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My generation ship aliens have 2 thumbed hands. No, not the 2 thumbs on the same side that some humans have but the 2 thumbs on opposite sides. So it would be kind of like this:

enter image description here

Except more reptilian and less human as far as the skin but this picture right here is of a hand with 2 thumbs and the thumbs on opposite sides of the hand.

The reason my aliens have 2 thumbs on each hand is to increase their grip strength. The grip strength of the pinky may be 50% of the total grip strength of the human hand but we wouldn't have much grip strength without a thumb. Having a thumb makes us better at grabbing on to things. I would imagine that having 2 thumbs would be even better. Plus, with a 2 thumbed hand you could cross your 2 thumbs while having the other 4 fingers relaxed. And counting in base 12 would be easier too.

Anyway, would having a second thumb on the opposite side of the hand increase the aliens grip strength significantly? I think it would, but would my aliens really have significantly better grip strength than humans with a second thumb on the opposite side of both hands?

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The Moties are an alien race from a series by Pournelle and Niven who have hands like those you describe, with a thumb on each side of the hand. https://en.wikipedia.org/wiki/The_Mote_in_God%27s_Eye

enter image description here

from http://javidluffy.deviantart.com/art/Meet-the-Moties-326500764

As regards real things, koalas have two thumbs but on the same side. Thus the grip is still a pincer with thumbs on one side and fingers on the other.
enter image description here

The problem with a hand like you depict in your photoshop image is that the fingers cannot contribute to a pincer like grip with both thumbs at the same time. They become superfluous.

Possums come closest to a real thing that you describe except the fingers are rotated to work with the "thumbs". Each thumb has an allied finger. That still leaves the possum with a superfluous middle finger, which might be used to signal other possums. That on the front feet only - back feet are like our hands. I think the Moties might have had 6 fingers and probably that gives 2 fingers allied with each thumb and no spare middle.

I was interested to see the palm grip pads on the possum which I did not know about and which are pretty cool.

enter image description here

this possum hand from http://www.hsu.edu/Academics/ARNatureTrivia/mammal-marsupial.html

My proposition: for your alien, copy the possum hand with its palm grip pads.
Maybe the marsupial angle could be used to design other aspects of your aliens too.

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  • $\begingroup$ Very interesting, never noticed this yet used to hunt possums $\endgroup$ – Kilisi Apr 18 '17 at 20:48
  • $\begingroup$ Plus one for the Koalas. $\endgroup$ – a4android Apr 19 '17 at 8:11
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Well the main purpose of the thumb is in giving something for all the other fingers to push against while gripping.

The technical term for the gription provided by a finger is the coefficient of static friction. The force that a finger provides is directly related to this coefficient. Assuming then that your aliens thumbs and fingers are similar on coefficient and contact area, your getting a 1/5, or 20% increase in gription.

Hmm. Not the fifty percent we need. Well, there is of course the other factor in the gription equation, force applied. Here is where the extra thumb really shines. The equation is F=uN, where u is that coefficient and N is the normal force applied. With one thumb we assume the limiting factor for holding gripping an object bigger than why we can just curl our fingers around (where a thumb doesn't help much) is how much normal force applied by the fingers that the thumb can counteract. With two thumbs, you can counteract twice the force (assuming enough finger musculature to supply that force). That's a 1/2, or 50% increase in gription. Note, however, that this is only in the thumb required for gripping case. Think a little about the times you hold onto something really tight, and I think you'll find that thumbs aren't usually all that helpful. They're useful for opening doors, but consider how your aliens will have to grip things to make full use of this thumb.

As a side note: to us one-thumbers, that extra appendage would be pretty weird. The utensils etc. that the two-thumbers use will seem pretty weird two.

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  • $\begingroup$ Towards the end of the third paragraph I clarify that this analysis is purely centered around the case where the thumb is used for counter-pressure, as this is the only case I see that extra thumb improves grip strength. I did note that some thinking about how this alien grips things would need to be done to use the extra thumb to full benefit as we don't use the thumb that much in our deathgrip situations. $\endgroup$ – user29032 Apr 18 '17 at 11:32
  • $\begingroup$ Ok makes more sense now, I'll delete my comment $\endgroup$ – Kilisi Apr 18 '17 at 11:36
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    $\begingroup$ Thanks for the feedback! In the future I'll try to make sure that the case of analysis in question is easier to recognize. $\endgroup$ – user29032 Apr 18 '17 at 12:13
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For grip strength feats the fingers push against the gripped object against the palm, not the thumb. Try hanging off a branch gripped between fingers and thumb and you may get a chance to see what happened while you're falling. Whereas you can hang from a branch, take your thumb off entirely and wiggle it around and still hang there for a while. Other primates have tiny thumbs relative to ours, they stabilise and help a bit, but overall it's the fingers and palm doing the work.

So grip strength isn't so much about the thumb. The thumb comes in to it's own and is particularly useful for fine gripping and manipulation rather than strong grip. You can see this with powerlifters, they don't grip around a bar, their thumb is on the same side as the fingers for many lifts. And when disarming someone holding a weapon, or breaking a hold, the thumb is the weak point in their grip, not the fingers.

You extra thumb would actually get in the way for some bar lifts. Other situations such as martial arts it will be an advantage in some ways but bad in others since you lose the ability to chop with the edge of your hand and instead would damage the outward thumb trying to do so. Human hands are actually pretty well designed.

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Reptilian with multiple opposing digits? Sounds like chameleons:

enter image description here

photo source

Yes, our little Madagascan color-changing friends already have the arrangement you are looking for. Why? Because their arboreal life almost demands it.

While moving along from branch to branch, from tree to tree, even over flowers and leaves, you need to be able to hold on. The mouth and tongue capture the prey, and the tail aids with balance and can grip a little, but for true twig-grabbing strength, they rely on that second opposable digit.

Your aliens started from something very close to Earth's chameleon, and therefore likely arose in a similar environment or biome. They then went right on evolving, until arriving at sentience and space travel and all that.

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  • $\begingroup$ It would make much more sense for the two thumbs to be really opposite the fingers rather than off to the sides. $\endgroup$ – Kilisi Apr 18 '17 at 20:46
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Around cylindrical objects, yes

The human hand performs seven types of grips, the six listed below plus a fist. Your aliens should be able to duplicate all these motions though some will be more awkward than for a human.

Human hand grip categories

Of the six grip types, the cylindrical grip would benefit most from an additional thumb. Spherical grip might benefit from a second thumb. Tip, palmar, and lateral grip would improve in some circumstances. Hook grip would see no change. A fist might benefit from a second stabilizing thumb but I'm not sure how much (I'm don't have a good idea of how fist strikes work in order to make a guess about how a second thumb might help or hurt).

Where these aliens grew up

It seems clear that these aliens evolved in an environment where the ability to very securely grasp cylindrical objects was a significant selection pressure. Whatever the environment, the neurological costs associated with a second thumb provided the needed survivability over a single thumbed cousin.

Additional Reading

The Anatomy and Mechanics of the Human Hand

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The utility of a second thumb would probably be greater if the two thumbs were specialized, rather than identical mirror images of our current digits. Similar in spirit to the asymmetrically-armed Moties in Niven/Pournelle's classic first-contact novel, https://en.wikipedia.org/wiki/The_Mote_in_God%27s_Eye

I posit one thumb (probably our original one) becomes the 'power thumb' with stronger bones, tendons and muscles. The hand becomes elongated (longer carpal bones) and the "power thumb's" bones elongate to match. The 'dexterous' thumb stays at finger scale, for fine/delicate work.

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