Name: 爪対爪 - マシュー・ボース (Claws vs. nails - Matthew Borths)
Uploaded: 2021-01-14T08:30:24.000Z
Duration: 5 min 11 s
Description: VoiceTubeの動画で発音を聞きながら英語表現を覚えよう！学べる英語：

Frequently found on four-limbed  animals around the world,

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

Bears use claws for digging  as well as defense.

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

And lions can retract their massive claws  for easy movement,

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

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

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

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

claws had already been present for  over 260 million years

in the ancestors of mammals and reptiles.

But despite the gulf of time between  their emergence,

these adaptations are both part of the  same evolutionary story.

Both nails and claws are made of keratin—

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

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

this protein factory produces an  endless stream of keratin,

which is tightly packed into cells  called keratinocytes.

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

both adaptations produce keratinocytes  in the same way.

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

The primary difference between the  two keratin coverings

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

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

wrapping around the end of the digit  and radiating outwards

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

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

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

of primates evolving wider,  more dexterous fingers.

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

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

High in the forest canopy where  these primates lived,

wide finger bones and expansive  finger pads were ideal

And nails improved that grip even further.

By providing a rigid surface  to press against,

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

Additionally, nails improved the  sensitivity of their digits

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

This combination of sensitivity  and dexterity

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

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

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

And when our ancestors moved  down from the trees,

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

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

their sharp points would've  likely interfered

Claws are ideal for piercing,  puncturing, and hooking,

but their points make grabbing difficult,  and potentially dangerous.

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

and researchers think elephant toenails  may sense vibrations

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

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

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

When they hear an opening,  they burrow into the tree

and skewer grubs with their needle-like  middle finger.

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

But as for which of these  adaptations is better?

字幕リスト動画再生

爪対爪 - マシュー・ボース (Claws vs. nails - Matthew Borths)

grab

improve

potentially

structure