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When a cat is growing, his nails are growing with him to some extent. A grown cat has a fixed nail length that is not extending.

By contrast, human finger nails just keep on growing, so we have to cut them to keep an aesthetically pleasing look.

Nature clearly has a mechanism to control nail growth.

What is the most probable reason as to why this is not implemented in humans?

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Cat claws are growing all the time, like horse hooves, or human nails. However, cats and horses usually use their claws/hooves, so they get shortened through mechanical action.

An indoor cat may need their claws trimmed if it doesn't use them enough (that's why cats will want to scratch everywhere), or if has supernumerary toes that don't normally touch the ground. Similarly, cattle that doesn't get to move will suffer from hoof overgrowth, which appears to be very uncomfortable to the animal.

Consequently, if you were to use your fingernails more often for digging and scratching, you wouldn't have to trim them all the time.

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Thank you Jonas for clarifying it for me. I guess the mechanism for limiting the growth when an optimal length is reached is non-existent, since there is no probing mechanism to know how long the nail actually is at a moment of time. The same may apply to hair. –  Maxim V. Pavlov Dec 25 '11 at 13:12
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Hair has a maximum length. A hair grows to full length, then sits idle, then falls out and is replaced by a new hair. –  Jay Bazuzi Dec 25 '11 at 20:39
    
@Maxim Of course there are mechanisms for that. How else does a leg know when to stop growing? Or any other organ in the body. Most of the mechanisms behind that aren’t yet completely understood though. –  Konrad Rudolph Dec 26 '11 at 11:13
    
@Konrad, well, the leg don't regenerate in case it's part is removed mechanically, whereas nails are in a constant growth state. Leg is a complex organ that has a feedback. It can tell the brain how long it is by the lenght of a neural connection network. Nail does not have it, so there may not be a mechanism to know how long it is. –  Maxim V. Pavlov Dec 26 '11 at 12:04
    
@Jay, thank you. Good point. This proves that the hair provides an informational feedback to the brain about it's length also, unlike nails. –  Maxim V. Pavlov Dec 26 '11 at 12:06
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