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为什么没有巨型哺乳动物?

Why No Giant Mammals?

It’s true that there are animals today,
事实上 如今的许多动物
notably the blue whale is the largest creature ever to inhabit the earth,
都比恐龙的体型大
that are bigger than dinosaurs.
特别是蓝鲸这一地球上最大的生物
But, no creature that currently walks on land gets anywhere close
但目前还没有哪种陆栖生物的体型
to the size of the Super Dino’s.
跟超级恐龙接近
The largest recorded species of dinosaur probably is Amphicoelias Fragillimus,
据记录 最大的恐龙大概是双腔龙属的易碎双腔龙
which measured up to 60 meters and weighed 122 metric tons!
足足有60米高 122公吨
By comparison, the African savanna elephant,
相比之下的非洲稀树草原大象
the largest modern land animal,
近代最大的陆栖动物
clocks in at just three to four meters tall and five and a half tons.
却只有3-4米高 5.5吨重
Both herbivores.
同是食草动物
So why aren’t there land mammals today that are as big as the dinosaurs?
那为何现在没有跟恐龙一样大的陆栖哺乳动物呢?
The fossil record gives us some clues as to when and how,
我们无法由化石得知为什么哺乳动物体型有极限
if not exactly why, mammals hit their limit.
但能知晓其何时 以及怎样达到极限
After the last of the dinos disappeared around 65 million years ago,
约6千5百万年前 最后的恐龙消失后
the size and diversity of mammals went totally nuts until…it didn’t.
哺乳动物的体型和多样性一直都乱套了
Around 30 million years ago,
约3千万年前
a kind of rhino-giraffe beast topped out at 5 meters tall
一种巨型的长颈犀牛最高达到5米
and seven and a half meters long,
最长达到7.5米
grabbing the record for the biggest mammal to ever walk the earth.
创下地球上最大的哺乳动物这一记录
Scientists think that there are several limiting factors
科学家认为有几种因素
that have kept modern land animals from approaching super-saurus sizes.
限制了当代陆栖动物接近超级恐龙的体型
First mammals give birth to live young.
首先 哺乳动物很早就孕育新生命
A mama dinosaur could just lay a bunch of eggs and walk away.
雌恐龙可以只下一堆蛋就撒手不管
But a mammal has to gestate and nurse and rear babies
但哺乳动物得怀孕 照料 养育小宝宝
at a great energetic cost.
这需要大量精力
Large mammals also have long periods of gestation.
大型哺乳动物的孕期还很长
Giraffes and rhinos gestate for about fifteen months,
长颈鹿和犀牛要怀胎约15个月
and elephants are pregnant for nearly two years!
而大象几乎要怀胎两年!
And the longer a pregnancy is,
孕期越长
the more dangerous they are for both the mother and the young.
母亲和宝宝就越危险
So a mega-herbivore would probably experience an unfeasibly long and dangerous pregnancy.
所以大型食草动物可能会经历超长又危险的孕期
Another theory suggests the dinosaurs grew so large
另一种理论表明恐龙如此庞大
because of Earth’s climate and ecology at the time.
是因为当时地球的气候和生态系统
A warmer, muggy climate with a higher carbon content in the air
在空气中碳含量更高 更温暖闷热的气候下
made the vegetation grow off the hook.
植被疯狂生长
Sauropods basically lived in a massive smorgasbord,
蜥脚类恐龙基本上生活在大型杂草丛中
and they may have gotten so titanic simply because they could.
他们体型这么庞大仅仅是环境使然
But the environment changed over time
但时过境迁
and herbivores today don’t live in such prime conditions.
如今 食草动物不再生活于这种基本环境中
Reason 3: Today’s land mammals are further limited by their skeletal systems.
第三 现在的陆栖哺乳动物还受其骨骼系统的限制
Sauropod fossils show evidence of avian like air sacs
蜥脚类动物化石显示它们有鸟类般的气囊
that made for a much lighter skeletons than you’d expect in animals so large.
形成的骨骼比想象中大型动物的轻得多
Some of these dudes were 30 meters long but only weighed an estimated forty tons.
一些家伙长30米 却大概只重40吨
A modern land mammal that size would likely weigh nearly twice that
当代那种体型的陆栖哺乳动物会重近两倍
and basically it’d be too heavy to get around.
基本上会因为太重而无法四处走动
Finally the fourth and maybe coolest reason:
最后第四条或许是最酷的原因:
a mega-mammal would probably cook it’s own tissues.
大型哺乳动物可能会自行产生热量
Mammals as you probably know are endothermic or warm-blooded.
你们所知道的哺乳动物是温血的
Our bodies generate and maintain their own temperature internally.
我们身体内部产生热量并保持体温
Reptiles meanwhile are ectothermic or cold-blooded,
然而爬行动物是冷血的
absorbing heat from their environment like a snake sunning on a rock.
从环境吸收热量 比如蛇在石头上晒太阳
There’s actually debate as to whether some dinosaurs were warm-blooded like birds,
实际上存在某些恐龙是否像鸟一样温血的争论
or if they have some sort of immediate system
或者是否有某种中间系统
that was neither all cold-blooded nor all warm.
不全冷血也不全温血
But regardless we know a mammal uses away more energy
但不管怎样 我们都知道 跟同等体型的爬行动物相比
than a similar-sized reptile.
哺乳动物耗能更多
So it’s probable that modern land mammals couldn’t get much bigger than elephants
所以 如果近代陆栖哺乳动物无法体内自行产生热量
without actually cooking themselves from within.
就无法在体型上超过大象
That’s enough to make me perfectly happy being this size.
我这个体型已经够开心了
Thank you for watching this episode of SciShow.
感谢收看本集科学秀
If you have any questions or comments or suggestions,
如有任何问题 评论或建议
we’re on Facebook or Twitter and of course down on the comments below.
请上Facebook或者Twitter 当然下方评论区也行
And if you wanna keep getting smarter with us here at SciShow,
如果想在科学秀与我们一起不断获取知识
you can go to YOUTUBE.COM/SCISHOW and subscribe.
可以上YOUTUBE.COM/SCISHOW然后订阅频道

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视频概述

据科学研究,大致有三种因素限制了现在的哺乳动物的体型,所以他们达不到恐龙般的庞大体型。

听录译者

收集自网络

翻译译者

大刀肉

审核员

审核员XY

视频来源

https://www.youtube.com/watch?v=QHe1wmEaYWo

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