Here is the original caption for the image.

"A massive pack of 25 timberwolves hunting bison on the Arctic circle in northern Canada. In mid-winter in Wood Buffalo National Park temperatures hover around -40°C. The wolf pack, led by the alpha female, travel single-file through the deep snow to save energy. The size of the pack is a sign of how rich their prey base is during winter when the bison are more restricted by poor feeding and deep snow. The wolf packs in this National Park are the only wolves in the world that specialize in hunting bison ten times their size. They have grown to be the largest and most powerful wolves on earth."