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Predation

While often secure from predation because of their size and strength, in some areas, vulnerable individuals are regularly preyed upon by wolves. Wolf predation typically peaks in late winter, when elk migrates south and bison are distressed with heavy snows and shortages of food sources, with attacks usually being concentrated on weakened and injured cows and calves. Wolves more actively target herds with calves than those without. The length of a predation episode varies, ranging from a few minutes to over nine hours. Bison display five apparent defense strategies in protecting calves from wolves: running to a cow; running to a herd; running to the nearest bull; running in the front or center of a stampeding herd; entering water bodies, such as lakes or rivers. When fleeing wolves in open areas, cows with young calves take the lead, while bulls take to the rear of the herds to guard the cows' escape. Bison typically ignore wolves not displaying hunting behavior. Wolf packs specializing in bison tend to have more males because their larger size than females allows them to wrestle prey to the ground more effectively. Healthy, mature bulls in herds rarely fall prey.

Grizzly bears are known to feed on carcass and may steal wolves' kills. While grizzlies can also pose a threat to calves and sometimes old, injured, or sick adult bison, direct killing of non-calves is rare even when targeting lone and injured young individuals; attacking healthy bison is risky for bears, who can be killed instead.