Bipedal Digitigrades?

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Hwo Thumb
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Bipedal Digitigrades?

Postby Hwo Thumb » Sat Jun 14, 2014 2:10 am UTC

For a story I've been writing, I've come up with a few aliens to inhabit the universe - I'm choosing not to address pesky things like the odds of sophisticated alien species actually encountering each other in favor of a softer sci-fi - and an interesting question came up. One of the species I've created is a race of short, bipedal canines. Naturally, the numerous doodles of each fictional species eventually caught the eyes of a few of my friends, and one of them pointed out that there's no way a human-like biped would develop digitigrade legs. I hadn't really thought about this before, but it occurred to me that they might be right; without even noticing it, I had started drawing these doglike aliens with dog legs, but these legs are evolved for quadrupeds. Granted, digitigrade legs have appeared in other animals, like birds dinosaurs, but obviously a bird's dinosaur's skeleton is structured completely differently, so it makes sense. On the other hand, if we had legs like that, we'd have to walk considerably differently, and it would likely change the shape of our spines. However, if you made a practice of walking on the balls of your feet, you'd notice that it's only slightly uncomfortable, and I don't see any reason why a species with a slightly different leg structure couldn't walk like that without trouble. (Although I noticed that it's difficult to jog like that - I think a digitigrade biped would have to either full tilt sprint or walk, with little middle-ground.)

Even though my story's science is too soft, and it's written so that there's no real reason to address this, I can't help but be curious. My questions are:
1: Is there a way for a human-sized biped to walk comfortably with digitigrade legs?
2: If such a species were shorter, say, shoulder height, would it reduce the disadvantage from legs like this?
3: At what point/size do two digitigrade legs cease to become advantageous, if they ever are at all?

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Re: Bipedal Digitigrades?

Postby Dmytry » Sat Jun 14, 2014 10:52 am UTC

We can walk on our toes, and I don't see why it would be inherently problematic to have a longer foot, if the ankle was stronger. We can even run on things like this: https://www.youtube.com/watch?v=gYLdlTeO0NU . Picture a human leg with shorter tibia and longer foot, walking pretty much the same how we do when walking on our toes.

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Re: Bipedal Digitigrades?

Postby zukenft » Sun Jun 15, 2014 9:53 pm UTC

humans walk on their toes all the time (well, using high heels, but it's kinda the same thing). so yeah, a human-shaped alien could have dog legs just fine.

even if you say a dog won't evolve like that, what you have aren't dogs, but space dogs ;)

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Re: Bipedal Digitigrades?

Postby sehkzychic » Fri Jun 20, 2014 7:19 am UTC

Well, leaving aside the whole evolutionary shenanigans with having aliens with something very closely resembling a terrestrial tetrapod body plan (why not have only one or two limb joints? why have the whole business of tarsals, metatarsals, and phalanges instead of larger, sturdier feet with only two or three flat, hip-like bones?) there's no reason to think that digitigrade animals couldn't evolve a bipedal gait. I think you gave the best evidence already: therapod dinosaurs. Digitigrade, tetrapod predators. As an origin story for your canine species, they could even have had dog-like ancestors (quadrupedal). Dogs can already walk on their hind legs for a limited time, given the wide variety of extant dog species, it's not hard to believe that some society could breed exclusively/preferentially bipedal dogs if they wanted. Maybe to let them use their forepaws for other activities? Then either through civilization collapse or slave-dog uprising, the bipedal canines become the dominant species on their planet (or maybe they're just granted personhood by a progressive society). Presto!: space-faring, bipedal, doggy aliens! :D

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Re: Bipedal Digitigrades?

Postby Izawwlgood » Fri Jun 20, 2014 1:11 pm UTC

It's very plausible, but it would have additional adaptations to make it so. Such an animal would probably have larger hind paws than what we think of dogs today having.

Also, fwiw, it'd be inaccurate to call them 'canines', persay.

Also, there's plenty of reasons to assume homologies for limb use and walking.
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Re: Bipedal Digitigrades?

Postby Hwo Thumb » Mon Jun 23, 2014 12:59 am UTC

Izawwlgood wrote:It's very plausible, but it would have additional adaptations to make it so. Such an animal would probably have larger hind paws than what we think of dogs today having.

Also, fwiw, it'd be inaccurate to call them 'canines', persay.

Also, there's plenty of reasons to assume homologies for limb use and walking.

As far as phylogeny goes, I'm creating a system where instead of using Genus Species to refer to an animal, they use Genus Species Planet. That way, if animals highly similar to, say, insects evolve on a planet, we can still call them insects without worrying about it. In any case, I think I'll take your advice with the large feet and keep the digitigrade legs. Thanks!

While we're talking about the plausibility of their bodies themselves, would this work for an evolutionary background:
Since the surface of the planet is almost entirely ice and snow, (A major collision pushed the planet just far enough away from the sun to freeze all the oceans on the surface) life began below the surface, in massive underground cave systems, with thermophilic mosses that make glucose using noxious CO2 vents, the water that is readily available throughout, and the heat from vents from the mantle. While at first it may seem that evolving in darkness means that all the animals below would lack pigmentation, the mosses release excess energy in the form of light that makes the eye a very useful invention. Most of the animals are warm blooded chordates - the fluctuations in temperature make life too difficult for ectothermic animals. There are still animals similar to arthropods from the outside, but they are warm blooded, have lungs, and tend to be much larger. The dominant species is a bipedal, canine-like animal, the Farbian. Farbians have large ears to hear well, and to help regulate body temperature when too close to heat vents. They are short and stocky, with bodies built to resist the high pressures encountered below the surface. However, they are extremely adaptable, and some manage to live on the surface. Surface Farbians are white instead of black, and tend to be taller, with smaller ears. They are still alike enough to crossbreed.

Plausible?

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Re: Bipedal Digitigrades?

Postby Izawwlgood » Mon Jun 23, 2014 1:18 am UTC

Hwo Thumb wrote:As far as phylogeny goes, I'm creating a system where instead of using Genus Species to refer to an animal, they use Genus Species Planet. That way, if animals highly similar to, say, insects evolve on a planet, we can still call them insects without worrying about it. In any case, I think I'll take your advice with the large feet and keep the digitigrade legs. Thanks!
Maybe; the thing to remember is that they wouldn't be Canids if they arose elsewhere. They may LOOK like Canids, but unless they can biomagically interbred with Canids or are actually really Canids from Earth, they aren't Canids. I think it's fine to say they look like Canids! But Animalia Chordata Carnivora Caniformia Canidae Canis erectus they are not.

I think that's an important thing to remember about biology.

And yeah, I think it might be useful if you looked at hominid and Aves bipedal adaptations and applied it to your Canid-like species. All the bipeds I can think of have relatively large feet and pretty sturdy walkin' on structures therein. I found this handy image; as an anatomy exercise, imagine what sort of changes would need to occur to facilitate/allow bipedalism. That said, it is fiction, so, you know, biomechanical accuracy isn't terribly important.

Hwo Thumb wrote:ince the surface of the planet is almost entirely ice and snow, (A major collision pushed the planet just far enough away from the sun to freeze all the oceans on the surface) life began below the surface, in massive underground cave systems, with thermophilic mosses that make glucose using noxious CO2 vents, the water that is readily available throughout, and the heat from vents from the mantle.
Eh, maybe? I'd go with 'a long term cooling event locked the planet in a prolonged ice age, and life adapted/flourished underground'. Life as WE know it on Earth is/was dependant on photosynthesis to produce oodles of O2. I have a hard time imaging that being in reasonable equilibrium without ongoing photosynthesis. Oxygen as an electron acceptor is, of course, something that can probably (I think does?) happen sans photoactivation. I think? It's been years since micro.

Hwo Thumb wrote:While at first it may seem that evolving in darkness means that all the animals below would lack pigmentation, the mosses release excess energy in the form of light that makes the eye a very useful invention.
A la deep sea GFP? Instead of 'releasing excess energy in the form of light', I'd just go with 'bioluminescent moss'.

Hwo Thumb wrote: Most of the animals are warm blooded chordates - the fluctuations in temperature make life too difficult for ectothermic animals.
Devils advocate counter point; maintaining temperature is expensive, and maybe hibernating animals flourish?

Hwo Thumb wrote:The dominant species is a bipedal, canine-like animal, the Farbian. Farbians have large ears to hear well, and to help regulate body temperature when too close to heat vents. They are short and stocky, with bodies built to resist the high pressures encountered below the surface. However, they are extremely adaptable, and some manage to live on the surface. Surface Farbians are white instead of black, and tend to be taller, with smaller ears. They are still alike enough to crossbreed.
Neat. Echolocation for the dark, or are those light emitting mosses REALLY prolific? How vast are the caves? If there are hot vents, presumably the planet is geologically active; is this an ongoing danger to the subterranian life? If there are surface dwelling organisms, what is the surface like?

Have you read Endymion/Rise of Endymion? An ice planet reminds me a bit of one of the planets they encounter, with... pretty horrible surface/burrowing creatures.
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Re: Bipedal Digitigrades?

Postby Hwo Thumb » Mon Jun 23, 2014 2:34 am UTC

Izawwlgood wrote:Eh, maybe? I'd go with 'a long term cooling event locked the planet in a prolonged ice age, and life adapted/flourished underground'. Life as WE know it on Earth is/was dependant on photosynthesis to produce oodles of O2. I have a hard time imaging that being in reasonable equilibrium without ongoing photosynthesis. Oxygen as an electron acceptor is, of course, something that can probably (I think does?) happen sans photoactivation. I think? It's been years since micro.

Oh, good point. The ice age thing makes more sense. As for the O2, the mosses are replacing photosynthesis. Though I'm not sure how to justify the O2 getting turned back to CO2. Is it reasonable to say that aerobic life developed to change to O2 back before the CO2 ran out in the caves and everything got oxygen poisoning?

Izawwlgood wrote:A la deep sea GFP? Instead of 'releasing excess energy in the form of light', I'd just go with 'bioluminescent moss'.

Pretty much. I think I had that written originally, not sure why I erased it. (I think I just wanted to clarify that the energy to produce the light is whatever is too much to use, so it wouldn't seem like they're getting magical light from nowhere. Though bio luminescence isn't too taxing on energy, is it?)

Izawwlgood wrote:Devils advocate counter point; maintaining temperature is expensive, and maybe hibernating animals flourish?
Oh dear. Scratch that, then. Hibernating ectothermic animals live closer to the vents where teh heat fluctuation is greater. Endothermic creatures live higher up where the temperature is more stable and mile.

Izawwlgood wrote:Neat. Echolocation for the dark, or are those light emitting mosses REALLY prolific? How vast are the caves? If there are hot vents, presumably the planet is geologically active; is this an ongoing danger to the subterranian life? If there are surface dwelling organisms, what is the surface like?

They basically completely coat the wall of the caves wherever there's enough warmth to support them. As for the caves themselves, they are massive. The caves range from tight tunnels and corridors to massive cathedral-sized rooms.I didn't want to blab about geology, but yes, the planet is extremely prone to earthquakes. Cities can only be built in the sturdiest of caves and need supports to guarantee their safety. It took a long time for advanced city building to develop. Underground volcanos are a constant threat. Therefore, the safest parts of the planet are in the middle of the caves - close enough to the vents to have energy and food, but far away enough to avoid the worst geological activity. The surface is mostly barren, populated by species that live both above and below the ground. Cool idea: What if some surface animals -not plants - developed photosynthesis? Since the surface is too cold for plants, this is the only way I can think of to justify anything living permanently on the surface. The development of advanced agriculture allows food to be transported up to those on the surface. In return, they provide clean water, and exotic meat from those few species that live above ground.

Oh, I forgot to mention this: The Farbians have weak infrared vision; enough to detect other warm-blooded animals. The trade off is a slight loss in eye resolution.

Have you read Endymion/Rise of Endymion? An ice planet reminds me a bit of one of the planets they encounter, with... pretty horrible surface/burrowing creatures.[/quote]
I might give it a try. It doesn't sound like my kind of sci-fi, but I've been meaning to read more diverse literature lately anyway. (Mostly the only Sci-fi I read is dystopians and space operas like S.A. Corey's The Expanse) Thank you so much for your insight! It helps to have someone to bounce ideas off of. I've been working out the logistics of some story elements so long, I need a second opinion to tell if they still make sense.

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Re: Bipedal Digitigrades?

Postby ahammel » Mon Jun 23, 2014 4:59 am UTC

Hwo Thumb wrote:As far as phylogeny goes, I'm creating a system where instead of using Genus Species to refer to an animal, they use Genus Species Planet. That way, if animals highly similar to, say, insects evolve on a planet, we can still call them insects without worrying about it.

I think that if the situation arose, sysyematists would probably add a taxonomic rank above Domain indicating a group of organisms that share a single origin of life. Call it a Dominion or something. All life on earth would be under the Terra Dominion, and all life on your fictional planet would be under the Fictalonian Dominion, which would be subdivided into Domains, Phyla, Orders, etc.

As for calling them Canines, there's already a precedent for sharing Genus names among Kingdoms, so you could share higher order taxonomic ranks among Dominions if you really want to, but I imagine biologists would take something weird about the new Species' anatomy and translate it into dog Latin/Greek to get their Order names.
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Re: Bipedal Digitigrades?

Postby PM 2Ring » Mon Jun 23, 2014 6:16 am UTC

Izawwlgood wrote:
Hwo Thumb wrote:While at first it may seem that evolving in darkness means that all the animals below would lack pigmentation, the mosses release excess energy in the form of light that makes the eye a very useful invention.
A la deep sea GFP? Instead of 'releasing excess energy in the form of light', I'd just go with 'bioluminescent moss'.


Yeah. Light energy in the visible frequencies tends to be too useful in driving chemical reactions to just throw it away for no good reason. So waste energy is far more likely to be in the form of heat, which is lower frequency than visible light. Also, waste energy is high entropy energy, IOW, energy you can't use to do work. Heat can be used to increase chemical reaction rates, but it's of limited use if you don't have a temperature difference to exploit.

So what evolutionary advantage does the moss gain in being bioluminescent? I guess being easier to find by animals who come to feed on it may help the moss to get spread around.

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Re: Bipedal Digitigrades?

Postby Izawwlgood » Mon Jun 23, 2014 8:00 pm UTC

Also, think about what the vision of creatures would be like having evolved to pick up outlines against a diffuse, pointless light source. They'd probably be TERRIBLE at differentiating features, but be exceptional at seeing movement and changes to light levels. Or something. LIke you said.

Hwo Thumb wrote:Oh, good point. The ice age thing makes more sense. As for the O2, the mosses are replacing photosynthesis. Though I'm not sure how to justify the O2 getting turned back to CO2. Is it reasonable to say that aerobic life developed to change to O2 back before the CO2 ran out in the caves and everything got oxygen poisoning?
My thinking was 'life emerged as on Earth, ice age happened, most life moved underground, photosynthesis taken over by mosses (or other weird things!) that release O2 (maybe from the heat + CO2 around these vents?), and lots of other life is aerobic. Presumably the interior/exterior of the planet aren't sealed from one another, so you have these massive cold canyons where something neat is happening. Maybe there are giant flying behemoths that photosynthesize!

Hwo Thumb wrote:Pretty much. I think I had that written originally, not sure why I erased it. (I think I just wanted to clarify that the energy to produce the light is whatever is too much to use, so it wouldn't seem like they're getting magical light from nowhere. Though bio luminescence isn't too taxing on energy, is it?)
I don't think so, relative to other biological functions. But what do you mean by 'produced light as a byproduct'. That doesn't make sense to me. Life doesn't really have 'excess energy' that it needs to offload, it would instead store that as something else. Fat stores or whatever.

Hwo Thumb wrote:Oh dear. Scratch that, then. Hibernating ectothermic animals live closer to the vents where teh heat fluctuation is greater. Endothermic creatures live higher up where the temperature is more stable and mile.
Boom, easy solution. Or maybe they all live together, struggling with different issues. We have cicada's that come up every 17ish years - maybe this planet has an outpouring of Flumrians every time a gas vent burrows up and ugh those fucking Flumrians are everywhere for a few months.

Hwo Thumb wrote:What if some surface animals -not plants - developed photosynthesis? Since the surface is too cold for plants, this is the only way I can think of to justify anything living permanently on the surface. The development of advanced agriculture allows food to be transported up to those on the surface. In return, they provide clean water, and exotic meat from those few species that live above ground.
Giant photosynthetic flying whales! Or maybe there's something else crazy on the surface, and the surface is only survivable by the exchange of goods with the underworld. Lots of possibilities!



Remember though, it's fiction. Sticking to feasibility improves the willing suspension of disbelief, but isn't required for it.
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Re: Bipedal Digitigrades?

Postby sehkzychic » Mon Jul 21, 2014 5:02 am UTC

Hwo Thumb wrote:
Izawwlgood wrote:Eh, maybe? I'd go with 'a long term cooling event locked the planet in a prolonged ice age, and life adapted/flourished underground'. Life as WE know it on Earth is/was dependant on photosynthesis to produce oodles of O2. I have a hard time imaging that being in reasonable equilibrium without ongoing photosynthesis. Oxygen as an electron acceptor is, of course, something that can probably (I think does?) happen sans photoactivation. I think? It's been years since micro.

Oh, good point. The ice age thing makes more sense. As for the O2, the mosses are replacing photosynthesis. Though I'm not sure how to justify the O2 getting turned back to CO2. Is it reasonable to say that aerobic life developed to change to O2 back before the CO2 ran out in the caves and everything got oxygen poisoning?

You don't really need an excuse to turn O2 into CO2. Molecular oxygen is very reactive, and it takes a lot of energy to produce. On Earth, nearly all of our oxygen is produced using the radiated energy of an enormous fusion reactor (colloquially known as "The Sun" lol). Mars doesn't have photosynthetes, so whatever oxygen was around and produced geochemically is stuck in the rocks as iron oxide. If you've got enough carbon lying around, and not too much metal easily accessible, getting CO2 is just a matter of time. Getting rid of oxygen is much easier than producing it, and even if life has to go through several cycles of producing oxygen and nearly dying out from oxygen poisoning, the oxygen will go away on its own, and the remaining organisms will be better adapted to handling oxidative stress. It might take millions of years to actually have oxidative reduction emerge, but it's definitely not unreasonable to predict over evolutionary time-scales.

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Re: Bipedal Digitigrades?

Postby diamante » Wed Aug 05, 2015 7:54 am UTC

This is a pretty late response but if you or anyone else is still interested I have a few points to make.

First of all, I believe you and your friend are falling into the often seen trap of assuming the human body is ideal for what it does (it is most definitely not). The ideal leg formation for large bipeds (large here is considerably smaller then humans and up, I'm not sure where the actual cutoff is and there would be a potential different class of super large that this may not apply to) is digitigrade not plantigrade. While many people would assume the lower load bearing capabilities would not allow us to walk on two legs if we had digitigrade legs instead of plantigrade, the weak point that would determine total load bearing capabilities however would still be our spine by a large margin (note this applies only to humans who have a terribly set up spinal column).

I am assuming your bipedal canines are carnivores? If that is so they would almost certainly be digitigrade as that would give them faster and more importantly quieter movement, two traits that are very important to carnivores.

Another point to consider is that the animal on earth with by far the best bipedal movement is the ostrich, which like all other birds has digitigrade legs.

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Re: Bipedal Digitigrades?

Postby Izawwlgood » Sun Aug 09, 2015 12:15 am UTC

diamante wrote: the weak point that would determine total load bearing capabilities however would still be our spine by a large margin (note this applies only to humans who have a terribly set up spinal column).
... I think you have this 100% backwards actually. There's a reason people gathering and transporting water great distances do so by balancing it on their heads instead of holding it in front of them - our spines are immensely immensely good at supporting downward weight. That's part of the advantage of our bipedal layout, in fact.
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Re: Bipedal Digitigrades?

Postby sevenperforce » Tue Aug 11, 2015 5:20 pm UTC

Izawwlgood wrote:There's a reason people gathering and transporting water great distances do so by balancing it on their heads instead of holding it in front of them - our spines are immensely immensely good at supporting downward weight. That's part of the advantage of our bipedal layout, in fact.

Indeed. This idiot holds a variety of world records for the most weight balanced on his head. The greatest weight, I think, was 416 pounds of bricks. Which is kind of scary.

Digitigrade gait is inherently unstable. A species of bipedal digitigrades would either develop extraordinarily good balance, rearrange some portion of their hips in order to stabilize, or have a tendency to spend a great deal of time sitting/resting. Humans have no difficulty standing in a stable position for long periods of time, but a bipedal digitigrade would likely have a "standing" position that is closer to a half-crouch, so they would rise when they wanted to move around.


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