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For millennia, caribou have made annual migrations.
数千年来，北美驯鹿都会进行年度迁徙。
Every summer, driven by the seasonal changes of the far north,
每年夏天，因远北地区季节转变的驱使，
and following long-established pathways, they have trekked almost 400 miles north.
循着早已确定的路线，它们已往北跋涉将近四百英里。
This herd alone in north-east Alaska numbers over 200,000.
光是在阿拉斯加东北部的这一群数量就超过二十万头。
Their destination is a vast wilderness known as the arctic national wildlife refuge.
它们的目的地是一片无垠荒野，也就是所谓的北极国家野生动物保护区。
These summer pastures provide rich grazing, just what's needed to raise newborn calves.
这片夏季草场提供了丰富的食物，正好能满足养育新生小驯鹿所需。
This one, fattening up on its mother's milk, would normally double its weight in just a few weeks.
还在依靠母乳长胖的这一头，通常只要几个星期体重就能翻倍。
But now grazing time is being disrupted.
可是如今吃草的时间受到了干扰。
In summer, the tundra becomes waterlogged.
在夏天，冻原变成泡在水中。
These are excellent breeding conditions for mosquitoes and other biting insects.
这种环境最适合孳生蚊子和其他会叮咬的昆虫
But as the climate warms, there are concerns that insects will breed earlier
气候暖化的同时，也有昆虫提早繁殖的忧虑，
and eventually reach plague proportions just as the caribou need to feed.
最终在北美驯鹿需要进食时达到致灾量。
Individual animals could lose a quarter of a pint of blood a day to biting insects.
一个动物一天可能因为昆虫叮咬而失去四分之一品脱的血。
So the caribou may end their grazing early and head for cooler, mosquito-free pastures.
因此北美驯鹿可能提早结束进食，前往较凉爽、没有蚊虫的草场。
But to get to such places, they will have to cross rivers swollen by seasonal runoff.
然而若想要抵达，它们必须渡过因季节性迳流而上涨的河水。
For a young calf, a deep river crossing is very dangerous.
对于幼鹿来说，渡过深河非常危险。
He does what he can to stay close to his mother.
它想尽办法紧跟在母亲身边。
But he doesn't have the strength to resist the fast-moving current.
但是它的力量不足以抵挡快速的水流。
He is one of the lucky ones. Reunited with his mother, he can continue on his journey with the herd.
它是幸运的一个。与母亲重逢的它，可以跟着鹿群继续旅程。
But other calves are less fortunate... and must now fend for themselves.
可是其他幼鹿就没这么幸运，现在必须自己设法谋生。
Some are too exhausted to take another step.
有些已经太过疲累，无力再走一步。
But this is not a safe place to take a rest.
但这里不是能安全休息的地方。
