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So... I'm fiddling with Yet Another Bipedal Digitigrade Species. There is tons of material on the legs, but seemingly not so much on the hands.

Short Version

So... I'm wondering; is it possible to design a "hand" that can function as both a digitigrade 'foot' and be useful to a civilized tool-user?

Longer Version

As we know, digitigrade forepaws have extended metacarpals, and the range of motion between the metacarpals and proximal phalanges is from roughly in-line to about 90° forward/out. In humans and most other species with grasping fingers, the range is from roughly in-line to about 90° backward/in.

Another challenge is claws, especially the retractile variety, which effectively 'eat up' a useful joint. That said, I'm not sure how much it would really inconvenience most people if the last two digits of their fingers were fused.

An obvious solution is to provide the MC/PP joint with a greater range of motion. Is this plausible? If not, is it plausible that a species with such 'fingers' would be able to grasp objects anyway? (What might the thumb look like?) Or is there any other 'hand' design that could be used?

Notes

  • 'Hands' should be able to grasp objects well enough to plausibly be able to survive in any period of development (assume technological progression similar to human).
    • Should be able to grasp a pencil / thin rod. Bonus points if they can use chopsticks.
    • Should be able to grasp a tool handle / thicker rod.
    • Should be able to grasp a ball / apple / etc.
    • Okay if they need to use two hands more often than humans.
  • Should be able to 'stand' on forelimbs with weight partially carried by the metacarpals. Should ideally be able to walk like this, but okay if they can't run (on all fours) very well.
  • Phalanges are probably longer than otherwise; closer to human proportion.
  • There is definitely a thumb, although I'm not entirely sure what it looks like or how it stays out of the way for walking-on-hands. Ideally it is opposable, at least to the tips of the other fingers when said fingers are curled. (Maybe not to each finger joint.)
  • Should be able to have "retractable"¹ claws.

(¹ For you pedants out there, yes I know claws are really protractile.)

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    $\begingroup$ The features that make digits good for support when running make them bad manipulators. loose flexible joints make for the best fine motor control and are also the most likely to dislocate if used for running. the faster you want your creatures to be able to run the poorer their fine motor skills will their hands has to be. $\endgroup$ – John Feb 23 '20 at 4:13
  • $\begingroup$ @John, sure, but to clarify, the intent is that they are primarily bipedal but with 'hands' that have visible digitigrade features, as opposed to just giving them hands that are essentially human. Being able to run (on all fours) well would be nice, but is not required. (They should be able to walk, slowly, at least.) $\endgroup$ – Matthew Feb 23 '20 at 5:30
  • $\begingroup$ (Ack, did I not mention they are primarily bipedal? I thought I did; sorry! Edited...) $\endgroup$ – Matthew Feb 23 '20 at 5:39
  • $\begingroup$ @BLT-Bub, squirrels aren't digitigrade though? There are plenty of examples of plantigrade hand-like paws. (If you ignore primates, raccoons are the next best known example... and yes, squirrels too.) $\endgroup$ – Matthew Feb 23 '20 at 5:54
  • $\begingroup$ if that is all they need to do with them human hands can do that, so what exactly are you looking for, cosmetics? $\endgroup$ – John Feb 23 '20 at 12:17
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Have them knuckle-walk like the great apes do.

Like John mentioned in a comment, having fingers that can flexibly bend in both directions is likely to result in fingers that are either bad at being used for walking, or bad at being used for gripping.

So, you can split the circle by only having them bend in one direction: inwards. They'd then walk around on their knuckles and the backs of their fingers, like a backward-pointing foot, and use the insides of their fingers for gripping things.

This is an inverted posture from the positions you see the "fingers" used in animals like dogs or cats, but which preserves the full functionality of the fingers in this context.

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