PLANET EARTH From Pole to Pole
Summer brings 24 hours of sunlight and the thawing（化冻） shifting landscape. Further south the winter snows have almost cleared from the Arctic tundra（冻土带）. Northern Canada's wild frontier. Here nature stages one of her greatest dramas - Every year three million caribou（北美驯鹿） migrate across the Arctic tundra. The immensity of the herd can only be properly appreciated from the air. Some herds（兽群） travel over 2,000 miles a year in search of fresh pastures（牧场）.
This is the longest overland migration made by any animal. They're constantly on the move. Newborn calves have to be up and running the day they are born. But the vast herds do not travel alone. Wolves. Packs of them, eight to ten strong, shadow the migration. And they are hungry.
It's the newly born calves that they are after. Running directly at the herd is a ploy to generate panic. The herd breaks up and now it's easier to target an individual. In the chaos a calf is separated from its mother. The calf is young, but it can outrun the wolf if only it manages to keep its footing. At this stage the odds are even - either the caribou will make a mistake or after a mile the wolf will give up.
