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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, but the arm bones suggested a larger animal, one about three meters long.
那又长又尖的爪子让他想起狮子，但它的臂骨显示其为更大型的动物，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 to keep an eye out for this mysterious predator.
杰斐逊提醒探险者刘易斯和克拉克要注意这种神秘的猛兽。
But Jefferson's box of bones didn't come from a lion. They came from an extinct giant sloth.
但是杰斐逊的那一箱骨头并非来自狮子，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 and giant armadillos.
同时生活在那里的还有其它远古生物，比如乳齿象和巨型犰狳。
Some ground sloths, like the megalonychid, were cat-sized, but many were massive.
一些陆地树懒像猫一样大小，如Megalonychid，但很多体型庞大。
Jefferson's sloth, Megalonyx, weighed about a ton, and that was small compared to megatherium,
杰斐逊收到的树懒，巨爪地懒，大约有一吨重，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 and sharp claws to uproot plants and climb trees,
它们用强壮的手臂和利爪缓步穿过森林和热带稀树草原，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, and they spread avocado trees far and wide.
但史前树懒可以，它们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 along with the Western Hemisphere's other giant mammals.
却在一万年前开始灭绝，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 by an oncoming ice age,
研究者认为，大懒兽的灭绝可能是由于即将到来的冰河时代，
or competition with other species, maybe humans, who arrived in the region around the time most of the sloths went extinct.
或是由于与其它物种间的竞争，也许是人类，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 of Central and South America.
如果，有六种树懒住在热带雨林的顶端，它们都集中在中南美洲。
Hanging out in the trees is a good way to avoid predators, and there are plenty of leaves to eat.
挂在树上是一个避开捕食者的好方法，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, keep their organs working, and all the other activities necessary for survival.
来保持体温，保持器官正常运作和维持生存必要的其它活动。
But leaves don't contain much energy, and that which they do have is tough to extract.
但叶子的能量含量并不高，这仅有的能量又很难提取。
Most herbivores supplement a leafy diet with higher energy foods like fruit and seeds.
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，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, they can spend five to seven days, or even weeks, processing a meal.
不同种类的树懒需要五到七天，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, or metabolic rate.
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，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, and two-toed sloths come in third.
大熊猫是第二慢，双趾树懒是第三慢。
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，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, but that doesn't make them any less remarkable.
树懒可能不再是巨型猛兽，但那并不代表它们不够酷。
