The northern range is home to elk, bison, pronghorn, bighorn sheep, mule deer, wolves, and coyotes. Elk, sometimes called wapiti, are often seen in large herds grazing on grasses and forbs in open areas, and bull elk shed their antlers every year, with new antlers growing larger and more branched as they age; calves born in June are spotted for their first few months, helping camouflage them from predators. Bison, also called buffalo, can weigh up to one ton, and both sexes have horns; their reddish brown calves are born in late April and May, and although bison may appear tame and slow, they are unpredictable and dangerous. Pronghorn, unique to the plains of western North America, have horns in both sexes, while males also have black bands from their eyes to their snout and on their necks; they have keen eyesight and can sprint at nearly 50 miles per hour to escape predators. Bighorn sheep live in rocky, cliffy terrain, and the concave bottoms of their hooves make them sure-footed there; rams have distinctive curved horns that grow larger and more curled with age, while ewes have smaller horns that curve backward over their heads. Mule deer are named for their large mule-like ears and also have black-tipped tails; bucks have antlers, does do not, and mule deer are browsers that eat more leaves, twigs, and shrubs than grasses. Wolves, reintroduced into Yellowstone in 1995, have made a dramatic recovery; they live in family groups called packs and hunt elk, bison, moose, and smaller animals cooperatively within their territory, and they range in color from black to gray to white, with long legs, bushy tails, and short rounded ears; five to seven pups are born in the spring and are cared for by the entire pack. Coyotes, often seen in the park and often mistaken for wolves, are smaller and more slender than wolves, with larger pointed ears; they are most often seen alone or in pairs hunting mice and voles in open meadows, although cooperative pack hunting for larger animals like deer and elk also occurs in Yellowstone.