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Off the rugged coast of the pacific northwest, pods of killer whales inhabit the frigid waters.
在崎岖的太平洋西北部海岸线，一群虎鲸生活在冰冷的海水里。
Each family is able to survive here thanks mainly to one member, its most knowledgeable hunter: the grandmother.
每个虎鲸家庭之所以能够在这里生存，是因为它们的一个家庭成员是经验丰富的猎手：它们的祖母。
These matriarchs can live eighty years or more, while most males die off in their thirties.
这些女族长们可以活八十年或者更多，而家庭中的男性成员通常在它们30多岁时死去。
Though killer whales inhabit every major ocean, until recently we knew very little about them.
尽管虎鲸分布在世界各处，直到最近我们对它们都知之甚少。
The details of their lives eluded scientists
学界中一直缺乏对它们生活习性的研究，
until an organization called the Center for Whale Research began studying a single population near Washington State and British Columbia in 1976.
直到1976年，一个叫做鲸类研究中心的组织开始对一群生活在华盛顿州和英属哥伦比亚的虎鲸进行研究。
Thanks to their ongoing work, we've learned a great deal about these whales, known as the Southern Residents.
得益于科学家们的工作，我们能够了解这群被称为“南方居民”的鲸鱼。
And the more we learn, the more this population's elders' vital role comes into focus.
而我们越深入研究，虎鲸族群中的女族长的角色越显得重要。
Each grandmother starts her life as a calf born into her mother's family group, or matriline.
每一个“祖母”都从幼崽长大，出生在它们母亲的家族，或是说，一个母系氏族中。
The family does everything together, hunting and playing, even communicating through their own unique set of calls.
家庭成员聚在一起生活、打猎和玩耍，甚至通过它们独特的鸣叫进行交流。
Both sons and daughters spend their entire lives with their mothers' families.
鲸鱼公崽和母崽的一生都会生活在它们的母系家族。
That doesn't mean a young whale only interacts with her relatives.
但这不意味着年轻的鲸鱼只和它的亲属们互动。
Besides their own special calls, her matriline shares a dialect with nearby families, and they socialize regularly.
除了一些特殊的鸣叫，一个母系氏族通常与邻近的家庭们共享方言，它们中的成员因此可以频繁社交。
Once a female reaches age fifteen or so, these meetings become opportunities to mate with males from other groups.
当雌性鲸鱼到了15岁左右，这样的邻里聚会就成为了它们的求偶机会。
The relationships don't go much beyond mating -- she and her calves stay with her family, while the male returns to his own mother.
这种跨家庭的关系通常仅仅是为了交配，交配过后，雌性和雄性鲸鱼，会分别回到它们的母亲身边。
Until approximately age forty, she gives birth every 6 years on average.
从15岁到40岁，雌性鲸鱼平均每隔六年产子。
Then, she goes through menopause -- which is almost unheard of in the animal kingdom.
之后，它就会绝经，这在动物王国中基本上绝无仅有。
In fact, humans, killer whales and a few other whales are the only species whose females continue to live for years after they stop reproducing.
事实上，只有人类、虎鲸和一些其它鲸类的雌性，在绝经之后仍然会存活一段时间。
After menopause, grandmothers take the lead hunting for salmon, the Southern Residents' main food source.
在绝经后，祖母们领头带着家庭捕猎鲑鱼，“南方居民”的主要食物来源。
Most of the winter they forage offshore, supplementing salmon with other fish.
大多数冬天它们在海上觅食，既捕捉鲑鱼也捕捉其他鱼类。
But when the salmon head towards shore in droves to spawn, the killer whales follow.
但当鲑鱼成群结队地游往岸边产卵时，虎鲸会跟着它们。
The matriarch shows the younger whales where to find the most fertile fishing grounds.
女族长会向年轻的鲸鱼展示哪里是最肥沃的渔场。
She also shares up to 90% of the salmon she catches.
女族长还将它高达90%的猎物分享给其它成员。
With each passing year, her contributions become more vital:
每过一年，她的贡献都变得更加重要：
overfishing and habitat destruction have decimated salmon populations, putting the whales at near-constant risk of starvation.
过度捕捞和栖息地破坏导致鲑鱼数量减少，使得鲸鱼面临着持续饥饿的危险。
These grandmothers' expertise can mean the difference between life and death for their families -- but why do they stop having calves?
这些祖母的经验技巧是它们家族生存的关键，但是它们为何会停止产子呢？
It's almost always advantageous for a female to continue reproducing, even if she also cares for her existing children and grandchildren.
对雌性来说，维持生育能力通常是一个优势，即使她们需要照顾自己的孩子和孙子们。
A couple unique circumstances change this equation for killer whales.
几个独特的环境导致这个定律在虎鲸群体中不存在。
The fact that neither sons or daughters leave their families of origin is extremely rare in almost all animal species, one or both sexes disperse.
首先，无论是儿子还是女儿，离开它们原生家庭的现象都极为罕见，在几乎所有的动物物种中，两性之一或之二会离开原生家庭。
This means that as a female killer whale ages,
这意味着当雌性虎鲸长大时，
a greater percentage of her family consists of her children and grandchildren, while more distant relatives die off.
它直系亲属（包括它的孩子和孙子）的占比，会越来越大，而其它的旁系亲属的占比，会不断减少。
Because older females are more closely related to the group than younger females,
由于年老的雌性虎鲸和家庭的纽带更为紧密，
they do best to invest in the family as a whole, whereas younger females should invest in reproducing.
它们也愿意为家庭付出更多，而年轻雌性会将更多精力用于繁殖上。
In the killer whale's environment, every new calf is another mouth to feed on limited, shared resources.
在虎鲸的生存环境中，多一个幼崽就多一张口，而食物资源却总是有限。
An older female can further her genes without burdening her family by supporting her adult sons, who sire calves other families will raise.
年长的母鲸可以通过支持它的成年儿子来进一步维持她的基因，而不给自己的家庭带来负担，因为它成年儿子的后代将由其它鲸群抚养。
This might be why the females have evolved to stop reproducing entirely in middle age.
这也许就是雌性鲸鱼在中年后绝经的原因。
Even with the grandmothers' contributions, the Southern Resident killer whales are critically endangered, largely due to a decline in salmon.
即使祖母们付出了十分努力，“南部居民”虎鲸的生存仍然岌岌可危，鲑鱼的减少是主要原因。
We urgently need to invest in restoring salmon populations to save them from extinction.
我们迫切需要加强恢复鲑鱼种群的投资，来保护鲸鱼使其免于灭绝。
In the long term, we'll need more studies like the Center for Whale Research's.
从长远来看，我们需要更多像鲸类研究中心的研究。
What we've learned about the Southern Residents may not hold true for other groups.
我们了解的“南方居民”的生活习性，可能不适用于其它虎鲸群体。
By studying other populations closely,
只有通过详细研究其它鲸鱼族群，
we might uncover more startling adaptations, and anticipate their vulnerabilities to human interference before their survival is at risk.
我们才可能在它们的生存面临危险之前，了解更多虎鲸为了应对人类干扰的变异和预测它们在哪些方面对人类行为会比较脆弱。
