'WUUUGGH!' That's my penguin impression, which everyone's been thoroughly enjoying for the past week. Tonight, while the loveable penguins are no longer falling over or belly flopping onto dry(ish) ice, they're huddled together over their eggs, David Attenborough examines how the arrival of winter. With temperatures plummeting to -70C, winds reaching 120mph and complete darkness - sparks a ruthless battle for survival. As a blizzard rages, a female polar bear treks into the Arctic mountains to give birth, while the entire world's population of spectacled eider ducks braves the weather in a giant ice hole. Wolf and bison engage in battle, but alliances are also made, and beneath the snow, voles try to dodge the attacks of great grey owls and weasels.