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  • Consider the claw.

  • 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,

  • before flicking them out to hunt.

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

  • until their claws evolved into nails.

  • 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.

  • Rich in blood vessels and nutrients,

  • 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.

  • Since nails evolved from claws,

  • both adaptations produce keratinocytes in the same way.

  • The cells grow out from the matrix,

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

  • The primary difference between the two keratin coverings

  • is really just their shape,

  • 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

  • to form a cone-shaped structure.

  • 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

  • for gripping narrow branches.

  • 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

  • are closely linked.

  • 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

  • with these primates' regular tasks.

  • 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.

  • Manatees use nails to grasp their food,

  • and researchers think elephant toenails may sense vibrations

  • in the ground to help them hear.

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

  • have re-acquired claws.

  • 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

  • are used throughout the animal kingdom.

  • But as for which of these adaptations is better?

  • That's an answer we may never nail down.

Consider the claw.

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B2 中上級

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

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    shuting1215 に公開 2021 年 01 月 14 日
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