A long, sharp claw reminded him of a lion,

又長又尖的爪子讓他想起獅子

but the arm bones suggested a larger animal,

但從臂骨判斷，應該是身形更長的動物

one about three meters long.

身長約莫有三尺長

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

猜測可能是隸屬北美獅類的大型不知名品種

Jefferson warned explorers Lewis and Clark

傑弗遜警告路易斯與克拉克兩位考察家

to keep an eye out for this mysterious predator.

多留心注意這神秘的掠食者

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

但傑佛遜盒子內的骨頭來源並非是一頭獅子

They came from an extinct giant sloth.

而是來自已滅絕的巨型樹懶

Prehistoric ground sloths first appeared around 35 million years ago.

史前陸地樹懶初次出現於三千五百萬年前

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

當時住在北、中、南美的許多種

alongside other ancient creatures like mastodons

與其他古生物像是乳齒象

and giant armadillos.

與巨型犰狳

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

有些陸地型樹懶，像是大地樹懶，體型似貓

but many were massive.

但多數體型龐大

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

傑弗遜收到的樹懶遺骨 — 巨爪地懶，體重約一噸

and that was small compared to megatherium,

和大地樹懶相比，巨爪地懶體型已經算小了

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

大地樹懶可達六公噸，近似於一隻大象的重量

They ambled through the forests and savannas using their strong arms

大地樹懶在森林、草原漫步，使用強而有力的手臂

and sharp claws

及尖銳的腳爪

to uproot plants and climb trees,

把植物連根拔起與爬樹

grazing on grasses, leaves, and prehistoric avocados.

採食青草、樹葉與史前酪梨

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

事實上，假如沒有巨型樹懶，我們今日可能沒有酪梨

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

小型動物無法吞食酪梨的大種子

but the sloths could,

但樹懶辦得到

and they spread avocado trees far and wide.

牠們不斷散播酪梨子，散播得又遠又廣

Ground sloths flourished for millions of years,

大地樹懶世代綿延數百萬年

but around 10,000 years ago, they started disappearing

但差不多在一萬多年前，大地樹懶開始漸漸消失

along with the Western Hemisphere's other giant mammals.

跟著其他西半球的大型哺乳類一同消失在地球上

Researchers think that ground sloths could have been pushed out

研究者認為，大地樹懶可能遭到

by an oncoming ice age,

接續的冰河時期所排斥

or competition with other species, maybe humans,

或是跟其他物種競爭，可能是人類

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

人類約莫在這個時期到達樹懶生存區域，與樹懶絕種的時期恰好吻合

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

有些小型樹懶存活下來，但移居到樹上

Today, there are six species left living in the rainforest canopies of Central and South America.

如今，六種樹懶族群生活在中南美洲雨林的樹冠層

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

在樹上遊蕩是避免天敵的好方法

and there are plenty of leaves to eat.

而且樹冠層上有許多樹葉可以吃

But this diet has its drawbacks.

但這樣的飲食方式也有缺點

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

動物從食物中萃取能量，能量讓動物有力量移動

maintain their body temperature,

維持體溫

keep their organs working,

保持器官運作

and all the other activities necessary for survival.

以及其他涉及生存相關的必要活動

But leaves don't contain much energy,

但樹葉並無法帶給樹懶的身體許多能量

and that which they do have is tough to extract.

而樹懶攝取的食物又很難萃取出能量

Most herbivores supplement a leafy diet with higher energy foods

許多草食動物攝取的「素食餐」以高能量食物為主

like fruit and seeds.

像是水果和種子

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

但樹懶，特別是只有三指樹懶，大多只攝取樹葉而已

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

樹懶演化成能完美調整的策略來應對如此拘束的飲食方式

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

首先，牠們盡其所能從食物中萃取能量最大值

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

樹懶擁有多腔胃，幾乎佔整個身體的三分之一空間

and depending on the species,

不同種類的樹懶

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

可能花五到七天，甚至幾個禮拜，來消化一餐

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

另外一個秘訣是使用越少能量越好

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

其中一個樹懶運用的方法，毋庸置疑的就是，盡量不要移動

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

牠們大多數的時間都用在吃、休息與睡覺

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

牠們每週只會從樹冠層下來一次，只為了洗澡上廁所

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

樹懶移動的時候，速度並不快

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

樹懶會花費將近五分鐘橫越平均一條道路

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

如此不疾不徐的生活方式讓樹懶不需常常用到肌肉

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

事實上，和同樣體型大小的動物相比，樹懶的肌肉量少約百分之三十

Sloths also use less energy to keep themselves warm

樹懶也運用很少能量來維持體溫

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

因為牠們的體溫能夠有攝氏五度左右的變動調整

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

比冷血爬蟲類變動幅度小，但比多數哺乳類還要大了

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

這些生理與習慣適應縮減了樹懶的能量支出

or metabolic rate.

或代謝率

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

三指樹懶的代謝率是所有哺乳類中最慢的

The giant panda is second slowest,

巨型熊貓則排名哺乳類第二慢

and two-toed sloths come in third.

而兩指樹懶則名列第三慢

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

移動緩慢讓樹懶得以在樹冠層棲地繁榮發展

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

但如此一來也為其他有機類生物提供良好的棲地

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

包含藻類，提供樹懶一點額外的掩護，甚至成為小點心

Sloths may not be giant anymore,

或許樹懶不再如此巨大

but that doesn't make them any less remarkable.

但這並未減損樹懶引人注目的特質