英语听力：自然百科 动物大迁徙

Every fall, this herd of pronghorn antelope treks more than 100 miles from Grand Teton National Park to their winter habitat in the Upper Green River Valley of Wyoming. This phenomenal migration, one of the longest in North America, has been taking p

英语听力：自然百科 大迁徙之繁衍生息 Need to Breed—8

In the war-racked country of Sudan, lies Boma National Park, the largest intact savanna in East Africa. Mile upon mile of what looks like pristine wilderness unfolds. But much of this is an empty paradise. During the Sudan's decades of ruinous civil

英语听力：自然百科 大迁徙之繁衍生息 Need to Breed—7

But they can't migrate to greener pastures until the last of the youngsters has been winged. So mother has become stingy and edgy. Readying for the migration ahead, she tongues oily secretions over every bit of her flight equipment. A bat's wings are

英语听力：自然百科 大迁徙之繁衍生息 Need to Breed—3

From tip to tip, its wings stand eight feet. This superb blighter can soar almost e* 24 hours a day. Waiting in this bustling, the troublous of hundreds of thousands of birds, her mate shores up the nest from last year's breeding. And after seven lon

英语听力：自然百科 大迁徙 Great Migrations—10

A quarter of a million wildebeest will die in their yearly odyssey. In the sun burnt Serengeti, hunger keeps the wildebeests thundering northward, following the rain. A collective intelligence keeps the herd on track, neighbor leading neighbor, along

英语听力：自然百科 大迁徙 Great Migrations—9

Females travel in family groups, sometimes leading their young on yearly wanderings of up to 20,000 miles. Each day, they gather to reaffirm their bonds. They seem to crave physical contact. Finally, the mothers disperse to feed in shifts. They leave

英语听力：自然百科 大迁徙 Great Migrations—8

For the delicate monarch, it may as well be a trip to the moon. And yet somehow they do it, their tiny bodies attuned to a seasonal sundial and resonating to the invisible signature of the earth's core--its magnetic field. Around the planet, animals,

英语听力：自然百科 大迁徙 Great Migrations—7

Then the brilliant masses take to the wing, heading to a place they've never been. Their only map, imprinted on their genes. A mysterious interplay of sun on eyes and antennae, an uncanny sense of magnetic north guides the way. It will take five mont

英语听力：自然百科 大迁徙 Great Migrations—6

Ravenous triggerfish seize the luckless, an unceremonious end to an epic quest. For the surviving males, the odyssey has just begun. The females will arrive within days, urged on by celestial forces demanding they reach the males and the sea in time.

英语听力：自然百科 大迁徙 Great Migrations—4

Only the fit and the lucky will inherit the relay race of their lives. It is a race that is increasingly endangered like many of the world's great migrations. Climate change disrupts weather patterns and exploding human populations encroach on their

英语听力：自然百科 大迁徙 Great Migrations—2

What on earth could drive this headlong rush to slaughter? Only earth herself with her insistent clocks. This story begins in the Great Rift Valley, where the continent is tearing itself apart and being reborn. The plains in the shadow of the rift, t

英语听力：自然百科 大迁徙 Great Migrations—1

They're born to move, to move or die, forging ahead against terrifying odds, navigating the first of millions of miles, spanning continents on fledgling wings, clattering ever onwards. Right now, our planet is on the march, on the wing, on the run, a

英语听力：自然百科 大迁徙 Great Migrations—12

It takes four days for the larva to emerge. It gorges on its own egg case, before turning to tender milkweed. It must grow fat, build up its reserves for the long migration ahead. After two weeks of manic feasting, its body mass now 2,000 times what

英语听力：自然百科 大迁徙 Great Migrations—13

On Christmas Island, a different kind of home-coming preparation has begun. The males, though exhausted by their long pilgrimage, are readying snug burrows before the demanding females arrive in search of nursery-ready homes. Having dipped in the oce

英语听力：自然百科 大迁徙 Great Migrations—14

Many will drown in their frenzy to launch their eggs seaward. Most years, only a handful of their offspring survive. But sometimes, if wind and sea and luck are with them, a tidal wave of tiny crablings will return to these shores. There is no way to

英语听力：自然百科 大迁徙 Great Migrations—15

Soon there may be violence as males joust to win female affections. But for now, everyone enjoys a touching reunion. The calf, out of his depth among these goliaths, returns to the mentoring eye of his mother. He may not leave her for another 10 year

英语听力：自然百科 大迁徙 Great Migrations—16

Finally, they can rest. For another five months, they will cling to this winter refuge, entering a sort of waking trance of energy conservation. Until the longer days of spring awaken them again to mate and pass along to the young--their migratory im

英语听力：自然百科 大迁徙 Great Migrations—17

The merciless yellow crazy ants take a terrible toll. But by sheer force of numbers and the inexorable will of their genes, they prevail. That is the fierce logic of migrations. But what is logical for the many can be madness for the few. At the Mara

英语听力：自然百科 大迁徙 Great Migrations—18

But there is desperation here for the crocs well. Eat well now, and they may survive leaner times for months even a year. It's easy to forget that death is as vital to the herd as life. The young emerge from this brutal baptism stronger and wiser. Ba

英语听力：自然百科 大迁徙 Great Migrations—11

It's a grim end to a short life, but many of the surviving calves will face a more terrifying death. In the Mara River, another migration is culminating. Crocodiles have memorized when and where the wildebeests will try to cross. They have gathered i