Centaurs

[So Haleza Grise]

In the past I toyed around with the idea of a conculture of centaurs, but I decided the biology was too implausible: why would a species evolve an L-shaped spine and a special pair of forward-facing arms? Six limbs I could imagine, but not a specific bodyplan that split like that. But biology is not my strong point. Has anyone ever had centaur species in their conworlds?

[mèþru]

I'm pretty sure six limbed animals with two "arms" and four "legs" is plausible, just not the classic centaur body.

[nebula wind phone]

It's interesting that on Earth we've got precedent for bony creatures losing limbs, but not gaining them. If that's not a coincidence -- maybe it really is harder to make your body plan more complicated once you've got a skeleton? -- then centaurs would need to be a very archaic branch of the family tree, sort of the coelacanth of that world.

[mèþru]

There is an easy way to gain limbs: if every segment of type x has y limbs, then evolve so that the segment is copied. The problem is that most creatures eventually evolved fused segments, making it harder to copy the segment. Vertebrates on Earth don't even have this kind of segmented body plan, so they are out of luck. That said, vertebrates on a conworld could hypothetically be segmented, although segment fusion would evolve way before reaching the size of a rat because bone is much denser and segmentation becomes less advantageous the heavier the mass of the segments.

The easiest way is to just start with a six legged creature; evolving new limbs at the size of a human is very improbable. (I guess a boneless flap of skin of some sort could evolve and then a genetic disease causes the bone and then muscles to grow there, but it would be disadvantageous because it's just a bone sticking out where it shouldn't rather than a specialised structure that is actually useful.)

Another solution is that you have cartilage or something else lighter than bone as the material of the skeleton.

[Bessunire]

I think the most likely way for a centaur-like creature to evolve is to start out with a hexapodal animal that uses all its limbs for locomotion. It probably have be fairly small and generalized in its lifestyle so that its legs don't become specialized for running or some other use inimical to grasping. This creature's front legs could then become specialized for grasping pray or plant life depending on its diet, then, as its brain becomes bigger and heavier, assuming the brain is located in its head, which is likely since that's where most of the sense organs are likely to be, it might be more advantageous to carry the head upright rather than horizontally so that the creature is less front-heavy, which would result in an L-shaped spine.

If you're into some really serious speculative biology, I believe Gert van Dijk has at least one post on his Furaha blog about the evolution of centaurism, but you will have to search through his archives to find it, since it is a rather old post.

[ˈkuː.ɑːnˠ001]

There's no particular problem with a vertebrate with two arms and four legs, as long as your planet has a long line of six-legged vertebrates for them to evolve from. The physiology of a large land vertebrate is not going to play well with the mutation required to add a new pair of limbs.

I think you're right about the L-shaped spine though. Off the top of my head, I can't think of any example in biology of a major structure evolving a right-angle. If I was going to have 'centaurs', they would look a lot more like giant mantises than the classical centaur shape.

[Pabappa]

Off the top of my head, I can't think of any example in biology of a major structure evolving a right-angle.

There are some examples ... snails, larvacea, and hermit crabs all have asymmetrical anatomy involving a 90 degree curl of a previously straight segment. However, two of these are aquatic, and none of them are vertebrates. I agree that spontaneously developing from a bridge-like spine into an upright one is unlikely unless there's a sudden new advantage that would come from that mutation just by itself. For example, eating leaves from tall trees helped the giraffe grow a neck that would have otherwise been a huge disadvantage for its heart.

I think this discussion has come up before and people concluded that getting a six-limbed fish to climb out of the water would be the best hope for developing six-limbed land animals later on ... the focus of that endeavor was to get four-legged birds rather than centaurs, but the situation is the same early on in development.

Fish have spontaneously developed new fins over time. the earliest fish had only two fins and some modern fish have up to ten. These fins can come from various mutations:
1) duplication of a previous structure (bass developing several dorsal fins instead of just one)
2) a new, boneless fin (adipose fin of trout)
3) paired fins on the underside of the fish .... theese are the ones that developed into our limbs, while the other two types of fins were completely lost. Im not sure how these fins got started, but they are the ones that are clearly the most important.

If there can be seen to be an advantage, even a small one, for a fish with six paired fins over one with four paired fins, i'd say the future evolution of this lineage into a centaur-like creature is guaranteed despite all of the other potential roadblocks. However there would also be six-limbed versions of many other creatures as well, possibly including birds, which would gives the centaurs some pretty stiff competition for resources and habitats. Six-limbed birds are essentially dragons, and even at a large size they would be more maneuverable on the ground than any birds alive today.

[Salmoneus]

There are three elements here:

- having six limbs
- having a centaurid limb plan (four legs, two arms)
- having a centaur body shape (vertical trunk, horizontal torso)

On the first: we shouldn't expect hexapody to be common. It's a terrible idea. Large six-limbed animals will be out-competed by four-limbers*. However, six-limbed animals could arise early on (probably due to smaller animals being supersized due to lack of competition in new environments), and survive by finding niches where they escape competition. The obvious way to do that is to find a new use for their superfluous limbs.

On the second: if you're going to repurpose two limbs, the front pair are the most obvious option - they're closest to the brain and the mouth. Having a pair of limbs dedicating to holding, grabbing and manipulating makes sense. On earth, such limbs are found in crabs and scorpions, and some invertebrates also have evolved mouthparts that are virtually an additional pair of grabbing limbs.**

On the third: if you're going to have two limbs, having them on a trunk above the forelegs makes sense too. This allows the weight of the foretrunk to be held directly on the forelegs. The alternatives are a rigid spine to allow the foretrunk to extend horizontally, or a complex double-shoulder in which there's basically no foretrunk and the head fits straight onto the shoulder from which both legs and arms emerge. The disadvantage of the vertical foretrunk is that it makes reaching the ground harder; the advantage is that it makes reaching high things easier. The obvious environmental niche of the centaur is in forests, where their upper arms allow them to reach and manipulate high branches - picking fruit, for instance, or just tender leaves.

Indeed, we have something similar to the centaur's body-plan on Earth: look at giraffes. If they don't look entirely 'L-shaped', it's because their skeleton is obscured by the mass of muscle over their shoulders (likewise, 'real' centaurs would not be pure man+horse; they'd need some muscle in between for bending).

So I think it's viable; but it's certainly not particularly likely!




*this has happened at least once on earth: six-limbed invertebrate fliers were completely out-competed by tetrapods once they developed flight, and survived only at small sizes.

**another option would be digging limbs, like a badger's forelegs. An animal that tunnelled extensively would also reduce the cost of extra limbs (because gravity would be critical). My SF setting has a horizontal hexapod species sort of like a cross between giant badgers and cockroaches.

[zompist]

As others have said, centaurs are more likely if six legs is the norm for your dominant animals.

Six legs also offers the option of having dragons-- i.e. four-legged winged creatures. (The usual dragons shape makes no sense, except perhaps on a planet with minimal gravity. But birds with extra arms could work.)

Most attempts at centaur anatomy (do an image search!) founder on the problem of melding human and horse anatomy-- since you're duplicating the parts of the body that have rib cage / heart / stomach / intestines. It honestly looks pretty dumb. So I like the giraffe idea: don't put a whole torso in place of the horse's head; place an extended neck.

We still have a lot of segmentation in our body plan-- that's what the vertebrae are, after all, as well as the ranks of muscles in a six-pack. But yeah, it's all messed up by putting all the other organs in there.