loosens its grip on the land.

This is the tundra.

Open, windswept hills too cold for trees to grow.

It's late summer now.

The muskoxen shed their warm, woolly under-layer.

[bird chirping]

The males prepare for the rut as last season's

calves stay out of the way.

With hormones pumping through their veins,

the moody males go through the motions of battle

and practice their technique.

Their headgear creates a natural crash

helmet and their thick horns and skulls

act like shock absorbers.

[snorting]

As the males face off, they fight to establish dominance.

The lower bulls soon learn to back down.

Then the big boys can get serious.

[snorting]

The rest of the herd clears the arena

and makes the most of the lush and ice-free open meadows.

[music playing]

[birds chirping]

[snorting]

[dramatic music]

Charging at 25 miles an hour, the impact of the muskox

bulls echoes out across the tundra--

[bang]

-- while the mother muskoxen keep

their calves safe and well-fed before the weather turns again.

[birds chirping]