Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

All legs and thumbs...?

  1. Oct 15, 2015 #1
    What are the advantages and disadvantages of hexapedal movement as opposed to bipedal or quadrapedal?

    I am designing some sapient/sentient alien species (and the requisite environments that would have shaped them). I am attempting to avoid falling into the too-easy trap of just going the Star Trek "humanoid with knobbly faces" and break the mold a little more. And because I am far to pendantic fo my own good, I'm never content to go "because it's cool" without at least making some attempt to understand the real science.

    My current contender is a species that would come from a six-limb pattern dominant (as opposed to a terresial large verbrate four-limb pattern). However, I don't actually know (nor have I been able to Google-fu) how insect walking patterns would scale up to larger creatures and how the gaits compare with each other.

    To my understanding as a lay student of natural history, the number of limbs a group developed evolutionarily was something of a "it just turned out like that" (like the number of digits on a vertebrate hand - more on this further down) chance. What I don't know is whether that would apply to a macro-scale change like this or whether, like trachea, hexpedal (or more) patterns are something that only works on smaller creatures. I.e. could you have essentially a six-legged lion (obviously the legs would be in different places, but you get my drift) or a four-legged ant that woudl function comparably to their terrestial counterparts?

    Further complicated this is the nominal the species would have developed a quadrupedal stance (ala Preying Mantis) wherein it would have an upright body and the forelegs would have become manipulative limbs. I'm uncertain as to whether this would mean it would have developed a (say) mammal-like limb positioning/stance, or whether, like the mantis, it would still have an insect-like one and what the relative pros and cons of the two patterns would be.

    (I am envisioning said creature to have evolved from a group of primarily scavengers and generalists and the upright stance would have arisen as a way to carry things over frequent floods - a bit like some theories of how human upright stance arose.)

    Finally, there is the question of digits. While to the best of my knowledge, as I say, the number of fingers is essentially "random chance," what I'd be most interested to learn is about thumbs. Specifically, are there any mechanical reasons why the thumb is in the inside/top of the hand and not, say, on the outside/bottom. Would having the thumb on the opposite side of a hand make any difference at all to its function?

    If anyone can answer some of these questions - or point me in the right direction where I might find them, I would be greatly appreciative.
  2. jcsd
  3. Oct 15, 2015 #2


    User Avatar
    Gold Member

    I think you're pretty safe in going with what works for you. I don't think there's so strong a case for mammal-like superiority when it comes to limbs. However, we believe that reptiles would have a tough time evolving to an upright stance.

    Again, you're probably safe with any design that allows a grip.

    There is much suggestion that mammals have been able to grow large enough to have a large brain partly because of the superior anatomy i.e. endoskeleton. Insects have an exoskeleton, which restricts their. growth unless they molt. It also affects the attachments of their muscles and ligaments, though I can't rmember the specifics.
  4. Oct 18, 2015 #3
    The key is going to be energy efficiency and its environment.

    For sentient creatures (like ourselves) about 20% of total calories used goes to the brain. The limiting factor for our brain size is really the size of our heart and circulatory system (along with supporting organs). Adding extra limbs only complicates that energy budget.

    Not only does it consume more calories for muscle and tissue, but it increases the load on the brain (primarily the motor cortex), which would mandate a larger brain size and its support systems.

    I haven't even touched on dexterity and innervation of limbs, but I remember from my anatomy class a distorted picture of a human that represented body size size with respect to innervation. Hands, feet, and face were abnormally large in that picture. Adding more limbs taxes the brain even more. not just more motor nerves, but from a sensory perspective, too.

    Nature seems to adore efficiency and approaching the problem from that direction will be a help. Also, you might research exobiology as a guide.

    Of course, if it isn't critical that your creatures be that realistic, then do what you like.

    One less critical factor are subtle twists in anatomy such as pentadactyl versus hexadactyl hands and feet (number of fingers and toes). Here it appears evolution simply selected pentadactyl more by chance. There is plenty of fossil evidence to hexadactyl and even octadactyl digits some 350 million years ago with no known complications. We simply ended up with five digits by the luck of the draw, which was a good thing because I am perfectly happy with working in base 10. :)
  5. Oct 19, 2015 #4


    User Avatar
    Gold Member

    And cats are busy breaking the rules on this one once again.

  6. Oct 19, 2015 #5
    There are no rules for cats except that they do rule. :smile:
  7. Oct 20, 2015 #6
    As for the thumb, no, I see no reason that the position of our opposable thumb or the number of digits on our hands would make any difference. There are other ways to grip an object, look at an eagle's talon.
  8. Oct 20, 2015 #7
    I wonder if the talon would serve as well as our digits. The talon is optimized for grasping prey, but not so good for manipulation compared to the opposable thumbs.

    Our thumb can nimbly touch each finger. Actually, no other primate has the level of dexterity our hands provide. We have superior thumbs.

    That dexterity provides for advanced tool making, something a talon would not be able to do. An eagle, no matter how sentient you make it would not be my choice for a surgeon.

    I will admit I am a fan of convergent evolution.
  9. Oct 20, 2015 #8
    As an eagle's talon is right now, I agree, however: remove the huge sharp claws and provide slightly more side to side motion to the "thumb" and you have a hand that seems like it could do anything we can do.
  10. Oct 20, 2015 #9
    An osprey might be a better choice as they have a true opposable digit (Eagles do not do well catching fish like osprey), but the number of joint segments in that foot are fewer than our hands, which limits dexterity.

    I am thinking you will be hard pressed to find anything as versatile as the human hand. If you do it is not likely to have a radical difference.
  11. Nov 2, 2015 #10
    I haven't abandoned this, despite not making any more recent posts until (I've been rather busy and my time to spend on this particular facet has been a bit limited over the last two or three weeks!)

    Right. So, general concensus seems to be that, for the formation of digits (and opposable digits), I can pretty much take carte blanche to do whatever comes to mind, within reason (as I half-expected, but it never hurts to pick a few other brains). So im Iwanted to do, say, seven fingers hands including a thumb which is opposite to the way humans thumbs are I'd be okay. (Not that I've actually thought hard about that yet, as irst I wanted to ascertain if there was any mechanical convergeance factors I needed to bear in mind.)

    That in an of itself is not necessarily a problem - it just would mean the resultant creature would require a highly efficient metabolism and/or Be Smarter than humans - though it is a consideration I had not noted. (Actually, I had intended the nominal alpha plan would have a large head to accomodate a larger mouth, so adding a big brain to that would not be a stretch!)

    I have done a little bit; thus far I have read "Evolving the Alien" by Jack Cohen and Ian Steward, which while interesting, was not massively helpful in my general thrust, and whatever I can manage to turn up on google. If you have any good sources, recommendations or suggestions (bearing in mind I can cope with a modest level of technical details, but I'm an engineering-trained CAD jockey with a keen lay interest in natural history and such rather than an actual professional zoologist!), I would appreciate it.

    Which brings me back to the major question, which is whether or not a hexapedial (or octopedial etc etc) body structure is efficient at larger than insect sizes.

    (I will be assuming that the creatures, while having a hexapedial body-structure, will be closer to a mammalian physiology, i.e. probably endo skeleton, endothermic, rather than just a big, smart insect.)

    If it isn't, then the question becomes which limb would be lost and/or is there another use for it that might have instead have been developed. (I.e., could a phylum/superclass/class have turned a third pair of legs into some sort of manipulator (e.g. crab claws or something) and move around tetrapod style on the other three), or is tetrapedial body-shape just the optimal convergeant solution?
  12. Nov 2, 2015 #11
    Which goes back to one of my previous posts on brain size versus support organs. Extra limbs are just evolutionary baggage for sentient creatures.

    However, it's your story and you can make aliens any way you want. You would not be the first to walk on the wild side here.

    Specialization in limbs will be dictated by the environment. If something in the environment favors that new trait, that creature will have a statistically better procreation result and ultimately displace creatures that lack that trait over time.
  13. Nov 3, 2015 #12

    So, let me approach this from another couple of angles.

    What would happen with a sentient/sapient octopus, then? (Which already have pretty complex brains.) Would it lose tentacles, or are they a facet of the aquatic environment (so you would not get land-octopi) and/or the fact it has no fingers?

    Going back to the hexapods:. First off, then, are you even likely to get an endothermic hexapod? Or is tetrapod a convergent universal, not a parochial (i.e. one like the number digits which is because it just turned out that way) trait for large-land-based creatures?

    Assuming the answer to the first question is yes (because if not, it fundementally changes the entire set-up and pretty much garentees that you basically have to have humanoid sentent/sapient creatures) then how does your sentient/sapient creature develop and what is it's "primate" group like? Given that it would be moving from a purely hexapedal movement system to a mantis-like one (where it would have two not-quite-arms and still move around on six limbs) as the arms develop, how do the legs change? I'm having a bit of difficulty imagining what the mid-stage creature looks like; I can get how it would go from "walk on sex legs" to "walk on four legs with two arms", but going from there to "walk on two legs with two arms" I can't quite wrap my head around how that would work. Would they first have to develop an ape-like stance while the middle legs disappear, and then move to a more upright stance? Or would one pair of legs... "take over" somehow...?

    What about if it is arboreal? (Thinking of how many arboreal mammals re-develop a fifth limb in the form of a prehensile tail.) Would it be likely to retain both pairs of legs then?
  14. Nov 3, 2015 #13
    I think vertebrates were spawned from fish, so the number of pectoral fins really defined the number of limbs from the start.

    Adding additional limbs is going to be evolutionarily more difficult than losing them (i.e., snakes). Why? because additional limbs require larger cardiovascular system, additional nervous systems through the spine, and additional regions in the brain.

    The probability of a genetic mutation suddenly pops up sporting these attributes and successfully overtaking the population that it spawned from is probably close to nil.

    So, I guess that your best chance for such a creature (six legs) to arise in the first place begins in the sea and assumes that there is some evolutionary advantage to additional fins. I'm not a ichthyologist, so I really can't help you there, but it might give you a direction to pursue if you could find a biologist in that field.
  15. Nov 4, 2015 #14
    As it is, the class of cephalopods is restricted to full salinity oceanic waters and don´t occur on land nor in fresh or even brackish waters, because they happen not to osmoregulate.
    That´s not a general trait of mollusca. Snails occur in sea, in fresh water and on land. So do slugs.
    If gastropodes make a good use of their one foot, I see no reason why a land octopus might not use all eight tentacles.
    How would land animals compare between fleshy legs with no skeletons (snails, slugs, land octopodes), 6+ legs with exoskeletons (insects, crabs) and legs with endoskeletons (tetrapodes)?
  16. Nov 4, 2015 #15
    I had an interesting argument with a retired doctor on a similar thread. Basically, we were discussing the advantages of bones (endoskeleton) for limbs. My point was that they provide excellent leverage to increase the effective use of muscle.

    Probably a good example would be standing, which takes relatively little energy due to the bone structure. Imagine if your legs were structured like the trunk of an elephant. It would take much more effort to stand because the muscles would need to be contracted to maintain a stiff appendage.

    That argument doesn't even discuss the advantages of how bones protect vital organs.

    For land dwelling creatures of any size you need something to protect organs and bones do a very good job of that. However, they also make movement energy efficient.

    For water based organism like the octopus it is not such an essential trait, as they ambulate well in that environment, but it would be very difficult for any sentient creature to create any kind of technology restricted to water.
  17. Nov 4, 2015 #16
    What makes you say that?

    There are a few sea-creatures that use tools (one octopus reportedly takes them from the shore). I find it difficult as an engineer to believe that, in the right circumstances for sapent/sentient creatures to evolve, it would be that implausible. Granted, you would probably find their "tech tree" as it were would be quite different in many ways (aside from the undeerlying principles). (The right conditions for sapience/sentience would seem to be the more difficult lightning to bottle, as its emergance in humans is still not that well understood.)

    This is something of a side-question: I was, in general, considering an endoskeleton creature (if for no mother reason than to avoid the mammal/avian/reptile/amphibian or insect paradigm you often find you get in scfi). But rather than slavishly going with tetrapod (as I have done with most alien races thus far) I wanted to at least examine the possiblity of what other sorts of body-forms a dominant phylum might feasibly take.

    (One of my long-term projects is an entirely alien planet built from the ground up (strictly no humands allowed!) and I want to very carefully strike the best balance between "not Earth analogue," "reasonably plausible" and "not too alien that a RPG player won't go funny trying to wrap their heads around playing the sapient/sentient species.")
  18. Nov 4, 2015 #17
    thumbs - inside/top - orientation ? Palms up, the thumb is on the outside. palms down it is on the inside. Difficult to use with palms outside, thumbs down.

    But at most, you can get about 270 degrees rotation of the hand with the use of the shoulder, arms and wrist. The wrist rotates very little. The shoulder about 90 degrees and the rest 180 from the forearm.
    The elbow joint flex is inwards.
    Hands, with fingers and thumbs, just jutting out from the body aren't much useful - the whole thumb-fingers-wrist-arm-shoulder thing seems to be neglected as having importance to human survival.

    With that, and without fingers/ thumb speciality, a tool at the end of the wrist has very good mobility to locate an object, clasp it, and return it to the mouth, as I suspect, as a general statement, that any organism at seeing an object would be to decide whether it is food or not. ( Maybe we still think that way but have learned to supress such immediate inclinations as we grow up ). The tool, could be just two prongs that move apart, and that could be sufficient for many purposes.

    A better tool has extra digits to overcome certain deficiencies of a two pronged tool. The grip on particular objects, such as a club, becomes better with the extra digit tool, for one reason being that less pressure, or less force, per digit is needed to hold the object, and the wider grip allows some better manipulation of the object in space.

    With extra digits, the thumb speciality is in its location for the benefit of the organism. Objects picked up and brought to the mouth, or for examination, suffer little or no interferance from the other digits.

    A thumb say on the inside bottom - the other digits are in way of vision so while eating, when the food is brought to the mouth; maybe you may miss a predator movement. Also, energy expenditure is important. One may require a full 180 degree twist of the hand to put the object in your mouth, in which case the added expendature of energy necesitates eating a while longer, or starving for longer if required supply of food is not available.
  19. Nov 4, 2015 #18
    Aha! Thanks.

    I was thinking that it wouldn't make a lot of difference - and it probably wouldn't - so long you were eating something that you grasped with the whole hand. (e.g. carrot or something) because you'd raise it to your mouth same as we do. But as I worked through what you said, I realised that it DOES make a difference if it's a small thing, something you'd have to pick up with finger and thumb, doesn't it? Because that would be like eating something held between your thumb and your little finger. Then you would have to bend your hand in more awkward position*. I suspect that maybe if your joints naturally were set up that way it might be slightly easier (though without a basis through which to test that, I wouldn't presume to do anything other than theorise) - but we have established it DOES make a difference.

    So the the sake of sanity, I think I will abandon that idea, and either stick with humanoid hand-structure - or, as I have done before, have creatures with more than one "thumb" if I want to gret more creative. (Or have small tentacles instead of fingers or something...!)

    *Y'know, assuming your mouth was in a head position and not on your belly or something...
  20. Nov 4, 2015 #19
    Well, try to light a fire underwater and see what happens. Call me when you get it started. I'll bring the marshmallows and beer. :-)

    Developing a real technology underwater is never going to be as easy as in air. Clearly electronics are out of the question unless you do that on land, but if you can do that on land, why return to the sea? You know how rugged marine equipment needs to be.

    Do you have an environment already in mind or are you designing from the other direction (i.e., life forms first then design an environment that supports it)?
  21. Nov 4, 2015 #20

    Yes, fire, obviously. (It's been a long couple of weeks short of sleep because of the builders, that's my excuse...!) I was thinking both too advanced and too simple. Yeah, that does make it a fair bit harder, don't it? (Thought to be fair, also for creatures not in an oxygen atmosphere, too.)

    This thread is supporting basically a double-pronged, but related exercise.

    (Long winded explanation inbound...)

    The first one is to create an (ideally fairly nonhumanoid) alien species to be "like humans but not" for my wargames starfleets. In the sense that humans tend to get loads of factions, and alien races tend to be lumped into "this alien species" and while I've made a few nudges in that direction up until now, this will be the first time I've actively gone out of my way to set it up from the start with two factions.

    This is the nominal "hexapod" thing I'm contemplating. The reason for that is simply that the race avatar I used during a playthrough of Space Empires 5 where the first faction (the Xyriat Hegemony) was first named was some sort of big-jaws massive head on spider-y quaduped legs (with no arms...!) Obviously not quite suitable, but I liked the idea enough to explore a four-legged, two-armed race for proper use. So for this species, it is sort of fit the environment to the creature, though the ideas are no concrete in my head at the moment, so there's room to play.

    (This is strictly actually part of my day job, since it's fluff for my CAD 3D print starship fleets!)

    The second prong is rather more complex and in-depth. This is the aforementioned entirely alien world, which is (in principle) a new campaign world for my roleplaying. After doing a campaign world for D&D where I chucked the monster manual and a fair bit of the rules out the window and re-built the bestiary from mythical sources from scratch, I decided after tene years or so, that wasn't stupid or hard ENOUGH, and it was now time to start looking at doing something I've always wanted to do: a completely, totally alien fantasy world.

    This obviously is requiring me to completely build up everything from the whole cloth! One of the major early concerns is obviously what sapient/sentient creature will be the "human" of the world (though like high fantasy, the idea is to have a fair number eventually, as variants on the theme "elves" to the "humans" as it were). In building that species, it will also thus determine the phylum/superclass/class it will belong to. (Sorting stuff into taxonomic orders is a Thing I do; I even started classifying the creatures on the aforementioned fantasy campaign world...!) So it's not just the race in question I'm looking at, it's what it evolved from right back to the kingdom, nearly. Hense this thread, since number of limbs/digits is going to be a big determinator to the body plans of the most dominant species on the planet.

    (My years of building experience have taught me to examine very carefully what I'm doing, because I'm such a damn pedant, rather than just go with rule-of-cool. I am prepared to slather "magic did it" onto top of things when necessary, but I try to keep those instances to a minimum and always with some idea how magic did it and preferably on top of science. (I always right the material as "in-universe" from the perspective of some future historian - for which you can solidly blame the influence of Stewart Cowley's Spacecraft 2000-102100AD, so I am not averse to saying "we don't know, but there are theories that...") As an appropriate analogy for this thread, I try to only use "magic did it" (literally or metaphorically) as flesh on the underlying bones of known science and keep the number of metaphorical "magic invertbrates" to a minimum.)

    So, this planet in question is roughly Earth-sized, with an Earth-like atmosphere (since to do otherwise would have screwed up what data I could scrounge up). It's tidelocked to its primary at an usually far distance out. (I spent quite a bit of time fumbling around - with considerable help from this forum, actually - in setting up all the astrophysics.) The primary is an RCB variable star, which means every so often, the (principally visible, but not so much IR) light from the star gets occulded by clouds of emitted carbon dust. Because of stabilising cloud feedback and the distance, the planet only gets a little more solar energy than Earth.

    The concession of rule-of-cool over physics (and because this wasn't complex enough yet!) is the life of the star and the spacing and duration of these period have been exponentially extended from the real-world basis, so that what happens is these dimming period come along, causing a major climate shift that, while natural, catches the existant civiliations by surprise and they essentially collapse rapidly as the darkness falls1; and no-one remembers it by the time the next civilisation arises. Which as a narrative device, allows you to have plenty of old ruins around, a bit of tech as well as magic and potentially a literal "the time of darkness is upon us" fantasy sort of game, where the twist is the good guys can't stop it, only stop themselves being overrun by the shadowy creatures from the nightside...! (THOSE are definitely bovine-excremented up. With still some attempt at explanation2, yes, but still very much laughing in conventional physics and biology's faces!)

    The planet has an arbitarily strong magnetic field (since despite my best efforts, I could find to way to calculate what the actual solar wind would be), required to stop the atmosphere being blown off. This is generated by *cough*cough*cough*some material in the planet core*cough*cough*cough* (because I can't Physics a credible answer to that one...!) As a tidelocked planet, it has a steady breeze blowing from night to sun side at ground level (and the reverse at high level), which has some interesting implications for plants and flying creatures. There is thus a planetful of regions and habitats for a sentient creature to have come from. Obviously, the nightside will have to be largely reliant on non-solar power like deep-sea vents for life (a night-side ocean - possibly frozen at the surface - is one of my nominal ideas), and the sun-side is mostly (tiny) extremeophiles

    So in this second case, it is a bit of everything. I need to come up with a "protagonist" species that is more alien, but not so alien that it makes their heads bleed or my job as DM too complicated to arbitrate. So "humanoid" is likely to be the end result, but I want to examine how far I can bend that and remain feasible. At the moment, I have no preconceptions (other than a hazy lumpy-headed humanoid I'm deliberately not putting any stock into) aside from that until I've weighed up all the options and possibilities.

    (The first idea for an actual game in anger on this world in my head is to start the players as a very primitive tribe, forced out of their home by a flood (well, it's a bi more complicated than that, but I think I've rambled enough...!), having to trek down the Long Walk, a roadway made by some fallen civilisation and the adventures they have as they lead the tribe to a new home.)

    1The idea being what would happen if tomorrow, the sun suddenly went dim and all the plants started changing colour and not producing as much edible material - and probably some of the animals changing too - and nobody had any idea, since it was ten thousand years since it last happened (we just got lucky). Earth would not fare that well, especially if it happened very quickly and suddenly.

    2Which at present time runs as follows as "can feed on the latent EM emissions of the planet almost indefinitely, but need EM from the IR and lower range to be able to grow, reproduce and think beyond animal instincts, but are fatally vulnerable to high-frequency EM (UV and up), so can't storm the dayside except when the sun is dimmed." It's a start...!
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook