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all,

I was reading another post on this forum about humans with digitigrade legs, mostly noting that they were relatively inefficient due to constant exertion to remain standing. On a planet with lower gravity, like the Moon or Mars, this would be less of an issue, right? Would evolving digitigrade legs be useful compared to plantigrade in 1/6-1/4 Earth gravity, particularly for a predatory, bipedal animal? I know “running” in the traditional sense was impossible for astronauts on the Apollo missions due to the lack of gravity, forcing them to move around by hopping or bounding, so maybe digitigrade legs wouldn’t be worth the instability (?) I’d be curious to see how high a digitigrade Martian could jump. Any input?

Thanks!

The digitigrade legs post: Humanoids with digitigrade legs?

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  • $\begingroup$ On this and my last question, the first two words got cut off. Not really sure why. It said “Hello again, all,” instead of just “all,” when I posted it. Not a huge deal, just confused lol. $\endgroup$
    – Mark Price
    Sep 4, 2021 at 4:59
  • $\begingroup$ Someone more experienced with SE might chime in with more but as I understand it we try to avoid pleasantry's that do not enhance the question or answer. That included hellos and thanks you. Just a FYI :) $\endgroup$
    – Kezat
    Sep 4, 2021 at 13:17
  • $\begingroup$ @Mark Price Digitigrade feet would not be much advantage in motion on a low gravity world due to lack of oxygen and thus death. Unless, of course, they were wearing spacesuits outside, or moving around inside an enclosed habitat with an artificial atmosphere. Or maybe their society had terraformed a low gravity world to give it a breathable atmosphere. Andyou might want to use the search function in the black bar above to search for quesitnsand answers about hypothetical low gravity worlds with dense atmospheres. $\endgroup$ Sep 4, 2021 at 16:38
  • $\begingroup$ @M.A.Golding low gravity doesn't equate lack of oxygen, especially when none of 'science' tags are involved. And Mark's question is about the superiority (or lack of it) of digitigrade in no relation with the atmospheric pressure - his question works just fine in an "Asimov's Cave of steel just on Mars" scenario. $\endgroup$ Oct 4, 2021 at 6:40

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It depends.

I'll assume here that we're looking at planet earth identical in almost every way but with weaker gravity.

Basically, let's begin with the differences: what is up with each arrangement?

Basically, plantigrade legs offer much more stability and higher surface area, this can still allow a creature to be efficient at walking and have much better balance. However, plantigrade legs come short when it comes to running at higher speeds. It's generally better if you need to move around in 3 dimensions, if you need to have sure footing or if you're using your legs to swim. They can also allow for a more flexible leg.

Digitigrade are the opposite basically. They're great for running faster and with good efficiency, but offer less surface area and are generally less stable when you need to balance yourself. They're great for running fast in 2 dimensions and can allow for very strong kicks,but being structured for running means that, by default, they'll be less flexible.

So, plantigrade or digitigeade predator? It's not a straightforward answer. Depending on how and where they hunt, things can change quite a bit. Let's go over 2 examples:

1- Your creature can fly (on such a scenario, lower gravity means you can be bulkier than you normally could, as the pull of gravity being weaker makes taking it to the skies much easier). On such a scenario, how you'll take down your prey will also tell how your legs are. If it has the body plan of a bird, then it's probably going to be digitigrade with eagle or owl-like legs. Of however it uses its Jaws like a spectral bat, it's legs might be kept plantigrade since they aren't as needed. The type of wing might also interfere with that, since as far as I know all bats and the majority of larger, more human-sized pterosaurs were plantigrade (which would make in your bipedal creature, since it'll need more stability without the forelimbs to help walking) while all birds descend from dogitigrade ancestors and thus kept such a limb arrangement.

2-your creature lives and Hunts in dense forests and can't fly. Plantigrade. In such an environment running is pointless, much like a flying creature, when you move, you do so in 3 dimensions, and with less gravity to pull you down you can still hold onto weaker branches without falling. In such a scenario you don't want to be good at running on the ground, you want to be nimble and good at grabbing, things you can do much better with plantigrade hind limbs (which you can litterally turn into a second pair of dexterous hands to move around).

3- your creature lives on plains and runs after prey. By default I'd say digitigrade, but unlike the other 2 cases, in here the lower gravity is a problem. You'll take longer to reach the ground, and while that can mean some long running gates, it also means there's a good window of time between each time you get to push the ground, meaning running creatures will probably need better ways to get as much traction as possible. Claws that dig into the ground like those of an ostrich come to mind.

I'd still say that digitigrade would be overall better than plantigrade, but this should illustrate my point well: lower gravity negatively affects you specializations for running while buffing adaptations to fly and climb. If your Martian needs to run after prey, it'll probably still be digitirade (like an ostrich, emu or cassowary, not like a cat or a dog. Mind how the feet of the latter ones are adapted to a quadruped).

Overall it depends a lot on what your creature Hunts, how and where. You want to awkwardly pursue prey on the ground? Digitigrade might be best. You want to climb at them at terrifyingly efficient speeds below the forest canopy? Platigrade. You're an aerial predator that hits what's more stuck to the ground than you? It might vary on what you use to kill them (your legs, with feathery wings? Probably digitigrade like a bird's. You use your Jaws and membranous wings? Plantograde might be a better fit).

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    $\begingroup$ Digitigrade legs are also much better at jumping, and coupled with a lower gravity might allow for some truly stupendous leaps. But they also tend to put your center of mass higher above the ground, which will make for traction/balance problems when accelerating, other than the initial leap. $\endgroup$
    – PcMan
    Oct 4, 2021 at 6:32
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On average a digitigrade "foot" has a lower contact area than other types.

A lower contact area coupled with a lower weight (because of the lower gravity) means also a lower friction, which is essential for actual motion control.

I have the impression that, unless the ground has some very special conditions, a digitigrade foot won't be that much of an advantage.

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  • $\begingroup$ Assuming a sandy or dusty terrain, could some kind of big claw or “scoop” on the toe help with motion control just by digging into the sand? Not sure if gravity would be strong enough for the scoop to penetrate. $\endgroup$
    – Mark Price
    Sep 4, 2021 at 4:57
  • $\begingroup$ @MarkPrice "help with motion control just by digging into the sand?" Ok, now do it on gravel or rocks. 'Cause you didn't ask your question with "in the regoliths of Moon or Mars", so I can't exclude a "on the corridors of Martian Council" environ :) $\endgroup$ Oct 4, 2021 at 6:49

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