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I have a number of intelligent alien species with a high number of limbs (from 6 up to 18) some with radial symmetry and some bilateral, in most cases the creatures can crawl using all limbs but when doing task the hind legs (3 to 6 depending on limb number) become the legs supporting their weight as the raised up front of the body uses the rest of the limbs as arms. I also have other species with very high numbers of simple appendages, similar to echinoderms.

I have some species which I have elements of their physiology but I have not decided on the limb number (although I do prefer more than 4 with 2 as arms). I believe there is a point where more is not better, as they will involve more brain power, blood/nutrient supply and sensory methods, and there comes a point where less limbs can reach all the places and carry out the tasks that more can.

These alien designs will be for my technologically advanced species, they will be spacefaring and will need to complete tasks and progress in a way that is similar to humanities path to becoming technologically advanced.

Is there theoretically a most efficient number of arm-like appendages, where anything more requires too much additional support systems that it would in efficient for a creature to evolve and supply necessary requirements for survival?

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    $\begingroup$ The way the question is asked, I am inclined to say, that the creature will have the number of limbs evolution requires for it to overcome the problem the the environment has placed on it. Is there an upper limit to the number of limbs? Millipedes! $\endgroup$
    – Gillgamesh
    Commented Jun 7, 2022 at 14:25
  • $\begingroup$ Tell us more about your creatures. What do they use their many limbs for? $\endgroup$
    – Daron
    Commented Jun 7, 2022 at 14:28
  • $\begingroup$ @Daron I have added details to the question. $\endgroup$
    – user96146
    Commented Jun 7, 2022 at 15:07
  • $\begingroup$ @Gillgamesh Evolution will decide their physiology but efficiency will dictate how many limbs are needed. Millipedes are not intelligent (by my questions standards) nor do they use their limbs as arms and arthropod ancestors had many limbs like millipedes but most lost the majority of limbs, it show that for the majority of arthropods the high number was not efficient. $\endgroup$
    – user96146
    Commented Jun 7, 2022 at 15:11
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    $\begingroup$ For any "Most" or "Best" question, the correct response is almost always "For What Purpose?" The reason that different creatures have different designs for their appendages is that they occupy different ecological niches and have different priorities for their appendages use. The most universally efficient shape is a sphere, but that works best if you are a star. I, for instance, am not a star, so I have a different body shape, more suited to trolling SE sites with relevant/annoying comments. $\endgroup$ Commented Jun 8, 2022 at 13:17

6 Answers 6

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There is No Number

There is no ideal number of appendages because there are too many demands made of limbs in various circumstances.

Evolution is not the "survival of the fittest" but "survival of the barely adequate." If nothing is actively selecting against or for a trait, creatures with that trait will stick around (all else being equal). This is why we have creatures with many appendages, detachable appendages (sea stars), none (some parasites), and more in between!

Major Factors

Many factors for appendage number and type exist, but some of the big ones and things to consider are are:

  • Movement: does it like swimming, mud-skipping, running, climbing...?
  • Object Manipulation: has this critter the need to build shelter, hold offspring, groom themselves, contribute to world building overflow, etc...?
  • Food Acquisition: can the appendage help with cracking shells, neutralizing prey, collecting floating particles, grabbing a berry, etc...?
  • Predator Evasion: can the appendage help with running away, hiding, intimidating...?
  • Body Plan: is this limb part of a repeating pattern?

There can be more factors, but consider these when creating the initial design of "realistic" creatures. Going lower may make sense from a locomotive perspective but not from a body plan or object manipulation perspective.

In all, limbs need to be only "good enough" for survival in whatever environment the creature finds itself in and the lifestyle it leads.

In terms of these aliens, long term spaceflight and tool use are simply more evolutionary inputs. If an adaptation increases their evolutionary fitness, you'll see more of it. It does not really matter if the adaptation is diminished or lost limbs, better beaks for opening walnuts, finer motor skills, or what-have-you: evolutionary pressures still apply!

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  • $\begingroup$ Good answer on the matter of creatures generally. I enjoyed but it's not the question. Very intelligent creatures need to benefit from modularity early in their evolution. Don't throw more arms at the problem, make a tool. But it's possible for a creature to become advanced with design suboptimal for civilized society. $\endgroup$
    – Dor1000
    Commented Jun 8, 2022 at 2:14
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    $\begingroup$ "If nothing is actively selecting against or for a trait, creatures with that trait will stick around" - while this can be true, a big but useless appendage will automatically be selected against, because it uses up nutrients while contributing nothing. They will then become vestigial, and shrink considerably, and even (mostly) disappear over time, with barely a small trace of it left. $\endgroup$
    – vsz
    Commented Jun 8, 2022 at 5:37
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    $\begingroup$ @vsz: Yes, but extant organs, especially surplus limbs, can also be repurposed for new tasks. Deciding whether a limb becomes vestigial or exaptive is a race between function loss and evolutionary co-option, and co-option can often win. $\endgroup$ Commented Jun 8, 2022 at 7:10
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    $\begingroup$ Thank you for getting evolution right! $\endgroup$
    – Hobbamok
    Commented Jun 8, 2022 at 14:41
  • $\begingroup$ For those reading the comments... I take this lively discussion as proof that limb count and body arrangement is complicated and my original statement that there is no ideal number is totally justified. $\endgroup$
    – PipperChip
    Commented Jun 8, 2022 at 15:19
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Definitely Four!

I, as a human who sometimes uses a soldering iron, would find it incredibly useful to have four hands. Two to hold the items being soldered together, one to hold the soldering iron, and one to stop my glasses falling forward.

The limbs doing the finer work would be higher up the trunk and nearer to the eyes so as to be seen clearly. The eyes would be above all the limbs as light generally comes from above.

In fact two very strong, vice-like appendages for rough and tough applications, plus two appendages similar to human arms and hands for doing finer work would be ideal. Beyond that number, I think that central coordination would simply be too difficult to achieve with a multi-tasking brain. The complexity of synchronisation grows more than linearly. Already we have trouble rubbing our stomachs and patting our heads at the same time. The two lower arms could be mostly static and used for holding workpieces in place.

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    $\begingroup$ +1, because I really want the top answers to be "this is unknowable or four". Also, this answer supports carcinizaton (en.wikipedia.org/wiki/…) $\endgroup$
    – PipperChip
    Commented Jun 7, 2022 at 18:33
  • $\begingroup$ Lol. I quite often use my feet or knees as vice so I could use the extra pair of arms for soldering a second item at the same time. There are some very talented people that can draw excellent pictures with both hands at the same time, or military that use each eye for different tasks, I think your underestimating human capabilities. $\endgroup$
    – user96146
    Commented Jun 7, 2022 at 18:42
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    $\begingroup$ @RayHammond Yeah but then you also can't walk or stand as you work which is an evolutionary disadvantage. And to be honest, the times you need the most hands are when desoldering something. You need enough hands to hold enough irons for each pin, an nth hand to hold and brace the board, and an nth+1 hand to lift the chip. Maybe an nth+2 hand to add extra flux if required or scratch your nose. $\endgroup$
    – DKNguyen
    Commented Jun 7, 2022 at 20:27
  • $\begingroup$ @DKNguyen Sounds like a lot of soldering fans on here and people who can't pat their head and rub their stomach at the same time lol. There are many tasks that extra arms would help. On a side note I forgot that I used to play as complex guitar styles as I could, fast Spanish styles, metal solos, double tapping with harmonics whilst tapping the body in different areas and tapping my feet. I bet with I could manage 3 or more sets of strings with enough hands, although this is more muscle memory as many tasks are. $\endgroup$
    – user96146
    Commented Jun 7, 2022 at 20:54
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    $\begingroup$ What if, while soldering, you need to scratch an itch and also hold your drink while pushing your glasses back into position? 😁 $\endgroup$ Commented Jun 11, 2022 at 3:50
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I most closely agree with PipperChip, but not quite.

Points:

  • Best and second best are not too far apart. The advantages are not going to make pinnacle predator and extinct as much as give one more advantage in one terrain and another more advantage else where.
  • In space the costs are going to be very high, and radiation is going to be the #1 adversary (by far far far). Cosmic rays are particles that have 10^20 eV and neutron radiation from cosmic-rays interacting with stationary matter kills organic material, and semiconductors, like a literal plague.

On earth we have a lot of examples, and while they may not be "everything" they are more than nothing and they tell us some important things.

  • Fewer is better, because simplicity is better. Costs can be high. Things with more than 2-arms and 2-legs or even 4-legs tend to be very small, or require much higher oxygen and a more soupy atmosphere to be large.
  • Biology seems to find a "good enough" in the larger structure and work with more small-scale variations within it, consider how many many many insects are 6-legged (3-segmented) creatures, and consider how many insects there are.
  • Biology finds bilateral (or more) symmetry to be efficient (likely because of error checking, and denser dna), so creatures tend to work in multiples of twos. (4-mammal, 6-insect, 8-arachnid, 10-decapod, ...)

There are simulations of evolution where creatures try to evolve to do mechanical tasks. OpenAI and google had a bender on them a while back. Might look at dusting that off, and trying it out on a mock-satellite or such and see what mechanical configurations show up as most useful. Let the computer surprise you, and if it does a decent job, then don't just put it in a novel, do what the "Interstellar" folks did and get it published in peer-reviewed journals. ;)

EDIT: More thoughts.

Astronauts treat water like it is zero-g, because buoyancy helps it act like that, though without radiation and with a pressurized operating fluid around it.

  • You could look at aquatic micro-organisms and diatoms. link
  • the burgess shale has some very intersting paleolithic organisms link Obapinia, Hallucigenia,
  • you could replace fins with solar-sails or ion engines of some sort.
  • solar sails would collect atoms for use as material or fuel
  • Ive thought for some time of a "universe of mind" where the brain is a quantum antenna and consciousness isn't intrinsic to biology, but it allows the consciousness in another sort of universe to act out, somewhat like inter-dimensional "surrogates". It is a different take on "soul". A space-based organism could have a very different neurology, and maybe couple to elements of "universe of mind" that are very different from terrestrial ones.
  • scale is interesting. big things on earth need lots of food. I've wondered if suns/stars are conscious and talk to each other over millions of years. Are there carefully structured whorls and solitonic wakes in the solar ejecta that are electromagnetically active, and count as conscious mega-sized, vastly empty, gas and dust structures?

Links:

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    $\begingroup$ The AI simulated creatures is a good link. I have been searching a lot of multi legged robots and ones with strange gaits, I have seen some AI evolution design but haven't come across this or thought of searching AI designs deeper. Thanks. $\endgroup$
    – user96146
    Commented Jun 10, 2022 at 16:58
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The optimal number isn't necessarily even, and they may not all be equivalent.

Niven and Pournelle described a roughly humanoid alien species (“Moties”) with three differently-sized arms in The Mote in God’s Eye. The Moties were described as being significantly more adept than (bilaterally symmetric) humans at grasping and manipulating objects due to their having a range of different tools at hand, so to speak. They could exert a large force to hold or bend an object with their largest hand (the gripping hand), and work delicately with fine motor control with their smaller hands.

Most animals we know of are approximately symmetric (radially or bilaterally) with a few noteworthy exceptions like fiddler crabs but note that the 8 arms of an octopus are not identical, any more than the 5 digits on your hand are identical. Each pair of appendages of bilaterally symmetric creature is likely to be somewhat different from the other pairs, specialized to some degree for different tasks.

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  • $\begingroup$ Hi Scott, welcome to Worldbuilding. I'm not certain I understand how this answers the question what the most efficient number of arm-like appendages is. $\endgroup$
    – Joachim
    Commented Jun 9, 2022 at 9:56
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Two

Regarding 2 vs 4 limbs for advanced species. With 4 limbs each is at half the strength and coordination: for the same overhead you could have 2 arms that are better. The better arms will outperform when tasks can't benefit from extra limbs. When you need 4 limbs get an assistant, improving social bonds. If extra limbs helps it needs to help the species pre-technology. After that they access mechanical augments, genetic engineering and advanced interface tech. We need only our mind and some reliable signals sent out. Thought control sounds unstable. Touch interface will stick around. Number of fingers is important.

Regarding life generally. As child I drew creatures and assigned them many abilities. Wondering why don't all creatures have venom, flying, scales, lungs and gills, excellent senses in all conditions, and being completely massive of course. Selection doesn't follow the rule of cool. It's better to cut junk. Successful species are often minimalist in design. Special abilities have overhead and force you into a niche. Multi-purpose is good. I like rats, they use their teeth to eat, carry, tunnel, fight. Hooved animals use their whole body to fight: biting, kicking, body slamming.

How good an arm is could max out. More arms will sometimes outperform less. Many tasks are muscle memory: 4 arms could easily be trained for repetitive tasks: memorizing a harp piece. Weaving or sewing leads to biting the string or cinching in knees: I always feel like a 2-armed loser.

Feet that double as graspers is pretty good. I feel like a clumsy-toed freak.

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The needed for the moment.

The best body would be the one that can adapt to every situation, if he needs to grab something then he will split his arm in two, if he needs to work with something that requieres four arms then he split his arm in four.

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