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为什么有时候动物会变得巨大?

Why Do Some Animals Get Gigantic?

我是来自《分钟地球》 的凯特
Hi, this is Kate from MinuteEarth.
动物通常都大小不一
Animals come in all different sizes, but usually,
但是经过演变 每种动物
over evolutionary time, each type of animal
都保持大致相同的体型
stays roughly the same size.
每次
Every once in awhile, though,
一些不可思议的事发生时
something crazy happens that
动物的体积会变得巨大
allows an animal to get truly gigantic.
以昆虫和节肢动物为例
Take insects and other arthropods,
它们的小部分是它们的身体
which have tiny bodies in part
因为它们通过它们的外骨骼来呼吸
because they breathe by sponging up air through their exoskeletons,
空气中可利用的氧气
and the available oxygen
在被利用之前会分散开来
can only diffuse so far before getting used up.
如果这些昆虫和节肢动物有更大的身体的话
if they had bigger bodies,
氧气也会更容易进入到它们体内
oxygen wouldn ’ t reach far enough inside.
但是 大约30亿年前 地球上的大气层是成锥形的
But about 300 million years ago, Earth’s atmospheric oxygen levels spiked.
空气中的氧气所占的比例越高
With more oxygen in the air,
节肢动物的身体也会长得更大
arthropods ’ bodies could grow way bigger,
导致像蜻蜓那样的大昆虫变得像老鹰一样大
leading to mega-bugs like a dragonfly the size of an eagle
而千足虫则可以达到一艘爱斯基摩人的双人划子大小
and a millipede the size of a two-person kayak.
恐龙 在另一方面
Dinosaurs, on the other hand,
则在没有任何外界帮助的情况下 变得异常的大 但是在某些情况下
got pretty darned big without any outside help – but at some point
因为平方立方定律 它们的高度会受到限制
they hit a limit due to the so-called square cube law:
身体的力量由骨头和肌肉的横截面面积决定 但是重量是由体积决定的
body strength is based on the cross-sectional area of bones and muscles, but weight is based on volume.
这个与 把立方体的高度增高两倍
And just like doubling the height of a cube
正方体的横截面会增大四倍
causes its cross-sectional area to get four times larger
而其体积会增大八倍的道理是一样的
but its volume to get eight times larger,
当动物变大时
when an animal gets bigger,
它们确实会变强壮 但它们同时更会变重许多
it does get stronger but it gets waaaay heavier.
从恐龙化石可以看出
Fossil evidence suggests that dinos were nearing the size
恐龙已经大到不能缠绕自己身体了
at which they could no longer lug around their own bodies
在恐龙经历了一次有突破性的进化后
when they stumbled upon an evolutionary breakthrough.
一系列气囊遍布它们的骨架
A system of air pockets and air sacs throughout their skeletons
这使得它们在重量没有增加的情况下体积变得更大了
that allowed them to get bigger without getting heavier
并且长出了极其长但是轻巧的脖子
and have incredibly long but light necks
这使恐龙能够吃到树上的叶子
which granted them access to a huge bounty of leaves.
最终
Eventually, though,
陆地上一群群的动物都遇到了平方立方定律问题
a group of land animals got around the square-cube problem altogether
它们通过回到水里 来使自己漂浮在水上
by climbing back into the water, which buoyedtheir weight.
因为它们的肺随着它们一起进入到水中
And since they took their lungs with them into the water,
所以它们能呼吸到含氧量很高的空气
they could breathe oxygen-rich air,
而不会被含氧量低的水给困住
rather than being stuck with oxygen-poor water,
这也使得这些哺乳动物长到世上最大的鱼体积的两倍大
allowing these mammals to grow almost twice as big as the biggest fish.
但是因为没有足够的食物 所以这些巨大的生物不能够再继续变大
But these giant creatures just didn’t have enough food to get any giant-er than that.
几百年前
Then, a few million years ago,
洋流的变化将海底上吨的营养物带到海洋表面
changing ocean currents brought tons of nutrients up from the depths,
这些营养物使上升处附近的浮游植物迅速繁殖
which fueled huge, localized phytoplankton blooms,
浮游植物的增加反过来也吸引了大密度的浮游动物
which in turn attracted enormous concentrations of scrumptious zooplankton.
蓝鲸形成了新的饮食习惯
With this new krillion calorie diet,
它们的肺部可以呼吸空气 身体可以漂浮在水上的
together with their air-breathing lungs and water-supported bodies,
这使他们的体积迅速地增加了三倍 变得不一般大
blue whales quickly tripled in size to become not just gigantic
而是成为有史以来最大的动物
but truly the largest animals to have ever lived.
我认为 一定有理论可以解释这种动物变大的现象
And THAT is certainly something to spout about.
嗨 我是大卫
Hi this is David.
因为我孩子总是不停地问我关于为什么一些动物这么大的问题 所以我做了这个视频
I wrote this video because my kids kept asking me questions about why some animals get so big.
来帮助培养他们对这个世界的好奇心
To help foster their curiosity about the world,
这些孩子的奶奶帮他们订购了Kiwi公司的产品
their Nana got them subscriptions to KiwiCo,
叫做KiwiCrate 每月给孩子们一盒科学 艺术 数学项目 让他们动手做
which sends them a box of hands-on science, art, and math projects called a KiwiCrate every month.
当我知道Kiwi公司要赞助我这个视频的时候 我十分激动 因为我们都喜欢他们公司的那些盒子
I was super excited when Kiwi offer to sponsor this video because we love the crates
并且 我很确定你们家也会喜欢的
and I’m sure your family will too.
只要在Kiwico.com/minuteearth 上注册账号
Sign up at KiwiCo.com/minuteearth,
Kiwi公司就会给你免费提供第一个月的产品
and they ’ ll give you the first month for free
但是只有在假期注册才有用哦
just in time for the holidays.

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视频概述

对 动物体积变得如此大感到好奇? 快来看视频吧 视频为你一一解答

听录译者

收集自网络

翻译译者

Jasper

审核员

审核员GK

视频来源

https://www.youtube.com/watch?v=-B_ZZjD2ZeU

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