为什么树懒动作这么慢?/Why are sloths so slow?

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

1796 年,汤玛斯·杰佛逊收到了一只不明生物的骸骨

A long, sharp claw reminded him of a lion,

又长又尖的爪子让他想起了狮子

but the arm bones suggested a larger animal,

但它的手臂骨头看起来像是更大只的动物

one about three meters long.

一个大概三公尺长的生物

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

杰佛逊认为这可能是一种新发现的巨大美洲拟狮

Jefferson warned explorers Lewis and Clark

他特别警告了探险家路易斯和克拉克

to keep an eye out for this mysterious predator.

一定要特别小心这种神秘的掠食者

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

但是这些骸骨并不属于狮子

They came from an extinct giant sloth.

它们来自于已绝种的巨大地懒

Prehistoric ground sloths first appeared around 35 million years ago.

史前地懒最早是在 3 千 5 百万年前出现

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

史前北美、中美及南美有数十种地懒居住

alongside other ancient creatures like mastodons

与许多古老生物同时期比如乳齿象

and giant armadillos.

还有大犰狳,

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

一些地懒,像是二趾树懒,大小和家猫差不多

but many were massive.

但也有许多地懒体型巨大

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

杰佛逊收到的骨头属于巨爪地懒体重约一吨

and that was small compared to megatherium,

而大地懒的体型更加巨大

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

可以达到 6 公吨,相当于一头大象的体重

They ambled through the forests and savannas using their strong arms

它们漫步于森林及莽原之中

and sharp claws

强壮的手臂及尖利的爪子可以将植物连根拔起

to uproot plants and climb trees,

或是用来爬树

grazing on grasses, leaves,and prehistoric avocados.

它们吃草、树叶和史前酪梨

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

事实上,我们今天能有酪梨可吃还得归功于大地懒

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

体型较小的动物无法吞下酪梨的种子

but the sloths could,

但是地懒可以

and they spread avocado trees far and wide.

它们将酪梨的种子散布地又广又远

Ground sloths flourished for millions of years,

地懒繁盛地生存了数百万年

but around 10,000 years ago,they started disappearing

但是在大约一万年前它们开始渐渐消失

along with the Western Hemisphere's other giant mammals.

其他西半球的巨大生物也是如此

Researchers think that ground sloths could have been pushed out by an oncoming ice age,

研究人员认为地懒可能是因为冰河期而开始灭绝

or competition with other species,maybe humans,

或是因为和其他生物竞争而消失,地懒的竞争对手可能是人类

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

人类正好是在大部份地懒消失的时期出现

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

一些体型较小的树懒存活了下来,并且迁移到树梢生活

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

时至今日,仍有六种树懒

of Central and South America.

生活在中美及南美的热带雨林林冠之中

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

在树上生活可以避开掠食者

and there are plenty of leaves to eat.

而且还有很多树叶可吃

But this diet has its drawbacks.

不过这种饮食习惯也有缺点

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

动物透过从食物摄取的能量供自己移动、

maintain their body temperature,

维持体温、

keep their organs working,

使器官正常运作、

and all the other activities necessary for survival.

以及其他所有生存必须的活动

But leaves don't contain much energy,

但树叶含有的能量少之又少

and that which they do have is tough to extract.

而树叶之中仅有的能量也很难被摄取利用

Most herbivores supplement a leafy diet with higher energy foods

大部分的草食动物除了树叶以外还会补充高能量的食物

like fruit and seeds.

像是种子还有水果,

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

但是树懒,尤其三趾树懒几乎完全只吃树叶而已

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

它们演化出了专门应付这种饮食的策略

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

首先,它们尽量从树叶中摄取足够的能量

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

树懒的胃占了身体的三分之一,而且有许多隔间

and depending on the species,

根据品种的不同

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

树懒可以花上五至七天甚至是数周来消化胃里的食物

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

另一个策略就是减少能量的消耗

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

其中一个达成目标的方法很明显地,就是尽量少动

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

树懒大部分的时间都在吃饭、休息或者睡觉

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

它们一个礼拜只会从树冠上下来上一次厕所

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

当树懒移动的时候,通常都不是太快

It would take a sloth about five minutes to cross an average nei***orhood street.

让一只树懒跨越住宅区的马路会花上它整整五分钟

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

这种超缓慢的生活步调使树懒不需要太多的肌肉

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

事实上它们比其他体型相近的动物少了 30% 的肌肉量

Sloths also use less energy to keep themselves warm

树懒们也尽量减少消耗在保暖方面的能量

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

它们的体温变化范围可以达到摄氏五度

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

体温变化较冷血动物的爬虫类小,但是比大多数的哺乳类动物要大

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

这些生理上或者行为上的适应策略

or metabolic rate.

将树懒消耗的能量代谢速率降到最低

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

三趾树懒的代谢速率在哺乳类之中是最慢的

The giant panda is second slowest,

大猫熊的代谢是第二慢的

and two-toed sloths come in third.

而二趾树懒则是第三慢

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

缓慢的动作让树懒得以在树冠层栖地中繁衍昌盛

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

但这也使得树懒自己变成了其他有机生物的栖地

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

比如长在树懒身上的藻类能帮助树懒伪装,还能当零食吃

Sloths may not be giant anymore,

尽管树懒的体型不再巨大

but that doesn't make them any less remarkable.

人们对树懒的兴趣还是丝毫未减

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