By May, the sun is growing in strength.
到了五月份，太阳光照在逐渐增强。
In the North American tundra the reindeer that have survived the extreme winter weather are hundreds of kilometers into their epic migration.
在北美洲的苔原上，在极度寒冷的冬季里，幸存下来的驯鹿们己在它们史诗般的迁徙之旅中跋涉了几百公里。
The porcupine herd are now approaching their calving grounds near Alaska's northern coast.
格兰特驯鹿群如今己临近了位于阿拉斯加北部海岸周围的繁殖地。
But the tundra still has many obstacles for the reindeer to overcome.
但苔原上仍有重重困难有待驯鹿们去克服。
This urge to migrate thousands of kilometers defines the lives of the reindeer.
这种进行千里大迁徙的自然本能支配着驯鹿的一生。
Much of the reindeer's food is still covered by snow, but that's all about to change.
如今驯鹿的大多数食物仍然在冰雪覆盖下，不过这一切即将改变。
Spring has arrived. Right across the Arctic, the planet's most dramatic change of seasons has begun.
舂天己经到来了，整个北极地球上最引人注目的季节更替开始了。
The sun is now high enough in the sky to raise the temperature above zero for the first time in months and the ice turns to water.
现在，天空中太阳的高度几个月以来首次足以使气温升至零度以上，冰融化为水。
The reindeer have finally reach their calving grounds.
驯鹿们终于抵达了它们的繁殖地。
It's time for the females to give birth in the same place they themselves were born.
现在是雌性们在自己出生的地方进行分娩的时候了。
In the incredible synchronized birth, almost half the young of the herd can be born within twenty four hours.
在这场叹为观止的分娩潮中，鹿群里几乎有一半的幼崽会在二十四小时内出生。
The calves can stand in minutes and, within a day, will have to keep up their mothers. As the herd continues north towards the summer grazing lands.
小鹿几分钟内就能站立了，它们必须在一天之内学会赶上母亲的脚步，而鹿群仍然在继续向北面的夏季牧场进发。
Once it stands, the calf will hardly ever stop moving for the rest of its life.
小鹿一旦学会站立，它一生中就几乎不会再停止前进的脚步。
Across the tundra, things are changing dramatically, even for the wolves who now have pups of their own.
整个苔原上的一切都在发生引人注目的变化，甚至连狼群现在也繁殖了自己的后代。
Their pups are just a week old, and far too young to join the hunt for reindeer.
它们的幼崽们现在只有一星期大，还远不能加入捕食驯鹿的行动。
The mothers too must stay put to feed the young pups. But the males are on the hunt.
妈妈们也必须留下来哺肓年幼的孩子，而雄性们则负责捕猎。
This pair have traveled many kilometers in search of the reindeer.
这群狼为了寻找驯鹿己经跋涉了很长的距离。
Ahead of them, there are thousands of new born calves, and if the wolves don't make a kill soon, their own pups will perish.
如今出现在它们面前的就是几千头新生的幼鹿，而如果狼群不尽快猎杀一些的话，它们自己的幼崽就会饿死。
But at this time of the year, it's not only the reindeer whose calves are target for the hungry wolves.
但在每年的这个时候，不仅a驯鹿的幼崽成为饥饿的狼群捕食的目标。
These Canadian musk ox calves are just a few days old and small enough to be overpowered by single wolf.
这些加拿大麝牛幼崽只有几天大，弱小得只要一头狼就能将其制服。
But with so many adults surround, each weighing nearly five hundred kilos, the odds are stacked against the wolves.
不过有这么多成年麝牛在四周保护，而每头都重达近五百公斤，形势对狼群是不利的。
The moment the musk ox spot the wolf, they instinctively form a defensive ring to shield their young.
麝牛一发现狼，它们就本能地组成防护圈保护幼崽。
It will take the entire wolf pack to take on this herd.
这将需要整个狼群的合力才能应对这个兽群。
