eli5 Why do human fingernails grow so long?
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Fingernails are made of Keratin which is very strong, strong enough not to wear or crack under most our exertions. So it just keeps growing until it eventually cracks and breaks off, but that means it can keep going a very long length if a person is very gentle with their fingernails. But most opt to simply use a trimmer to manage the length.
Which is the same with almost all keratin protrusions. Most animals will have instinctive behavior to wear or crack their nails(claws) to keep them from getting too loud. Typically involving scratching at hard objects like trees or hard-packed dirt. Cats may enjoy scratching a pole, but that enjoyment is from a evolved behavior that keeps their claws at a near optimal length.
It’s not just human fingernails. The nails of most animals just keep growing. The difference is that animals constantly use their nails (walking, climbing, eating, killing etc.) Which wears them down.
Inactive pets often grow too long nails, which can be a health issue. Inactive wild animals don’t really live long enough to grow their nails.
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