By Jonathan Amos
BBC News science reporter
It is an extraordinary climb even for the most able of mountaineers.
It can be a hazardous business on the scree slopes of the high Rockies
Every summer, grizzly bears clamber up the scree slopes of Wyoming's Rocky peaks in search of food.
At first sight, their rummaging among the shattered stones seems pointless.
Why would they waste their time in this barren landscape when the race is on to fatten up before the long winter hibernation?
But the grizzlies know these talus fields hide a rich meal: there are huge aggregations of moths resting in between the cool, jagged fragments.
The insects have flown up from the Great Plains to escape the heat of the prairies - only to find the mouths of hungry bears.
A grizzly (Ursus arctos horribilis) can eat 40,000 army cutworm moths (Euxoa auxiliaries) a day.
One study found that a bear feeding extensively on the insects over a period of a month could consume up to 47% of its annual energy budget of 960,000 calories.
"I've actually tried one of these moths myself," says Jonny Keeling, a BBC Natural History Unit producer. "They taste like a juicy little packet of butter."
Keeling has filmed the bear feast with Wyoming cameraman Shane Moore for the BBC's spectacular new series Planet Earth.
"This is probably the most comprehensive effort to film this behaviour," Keeling said. "I tried on three other occasions and failed on all of them, so it was nice to go back and succeed."
The bears are operating at 10-11,500ft (3,000-3,500m); they will have gone up a good mile to get to the moths. The slopes are sharp and large boulders frequently tumble from above.
"It's very remote; we had to pack in on horses and in one very sad situation it was so steep a mule fell off a cliff and died, despite all our precautions," recalled Moore.
Grizzly numbers are now holding up
"It's also very hazardous because you get these lightning storms that go by every afternoon and hit on the peaks. It's a pretty crazy place to film and see bears."
Planet Earth is the first natural history series to be filmed entirely in high definition (HD).
It is the only TV technology that could come close to conveying the true majesty of the featured landscapes - from the mightiest mountain to the deepest gorge.
"The HD cameras are bigger than the film cameras we used to use; they're heavier," explained Moore.
"These bears, despite their ferocious reputation, are actually very shy, so we mainly had to avoid being winded. At the start of every day, we'd work out which way the wind was blowing and hatch a strategy to get into a good place to shoot the bears."
The bears' story takes place over July and August. The moths have come up from the plains to mate and feed on alpine flowers. Those that survive the foraging grizzlies will return to the plains in late summer to lay their eggs.
A grizzly can consume thousands of moths a day
The resulting caterpillars will then battle with farmers who will do all they can to clear the army cutworms from their crops. And that, of course, has implications for the bears: some years the talus fields will offer meagre fare.
There is even concern that some of the chemical residues picked up by caterpillars in the crop fields may eventually find their way into the bears.
"To me, it is so stunning to see how connected the world is," said Moore. "What the grizzlies do in the peaks of the Rocky Mountains is dependent on what the farmers do on the Great Plains."
Planet Earth is the latest series to release material under the BBC's Creative Archive Licence. The scheme allows people within the UK to watch, download, edit and mix clips and programming for non-commercial use.
The new Open Earth Archive, as it is known, contains some content not featured in the final edit of Planet Earth.
The first episode of Planet Earth is broadcast on BBC One this Sunday at 2100 GMT. The grizzly bears are featured in the second episode on 12 March.