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Consider a race of humanoid, winged tree-dwellers. They can fly and generally spend most of their time either in the air or climbing around their tree-homes in the upper reaches of a forest. In order to climb, land, and perch effectively, they need claws capable of sinking easily into hardened tree bark and occasionally even stone. At the same time, in order to create and manipulate tools, they need humanlike hands with nimble fingers and opposable thumbs. They must be able to extend or retract their claws quickly, including in response to being startled, or as a weapon in a fight.

Given all this, what is the most likely digit structure for retractable claws in humanlike hands? Sitting at, and retracting into, the tips of the fingers? Or situated between the fingers and retracting into the palm?

What problems would a race with this kind of retractable claws have? Could a claw get "stuck"? How would the muscles support the weight in both configurations? How would the curvature of the claws affect the shape of the hand - the fingers for fingertip claws, or the palm for between-finger claws? What happens if a claw is broken? Are there any benefits to one claw structure over the other?

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  • $\begingroup$ Keep in mind that claws and nails are basically the same thing. $\endgroup$ – a CVn Nov 1 '16 at 10:48
  • $\begingroup$ Rather than retractable claws, could you give them strong, sharp claws on their feet, and claws reduced to mere nails on their hands? If the purpose is to allow them to land and perch effectively, their hands wouldn't necessarily be needed. Anchoring the feet would work just as well. $\endgroup$ – Werrf Nov 2 '16 at 20:17
  • $\begingroup$ What about claws only on two of your fingers? $\endgroup$ – Diego Sánchez Nov 3 '16 at 14:31
  • $\begingroup$ @Werrf Hands are necessary for climbing and perching on non-horizontal surfaces (such as the side of a tree). I mean, I know there's some rock climbers out there who can grip the tiniest of protrusions with bare fingertips, but it's probably not ideal for a race of tree-dwellers. :) $\endgroup$ – thatgirldm Nov 3 '16 at 20:04
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    $\begingroup$ If a couple of fingers have big, hooked claws and can somehow flex out of the way, they could use those fingers to hold on to surfaces and the other, unclawed fingers for fine manipulation. You'll be basically segregating the two functions in two different types of fingers. $\endgroup$ – Diego Sánchez Nov 3 '16 at 20:07
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How about, rather than extending the claws, you retract the skin around them?

Keep the claws firmly attached to the bone, but add a muscular pad, something like a tongue or an elephant's trunk, that can close around the claw when it's not in use. That would allow fine manual dexterity - perhaps even better than our own fingers, if this muscle could be controlled precisely enough - but also keep the claw available for grasping tasks.

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Retractable claws are tricky business - consider cats, for example. Their claws don't really retract all that much - it's more that the skin around them is flabby enough to cover them unless stretched. The claws are always there. And this extra skin around the "fingers" doesn't lend itself to dexterous digits.

If I might suggest an easier solution - smaller claws at the base of each finger, around the palm of the hand. These would be perfect for gripping trees, while leaving the fingers themselves free to manipulate objects.

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    $\begingroup$ An excellent alternative solution. Plus one from me. Good for gripping trees and leaves the fingers free for manipulation. $\endgroup$ – a4android Nov 1 '16 at 11:53
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    $\begingroup$ I'm aware retractable claws are tricky, but small claws at the base of the fingers doesn't sound any better. It's actually quite hard to manipulate objects when something is sitting there - your fingers lose range of motion, you can't grip effectively, etc. I'm looking for answers that specifically address the issue of retractable claws in humanlike hands. $\endgroup$ – thatgirldm Nov 2 '16 at 16:02
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An alternative would be segregating the two functions into two different kind of fingers.

Your creatures could have two big hooked claws in two of their fingers and nails in the others.

The hooked claws would need to flex back when using the other fingers for fine manipulation, so they don't get on the way. As they don't need such fine control as the others, they can have bigger muscles and a wider range of movement.

Something similar to Spielberg's velocirraptor's feet.

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Perhaps this could work if the claws were made more finger-like. Instead of claws which retract into the hands/fingers, how about small claws between fingers which can be pulled towards the back of the hands (putting them out of the way behind the fingers) or forwards, extending in front of the palm in the same way that regular fingers move at the knuckles?

I imagine the hands would be more brittle and less strong, needing space for muscle and bone for such structures.

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What about something like a spur growing backwards from the back of the hand like from the knuckles. By backwards I mean it would curve away from the hand. This would reduce the need to have it retract and still allow fingers to be regular. The hand could be twisted around when using it to climb and for fighting could be used in a back handed striking style. Arms in general might be double jointed and/or more flexible to allow for ease of rotating and for hugging a surface when using the spurs.

They could be a mutation where the fingers split into two fingers at the knuckle but the backwards fingers didn't fully develop and became spurs. The could also maybe develop claws at the end of the spurs as a remnant of being fingers. Or maybe the culture developed a tradition of sharpening the spurs.

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