The Wildlife of Yellowstone | BBC Earth

As it gets colder, one animal here gets stronger. Wolves. The winter is their time. Gradually, it weakens their prey. This is the druid wolf pack. One of the largest and most powerful in Yellowstone. The pack have this bull elk surrounded. But there's a problem. The pack won't follow the bull into the river. They won't risk freezing to death in the ice cold water. What's more, now the elk's antlers are at just the right height to keep the wolves at bay. It's stalemate. But it's now the elk that has a problem of his own. Although it's only knee deep, he can't stay in this freezing water forever. A young female is not prepared to let him go. But the elk is strong. One on one, he has the advantage. Her only support is another youngster. They are neither strong or experienced enough to bring this elk down. But it's enough to make him turn and run. Back to the river, where he knows they won't follow. But the longer he stays in the freezing water, the weaker he will get. Others before him have waited here too long. And wolves are patient. Right now, his strength is his only advantage. He has to try again. This time, even the young wolves stay put. Without the support of the pack, they never really stood a chance. And the pack have already decided that this early in the winter, a bull elk in his prime is just too strong. But as the winter gets colder and the snow gets deeper, the tables will turn. On the open plateau, right in the middle of this frozen volcano, is an animal that has lived here since the last ice age. Bison are exposed to the worst of the Yellowstone winter. But they are built for it. Their thick coat is such good insulation, that they only need a tiny amount of energy to keep warm. So they slow their metabolism right down and concentrate on feeding. With massive neck muscles, they sweep their heads down through the snow to get to the grass beneath. But the grass has long ago put its summer goodness down into its roots, and now has about the same nutritional value as cardboard. They will need to do all they can to save energy if they are to ward off starvation until spring returns. As the winter strengthens its grip, elk move into more sheltered valleys at the edge of Yellowstone. They don't have the bison's ability to move deep snow. But this brings them into the territory of the druid pack. As the grazers are beginning to weaken, life for the wolves is getting easier. They are now successfully hunting about twice a week. They even have the energy to play. But their play has a purpose. It fine-tunes their hunting skills and helps bond the all-important pack structure. Though there are 16 of them, they can only hunt an animal as large as an elk if they hunt as one. The strength of the pack is what will get them through the winter. Bald eagles spot carcasses from miles away. But there is strong competition for a kill like this. A coyote. He has been shadowing the wolves and moves in now they have gone. It's now February, and when almost everything else in Yellowstone is on its last legs, the druid pack is reaching peak condition. And it's now that the young females come into season. Hanging back from the pack is a lone male wolf. He has no territory of his own, but follows the pack, scavenging from their successors. But right now, food is not his priority. The young females won't mate with the pack's alpha male, as he is their father. So the intruder could well be in with a chance, as long as the alpha doesn't see him. Whilst the pack are distracted, one female sneaks away. She won't give up the security of the pack for him, so they meet close by in secret. But the pack are now coming their way. The alpha male is on to him. He won't tolerate any other male in his territory, let alone with one of his females. When wolves mate, they become locked together for up to half an hour. The intruder can't break free. For now, the alpha male has done enough. He's seen the intruder off, and he needs to return to reassert his position in the pack. The intruder retreats to a precarious life in the shadows. But whatever happens to him, so long as he was coupled for long enough, he will have young, brought up in the security of one of the strongest packs in Yellowstone. These clouds don't signal rain. They are vast smoke plumes. Yellowstone is burning. Throughout the summer, the dry lodgepole pines have become like a tinderbox, and lightning has struck the match. The worst wildfires will burn for weeks. In 1988, a third of Yellowstone burnt in a single summer. Devastation. Animals that depend on these forests will starve this winter. But Yellowstone itself has a longer perspective. Ashes fertilize the soil, and fire opens it up to sunlight. As the forests regenerate, new life finds opportunity. In August, fledgling hummingbirds gorge in fields of fireweed that have risen from the forest's ashes. For them, the summer is already nearly over. They must chase the sun south before winter returns. The forest may be parched, but there is still water here, in lakes formed by ancient glaciers. A visiting male otter woos a female on the lake's shore. But she seems a little distracted. Her two pups are nearby. They are about 10 weeks old, and the time is right to lead them from the holt. Males are not above stealing food from cubs, so she makes it very clear that he is not going to be joining them. This female grew up on the lake, so she knows where to take the cubs for a very special trip. Yellowstone cutthroat trout are massing in the mouth of a stream that feeds the lake. They are preparing for their annual spawning. But the trout must wait. The stream is still running fast, too turbulent to lay their eggs. It's a nervous time for them, and for good reason. This osprey has a brood to feed, and will be back. Now the otters are here too. She chooses her moment to lead the pups upstream. Then leaves them to watch, and gives them a lesson in hunting. A large trout like this, full of eggs, is a trophy catch. She chews its tail first, to make sure the fish can't get away. While the pups are still demanding milk, the mother otter will often keep the whole fish for herself. Her pups are happy enough with caviar. As the flow becomes slow enough for the trout to lay eggs, there is a brief window of opportunity to spawn, before the river gets too shallow to swim upstream. The female digs a trough with sinuous flicks of her body, and the attending male fertilizes the eggs as they are laid. Millions of new lives. Yellowstone sits above the molten core of a volcano. Underground water becomes superheated and erupts into the air. Strangely, geysers and hot springs can creep around the landscape. The curious shapes of grotto geyser are actually trees, entombed and petrified by minerals dissolved in the water. Hot water scorches and drowns the roots of trees encroaching on the valleys. It can keep the forests in check, and the rich pastures open. Those pastures now produce the new grass needed to make rich milk and feed a bison baby boom. The few calves born on the migration are now joined by hundreds of new youngsters. The predators have been waiting for this. Coyotes have a voracious reputation here, but on its own this one is certainly no match for a healthy bison and her calf. Pronghorn give birth to twins, each carefully cleaned to hide their smell from predators. While they are with her, they are relatively safe, and for now the coyote is content with the afterbirth. But the mother pronghorn must leave her calves alone while she grazes, and with legs as wobbly as stilts, their best defence is to lie low. She has young of her own to worry about, a demanding litter of nine, whose arrival has been carefully timed to coincide with an abundance of food. But they are not top dog here. Yellowstone's top predator, the wolf, dominated during the winter, but is far less visible in summer. They prefer to stay close to well-hidden dens to protect their pups from rival packs. This pack seem prepared to allow their young a brief foray to experience the world beyond their den. Even in Yellowstone, it is extremely rare to see wolf pups like this. Before long, they are summoned back into the forest by the pack. It will be winter before they are regularly seen again. For the last six weeks of strutting and herding, male elk have eaten almost nothing. They are exhausted. This bull has done well. He has successfully held on to his harem, and now the females are finally coming into season. But they are being distracted by another male. If a bull elk can't dominate all rivals, he can't have access to the females, and all his effort will have been in vain. Now he must gather the last of his strength. The rival wants to take him on. The aim is to get an antler point into his neck. But they are evenly matched. Neither can penetrate the other's guard. Now it's all about power. A well-aimed thrust or a broken neck will kill. This challenger is lucky to get off with just a parting stab in the rump. The victor returns to his females. His young will be born next spring. But the prospects are not so good for a defeated bull. After all this effort, he will now have to wait until next autumn to try his luck again. That's if he even makes it. Bull elk, exhausted by the rut, struggle to survive the Yellowstone winter. It's now October, and the winter is catching up with the elk once more. An early flurry of snow is a sign that it's time to make a decision. To stay is to face the certainty of snow and wolves. To go offers the chance of an easier life, but the uncertainty of the world beyond Yellowstone. It's now November, and the elk have found their feeding grounds just in time. The snows of winter at their heels. Here they join other herds who come to this place every year. Where the snow will be less deep, and life a little easier. But today they graze on an island of grass, surrounded by development. As they run from the winter, the fate of Yellowstone's animals outside the national park is decided not by the cold, but by people. These elk are lucky. This refuge has been kept aside to give them some degree of sanctuary. So although the park isn't big enough to protect all its animals all the time, its influence can spread beyond its boundaries. And if even ranchers can come to tolerate wolves, then anything is possible. In the mountains of Yellowstone, where the elk's bugle signaled the beginning of autumn just two months ago, all seems deserted. But now, the final act of the season is about to take place. From out of apparently nowhere, come the bighorn sheep. The toughest of all Yellowstone's animals, they can stay here all winter on slopes and crags that the biting wind keeps clear of snow. Now they are coming together to rut. Like elk, the males battle for females. But where elk do their best to avoid fights, bighorn relish them. A quick test of horn size and of other important bits of anatomy, and the males get straight to the point of sorting out who is toughest. As the sound of their battles echoes across the Yellowstone wilderness, it marks the end of autumn. A beaver can fell a cottonwood tree in just a few hours, hundreds in a year. The beaver doesn't chew through the whole trunk, just enough to make the tree unstable. It then retreats and lets the wind do the rest. It cuts branches into more manageable lengths, and then swims them down a network of purpose-built canals towards the dam. The pond gives this beaver protection from predators, and the canals allow it to forage far into the forest, carrying many times its own weight with ease. Autumn is the busiest time of year for beavers. It won't be so easy to make repairs when the pond is frozen over. The sound of running water is their stimulus to shore up gaps with timber, and plug leaks with mud. But the dam not only serves the beavers. Moose come here from the forests around to feed on weeds that thrive in the beaver's shallow pond. The weed is rich in vital sodium that the forest can't easily provide. But now that winter is approaching, another essential role for the dam is revealed. These smaller branches are not for fixing the dam. They're for eating. The beaver secures them to the mud in the lake bottom. In just a few weeks, this lake will be frozen, and the beavers won't be able to cut and move trees. But they will be able to swim right under the ice to feed from this underwater larder. Moose also eat twigs and branches, and often try to take advantage of the beaver's hard labour. This young male is getting a little too close to the beaver's larder. Autumn is not a time for sharing.