The huge male polar bear patrols nearby.
一头庞大的公熊在附近徘徊。
The bear knows that whales must come up for air every twenty minutes.
它知道白鲸每隔二十分钟必定会探出水面进行呼吸。
The polar bear is the largest land predator on the planet and males can weigh up to eight hundred kilos.
北极熊是地球上体型最大的陆地掠食者，而公熊的体重可以达到八百公斤。
Each beluga weighs almost twice that.
每头白鲸的体重几乎是它的两倍。
If he can succeed in catching one, a beluga would provide this bear with the substantial meal.
如果它能够成功地捕捉到一头，那么就会是一顿丰盛的美餐。
But a four-meter long whale is a challenging prey to catch.
不过体长四米的白鲸却并非是唾手可得的猎物。
The bear rolls in the snow to remove salt crystals from its fur.
北极熊在雪地上打着滚以去除粘在它毛皮上的盐晶。
Salt makes the fur less effective insulated.
盐会使毛皮的隔热保暖功能下降。
These belugas have been attacked by many bears over the last six months. Some have been caught.
这些白鲸在最近的六个月里曾经遭到许多北极熊的袭击。有些甚至被捉到。
Belugas have a layer of blubber up to fifteen centimeters thick.
白鲸身上有一层厚达到十五厘米的鲸脂。
For the bear, this catch brings a meal rich in energy.
对北极熊来说这样的美餐能带来充足能量。
At this time of the year, gulls depend on scavenging from polar bear kills to survive.
每年的这个时候海鸥们都依靠捡食北极熊捕食后的残羹剩饭而生存。
Soon, the ice covering the Arctic Ocean begin to melt.
很快覆盖北冰洋的冰层开始融化了。
Only then will the belugas have a chance to escape to open water.
直到这时白鲸们才有机会逃往开阔水域。
It's mid March in the Arctic, and the days are growing longer.
时值三月中旬，北冰洋白昼逐渐变得更长。
In Svalbard, the mother bear and her cubs are emerging from their winter den.
在斯瓦尔巴德群岛，母熊和孩子们从过冬的洞中探出身来。
The cubs are now three-month old and they weigh over ten times as much as they did when they were born.
小熊们现在三个月大了，而体重也已经超过了刚出生时的十倍。
Rolling in the snow helps clean her fur, but the whole family seems to be enjoying the fresh air and sunshine.
在雪地上打滚有助于清洁毛皮，而这一家子似乎很享受新鲜的空气与灿烂的阳光。
The bears will stay near the den for the next two weeks, returning to its warmth if the weather closes in.
它们还会在洞穴附近再呆上两周以便在风暴来临时回到温暖的洞中。
Adult polar bears have perfectly adapted to life in this extreme environment.
成年北极熊已经完美地适应了这种极端环境下的生活。
Underneath the dense white fur, the mother bear skin is black to help absorb the meager heat of the Arctic sun.
母熊浓密的白色毛皮下面是黑色的皮肤，这有助于吸收北极非常稀少的太阳热量。
And beneath the skin, lies a layer of fat more than ten centimeters thick which helps maintain body heat.
而在皮肤下面则是一层厚度超过十厘米的脂肪，有助于维持体内热量。
But at this young age, the cubs have very little fat to insulate them.
不过小熊们年龄还太小体内脂肪也很少，不足以保持热量。
They rely on the energy of their mother's milk to keep them warm. But she hasn't eaten for months.
它们要依靠母乳供给的能量来保暖。但母熊已经数月没吃东西了。
Surviving only on her fat reserves, she has lost a third of her body weight.
它仅仅依靠体内贮存的脂肪生存，因此体重已经下降了三分之一。
Very soon, she must head for the rich hunting grounds of the sea ice.
不久后，它就必须前往海冰上富饶的猎场捕食了。