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Sleipnir is Odin's steed - an eight-legged horse in Norse mythology. How and where to place its legs to make it faster than four-legged horses? What other benefits or drawbacks may arise from the additional legs?

Usually one would take its existence for granted - the gods made it that way. However for the sake of the question: what selection pressure would make evolution invent a horse like that and what conditions would support this feature?

Clarifications: Assume earth-like conditions (gravity, air pressure and composition, etc). Main focus should not be on Odin handwaving the beast into existence but the characteristics of such an animal and its development assuming evolution (besides benefits and drawbacks of such a design which could be discussed no matter how the creature evolved).

enter image description here Odin rides to Hel, Source, released into the public domain


This question is part of the Anatomically Correct Series.

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    $\begingroup$ @dunc123 not a realistic evolutionary pathway. Could we perhaps be looking at a spider that has evolved to look like a horse? I don't think a spider evolving to look like a real sized, rideable horse is more realistic than having a horse evolve extra legs. $\endgroup$ – Renan Jul 28 '16 at 18:39
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    $\begingroup$ @Renan Well there would be slightly more precedence for an arachnid growing large than a land vertebrate gaining legs. Maybe it didn't look much like a horse but nobody wanted to argue with Odin about the species of his favourite pony... $\endgroup$ – dunc123 Jul 28 '16 at 18:47
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    $\begingroup$ On the news, some years ago, I saw a reference to a calf (I think) which was born with six legs. So Sleipnir could just be a 'mutant' horse with eight legs. Of course the calf's extra legs weren't much use, but Odin could have handwaved its nervous system to make them useable. $\endgroup$ – Sara Costa Jul 28 '16 at 19:08
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    $\begingroup$ Calves with multiple legs aren't that rare, and you sometimes see the same in humans. There's a lot to handwave when it comes to mythology, but Sleipner (as we spell it in Sweden) could simply be a "Siamese twin" horse which happened to survive early childhood. As with modern car enthusiasts, it doesn't have to be fast, it's enough if it looks fast. A horse with eight legs definitely looks fast! Twice the horse power. $\endgroup$ – pipe Jul 29 '16 at 4:25
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    $\begingroup$ Also relevant: youtu.be/gHZ6oneFTZw?t=261 "Only this morning, in the courtyard, I saw a horse with two heads and two bodies!" $\endgroup$ – pipe Jul 29 '16 at 4:28
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Something that evolution very seldom does is create extra limbs. One of the best reasons for this is that there's no viable intermediate forms; you have to grow the new limbs to the same length as the old ones before they can be useful, and that would take millions of years of having useless stubs.

So I propose that your eight-legged creatures actually have four split legs. To start off with, you can use an animal with a cloven hoof, like a cow or goat. These animals are already used as pack animals, so with a little bit of selective breeding they should be able to somewhat reliably carry humans.

From there, it's just a matter of cutting the rest of the leg in half. To do this with evolution, I'm imagining an environment with very uneven footing. A lot of holes where one toe could fall in, and then you break your ankle and have to be put down. The animals that can traverse this without breaking their ankles might be the ones with longer toes, or less connections leading up to the ankle, leading again to longer toes.

After that, luckily there are already two bones in the foreleg, so you just have to decouple them. This is going to be a big change that will probably take a lot of generations, and I doubt that there'd actually be enough of a benefit for it, but I still think it's more likely than spontaneously growing four extra legs (and who knows, sometimes evolution does stupid things).

At this point, around 2/3rds of the leg is split up, and I think that's good enough. It's still going to look really strange, and look a lot more eight-legged than any other comparable creature.

EDIT: hand-drawn illustration because people seem to like those: enter image description here

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  • $\begingroup$ Split legs is certainly an idea worth investigating. The linked picture shows an artist's impression that could be identified as such. $\endgroup$ – Ghanima Jul 28 '16 at 19:45
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    $\begingroup$ Yeah for hand-drawn illustration! $\endgroup$ – Ghanima Jul 28 '16 at 20:09
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    $\begingroup$ Interestingly, the wikipedia article for sleipnir contains some 18th century drawings with just this depiction. $\endgroup$ – Pork Jul 28 '16 at 20:59
  • $\begingroup$ Mollusks and arthropods are happy to evolve additional limbs at a whim. It's just vertebrates that find it hard to sprout them. $\endgroup$ – Daniel Darabos Jul 30 '16 at 19:47
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    $\begingroup$ I had a childhood friend with two thumbs on one hand, both the same size. So, I don't buy the argument that the mutation leading to an extra limb has to go via gradual growth of a stump over generations. Perhaps more of an issue is that an extra leg would need the structures that support it, make use of it, distribute weight to it appropriately, and allow for it's strength to be used. i.e. I imagine that development of the hips (or in your case knees) and muscular configurations is a bigger issue than the development of extra legs using a largely pre-existing genetic plan. $\endgroup$ – mc0e Jan 29 '17 at 6:47
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8 legs compared to 4 won't actually improve top speed very much...if anything the bulk of the extra appendages gets in the way. That said, there is a heavy advantage in the extra limbs: A 4 legged horse runs forward exceedingly well, however side to side movement or even stopping isn't the easiest, while an 8 legged creature could theoretically have the extra 4 legs designed to give it side to side movement, able to redirect the momentum of a straight forward run either left or right rapidly.

Ultimately the creature moves more spider like. Evolutionary pressures would need to favor the 8 legged creature. 2 needs:

1 - harsh terrain. 8 legs would be an easier balancing act than 4 legs...really rough terrain (volcanic for example) where straight forward movement is hindered due to unstable ground would mean the ability to step side to side quickly would allow for quicker movement through harsh terrain (look up devils golf course death valley for an example of this terrain).

2 - High speed predators. If a high speed predator existed that could rundown a horse a full gallop, then the horses would need to adapt to other mechanisms to survive. Quick agile side to side movement becomes this mechanism (IE, the 8 legged horse can jump to either side and change it's trajectory faster than the predator it's evading)...or the ability to enter and move quickly through a terrain as listed above.

The side to side movement could be a tremendous advantage if the regular 4 legged counter part was slower than the top predator in the area.

Quick edit:

The same tactic is employed when we see Orca and Sea-lion encounters. The Orca is far faster in a 2 dimensional plane, while the Sea-lion is significantly more agile. The Orca gets a first strike...a surprise where it swims at full speed at its intended target. If the Sea-lion (with a bit of luck) sees it, it can get out of the way at the last second. What then occurs is a tight circle, with the Orca being outmaneuvered as it simply cannot replicate the tight turn. Of course this will often end up as an eaten Sea-lion anyway, but this is also the only route this Sea-lion has for survival. Similar setup here, with our 8 legged equine capable of making tight turns at higher speeds to outmaneuver a quick yet less agile predator.

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  • $\begingroup$ Interesting aspect! $\endgroup$ – Ghanima Jul 28 '16 at 19:49
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The Sleipnir is a Creepy Crawly

The worlds fastest spider, The giant house spider, can run at speeds of almost 2 feet a second. Scaling this up we can assume that a spider the size of a horse would be able to run 177 miles an hour!

enter image description here

Another interesting spider species, Salticidae, imitates ants, as seen above. If on the strange world of Asgard, the gravity is lower, we can say that giant horse mimicking spiders evolved to hunt down equidae animals.

The specific limbs of the Sleipnir would look very close to the horse but would split in two, with the hooves acting as claws. Their mandibles would likely evolve to either go flush with or inside their face.

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    $\begingroup$ While I like the idea we have to make do with earth-like gravity as Odin is known to walk the earth and not just some ethereal planes. $\endgroup$ – Ghanima Jul 28 '16 at 18:50
  • $\begingroup$ @Ghanima considering Odin is a god, he can walk where ever he wants, regardless of gravity. Also we can just say that while on Earth Odin grants a small field of weak gravity around the Sleipnir, remember he's a god $\endgroup$ – TrEs-2b Jul 28 '16 at 18:54
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    $\begingroup$ Where did you get 20-40 miles an hour from? 2 feet per second is about 1.4 miles per hour. Wikipedia says the female's body length is about 18mm, while a horse length is about 2.4m (en.wikipedia.org/wiki/Horse_length), so scaling for body length would give 'Spider-horse' a top speed of 177mph. $\endgroup$ – arboviral Jul 29 '16 at 12:21
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    $\begingroup$ @arboviral I didn't have time to fully do the math, so I made a safe guess and planned on doing the math later, thanks for correcting my error. $\endgroup$ – TrEs-2b Jul 29 '16 at 16:22
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    $\begingroup$ Speed scaling is unlikely to be linear with body length. $\endgroup$ – user2943160 Jul 30 '16 at 19:06
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Equus sleipniricus evolved from our world's horses, perhapes with the help of radiation and teratogenics along the way. In all aspects they are just like ours, but they have the ability to regenerate lost limbs, just like newts do.

Which is why eight legs are beneficial to them: four of those legs are spare ones. Should a predator such as a bobcat or panther bite them in the leg, or should a leg break for any reason, they are capable of undergoing autotomy, that is, they let go of that leg. Having spares mean that they can keep running for their lives.

Notice that the spare legs don't just sit still, waiting for the original ones to break. Equus sleipniricus is able to use them to run as well at any time, thus being faster than a real Earth horse.

Also, four hind legs means twice the kick when they kick.

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  • $\begingroup$ But there are certainly drawbacks in having to carry the spares around all the time. You would want it to be the fastest horse on earth. $\endgroup$ – Ghanima Jul 28 '16 at 18:35
  • $\begingroup$ @Ghanima the biological, technical explanation is that you can't have your cake and eat it too. ;) $\endgroup$ – Renan Jul 28 '16 at 18:37
  • $\begingroup$ Well, you better make it possible ;) $\endgroup$ – Ghanima Jul 28 '16 at 18:39
  • $\begingroup$ @Ghanima ok, I rephrased it so that those legs don't wait for the original ones to be wounded to start working. $\endgroup$ – Renan Jul 28 '16 at 18:56
  • $\begingroup$ Well you've got my vote but I think there's much more possible here. Could there be a way that eight legs up its endurance (rest half the legs and switch now and then)? But I guess it would be not just the legs that tire. $\endgroup$ – Ghanima Jul 28 '16 at 19:28
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I'm not a biology major, a vet or anything that has anything to do with anatomy. I did do some googling in consideration of this question.

First - your evolution question. If you check the mythos, Sleipnir is actually one of a kind and is a child born of a horse and Loki (Loki was the mother). Greek mythology also had its share of weird things born when gods mated with animals. My 'evolution' story for them would probably be based on Sleipnir being the forefather and the existing ones descending from him. Or maybe there were a few progenitors if Loki really liked that horse.

If you don't like that, I see the next most probable answer as your world having mammalian species with more than 4 limbs scattered across the world, either with 8 being the standard or maybe with 4, 6 or 8 depending on the ancestor. I'm no evolution scholar but I would assume land creatures generally have 4 limbs +/- a tail due to that being what we first obtained when crawling out of the sea. If your world had a few different things crawl out, you could wind up with different limb counts.

At this point, I think adding a limb to something would require extremely selective breeding over a course of hundreds, if not thousands, of generations. Adding 4 limbs...well, we'll be able to do that with gene splicing (or be extinct) faster than nature would be able to add them.

Second - How do the limbs work? I'm going to skip spider-shaped bodies, the implications of trying to set 8 legs in a circular fashion on an entity with a spine are not something I want to get into right now. If you desire a spider-horse solution, let me know and I'll ponder it. My knee-jerk answer is multiple spines.

I'd go for setting it up so that the front shoulder joints are replaced with small versions of hips - rather than a single joint, you have two side by side. The hip bones in the back would have evolved to have a similar arrangement with 4 joints coming off of them in 2 pairs. The stomach would have some sort of pouch and one set of 4 legs would generally be left in the pouch. This gives the animal the opportunity to allow the leg muscles in half the legs to be at rest at any given time, increasing stamina. The lungs and heart would expand for this, or maybe the horse actually has two sets of each to account for the extra oxygen and blood flow required to service an extra set of muscles. Spare organs is also generally an awesome feature for mythical animals.

The addition of 4 legs should add a decent amount of height + distance to the jumps, ask a physics major for specifics I'm going to go ahead and guess you'll get something like 50-80% extra.

With extra organs and legs, you're looking at a king of horses that isn't necessarily faster than another horse but will far outlast any other.

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  • $\begingroup$ a child born of a horse and Loki (Loki was the mother) Now that sounds like worthy of its very own set of questions, on many levels. $\endgroup$ – AnoE Jul 29 '16 at 14:16
  • $\begingroup$ @AnoE it is more prosaic than it sounds - Loki shapeshifted to a mare before conceiving. $\endgroup$ – rumtscho Jul 29 '16 at 18:20
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For anatomy, you can look at the Direhorse from the Avatar movie for at least the first 4 limbs. I'm sure some animator or artist has done an anatomical series on how the bones and muscles would work to allow parallel limbs to work as most aspects of that film got a pretty through scientific treatment. You can duplicate this for a rear set of legs.

As for how such a horse came to be, consider a bizarre, yet stable, conjoined twin that the gods somehow can replicate. Perhaps as a foal(s) the unwanted parts are excised (like a duplicate head), internal systems are hooked up as necessary, and somehow the beast can control all 8 limbs. Obviously this isn't something that would be likely to develop into a naturally occurring species, but then again, this is a steed for the gods, so maybe they can irradiate and inject teratogens into a normal pregnant mare in order to create a new Sleipnir when Odin loses his old one.

It is also quite possible that "8 legged steed" was just Norse slang for "a really fast horse that moves so fast it looks like it has 8 legs!", not something meant to be taken literally :)

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  • $\begingroup$ I really like that last thought. $\endgroup$ – Sara Costa Jul 29 '16 at 9:06
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Have you ever seen "Tölt" the 5th gear of the Icelandic Horse?

Youtube video of Tölt

They have another style of walking which does not compare to neither the canter nor the trot which are 3/4 or 2/4 rhythms. If Sleipnir was to do Tölt, it might actually give a speed benefit if the legs were approximately located as in the picture you provided, however they'd need to feature a full set of joints.

Tölting horses can get very fast, comparable to the average gallopping horse

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  • $\begingroup$ When you say "Tölting horses can get very fast, comparable to the average gallopping horse", do you mean that they can get just as fast as, or even faster? $\endgroup$ – MarqFJA87 Jul 3 '18 at 3:17

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