In the northerly reaches of the boreal forest, a strange phenomenon illuminates the night sky.
在北方森林的北部，一种奇怪的现象照亮了夜空。
The Aurora borealis.
北极光。
Electrically charged particles from the sun collide with the Earth's atmosphere, creating this otherworldly display.
来自太阳的带电粒子与地球大气层摩擦，创造了这个超凡脱俗的光影展示。
Viewed from space, the aurora brings light to the frozen poles, which otherwise would remain in darkness for almost three months.
从空中鸟瞰，极光为冰冻的两极带来光明，否则极地将在黑暗中停留近三个月。
This is the Arctic tundra, where temperatures can fall to -50 degrees Celsius.
这是北极苔原，温度可能会降至零下50摄氏度。
With little or no shelter, few animals can survive the winter here. But some do.
几乎没有庇护所，很少有动物能在这里过冬。但有些动物能够做到。
Muskox. Relics from the last ice age.
麝牛。上一个冰河时代的遗迹。
Two layers of fur protect them from temperatures of -30 degrees.
两层毛发能够保护它们免受零下30度的酷寒伤害。

This allows them to give birth in early spring.
这使麝牛可以在早春分娩
At six hours old, a calf is already able to stand, ready to follow the herd in the search for fresh grazing.
出生六小时的小牛已经能够站立，准备跟随牛群寻找新鲜的牧草。
But until they gain in strength, they are very vulnerable.
但在获得力量之前，小牛非常脆弱。
They're being stalked. Grizzly bears.
牛群正在被跟踪。一只灰熊。
Weighing up to 300kg, they have emerged after six months of hibernation, and are extremely hungry.
体重达300公斤，它们在冬眠六个月后出来了，而且非常饿。
One gets to within just ten metres, and then attacks.
一只熊就在十米之内，然后攻击。
Separated from their mothers, the newly born are unable to distinguish friend from foe.
小牛与母亲分离，刚出生的小牛分不清是朋友，还是敌人。
The bear instinctively kills every calf it can. Only a few escape.
熊本能地杀死了它可以杀死的每一头动物。只有少数小牛能够逃脱。
But the herd is now far away, and he is too weak to follow.
但是现在牛群已经走远了，它太虚弱了，无法跟随。
90% of calves do not survive their first year.
90%的小牛活不过第一年。
Even one without injury stands little chance without its mother. Reunited.
即使没有受伤的小牛，如果没有母亲，也几乎没有生存机会。重聚！
If just a few calves survive to adulthood, the future of the herd will be secure.
如果只有几头小牛能活到成年，牛群的未来，也将是有保障的。