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  • Off the rugged coast of the pacific northwest,

  • pods of killer whales inhabit the frigid waters.

  • Each family is able to survive here

  • thanks mainly to one member,

  • its most knowledgeable hunter:

  • the grandmother.

  • These matriarchs can live eighty years or more,

  • while most males die off in their thirties.

  • Though killer whales inhabit every major ocean,

  • until recently we knew very little about them.

  • The details of their lives eluded scientists

  • until an organization called the Center for Whale Research

  • began studying a single population

  • near Washington State and British Columbia in 1976.

  • Thanks to their ongoing work,

  • we've learned a great deal about these whales,

  • known as the Southern Residents.

  • And the more we learn,

  • the more this population's elders' vital role comes into focus.

  • Each grandmother starts her life as a calf

  • born into her mother's family group, or matriline.

  • The family does everything together,

  • hunting and playing, even communicating through their own unique set of calls.

  • Both sons and daughters spend their entire lives with their mothers' families.

  • That doesn't mean a young whale only interacts with her relatives.

  • Besides their own special calls,

  • her matriline shares a dialect with nearby families,

  • and they socialize regularly.

  • Once a female reaches age fifteen or so,

  • these meetings become opportunities to mate with males from other groups.

  • The relationships don't go much beyond mating

  • she and her calves stay with her family,

  • while the male returns to his own mother.

  • Until approximately age forty,

  • she gives birth every 6 years on average.

  • Then, she goes through menopause

  • which is almost unheard of in the animal kingdom.

  • 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.

  • After menopause,

  • grandmothers take the lead hunting for salmon,

  • the Southern Residents' main food source.

  • Most of the winter they forage offshore,

  • supplementing salmon with other fish.

  • But when the salmon head towards shore in droves to spawn,

  • the killer whales follow.

  • The matriarch shows the younger whales

  • where to find the most fertile fishing grounds.

  • She also shares up to 90% of the salmon she catches.

  • With each passing year,

  • her contributions become more vital:

  • overfishing and habitat destruction have decimated salmon populations,

  • putting the whales at near-constant risk of starvation.

  • These grandmothers' expertise

  • can mean the difference between life and death for their families

  • but why do they stop having calves?

  • It's almost always advantageous for a female to continue reproducing,

  • even if she also cares for her existing children and grandchildren.

  • A couple unique circumstances change this equation for killer whales.

  • 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.

  • This means that as a female killer whale ages,

  • a greater percentage of her family

  • consists of her children and grandchildren,

  • while more distant relatives die off.

  • Because older females are more closely related to the group than younger females,

  • they do best to invest in the family as a whole,

  • whereas younger females should invest in reproducing.

  • In the killer whale's environment,

  • every new calf is another mouth to feed

  • on limited, shared resources.

  • An older female can further her genes without burdening her family

  • by supporting her adult sons,

  • who sire calves other families will raise.

  • This might be why the females have evolved

  • to stop reproducing entirely in middle age.

  • Even with the grandmothers' contributions,

  • the Southern Resident killer whales are critically endangered,

  • largely due to a decline in salmon.

  • We urgently need to invest in restoring salmon populations

  • to save them from extinction.

  • In the long term, we'll need more studies like the Center for Whale Research's.

  • What we've learned about the Southern Residents

  • may not hold true for other groups.

  • By studying other populations closely,

  • we might uncover more startling adaptations,

  • and anticipate their vulnerabilities to human interference

  • before their survival is at risk.

Off the rugged coast of the pacific northwest,

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シャチの母系の内部 - ダレン・クロフト (Inside the killer whale matriarchy - Darren Croft)

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    Imee peng に公開 2021 年 01 月 14 日
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