Would there be any major disadvantages for a species to have six legs instead of four?

Question

I want a world in which animals roam the wilderness on four legs, yet at least some of them are able to do the kind of carrying and fine handling of objects done by humans. The planet is superficially similar to Earth, but it is not Earth, and there are no humans around (at least not yet; who knows what humans might do once they figure out interstellar spaceflight...)

There seem to be basically two ways to go about reaching that goal; either give the ancestor of land life on that planet six legs, allowing for two to evolve into arms and hands in a manner similar to how those of humans evolved; or give the ancestor four legs, and have their forelimbs serve both purposes, not unlike gorillas on Earth.

For the alien feel, as well as the additional options it gives, I'm currently leaning toward six legs, the foremost two of which could evolve (or not, depending on the species) into hand-like extremities.

This is intended to be a realistic world, so the normal issues of evolutionary selection pressure apply.

Suppose that an intelligent designer is faced with the choice of, for an ancestor species, ticking the "four legs" or the "six legs" checkbox on the requisition form. What, if any, would be good reasons for them to select four legs rather than six? Or, in other words, what disadvantages would be confered to the creature by having six legs as opposed to four?

For simplicity's sake, you may assume a single ancestor species for all relevant land life on the planet, so for example "some other species would be more energy-efficient in not needing to grow an extra two limbs" does not apply. Other species on the planet may have a different number of legs (compare to how there's also spiders and caterpillars on Earth), but can be ignored for the purposes of this question.

_{This question is basically the opposite of Why would an animal need six legs?}

Answer 1

There's at least one paper suggesting the reason for four limbs is because of the way embryos develop.

Basically, an embryo's cells develop early on into three layers: ectoderm, mesoderm, and endoderm. The endoderm becomes the digestive tract, the ectoderm the skin and nervous tissue, and the mesoderm everything else (bone, muscle, other organs). The mesoderm itself splits into two layers, one that lines the inside of the body cavity, the other that lines the gut.

The hypothesis is that the places the mesoderm will form bone and muscle that creates limbs is near the front and rear ends of the gut, because that's where the layers are close enough together to interact with the ectoderm and each other, but not close enough to merge completely, which leads to the formation of the tail and head, and not far enough apart, as they are along the gut, to prevent the tissue that will become limbs from forming.

If that's true, then in order to have six limbs, you need the early gut, for some reason, to be shaped kind of like an hourglass or peanut, some place where you get the right conditions for the proper distances to allow for limb creation between the front and rear ends. If this hypothesis is true, the the choice between four limbs and multiple limbs is set very early on in development, long before a body would have to deal with allocating resources for limb development.

That being the case, there's no particular reason why six limbs wouldn't develop if the early embryonic development wasn't just a slight bit different from ours.

Answer 2

Ultimately, the preference for 4 or 6 legs within a given environment would come down to the environmental pressures themselves. In its simplest form, the problem is one of agility v. strength.

In an environment where your animal has a survival strategy akin to 'tanking', or making themselves as strongly armoured and stable as possible, 6 legs will win because there are more feet on the ground, and that serves the double purpose of providing a more stable platform and more supportive strength for the armour plating you are going build on your animal. Think animals like Stegosaurus, Ankylosaurus, etc. These were heavy, slow creatures whose survival strategy was based on their ability to withstand attack long enough to give them time to damage or scare off their attackers with their spiked or clubbed tails respectively, and of course their scary roar (we assume).

Such animals would have actually benefited from 6 legs because they could have grown bigger (more legs supporting their weight) but the tradeoff is that they would have been slower. In other words, you're trading speed for toughness.

The extra legs add mass to your body, meaning that they adversely impact the square cube law. That is to say, as you scale up your animal it finds it more difficult to jump and agilely change direction faster because there is more mass by comparison to the relative size of torso for a creature with fewer limbs.

BUT; that's not the real impact of the square cube law for increasing limbs on creatures you want to give speed and agility.

More legs doesn't equal more speed
Adding more legs may increase your torque as a creature, meaning that you can climb steeper mountains or go for longer on the straight, but it simply cannot increase your upper speed. It might increase your acceleration from a standing start in some cases, but that's it. In point of fact, it can actually lower your upper speed because you have to find a gait that combines more feet in an efficient sequence and that's not easy to do.

Torso size can be smaller with fewer limbs
Fewer limbs mean that your torso only has to handle a smaller set of appendage points, so not only do the limbs you have get to be more spread out (and therefore not in the way of each other when running), but the torso can be proportionally smaller meaning that you get the real benefit of the square cube law by decreasing body size generally. Also, the shape of the torso can be squatter, allowing you to fit more organs in a more compact space without disrupting gait. All this leads to a leaner, faster creature suited to the open plains.

Standing
Ever seen a Praying Mantis holding its food with its front appendages? It's not standing but it's doing the closest thing it can to that. Why? Because when man's first ancestors started standing, they only had to tilt one pair of appendages; their legs. In doing so, the spine goes from horizontal to vertical immediately (oversimplification, but true enough for the purposes of this answer) because the hips served as a pivot point.

If your 6 legged creatures want the advantage of height as a vantage point from which to see their enemies approaching, which is largely what proto-man was doing, then you don't have a pivot point because you have 2 sets of hips to contend with. Unless you have a spine with a hinge in it, you're not going to stand upright. Remember, centaurs would have a very complicated spinal column that doesn't really work in the real world because you're essentially attaching an extra spine where the horse's neck normally goes. This is highly unlikely to develop in concert with using the forelegs as hands, and would be a separate and more complicated journey to uprightness than a simple pivot in the hips.

So; the general rule of thumb is;

6 legs - Tankers, slow and cumbersome, but heavily armoured and strong.
4 legs - Faster and agile, more likely to stand upright and develop 'hands' at some point.

As such, your 4 legs are more likely to develop on savannahs or in a world of ambush predators, your 6 legs will develop around large predators that prefer a fight before dinner.

Answer 3

Interesting question, Sleipnir approves. The short answer is, it sort of depends on your creature's body design.

From the reading it sounds like you are describing something that would function similarly to a centaur...possibly. It's also possible that the creature would travel on all six legs with the front pair functioning similar to a gorilla where they can be used for both travel and more fine motor skills.

The finer you want the motor skills to be the less likely your creature will use the front pair of limbs for locomotion.

What your creature is specialized for could range pretty widely as well.

I can envision a prairie/grassland creature with bison or rhino like characteristics using its size and bulk as a defense...maybe some horns just for fun.

• This creature would be low to the ground with strong legs. It would be very stable.
• The complications of extra legs in the creature's gait could be removed/mitigated by short stout legs and a slightly longer body than traditional prairie creatures
• This body design doesn't work particularly great for giving it precision digits, if you want to do that you probably need something more akin to a centaur body design with an upright portion that doesn't touch the ground.
• It likely wouldn't be all that fast but you can probably make an argument that it could move pretty quick in a strait line.

You could also have a tree-dwelling, monkey like creature, I can see an extra set of limbs being great for climbing.

• All six limbs would need to have gripping digits. Like monkeys these don't have to have opposable thumbs, the can simply hook to latch onto tree branches.
• If you are going to have thumbs it makes the most sense to develop them on the middle set of arms. Using the top and bottom set for climbing would be the most stable, then the middle arms could manipulate whatever while climbing.
• I see no inherent difficulty with gait in this setup.

Aquatic creatures also make a lot of sense.

• Extra fins could make the creature faster and more agile in the water.
• If you want an aquatic creature with actual hands you probably need the extra set of arms to make them remain decent swimmers.
• I find this creature less likely than the others to develop digits as survival, especially earlier in their evolution is going to depend on speed and agility in the water...not much need to evolve from that.

I struggle to see extra legs being beneficial in the mountains...but it could probably work, life finds a way to evolve in bizarre and unexpected ways all the time. Nothing immediately comes to mind on this one...the extra legs would be a hazard for a mountain goat as an example.

Predators are interesting with this setup...but complicated. I am immediately reminded of this guy from Avatar super awesome looking, and frankly a bit terrifying. Its actually what I was picturing though probably smaller in my sonic hunter question.

The weird thing about predators...rarely if ever are they the biggest creature.

• In Africa you have lions and cheetahs, they prey on animals much larger than themselves
• In North America you have wolves and cougars/bobcats which are quite a bit smaller than most of their prey
• Bears may be a decent example...but grizzly bears primarily go fishing, they don't really hunt deer or anything, they are not equipped to chase things for long
• In jungles you have a jaguar, they tend to hunt sloths, tapirs and monkeys. So this may be the best example of a predator that goes after smaller prey.

On a creature that is generally on the ground with a small frame, extra legs seem like they could be more a problem than a benefit...

• I suppose if you altered the frame a bit you could make it work. If you give the creature four front legs and two back legs I think you could get a gait that works. Plus the extra front legs would allow the predator to grip better when they latch onto their prey.
• When you specialize your digits to help you grab on and tear open something else it's not super likely you are going to evolve human manual dexterity.
• Predators are high energy creatures which makes size a liability, being bigger requires consuming more food more often, in lean years, be it from overhunting or a drought...that's not great, so a cool as the six-legged battlecat from avatar is...it doesn't seem real practical.
• It'd be strange but I can picture six legs where while hunting the middle legs are basically wrapped around the creature's abdomen but it looks creepy and weird in my head.
• As in avatar I can see six legged creatures working better in a world of larger creatures.

Answer 4

The only disadvantage that I can see is the extra consumption of resources.

We can see from Earth that six legs are not needed. Therefore the extra resources that are needed to grow and operate those extra legs could be put into other things (other adaptations or a slight increase in overall population).

I don't see this as a huge advantage but it is the only thing that I could think of that would answer this question.

Answer 5

A creature that walks on 4 legs and carries objects with the other 2 legs would need good support if their torso bends upwards.

Either strong muscles and/or bones arranged in a way to support most of the weight. Think about how easily humans pull out their backs when bending over to pick up something.

Such a creature might be better off carrying objects by bending down and under, holding their cargo against their bellies. This would require less effort, but would only make sense if their back or middle legs where for carrying. Otherwise how would they see where they were going?

Answer 6

Given the example of the development of the scorpion, it started out as a segmented worm with a pair of legs/swimming paddles per segment. These legs later developed into a variety of appendages, such as pedipalps, mandibles, pincers, and land traversing legs.

Beetles similarly developed palps, maxilla, mandibles, wing case lifting appendages, a set of four wings or wing counter weights, and six legs. If you consider the specialization of legs into structures as mentally nimble as a scorpion's pincers or a grasshopper's wings. I don't see any inhibition for a six legged terrestrial or possibly arboreal animal in development of manipulative appendages.

If the structure of any creature has a survival advantage the species tends to retain the structure. The concept that it is too mentally taxing to control large numbers of limbs is disproved by looking at the large number of multilegged creatures with rudimentary brains. A shrimp has four sets of manipulative maxillapeds, two sets of periopeds (walking legs) and five sets of pleopods (swimming legs) and two sets of uropods (fins) in its tail. With its undeveloped microscopic brain it can coordinate all twenty six of them to work in unison to locomote, maneuver and manipulate things in its environment.

Answer 7

Evolutionary selection pressure will always favor the solution that has the best cost/performance.

If having 4 legs is good enough to move around, then you will not see any animals with 6 legs.

Having 6 legs has upsides and downsides for animals

Upside:

• more stability, easier movement on hilly/accidented terrain

• can survive loss of a leg or two

Downside:

• movement speed is slower

• higher energy cost

On Earth the downsides were bigger than the upsides so 4 legs became the favored solution

You could see 6 legged animals with different evolutionary pressures. A planet with high gravity (think 2 or 3g) combined with a very accidented terrain with almost no flat land would probably favor 6 legged animals.

In those conditions, having more stability is worth the extra cost and lower movement speed.

On a planet with even higher crushing gravity, you d probably have animals with no legs. Crawling like snakes or worms would be the favored solution to move around.

Answer 8

There is a very real disadvantage to more appendages.

Controlling more appendages simultaneously takes much more concentration. Ask any fingerpicking guitarist. If you watch them they don't use the little finger on the picking hand. Because it's extremely difficult to control when you're already concentrating on the rest. Going from using 3 to four is relatively easy. 4 to five isn't.

So coordination of extra limbs would be a big factor in fine control scenarios.

Answer 9

If you use all your limbs for locomotion, then 4 makes sense because you can lift one while maintaining balance on the other three. If you only had 3 legs, you would have to maintain balance on two of them whenever you moved, and if you can do that easily then you don't need more than two legs for locomotion. If you had more than 4 legs, then you don't gain anything with the extras.

That said, it benefits bugs to have more legs because of their other functions, such as climbing/hanging/landing on ceilings, quickly navigating/braiding complex webs, or being able to lose segments without dying.

So, for a large creature to justify having more than 4 limbs, it would need to have some needful task or danger which merits the extra limbs. Some example ideas:

• The planet has creatures who are adept at attacking from behind (for some reason), and so the other creatures evolved an array of sensitive hairs and an extra set of limbs on their backside to detect and deflect such attacks
• The planet is covered in pools of fluid with high surface tension, so having more legs disperses the creature's weight enough to let them run across those pools with relative ease.
• The creatures fly and need to be able to land on very complex/porous/webbed terrain.
• The creatures primary source of food tends to hang off the edges of cliffs, so they need to be able to support themselves by gripping the cliff edge and food stalk, without putting too much pressure on the stalk itself.

I'm sure there are endless good reasons for having more than 4 legs, but given the ecosystem and environment on planet earth, most of our creatures have exactly the number of legs that they need; more would be wasteful and fewer would make survival difficult.

Hope this helps!

Answer 10

So from my understanding of the situation, the early dominant species to walk on land in this world were six legged and they went on to evolve into many other animals, some of those descendants went on to evolve into four legged animals with two arm like appendages.

Why would animals have evolved this way though? Most obvious answer would be speed. You don't need six legs to gallop.

Hopping would be an alternative (just think of fleas or other insects of that ilk), but that becomes problematic as the animals get bigger and probably not great in certain terrains (un-even ground or densely wooded areas).

I envisage that at some point a common ancestor of your six limbed (four legged, two armed) animals developed running on four legs and picked its fore limbs up off the ground to do this. Natural selection kicks in and the two pairs of back legs get further apart and the front legs become smaller. Animals which survive by pure strength/stability may stay on six legs.

Some animals may lose the use of the fore arms altogether if they find no evolutionary need for them.

But other animals could find different uses for those fore limbs like pulling plants up to get at the roots, latching onto prey, pick fruits from trees, mating rituals/displays or building things (nests, burrows, dams, etc.). These are the sort of activities that may spur evolution in the direction of fingers/thumbs and fine manipulation.

Of the animals that did find a use for their fore limbs, they could end up like your desired state animal of being four legged, with the ability to carry and manipulate things in a way similar to humans.

Only thing I would say, is that I can't see an obvious evolutionary path to have animals with an upright front half (centaur like). More likely that the animals limb joints (shoulders, hips, mid-hips?) would line up as opposed to forming an angle. As mentioned in another answer, a centaur would have great difficulty in picking something up off the floor. Although it's head would need to be above the shoulders so the arm like limbs don't interfere with peripheral vision.

Answer 11

There is no real disadvantage other than the material cost you get with any limb, but keep in mind tissue needs a use to be kept over prolonged evolutionary time so make sure your "missing link" species also has a use for them. The main reason modern tetrapods only have four is they evolved from fish that also had four limbs.

there is one notable exception for an animal with an upright posture, 4 limbs has a distinct advantage because six limbs start to get in each others way during the stride, so either you need a much longer (and thus more massive) creature or you need to be slow moving.

Answer 12

Your question reminded me of the 'hexapoda' (six-legged) dominant life forms on pandora, from Avatar (The movie, not cartoon). Some examples are the six legged horse, the 'dire horse' and the feline-like animal called a 'viper wolf'.

However to answer your question, four is the preferable number of limbs.(A good example of course being primates).Adding more limbs would just require more brain-power to control them, and would require a major change in configuration that would mean that it would have to have some benefit.

• While we don't have dire horses or 'viper wolves' on earth, we do know one species that have six limbs- insects. When a creature runs on two or four legs, only one or two feet remain in contact with the ground at any point in time. When an insect runs on a surface, three limbs usually remain in contact with the ground at all times. For insects like a fly, for example, this is an advantage because it means it has greater stability when clinging to a vertical surface or upside-down. That's not an advantage that other animals need walking upright on a flat surface.

• In many ways, six legs would hinder normal creatures. Insects on Earth that have six legs have them splayed out from the body so they don't get in each other's way. If your life form is anything like Avatar's, then all the creatures would have their six legs in line with their bodies, which would cause them to bump into or get tangled up together easily. As shown with our own history and science, four legs seems to be the best recipe for land-dwelling animals. Unless your creatures climb vertically etc, then four legs would be much more beneficial.

• Furthermore, creatures with more than four limbs have not evolved in land species. In fact the trend is to 'simplify'- as with snakes,
  which lost their limbs- and horses, which instead of having arms and hands, have hooves. Six, eight and even leggier arthropods (insects, arachnids and crustaceans) have, of course, been extraordinarily
  successful. But the fact that these creatures have exoskeletons
  rather than bones inside their bodies limits their size.

• Large hexapoda animals would need Large exoskeletons to support
  internal body tissues etc. However, it would be prohibitively
  cumbersome and heavy, and would demand lots of energy to move and
  grow. Because intelligence is strongly linked to how big, intricate
  and energy-consuming an animal's brain is, that sentience is unlikely to evolve in hexapoda like animals.

Edit: I concentrated on listing the disadvantages of having six legs, rather than directly comparing it with four legs.

Answer 13

There are 13 answers already given as I write this but I think I can still make a valid contribution.

Possible disadvantages of having more legs:

1. neurological complexity - not an issue
2. genetic diseases - Parkinson's etc as more damaging?
3. speed of movement - conflicting factors
4. agility - same factors as speed
5. energy budget - bigger obviously but weight/strength ratio?

First .. is the question really ONLY about legs? My take on the Questioner's intent is that he specifying that two of limbs will be for manipulation and the other 2 or 4 will be for locomotion. SO my answer is only about method of locomotion.

As such .. all of the points made about speed difference, neurological complexity etc are irrelevant in that the ontology of the species will be The One True Answer to its evolutionary situation. Whatever form it has will have been dictated by its environment, and that form will be the best for species survival. For me this settles the neurological issue as the critter's brain will evolve to meet the need.

The issue of whether six or four legs make an individual faster and/or more agile is for me reduced to: is one faster with two legs or four? The clear answer is four. Real World experience shows that any four legged animal can beat a human's speed and is far more agile.

However, energy budget is clearly higher with 4 legs - you have more muscles working to achieve the same distance over ground. My reference is North American native deer-runners who used their greater endurance to hunt down the much faster deer by relentlessly pursuing their prey until it was too tired out to run anymore. Thus, a four-legged disadvantage.

But .. bigger creatures might not be strong enough to have use this advantage. Strength to weight ratio puts a hard upper limit on how fast a creature can accelerate because of the inverse square effect of conservation of momentum. To wit, it takes 4 times the energy to accelerate a body two times bigger than your reference.

Individuals in a small species move faster because they use far less energy to move or change direction, regardless of the number of legs they are using.

Elephant sized four-leggers obviously use more energy to move, but I contend would still be able to sprint faster than a two legged critter of like size. In IRL elephants can reach 35 MPH running, but not for long. I think a two legged pachyderm might have better endurance, but only just. Someone with a better knowledge of bio-mechanics might prove me wrong here.

Now for the final point: certainly a disease or injury that interferes with coordination, like Parkinson's or ALS will be more severe for creatures with a more complex gait. Four legged creatures can certainly sleep standing up (locking knees) but once in motion need to coordinate well, and at speed even better. Whether such diseases would be prevalent or not is something that the author must decide upon. But the possibility weighs against the four legged option here.

Thanks to the Questioner for raising an interesting topic.

Answer 14

One disadvantage that I can think of is they would not be able to run fast. Take for example a Jaguar or a Deer. If you add two more limbs they would get in the way of running.

When a Jaguar runs it leaps with the front legs and catches up with the hind limbs where hind limbs cross the front legs/limbs. If you add two more in the middle they would get in the way.