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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. $\endgroup$
    – user
    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? $\endgroup$ 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. $\endgroup$ 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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It's clear that you're not talking about humans here, but I'm going to use human hands as the basis for my answer, as they are by far the most relatable hands for me personally.

I would definitely say they'd come out from in between the fingers. For one thing, there's actually space in between the metacarpals for claws to rest. The hands would however need to be a little wider to account for the claws as well as new muscles whose primary function is moving the claws, and a little fatter to account for the curvature of the claws.

Second, if there were to be a retractable claw coming from a fingertip, the final knuckle of that finger would need to be longer than the claw in order for there to be room for it to fully retract, assuming it actually can fully retract. Because if the retracted claw extended into the second knuckle, the fingers wouldn't be able to bend well, which would severely limit fine manipulation. On my fairly average-sized hands, a 2 to 2.5-inch claw could fit comfortably in between my metacarpals, but only a half-inch claw in my fingertips. For comparison, lions have roughly 1.5-inch claws, and tigers have roughly 4-inch claws. Since the primary function of claws in cats is grasping, these longer claws are very useful to lions and tigers because they lack fingers. Human fingers are already pretty good at grasping things, so a short claw at the end of a fingertip may be sufficient for most human claw-related needs. But longer claws in between the fingers would allow the fingers to rest, even when climbing or hooking prey, so they wouldn't be tired later when it's time to do a task requiring fine manipulation. On a related note, you would definitely want claws on a fingered hand to be able to extend while the fingers are extended for two reasons. One: For increased grasping capability. And two: Imagine trying to climb a tree with balled-up fists. There's no way your claws would reach the tree and you'd scrape your knuckles on the bark.

Third, human fingers contain no muscle tissue; only fat and connective tissue. If a finger were to sheathe a retractable claw, it would need to be a massive bratwurst finger in order for there to be room for a bone, fat, connective tissue, a claw, and muscles to hold the claw in place. And depending on how long you want the claw to be, the final knuckle of each finger would need to be very long. One possible solution to this would be to have the final knuckle of the fingers not have a bone, and instead only be a skin flap covering a claw. The skin flap could recede over the previous knuckle to reveal the claw. Theoretically this motion could be make with no muscles in the fingers, using only connective tissue connected to muscles in the hand. It doesn't sound very structurally sound to me, though.

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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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Cats' claws can retract because the last joint of each digit is able to bend completely backwards and has a fleshy sheath to go into.

The challenge with combining retractible claws with primate-like nimble fingers is the fingerprint. Our fingerprints serve an important purpose in gripping, the ridges allow for more friction and a firmer grasp. But that requires us to use the last joint of our digits for contacting objects, which is incompatible with retracting like a cat.

One possible solution would be to have gripping pads form on the second-to-last joint, instead of the last joint. If they have the same number of finger joints as mammals do, this would mean somewhat reduced finger dexterity, equivalent to if you used finger braces to make yourself unable to bend the last joint on each finger, but I've seen people with disabilities doing complex manual tasks with finger braces like that so I know it's workable. If you're not constrained to basal mammalian skeletal features, you could give them an extra joint in each finger. The biggest challenge they'd have, regardless of the number of joints, is that when their claws are retracted, the end of their digit will be wider front-to-back than a human's, causing difficulty with tasks that involve inserting fingers into small spaces.

Another solution would be to have claws and gripping surfaces on different digits, similar to many prosimians (lemurs etc). For example, tarsiers have fingernails on most digits, but have a "toilet claw" (used for grooming) on the second and third toe. The most important digits for manual ability in humans are the thumb and index finger - if they had a humanlike thumb and index finger but retractible claws on the middle and ring fingers, for example, that wouldn't have a huge impact on their manual ability.

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Claws should be split off from first knuckles from wrist. They only stick out when fist is made tight. add Abrasive palms, and microscopic suction cup cells to underside of fingers. Finally fingernails are permanently extracted small cat like claws. there you have it!

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