NARRATOR：

Listen to part of a lecture in a biology class.

FEMALE PROFESSOR：

There are certain animals that live high in the mountains...

at altitudes up above the tree line, where trees don't grow.

And moving around in this mountainous environment can be challenging,

because the mountain slopes can be very steep and rocky....

Besides that, there's often ice or snow, making these rocky slopes slippery-and dangerous.

So animals that live at high altitudes have special features that help them move around in this difficult, mountainous environment.

So, for one thing,

many mountain animals have strong muscles that help them climb up and down the steep slopes.

As they're moving around, looking for food, they have to climb up these really steep mountainsides, at a dramatic incline,

and get down again, too.

For many animals this would be a real problem,

requiring great strength, and balance.

Um, mountain goats, which live high up in the mountains in parts of North America...

mountain goats have a large, well-muscled chest and front-leg area,

and these big, developed muscles in this area help them pull themselves up these near-vertical slopes,

and balance in tight positions when climbing down.

Also, mountain animals often have specially adapted feet

that help them keep from slipping when they're walking over ice or rocks.

High-altitude mountain slopes can be very slippery, especially when there's snow or ice on them,

and it's a long way down if they fall,

so mountain animals need to have a safe, sure footing.

Take the bighorn sheep, for instance....

These sheep have special toes that move independently

so they can dig into the rock and ice.

And the back parts of their feet have special round, rubbery pads that help them grip the surface of the rock or ice,

to prevent sliding as they move around.