Yellowstone, volcanic wonderland. Two million acres of wild space right in the heart of North America. The heat of the summer has unveiled the full extent of the Yellowstone wilderness and, for a few precious months, it has blossomed. But, now, Yellowstone is changing. In just a few weeks, the snow and ice of winter will be back. As the animals of Yellowstone now turn to face perhaps the biggest challenges of their year, the true value of the world's first national park is about to become clearer than ever. It's late August and on Yellowstone's peaks there is already a dusting of fresh snow. Now, a new sound marks the new season. Fuelled by testosterone, the bugle call of this male elk is a boast of his strength. All over Yellowstone, male elk are challenging each other for dominance. The sound of Yellowstone's autumn. They are trying to win the admiration of females and gather them into a harem. Only then do they stand a chance at mating with them before winter. But the females are not yet in season, so they are not really that interested. And, for now, they have a more practical concern. Winter will soon be here. They are eager to head down to lower ground before the snow comes in earnest.