Polar Bears Can’t Just Switch to Terrestrial Food
“饿肚子”的北极熊

With a shorter season of sea ice, polar bears have less access to marine mammals. But switching to a terrestrial diet deprives them of the fatty seal meals they need to thrive.
随着北极海冰季节的逐渐缩短，北极熊捕食海洋哺乳生物的机会越来越少。然而捕食陆地生物并不能像猎食海豹一样为北极熊的生长发育提供足够的脂肪。

撰文/播音：贾森·戈德曼(Jason G. Goldman)
翻译：郭鑫鹏
审校：索红日

When the Arctic Ocean freezes over in the autumn, polar bears set off in search of their favorite meals: fatty ringed seals and bearded seals. By the summer, the sea ice begins to melt and break apart. Deprived of access to the tasty seals, polar bears spend the summer fasting. At least, that’s what’s supposed to happen. As the planet warms, the warmer ice-free season is getting longer and longer. So what’s a hungry bear to do?
秋季，当北冰洋进入结冰期，北极熊开始寻找它们的美餐：肥美的环斑海豹和髯海豹。到了夏天，海冰开始逐渐消融，因为难以捕获到美味的海豹，北极熊被迫进入夏季禁食期。至少，科学家是这么认为的。随着全球变暖，回暖的无冰季节也变得越来越长，一个饥饿的北极熊该如何捕食？

Scientists once thought that polar bears might survive by supplementing their pinniped diet by turning to terrestrial food like snow geese, their eggs and caribou.
科学家们曾认为以鳍足类动物为食的北极熊同样也可以靠捕食雪雁、雪雁蛋和北美驯鹿等陆地生物来维持生存。

“We’ve had this debate in the literature about whether terrestrial foods are nutritionally relevant to polar bears during the on-land season when the ice has melted.”
“在海冰消融的季节，我们就陆地上的猎物是否能够给在陆地活动的北极熊提供足够的营养 进行了学术讨论。”

University of Alberta biologist Nick Pilfold.
阿尔伯塔大学生物学家尼克·皮尔福德说，

“And we’ve known for a long time, going back to research in the early 70s, even going to back to early explorer logs, that polar bears will consume terrestrial based foods. But the debate was always whether those foods actually add up, energetically. “In other words, do these substitutes provide enough nutrition to make up for the lost seals?
“据我们所知，在过去很长一段时间，无论是上个世纪70年代早期的研究还是更早的探险者日志均表明北极熊会吃陆地食物。但是辩论的焦点在于这些食物是否能满足北极熊生长发育的需要。”换而言之，这些替代食物是否和海豹一样能够为北极熊提供足够的营养？

To find out, Pilfold and his colleagues estimated the weight lost each day by polar bears in the wilds of western Hudson Bay during the ice-free season, when they could ostensibly be chowing down on terrestrial foods. Then they compared that to the weight lost by bears that are temporarily held in Manitoba’s Polar Bear Alert Program.
为了找出答案，皮尔福德和他的同事们估算了西哈德逊湾野外的北极熊在无冰期靠捕获陆地生物的情况下每天损失的体重。然后他们将其与临时饲养在马尼托巴护熊中心的北极熊进行对比。

While captive, the bears do not eat. They drop about one kilogram each day. But the wild bears, who had access to the Arctic snack bar, lost the same amount of weight each day. The results are in the journal Physiological and Biochemical zoology. [Nicholas W. Pilfold et al., Mass Loss Rates of Fasting Polar Bears]
这些北极熊被捕获后，什么也不吃，体重每天会下降1千克左右。然而那些在禁食期有机会捕食陆地生物的的野生北极熊每天也失去相同的体重。该研究结果发表在了《生理学和生化动物学》上[Nicholas W. Pilfold et al., Mass Loss Rates of Fasting Polar Bears]。

“Which is really saying that the terrestrial foods do not have dense enough energy to offset the mass loss. So this idea that bears can just switch from eating sea ice-based food to land food and that’s going to help supplement against longer ice-free seasons, that really doesn’t show up in the data.”
“这确实可以证明陆地食物并没有提供足够的能量去补偿北极熊在禁食期所损失的体重，所以认为北极熊可以从海洋捕食转为陆地捕食的观点以及试着在无冰季向北极熊提供特定食物的提议都没有相应的数据支持。”

The problem is that polar bears evolved to rely on the marine diet.
问题是北极熊是在进化中逐渐依赖上海洋食物的。

“The food on land is protein and carbohydrate based, and the food the polar bears really focus on is fat. Fat is the name of the game for these bears; they’re highly adapted to absorbing that fat into their system and putting it onto their body as body mass, so they can fast on it later on. And the only thing that’s going to provide them with that type of fat are marine mammals. And they can only access those marine mammals when there’s sea ice.”
“陆地上的食物基于蛋白质和碳水化合物，而北极熊吃的食物大部分是脂肪。脂肪是这些北极熊狩猎的目标，它们非常适应于吸收脂肪并将其以体重的形式存储在自己身体里，正因如此它们才能在随后的禁食期活下来。唯一能够提供给它们这些脂肪的便是海洋哺乳动物，而北极熊只有在海冰季节才能够捕获到这些猎物。”

Adult males can go eight months without food. But younger, sub-adult bears, which are no longer nursing and must hunt, can’t last as long without sea ice. So as the Arctic warms, younger polar bears will disproportionately die out. Leaving the species’ survival in doubt.
成年雄性北极熊可以在8个月内不进食。但是对于那些不再依赖父母，必须独立狩猎的亚成年北极熊来说，它们无法度过漫长的无冰期。所以随着北极变暖，年幼北极熊将大量死亡，进而使得这一物种的存活堪忧。