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