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Each spring, animals travel up from the south to be ready for the rich grazing that will be unveiled by the spring melt. For the caribou the timing is critical, arrive too early and a winter storm could kill you, delay it too long and you may fail to lay down the fat needed to survive a polar winter.

Further south still, and stooped, shrouded figures end the flat monotony of the tundra.

This is the tree line, the first place on our journey with sufficient warmth and liquid water to enable a tree to grow. Surviving here is so crushingly difficult that it can take hundreds of years for a seedling to grow into a stunted shrub. But, even small trees can provide cover for a predator. - Wolves.

These, in northern Canada are the largest and most powerful wolves in the world, and they are setting out to hunt. The pack is 25 strong, a sign that the prey they are seeking is formidable. Bison.

Bison Herded by Wolves

These Bison are even bigger than their southern cousins, the largest land animals in North America. For generations Wolves and Bison, here, have been shaped by their battles with each other, making each the most impressive of its kind..

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The wolves are closing in, but their chance of ambushing the bison in the woods has passed, their prey are now in the open and grouped together for safety. The wolves will need to work together as a team if they are to make a kill.

They circle the herd trying to unsettle it and split it up. But, the bison are armed and dangerous, they will be safe so long as they stick together. The wolves up their game, harrying the herd, trying to trigger a stampede and split away the young. The bison form a defensive circle around their young with horns pointing outwards.

The wolves need a bison to break rank, but the tables are turning and the wolves have to retreat. The pack focus their attention on the rear of the herd and the bison begin to panic. A young bison falls behind. Even this yearling dwarfs the wolves. Running heads down, the herd's only thought, is escape. A stroke of luck for the wolves, the yearling is trampled by the stampeding herd..

The kill will feed the pack for several days. But, then they will have to resume the chase.