Claws vs. nails - Matthew Borths

譯者: Lilian Chiu 審譯者: SF Huang

Consider the claw.

想想爪子。

Frequently found on four-limbed animals around the world,

通常在全世界的 四足動物身上可以看見它，

it’s one of nature’s most versatile tools.

它是大自然中最多功能的工具之一。

Bears use claws for digging as well as defense.

熊類用爪子做挖掘和防禦。

An eagle’s needle-like talons can pierce the skulls of their prey.

老鷹像針一般的爪子 可以貫穿其獵物的頭骨。

And lions can retract their massive claws for easy movement,

獅子可以把巨大的爪子 縮回以方便移動，

before flicking them out to hunt.

要打獵時再將之彈出。

Even the ancestors of primates used to wield these impressive appendages,

就連靈長類的祖先，

在爪子演化成指甲前，

until their claws evolved into nails.

也都是運用這些了不起的附屬物。

So what in our evolutionary past led to this manicured adaptation,

所以，在我們的演化歷史中， 造成指甲的天擇適應演變為何？

and what can nails do that their sharper cousins can’t?

指甲又有什麼爪子沒有的功能？

When nails first appeared in the fossil record around 55.8 million years ago,

根據化石記錄，指甲最早 出現在 5580 萬年前，

claws had already been present for over 260 million years

而爪子已在哺乳類和爬蟲類身上

in the ancestors of mammals and reptiles.

有 2 億 6 千萬年了之久了。

But despite the gulf of time between their emergence,

但，儘管指甲和爪子的 出現時間差很多，

these adaptations are both part of the same evolutionary story.

兩者的適應演變 都出自同樣的演化故事。

Both nails and claws are made of keratin—

指甲和爪子的成分都是角蛋白，

a tough, fibrous protein also found in horns, scales, hooves and hair.

這種堅韌、纖維狀的蛋白質，

在角、鱗、蹄和毛髮中都有。

This protein is produced by a wedge of tissue called the keratin matrix.

這種蛋白質是由一種叫做 角蛋白基質的楔形組織所產生的。

Rich in blood vessels and nutrients,

這個蛋白質工廠富含血管和營養素，

this protein factory produces an endless stream of keratin,

可源源不斷地產製出角蛋白，

which is tightly packed into cells called keratinocytes.

這些角質被緊密地包裹在角質細胞中。

These high-density cells give nails and claws their trademark toughness.

指甲和爪子特有的強韌度 就是來自這些高密度的細胞。

Since nails evolved from claws,

因為指甲是從爪子演化而來，

both adaptations produce keratinocytes in the same way.

兩種的適應演變都是 以同樣的方式產生角質細胞。

The cells grow out from the matrix,

細胞從基質中生長出來，

emerging from the skin where they die and harden into a water-resistant sheath.

長出皮膚外後便會死亡、變硬，

成為防水的甲套。

The primary difference between the two keratin coverings

這兩種角質外觀的 主要差別只在於形狀的不同，

is really just their shape,

which depends on the shape of the bone at the end of the animal’s digits.

取決於足趾末端的 骨頭形狀而有所不同。

In claws, the bed of keratinocytes conforms to a narrow finger bone,

以爪子來說，角質細胞床 向上漸縮成比較窄的趾骨，

wrapping around the end of the digit and radiating outwards

包裏著足趾，向外放射，

to form a cone-shaped structure.

形成圓錐型的結構。

Animals with nails, on the other hand, have much broader digits,

另一方面，有指甲的動物 則有比較寬的手指、足趾，

and keratinocytes only cover the top surface of their wide bones.

角質細胞只會覆蓋寬骨頭頂端的表面。

It’s possible that nails have simply persisted as a side effect

有可能指甲只是靈長類在演化出

of primates evolving wider, more dexterous fingers.

更寬大、更靈巧手指時的 意外產物而已。

But given what we know about the habitats of our primate ancestors,

但，根據我們對於我們 靈長類祖先居住地的了解，

it’s more likely that nails came with their own powerful advantages.

比較可能的狀況是指甲 本身所帶來的強大益處。

High in the forest canopy where these primates lived,

在這些靈長類所居住的林冠，

wide finger bones and expansive finger pads were ideal

寬大的指骨和指墊是比較理想的，

for gripping narrow branches.

更適合抓住窄樹枝。

And nails improved that grip even further.

指甲對抓握有更大的幫助。

By providing a rigid surface to press against,

藉由一個可壓靠的堅硬表面，

primates could splay out their pads to create even more contact with the trees.

長靈類就能夠張開其指墊，

增加和樹木的接觸面積。

Additionally, nails improved the sensitivity of their digits

此外，指甲也會改善手指的敏感度，

by providing an extra surface to detect changes in pressure while climbing.

因為在攀爬時，指甲會提供 額外的表面積來偵測壓力的改變。

This combination of sensitivity and dexterity

敏感度和靈巧度結合起來，

gave our ancestors the precise motor control needed to snatch up insects,

讓我們的祖先有更精確的 運動控制，在抓住昆蟲、

pinch berries and seeds, and keep a firm grip on slim branches.

摘下莓果和種子， 及緊握纖細樹枝時都用得到。

The evolution of nails and the evolution of opposable thumbs and toes

指甲的演化與可相對拇指和腳趾的演化

are closely linked.

有密切的相關性。

And when our ancestors moved down from the trees,

當我們的祖先從樹上搬下來時，

this flexible grasp enabled them to create and wield complex tools.

因為有這種具彈性的抓握，他們 才能創造與使用這些複雜的工具。

Even if it was possible for wide fingers to sport claws,

就算寬式的手指能長出爪子，

their sharp points would’ve likely interfered

爪子的尖端也可能

with these primates’ regular tasks.

阻礙靈長類進行日常的工作。

Claws are ideal for piercing, puncturing, and hooking,

爪子很適合用來刺穿和勾抓，

but their points make grabbing difficult, and potentially dangerous.

但其尖端會讓抓握很困難， 也可能會造成危險。

However, both claws and nails are used in some unexpected ways.

然而，爪子和指甲都有 一些出乎意料的用法。

Manatees use nails to grasp their food,

海牛會用指甲來抓食物，

and researchers think elephant toenails may sense vibrations

研究者認為，大象的腳趾甲可能

可以感應到地面的震動 來協助牠們的聽覺。

in the ground to help them hear.

Meanwhile, some primates, like the aye-ayes of Madagascar,

同時，有些靈長類， 比如馬達加斯加的指猴，

have re-acquired claws.

則重新長回了爪子。

They use these extra-long appendages to tap branches and trunks,

牠們用這些特別長的附屬物 來輕敲樹枝和樹幹，

while listening for hollow sections with their bat-like ears.

同時用像蝙蝠一樣的耳朵 來聆聽尋找中空的部分。

When they hear an opening, they burrow into the tree

當牠們聽到空心的響聲時， 便會挖掘進去，

and skewer grubs with their needle-like middle finger.

用像針一樣的中指將幼蟲叉起。

We’ve only scratched the surface of all the incredible ways nails and claws

我們在這裡只有很粗淺地談到

在動物王國中指甲 和爪子的各種神奇應用方式。

are used throughout the animal kingdom.

But as for which of these adaptations is better?

但，至於何種的適應演變比較好呢？

That’s an answer we may never nail down.

我們可能永遠無法知道明確的答案。