Why are sloths so slow? - Kenny Coogan

翻译人员: Jack Zhang 校对人员: Tianyu Qiao

In 1796, Thomas Jefferson received a box of bones he couldn't identify.

1796年，托马斯·杰斐逊 收到一箱他无法辨认的骨头。

A long, sharp claw reminded him of a lion,

那又长又尖的爪子让他想起狮子，

but the arm bones suggested a larger animal,

但它的臂骨显示其为更大型的动物，

one about three meters long.

一个大约三米长的动物。

Thinking it might be huge unknown species of North American lion,

想着它可能是某种 未知的大型北美狮，

Jefferson warned explorers Lewis and Clark

杰斐逊提醒探险者 刘易斯和克拉克

to keep an eye out for this mysterious predator.

要注意这种神秘的猛兽。

But Jefferson's box of bones didn't come from a lion.

但是杰斐逊的那一箱骨头 并非来自狮子，

They came from an extinct giant sloth.

而是来自已灭绝的大型树懒。

Prehistoric ground sloths first appeared around 35 million years ago.

大约3500万年前， 史前陆地树懒开始出现。

Dozens of species lived across North, Central and South America,

许多物种生活在北美， 中美和南美洲，

alongside other ancient creatures like mastodons

同时生活在那里的还有其它 远古生物，比如乳齿象

and giant armadillos.

和巨型犰狳。

Some ground sloths, like the megalonychid, were cat-sized,

一些陆地树懒像猫一样大小， 如 Megalonychid，

but many were massive.

但很多体型庞大。

Jefferson's sloth, Megalonyx, weighed about a ton,

杰斐逊收到的树懒， 巨爪地懒，大约有一吨重，

and that was small compared to megatherium,

但还比不上大懒兽。

which could reach six metric tons, as much as an elephant.

大懒兽可重达六吨， 比得上一头大象。

They ambled through the forests and savannas using their strong arms

它们用强壮的手臂和利爪缓步穿过

and sharp claws

森林和热带稀树草原，

to uproot plants and climb trees,

拔除植物，爬树，

grazing on grasses, leaves, and prehistoric avocados.

以青草，树叶和史前牛油果为食。

In fact, we might not have avocados today if not for the giant sloths.

实际上，没有大型树懒 就没有今天的牛油果。

Smaller animals couldn't swallow the avocado's huge seed,

小动物吞不了牛油果的大种子，

but the sloths could,

但史前树懒可以，

and they spread avocado trees far and wide.

就是就能将其树种传播得更远。

Ground sloths flourished for millions of years,

大懒兽物种繁盛了几百万年，

but around 10,000 years ago, they started disappearing

却在一万年前开始灭绝，

along with the Western Hemisphere's other giant mammals.

一同灭绝的还有 西半球其他巨型哺乳动物。

Researchers think that ground sloths could have been pushed out

研究者认为，大懒兽的灭绝

by an oncoming ice age,

可能是由于即将到来的冰河时代，

or competition with other species, maybe humans,

或是由于与其它物种间 的竞争，也许是人类，

who arrived in the region around the time most of the sloths went extinct.

因为在同一时期， 人类到达了美洲。

Some of the smaller sloths did survive and migrated to the treetops.

一些较小的树懒确实存活了下来， 并迁移到树梢。

Today, there are six species left living in the rainforest canopies

如果，有六种树懒 住在热带雨林的顶端，

of Central and South America.

它们都集中在中南美洲。

Hanging out in the trees is a good way to avoid predators,

挂在树上是一个 避开捕食者的好方法，

and there are plenty of leaves to eat.

还有很多叶子可供食用。

But this diet has its drawbacks.

但这种饮食有其缺点。

Animals extract energy from food and use that energy to move around,

动物们从食物中提取能量， 并用这种能量来移动，

maintain their body temperature,

来保持体温，

keep their organs working,

保持器官正常运作，

and all the other activities necessary for survival.

和维持生存必要的其它活动。

But leaves don't contain much energy,

但叶子的能量含量并不高

and that which they do have is tough to extract.

这仅有的能量又很难提取。

Most herbivores supplement a leafy diet with higher energy foods

大多数食草动物在吃叶子的同时 也会食用其它更高能量的食物，

like fruit and seeds.

比如水果和籽类。

But sloths, especially three-toed sloths, rely on leaves almost exclusively.

但树懒，特别是三趾树懒， 几乎只吃叶子。

They've evolved finely tuned strategies for coping with this restricted diet.

为了以这样的饮食习惯生存下去， 它们进化出了特别的习性。

First, they extract as much energy from their food as possible.

首先，它们可以从食物中 提取出尽可能多的能量。

Sloths have a multi-chambered stomach that takes up a third of their body,

树懒有一个多腔胃， 占据他们身体的三分之一，

and depending on the species,

不同种类的树懒

they can spend five to seven days, or even weeks, processing a meal.

需要五到七天， 甚至几周时间来消化一顿饭。

The other piece of the puzzle is to use as little energy as possible.

它们的另一个秘诀 是尽可能少使用能量。

One way sloths do this is, of course, by not moving very much.

当然，要想避免消耗能量， 首先就要减少运动。

They spend most of their time eating, resting, or sleeping.

他们大部分时间 都在吃饭，休息或睡觉。

They descend from the canopy just once a week for a bathroom break.

他们每周从树顶下来一次， 只是为了上个厕所。

When sloths do move, it's not very fast.

当树懒动起来时，真的很慢。

It would take a sloth about five minutes to cross an average neighborhood street.

它们大概需要五分钟的时间 来过一条普通的马路。

This unhurried approach to life means that sloths don't need very much muscle.

这种不慌不忙的生活方式意味着 树懒并不需要很多肌肉。

In fact, they have about 30% less muscle mass than other animals their size.

实际上，它们的肌肉量 比其他同体型动物要少30%。

Sloths also use less energy to keep themselves warm

树懒使用更少的能量 也是为了保持体温，

because their body temperature can fluctuate by about five degrees Celsius,

因为它们的体温 可以上下波动约五摄氏度，

less than a cold-blooded reptile, but more than most mammals.

这个波动值低于冷血的爬行动物， 但比大多数哺乳动物要高。

These physical and behavioral adaptations minimize the sloth's energy expenditure,

树懒的这些身体和行为上的适应 能帮助它们减少能量消耗，

or metabolic rate.

降低新陈代谢率。

Three-toed sloths have the slowest metabolism of any mammal.

三趾树懒的新陈代谢率 是所有哺乳动物中最慢的。

The giant panda is second slowest,

大熊猫的第二慢，

and two-toed sloths come in third.

双趾树懒的第三慢。

Moving slowly has allowed sloths to thrive in their treetop habitat.

行动缓慢的生活方式 让树懒们在树梢的生活得意逍遥，

But it's also made the sloths themselves a great habitat for other organisms,

但也使它们自己的身体 成了其它生物的生活场所，

including algae, which provides a little extra camouflage, and maybe even a snack.

比如藻类，藻类让树懒的外形 显得更隐蔽，也可以被当作零食享用。

Sloths may not be giant anymore,

树懒可能不再是巨型猛兽，

but that doesn't make them any less remarkable.

但那并不代表它们不够酷。