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Short version: A given person’s arms are now legs. But somehow they are able to climb trees despite having no opposable thumbs or big toes. How is the anatomy of their arm-legs designed to make this possible?


Long version:

Alendyias recently asked a question concerning a creature called the Creeper, that is something like a big scary vampire bat. As per Alendyias rules, anyone who kills or eats a creeper acquires an "enchantment" that gives them some of the creeper's powers.

What the creeper enchantment does is to. . .

Turns your arms into legs!

Given a person who has undergone this transformation and has since adjusted to their life as a creeper, what kind of changes to their arm/leg anatomy would be required to make them proficient tree climbers? the level of improvement I’m looking for is that they could not climb trees for more than a few minutes before the transformation, and now it is as natural as walking.

One major constraint: I want to preserve plantigrade feet for all four of their limbs, and to not add opposable thumbs.

This brings me to my question -- what about their front legs makes this possible? How are the joints arranged so that the legs work for both walking on all fours and climbing through the branches?

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    $\begingroup$ Double spaces instead of "double enters" to avoid those huge gaps. Also, perhaps you could shorten a little the text towards the conclusion, notably below the section "Turn your arms into legs"; I fear people will give up midway or quick-vote before reaching the actual question ^^". $\endgroup$ Commented Oct 18, 2022 at 20:43
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    $\begingroup$ @Tortliena Good idea. I made a TLDR at the start of the question in a spoiler box. $\endgroup$
    – Daron
    Commented Oct 18, 2022 at 20:47
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    $\begingroup$ Don't make it a spoiler box. Just make it a clear statement of the question. If people want the fun of reading the (very) long version, they will. But every hurdle you place between users and your question simply turns users away. Spoilers are for hiding the answers in posts on Puzzling. $\endgroup$
    – JBH
    Commented Oct 18, 2022 at 20:54
  • $\begingroup$ Also, there are limited options for non-opposable-thumb four-legged creatures to climb things. Cats have claws. Geckos have setae. Without an opposable thumb your only option it to hang from something. I suppose we can include a prehensile tail just to be complete. This question seems awfully trivial. $\endgroup$
    – JBH
    Commented Oct 18, 2022 at 21:01
  • $\begingroup$ Edited to remove snide and passive aggressive mockery of another stack exchange member and self-indulgent rhapsodizing. $\endgroup$
    – Daniel B
    Commented Oct 18, 2022 at 21:17

6 Answers 6

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Claws

While humans and other primates use fingers to climb, there are many champion climbers that have no need for this. Cats and squirrels both come to mind; neither have much in the way of thumbs but they can pretty much run up a tree without difficulty.

One issue with this is that the square-cube law can be troublesome; the bigger you are the longer and sharper your claws need to be to climb effectively. But big cats like jaguars and lions can climb trees just fine (though not as well as smaller cats) so it shouldn't be an issue for human-sized Creepers, provided they have big, catlike claws.

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    $\begingroup$ Why would you assume Jaguars can't climb trees as well as smaller cats? Not only can they climb trees, some of them spend months in arboreal lifestyle during Amazon flooding season. And often enough they take their pretty large prey with them. Also, I've never heard of firefighters having to rescue a jaguar off a tree, so that makes them even better climbers xD $\endgroup$ Commented Oct 19, 2022 at 11:42
  • $\begingroup$ Don't forget the squirrels unique ability to rotate its hind feet 180 degrees backwards that makes it very easy for them to descend trees. $\endgroup$
    – Glen Yates
    Commented Oct 19, 2022 at 14:38
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    $\begingroup$ @OttoAbnormalverbraucher He/she isn't saying that they "climb better" in the sense of their technique or ability; rather, because of the physics involved, a smaller cat can climb the same distance faster, hang on longer, and change direction easier than the same cat scaled up to a larger size. Square-cube law means big danger kitty's claw surface area to body mass ratio is smaller $\endgroup$
    – automaton
    Commented Oct 19, 2022 at 20:28
  • $\begingroup$ @automaton That is not what I read. I mean, sure, physics give smaller bodies an easier time climbing, but that doesn't automatically mean smaller bodies are better at it. $\endgroup$ Commented Oct 21, 2022 at 5:49
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Pygmy Goat style

goats in a tree

https://www.treehugger.com/goats-really-can-climb-trees-4863877#:

Goat hooves can fold like folding a book, pinching things between the left and right halves. Thus they climb. Pinchy hooves plus goat intuition is all they need. The people with the Creeper enchantments are set up exactly like goats. For all intents and purposes they are goats.

Although I think Tom's idea has merit too. The Creepers are also hookers.

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    $\begingroup$ Goat Simulator meets the Matrix meets Bugsnax $\endgroup$ Commented Oct 19, 2022 at 14:42
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    $\begingroup$ These goats are more often than not placed in the trees by humans (as a means of attracting tourists). They rarely climb up there on their own. They're very good at staying balanced up there, but they would probably prefer to be on the ground. $\endgroup$ Commented Oct 19, 2022 at 15:08
  • $\begingroup$ Willk you should know you feature more prominently in the original version of the question before it got edited down for being too much fun. $\endgroup$
    – Daron
    Commented Oct 19, 2022 at 17:00
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Maybe they can have claws (prosthetical even) that they can latch into the soft wood of trees like sloths. enter image description here

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Strong hooking ability

I think it's impossible to truly "grasp" anything if your digits are not opposable. You've ruled that out, so I think we'll have to rely on climbing techniques that do not require grasping.

(It's worth noting that one alleged benefit of the Creeper enchantment is a much higher vertical leap, but I'm going to assume for purposes of this question that simply leaping from the ground directly to the top of a tree would not count as "climbing.")

So, my backup plan would be a part of the anatomy that can be hooked over (or around) a branch so that the creature can pull itself toward that branch.

As it happens, the human foot can kind of do this. Not by flexing, but by bending at the ankle. Bend your foot upward, bringing your instep closer to your shin, and you'll see what I mean. If you had two of those on the ends of your arm-legs, you could hook them around branches and pull.

But that anatomy is not well-suited enough for that purpose. Major problems are:

  • the muscle that performs that bend is not very strong in humans
  • the range of motion doesn't permit a sharp bend, which increases the risk that the hook will slip and the creature will fall
  • the instep is very poorly adapted for holding weight: the convex shape of the bone means that most of the force will be focused on the outermost portion of that bone, and I can tell you from experience that it doesn't take much force on that bone to cause a lot of pain

So, the features we'd want are:

  • flat or convex instep with some muscle between the skin and bone
  • rough skin on the instep for better grip
  • greater range of motion at the ankle
  • much stronger muscle that controls the ankle bend, at least on the side that bends the foot upward (might be the tibialis anterior)
  • probably change the kind of joint that the ankle is

What you really want is several features of the human hand. Even if you remove our opposable thumb and lock our fingers similar to how toes are locked, the hand would still be a decent hook because it doesn't have any of the problems I outlined.

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    $\begingroup$ In climbing, the method you describe is sometimes used. It's known as a "bat hang": you hook your toes behind something, and let go with your hands, hanging only on your toes. The typical objective in doing this is to rest your arms for a while. $\endgroup$ Commented Oct 19, 2022 at 13:01
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Panda Trivia: Giant Pandas are the only bears with grasping paws. Instead of opposable thumbs an elongated wrist bone acts as a sixth finger to let them hold bamboo more easily.

I wasn't sure if I believed this, but on another web page:

A panda’s paw has six digits—five fingers and an opposable pseudo-thumb (actually an enlarged wrist bone) it uses merely to hold bamboo while eating.

So perhaps hope is not lost, although I don't think pandas are especially good climbers.

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    $\begingroup$ Isn't that a "rose by any other name is still a rose?" What's the difference between an "opposable pseudo-thumb" and an "opposable thumb," which the OP has explicitly rejected? $\endgroup$
    – JBH
    Commented Oct 18, 2022 at 23:44
  • $\begingroup$ Because it is not adding an opposable thumb. But modifies the joint and wrist bones to have a different kind of grasping ability? $\endgroup$
    – UVphoton
    Commented Oct 19, 2022 at 0:00
  • $\begingroup$ I'm curious to hear from the OP, because that sounds like splitting hairs. I didn't come away from the question with the understanding that how the opposable thumb was expressed changed the fact that the mechanics of an opposable thumb were rejected. Daron? I'm curious to know if this answer meets your expectations? I'll upvote it if true (and delete all my comments). but I won't down vote it if not because it is a cool piece of information. $\endgroup$
    – JBH
    Commented Oct 19, 2022 at 0:42
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    $\begingroup$ Fun fact: Panda's actually use their heads to help them climb. "The head serves as a make-do extra paw, first pressed against one side of the tree and then against the other. This extra pressure helps the bear hold on as it releases and raises an actual paw." (sciencenews.org/article/…) $\endgroup$
    – E Tam
    Commented Oct 19, 2022 at 2:34
  • $\begingroup$ Pandas are actually not very closely related to bears, so it's not surprising they'd have differing physiology. Pandas are more closely related to raccoons and ferrets and bears are more closely related to dogs and seals than either of them are to each other. $\endgroup$ Commented Oct 19, 2022 at 15:12
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If the tree is not too thick and the creature can wrap it's arm-legs around it and hold on to it this way, it could climb by gripping with the lower legs, extending the upper ones, moving the lower ones up, etc. Kind of like how bears climb except it would rely only on holding tight as bears also have claws to help with grip.

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