But wherever grass eaters travel, predators lie in wait.
Here they are, Arctic wolves.
They must seize their chance while the caribou pass through their territory.
The wolf runs at the herd, trying to flush out the weak or the slow.
A calf is separated.
At full tilt, 40 miles an hour, the wolf is just faster.
But the calf has stamina.
Only a few weeks old, and this calf's will to survive is remarkable.
And it needs to be, for these young caribou have now started a journey that will last a lifetime.
Forever chasing the seasonal growth of the grass on which they depend.
Like all grassland creatures, they are at the mercy of these unpredictable but ultimately bountiful lands.
Grass can survive some of the harshest conditions on earth, flood, fire and frost, and still flourish.
So it is that grasslands provide a stage for the greatest gatherings of wildlife on planet earth.