As we follow the grip of winter over the course of six freezing months, we chart the fortunes of Yellowstone's wildlife in a finely balanced fight to survive. Bison and elk struggle to feed with the grass buried under some of the deepest snow in America, while the wolf grows stronger during the harsh winter months. All of their fates are ultimately linked to the power of Yellowstone's volcanic heart.

Life on the edge of a frozen sea is tough. Pack-ice at both poles is constantly on the move, and in winter freezes solid. Only in spring, does life begin again. Plankton blooms and feeds vast hordes of migrating fish, birds, whales, seals and polar bears. Walruses rake the seabed for clams. But it is a brief respite, for the ice soon returns.

Steve Leonard investigates the extremes of cold climates. On a snowy Japanese mountain at -15C, he strips off. Before he freezes to death, he sinks into a scolding volcanic spring with some new bath-time buddies, a troop of Japanese macaques also trying to stay warm. In South Georgia, Steve huddles with hundreds of king penguins. Steve also looks at how the blubbery body of the bearded seal allows it to survive the super-cooled Arctic waters.

The Arctic and Antarctic experience the most extreme seasons on Earth. Time-lapse cameras watch a colony of emperor penguins, transforming them into a single organism. The film reveals new science about the dynamics of emperor penguin behaviour. In the north, unique aerial images show a polar bear swimming more than 100km. Diving for up to two minutes at a time. The exhausted polar bear later attacks a herd of walrus in a true clash of the Titans.

In 2004, a team from the Planet Earth series captured the first ever film of a wild snow leopard in the mountains of Pakistan. For Nisar Malik, who led the expedition, these images sparked a passion that compelled him to return. With cameraman Mark Smith, he spent two years documenting the snow leopard's daily life, finally lifting the veil on the most elusive of all cats.

The summer melt of Arctic ice provides opportunities for millions of animals, including narwhal, beluga whales, families of Arctic foxes and vast colonies of seabirds. But for polar bears it's the toughest time of year. The melting ice makes it harder for them to hunt and threatens their survival. In the river shallows hundreds of Beluga whales gather to rub themselves on smooth pebbles in one of the most bizarre summer spectacles. While Guillemot chicks take their first flights from precipitous sea cliff nests to the sea 300 metres below.