英语听力：从南极到北极

They are constantly on the move. New-born 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, it’s 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, theodds are even—either the caribou will make a mistake or after a mile, the wolf will give up.