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The hungry wolves are not far behind.
饑餓的狼群就在后面不遠。
For the mother, it's too late and the herd must keep moving.
對媽媽們來說，現在為時己晚，馴鹿群必須繼續前行。
Of the tens of thousands of calves born in the tundra each year, only half will survive.
每年在幾萬頭苔原上出生的小馴鹿當中，只有半數能夠存活。
Even in summer, the fight for survival goes on.
即使在夏季，為生存而進行的斗爭也在繼續。
For the North American musk oxen, this is rutting season.
對北美麝牛來說，這正是求偶的季節。
These giant remnants of the last ice age have hardly changed in over fourteen thousand years.
這些最后一次冰河期的幸存者，在超過一萬四千年的歲月里幾乎沒有改變。
Males weigh almost five hundred kilos engaging bone-crunching battles for their right to breed with the best females.
公麝牛重達近五百公斤，為了爭奪與最好的母牛交配的權利而展開激烈的搏斗。
Their brains are protected by seventeen centimeters of skull and bone.
它們的大腦有厚度達到十七厘米的頭骨保護。
But if neither animal backs down, these epic duels can escalated to a fight to the death.
但如果雙方都不肯退讓，這種巔峰對決能變成生死之戰。
The musk oxen are one of the tundra's hardiest animals. But one mammal has mastered the tundra like no other.
麝牛是苔原上最頑強的動物之一，但另一種哺乳動物卻以不同尋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，它們以驚人的速度進行繁殖。這只雌性一個冬天就己生了兩窩幼鼠。
But she is heavily pregnant once again and is now preparing a nest to give birth once more.
而它現在又一次懷孕了，正為再次分娩準備著營造一個窩。
They have a gestation period of just three weeks and these newborns will be pregnant when they're just one month old.
它們的妊娠期只有短短三周，而這些新生的幼鼠也會在僅一個月大時就能夠懷孕。
There are five or six in each litter, and they're born deaf and blind.
每窩幼鼠有五到六只生下來又聾又瞎。
But they grow remarkably quickly on the diet of milk and constant attention from their mother.
但是它們在母乳的哺肓和母親持久的悉心照料下成長得極為迅速。
The young are covered in fur within days, and with such a startling reproductive rate, lemming populations can explode within weeks.
不出幾天幼鼠們全身就己長滿了毛發，而因為有如此驚人的繁殖速度，旅鼠的數量會在短短幾周內暴增。
Thanks to their sheer numbers and high visibility, lemmings are hunted by virtually every predator in the tundra.
由于它們數量驚人而且很容身被發現，旅鼠成為苔原上各種掠食者獵殺的目標。
Their underground tunnels are no longer hidden by snow and they're now at their most vulnerable.
如今它們的地下暗道也失去了積雪的掩護，所以它們現在變得最身遭受襲擊。
The number of predators on the tundra is also on the increase.
苔原上掠食者的數量也在不斷增長。
