TED演讲：重新定义字典(9)

So, enough scientists in other disciplines are really asking people to help, and they're doing a good job of it. 足够多的科学家在其它领域正寻求大家帮忙，而且大伙也干的不错。 For instance, there's eBird, where amateur bir

英语听力：自然百科 旱灾的成因与影响

旱灾的成因与影响 Its signs are subtle and slow: the earth dries; water levels fall; the rains do not come; and the land is gripped by drought. At its most basic, a drought occurs when more water is used than isreplenished. It is a balance bet

国家地理-2008-09-11 旱灾的成因与影响

Its signs are subtle and slow: the earth dries; water levels fall; the rains do not come; and the land is gripped by drought. At its most basic, a drought occurs when more water is used than is replenished. It is a balance between supply and demand,

SSS 2008-06-05

This is Scientific American's 60-Second Science, I am Karen Hopkin. This'll just take a minute. Biologists, traditionally track migrations by tagging animals with devices that allow their movements to be monitored. But Homo sapiens can't be fitted wi

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—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

美国学生人类历史第138期:封建制 度(1)

Chapter 31 Feudalism 第三十一章 封建制度 How central Europe, attacked from sides, become an armed camp 欧洲中部受到来自三个方向的敌人威胁，变成了地道的大兵营。 and why Europe would have perished without those prof