But that doesn't mean they're boring.

但這不表示牠們很無趣。

In fact, sloths are one of the most extreme animals on the planet, and not just because they poop only once a week.

事實上，樹懶是地球上最極端的生物之一，而這不只是因為牠們一週只大便一次。

First of all, sloths really are extremely slow.

首先，樹懶真的非常慢。

Their top speeds clock in at just 0.25 kph, making them the slowest mammal on the planet.

他的的最高速度是一小時移動 0.25 公里，這也使他們成為地球上移動最慢的哺乳類動物。

According to zoologist Becky Cliffe, that's because, well, sloths can't really see.

根據動物學家 Becky Cliffe，這是因為樹懶幾乎是看不見的。

60 million years ago, they lost the ability to see.

六千萬年前，牠們的視力就退化了。

They went almost completely blind.

牠們變得幾乎失明。

So, in the daytime, a sloth actually can't see anything cause it's too bright.

所以在白天，因為太亮了，樹懶是看不見的。

If you can't see where you're going, you can't run around because you're going to fall out of the tree.

如果你看不見的話，你自然也跑不快，不然你可能會因此掉下樹。

Moving slowly may sound like a liability, especially if you're nearly blind, but being sluggish actually comes with all kinds of benefits.

移動緩慢看似是弱點，尤其當你又幾乎失明，但遲緩的行為實際上給樹懶帶了不少好處。

For one, they save an enormous amount of energy.

第一，牠們省下了大量的體力。

In fact, sloths use about 90% less energy than the average mammal, which is vital, considering their diet is a low-energy snack consisting of leaves...with a side of leaves.

樹懶只需要一般哺乳類十分之一的能量就可以存活，而這是很重要的，畢竟他們的飲食也是低熱量的零食——樹葉，

And as it turns out, being slow also helps sloths avoid detection.

而且依結果來說，緩慢的動作也讓樹懶難以被察覺。

You see, sloths are so slow that predators like jaguars and eagles, which use movement to hunt, can't even find them.

樹懶實在太慢了，所以對像獵豹或老鷹這類的依獵物動作來覓食的獵食者來說，牠們根本找不到樹懶。

But some sloths take predator avoidance to the next level.

還有些樹懶甚至能把躲避獵食者這件事帶到另一個層級。

Thanks to rare extra vertebrae in its neck, the three-fingered sloth can turn its head 270 degrees, which allows it to smell incoming predators from almost any direction, and that's especially useful when you spend nearly all of your time motionless in a tree.

多虧了脖子上特化的椎骨，三指樹懶可以將頭旋轉270度，這讓牠們能夠聞到來自任何方向的掠食者的氣味，而這對幾乎一輩子都呆在樹上的樹懶特別有用。

In fact, sloths sleep, mate, and even give birth all while hanging upside down on a branch.

樹懶會在倒掛在樹上睡覺、交配，甚至生育。

And, as you might expect, that requires some pretty extreme adaptations.

沒錯，就如同你所猜想的，這需要非常極端的環境適應與演化。

They've got special tendons in their hands, which lock into place.

牠們的手上有特化的筋，它們是固定在一定位置的。

And, along with their long, hooklike fingernails, allow them to hang easily.

還有很長的、如鉤子一般的指甲，讓牠們能輕易地掛在樹上。

They're like a giant coat hanger.

它們就像大型的衣架掛鉤。

And to combat a rush of blood to the head, which might be familiar if you've ever hung upside down...

而為了防止頭部充血，就像我們倒吊時會發生的熟悉情景...

They have special sort of valves in their circulatory system that does actually stop the pooling of blood in the head.

牠們循環系統中特化的瓣膜，可以避免腦部充血的情形發生。

But if you think that's extreme, consider this: once a week, sloths risk their lives on a daring journey from the protective forest canopy to the ground.

如果你認為上述的演化很極端，聽聽這個：樹懶一週會需要冒著生命踏上一個危險的旅程，爬下保護牠們的樹林來到地上。

Their mission?

牠們的任務？

To poop.

要大便。

And it really is risky.

而且這真的非常危險。

We suspect about 60% of predator-based sloth deaths happen while they're pooping.

我們懷疑被掠食者殺死的樹懶中有六成都是在牠們大便時發生的。

They literally risk their life to come down to the ground.

牠們爬下樹真的是冒著生命危險的。

So they have to make it count.

所以牠們必須要讓這個風險冒得值得。

And, boy, they really do.

而牠們也的確做到了。

Sloths can lose an astonishing 30% of their body weight each time they go.

每次爬下數，樹懶都可以流失大概三成的體重。

Plus, they might get a mate out of it.

再加上他們有可能會遇到自己的交配對象。

How do you find a mate if you're solitary and you can't move very far?

如果你是獨居、爬得很慢、又走不遠，要如何找到伴侶？

Well, they're gonna have to use pheromones and scent markers.

牠們使用的是費洛蒙和氣味標記。

I think that coming down and leaving a little pile at the base of your favorite trees is sort of like a little signpost.

我認為在你喜歡的樹的根部留下一些你的記號就像留下一個指路牌一樣。

But if there's one thing that really pushes sloths over the top, it's their rough fur.

但樹懶真正驚人的其實是牠們的毛髮。

Sloth fur has its very own ecosystem.

牠們的毛就像一個獨立的生態系統。

Because they live in the rainforest, it's often wet, which creates the perfect environment for algae to grow and provide sloths with green camouflage.

因為牠們住在雨林中，毛髮通常都非常潮濕，因此為藻類創造了生存的最佳環境，這也使得樹懶擁有綠色的保護色。

But it isn't just algae that calls a sloth's fur home.

但不只藻類住在樹懶的毛髮裡。

You can also find beetles, cockroaches, and a species of moth that's found nowhere else in the world except for on a sloth.

裡面還有甲蟲、蟑螂，還有一個只存在於樹懶背上的蛾類物種。

And the sloths don't seem to mind one bit.

但樹懶卻似乎一點也不介意。

But that's not all.

但還不只如此。

Their fur might actually save lives.

牠們的毛髮可能還是救命仙丹。

Fungi in their fur have been shown to fight breast cancer cells, and they may also have properties for fighting malaria.

牠們毛皮中的真菌已被證明可以對抗乳腺癌細胞，而且它們還具有對抗瘧疾的特性。

So, despite their slow pace, sloths are far from boring, and in some environments, they aren't even slow.

所以，除了行動遲緩之外，樹懶一點也不無趣，而且在某些環境中，牠們其實根本不算慢的。

I mean, look at that daredevil go.

就看看這個冒失鬼吧！