For six months Yellowstone endures an unrelenting winter. An ice world of hunters. And hunted. But now footsteps on the slopes of a mountain herald a change in the season. The sun regains its strength. Water flows again. The race begins to recover winter's losses. To play. Learn. And breed. But the summer ahead will be fleeting and far from easy in this unpredictable wilderness. It's April. The land lies exhausted by winter. Everything is waiting for the return of the sun's warmth. The end of winter comes late to the mountainous northwest of the United States. And in particular, it lingers in Yellowstone. The heart of Yellowstone is a high, cold plateau 8,000 feet up, surrounded by the spires of the Rocky Mountains. After five months sleeping in those mountains, a powerful presence returns. This female grizzly bear has awoken early to find food for her two-year-old cubs. But she may have brought her family here too early. The Tetons, on the extreme southern edge of the Yellowstone plateau. It's where the spring thaw normally begins. But this year it's come late. Deep snow is still a novelty for her cubs. But she is much more focused. Led by her nose, she risks her 300-pound bulk on the thin ice of a bend in the Snake River. It doesn't look promising but her cubs follow with only a little hesitation. With paws that can knock down an elk, she feels delicately in the water. A fish frozen beneath the ice by this cold winter. One fish won't feed the family but she knows there are almost certainly more.