S1, E2: As snows melt and winter turns to spring, temperatures rise and Yellowstone bursts into life. Beavers feast on new green shoots, baby bison take their first faltering steps, and grizzly bear cubs emerge from dens to explore this new green world. But, as this fascinating documentary shows, spring is also a perilous time. Wolves are hungrier than usual and only the toughest and wiliest survive.