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  • [♪INTRO]

  • The blue whale is the largest animal that's ever existed,

  • heavier than even the biggest of dinosaurs.

  • But scientists have debated why whales are so huge for a long time.

  • The prevailing idea is that if you live in water, buoyancy allows you to get bigger than if you lived on land.

  • But it might not be quite that simple.

  • Marine mammals may actually have more constraints on their body size than land mammals,

  • and scientists now think they have to be big to conserve energy.

  • Land mammals have slipped back into the water four separate times.

  • Whales evolved from ancient hippo-like mammals; seals from a dog-like ancestor.

  • Manatees and dugongs are a branch of the elephant family tree,

  • and sea otters are basically just really big weasels.

  • And until 2018, most scientists thought that their buoyancy in water gave marine mammals the freedom to grow larger,

  • because it kind of literally lifted the weight from their skeletons.

  • But when researchers looked for patterns of change in body size in almost 4,000 living and 3,000 fossil mammals,

  • they found something more complicated.

  • In 3 out of the 4 marine mammal groups (whales, seals, and manatees) species converged on the same size:

  • an average of around 500 kg or 1100 pounds.

  • And when they tested various hypotheses to explain that, they found while buoyancy might allow marine mammals to get big

  • it's not the reason they grow.

  • That probably has more to do with keeping warm.

  • Ocean waters are colder than most mammals' ideal body temperature,

  • and water is better than air at conducting heat away from you.

  • Since larger and rounder bodies have less surface area in relation to their volume,

  • they lose heat less rapidly even in chilly seas.

  • You'd think that means marine mammals could just keep growing bigger and bigger,

  • but the researchers found that if they get too big, there's no way they could eat enough to feed all those cells.

  • 500 kg seems to be just the right size to stay warm but still be able to eat enough to maintain a huge body.

  • But there are outliers.

  • Baleen whales like blue whales managed to hack the equation a bit when they developed those fibrous baleen bristles,

  • because filter feeding is a very efficient way to get lots of calories while spending very little energy.

  • And sea otters were the oddball that didn't converge on that 500kg size.

  • They're bigger than their weasel ancestors, but not as big as other marine mammals.

  • That might be because there's something special about weasels and their relatives that limits their size,

  • or allows them to stay trim.

  • Or, it's possible sea otters are still growing, and that milennia from now, they'll be huge.

  • Wait, would sea otters the size of polar bears still be ridiculously cute, or utterly terrifying?

  • I'm going with terrifying.

  • Anyhow, the findings have got scientists rethinking what they thought they knew about the life in the seas.

  • And they just go to show that if you keep asking good questions,

  • you might find the answers aren't as simple as you thoughtbut they're also much cooler.

  • Thanks for watching this episode of SciShow, a Complexly Production.

  • If you find marine mammals fascinating, you might like the episode of our sister channel, Eons,

  • where they delve into how whales made the transition from land to sea.

  • [♪OUTRO]



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B1 中級 美國腔

为什么海洋哺乳动物这么大(Why Are Marine Mammals So Big?)

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    joey joey 發佈於 2021 年 05 月 11 日