The hungry wolves are not far behind.
饥饿的狼群就在后面不远。
For the mother, it's too late and the herd must keep moving.
对妈妈们来说，现在为时己晚，驯鹿群必须继续前行。
Of the tens of thousands of calves born in the tundra each year, only half will survive.
每年在几万头苔原上出生的小驯鹿当中，只有半数能够存活。
Even in summer, the fight for survival goes on.
即使在夏季，为生存而进行的斗争也在继续。
For the North American musk oxen, this is rutting season.
对北美麝牛来说，这正是求偶的季节。
These giant remnants of the last ice age have hardly changed in over fourteen thousand years.
这些最后一次冰河期的幸存者，在超过一万四千年的岁月里几乎没有改变。
Males weigh almost five hundred kilos engaging bone-crunching battles for their right to breed with the best females.
公麝牛重达近五百公斤，为了争夺与最好的母牛交配的权利而展开激烈的搏斗。
Their brains are protected by seventeen centimeters of skull and bone.
它们的大脑有厚度达到十七厘米的头骨保护。
But if neither animal backs down, these epic duels can escalated to a fight to the death.
但如果双方都不肯退让，这种巅峰对决能变成生死之战。
The musk oxen are one of the tundra's hardiest animals. But one mammal has mastered the tundra like no other.
麝牛是苔原上最顽强的动物之一，但另一种哺乳动物却以不同寻常的方式征服了苔原。
Lemmings. They breed at an astonishing rate. This female give birth to two litters during the winter.
旅鼠，它们以惊人的速度进行繁殖。这只雌性一个冬天就己生了两窝幼鼠。
But she is heavily pregnant once again and is now preparing a nest to give birth once more.
而它现在又一次怀孕了，正为再次分娩准备着营造一个窝。
They have a gestation period of just three weeks and these newborns will be pregnant when they're just one month old.
它们的妊娠期只有短短三周，而这些新生的幼鼠也会在仅一个月大时就能够怀孕。
There are five or six in each litter, and they're born deaf and blind.
每窝幼鼠有五到六只生下来又聋又瞎。
But they grow remarkably quickly on the diet of milk and constant attention from their mother.
但是它们在母乳的哺肓和母亲持久的悉心照料下成长得极为迅速。
The young are covered in fur within days, and with such a startling reproductive rate, lemming populations can explode within weeks.
不出几天幼鼠们全身就己长满了毛发，而因为有如此惊人的繁殖速度，旅鼠的数量会在短短几周内暴增。
Thanks to their sheer numbers and high visibility, lemmings are hunted by virtually every predator in the tundra.
由于它们数量惊人而且很容身被发现，旅鼠成为苔原上各种掠食者猎杀的目标。
Their underground tunnels are no longer hidden by snow and they're now at their most vulnerable.
如今它们的地下暗道也失去了积雪的掩护，所以它们现在变得最身遭受袭击。
The number of predators on the tundra is also on the increase.
苔原上掠食者的数量也在不断增长。