bison.
Wolves will always go for the prey that offers the most food for the least amount of risk. While bison are massive (definitely the most food), they are so dangerous that only some wolf packs will take them on. A bison can easily kill or critically injure a wolf and it takes a lot of hunting cooperation to bring one down. However, when their preferred prey (elk) are not around, some larger packs have turned to hunting bison – especially in the winter, since bison don't migrate. It takes a big wolf pack (5-10 wolves is optimal) to take down an adult bison. A successful bison hunt usually relies on having bigger, experienced males in the wolf pack.