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Could a mammal evolve 6 legs?

Is it possible for a mammal-like creature to evolve 6 legs? What are the downsides to having that many legs?

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Short answer: no.

Long answer: almost certainly not.

The four limbs and (optional) tail body plan is baked into vertebrates. ALL vertebrates. Not just mammals, but reptiles, amphibians, fish -- anything with a spine and bones (or even cartilage) has been four limbs and a tail (with the option to minimize or even eventually delete any of these due to evolutionary pressure) so far back into the history of life that you'd need to branch the evolution before the rise of cartilaginous and bony fish.

While not an impossible idea, what you'd wind up with would resemble a modern mammal only due to (posited) convergent evolution -- the way the "crab" body plan has apparently evolved more than half a dozen separate times. It would be less closely related to modern mammals than a slug is to a snake.

Downsides to more legs? Higher nutritional requirements to form, maintain, and operate more limbs, mainly. I don't know of any known science behind why vertebrates have only four limbs (plus tail) vs. six (plus tail), given that the majority of dry land life species are six-legged arthropods -- but it certainly seems there has to be a reason, and slightly higher nutritional requirements during development and adulthood might throw enough advantage to the smaller limb count to do the job over half a billion years of vertebrate evolution -- except that the only vertebrates with more than four limbs (plus tail) apparently died out before the split between cartilaginous fish (sharks and kin) and bony fish (all other kinds of fish, from which amphibians and land vertebrates descended).

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    $\begingroup$ "Zero fossil evidence (that I know of -- I'm no paleontologist) that six-limbs vertebrates ever existed, even in "prototype" forms": Acanthodii. Back in the Palaeozoic, before the split between bony fishes and cartilaginous fishes. $\endgroup$
    – AlexP
    Sep 24 at 14:11
  • $\begingroup$ Pictures in that Wikipedia article seem to show the same fin layout as a modern trout: four fins analogous to legs, twin dorsal fins, anal/ventral fin, and caudal fin. $\endgroup$
    – Zeiss Ikon
    Sep 24 at 14:15
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    $\begingroup$ Many of them had three or more pairs of fins. Moderately well-known for this. (The point being that they lived before out inner fishes acquired a skeleton for the limbs. The road not taken etc.) $\endgroup$
    – AlexP
    Sep 24 at 14:21
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    $\begingroup$ @AlexP "Moderately well-known for this" took me a couple years to come across it after seeing a question on it in Biology SE, no one there produced it in an answer to the question so perhaps not 'as' well known as it could be? & I searched Google extensively at the time, didn't find it. $\endgroup$
    – Pelinore
    Sep 24 at 19:35
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    $\begingroup$ The four limbs arose from fins on fish. I suspect the lack of a six-limbed endoskeletal body plan on Earth has less to do with nutritional requirements for limbs, and more to do with hydrodynamics and swimming efficiency for the lobe-finned fish from which Tetrapoda later evolved. $\endgroup$ Sep 24 at 23:42
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Yes, totally

The mammal body plan accounts for five limbs: either 2 arms + 2 legs + tail, or 4 legs + tail. Some mammals have evolved to have no tail at all, some oceanic ones have evolved to have no legs.

But just like they evolved to lose limbs, they might evolve to gain limbs. Kangaroos sometimes walk on four, and they are able to stand on their tail; if one had a second tail, and it helped him stand his ground on a fight, or have more balance... That trait might be passed on, and then millions of years later you have six limbed mammals. A bit more evolutionary pressure and those tails become functional legs.

This kind of thing has already happened in our world (with reptiles though, not mammals), and didn't even need an existing limb to be duplicated. Dragons (the drago genus of lizards, not Trog Dor nor Smaug) developed an extra pair of limbs from their ribs, which allows them to glide:

A Draco volans specimen gliding

With the right pressures, in a few million years those wings might become legs if that helps these critters survive and spread.

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Totally Possible

It seems that the underlying reason why terrestrial mammals have four limbs to begin with has to do with the paired limbs' evolution from the gill arches of our fishier ancestors.

In order for mammals to evolve with six limbs, you simply need six limbed proto-mammals that evolved from a six-limbed transitional reptilo-mammal and so forth back to the fishes that began churning out gilly limbs in the first place. Your ur ancestors of six limbed mammals will be fish that churned out an extra set of gilly limbs.

This may seem a bit odd, but given that evolution has transformed the bony-cartilaginous structures into limbs, we could make the case that mammals are already six-limbed, for the simple reason that our mandibles also evolved from the same structure.


Getting from a currently four limbed mammal, like a dog, to a six limbed mammal could still be done, of course. It would require considerable convolutions of evolution and natural pressures.

I'd argue that putting some of the ribs back into the business of locomotion might eventually lead to a six limbed mammal. Transformed ribs could similarly be coopted into other uses, like fighting or capturing prey.

Can you not imagine one of Fido's distant progeny, 48 million years from now, roaming some future forest, and having spotted his prey, rushes towards it and flings forward his spiny-feathered thoracic claws, trapping his prey in a hedge of spikes while he leisurely crushes its skull with his massive grinding teeth?

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Yes, elephants.

enter image description here

The way legs develop is that you have a long appendage which is leg like which can be used as a leg. Vertebrates' common fish ancestor had leg like appendages, which became legs. Elephants have tails and a trunk. Those could both develop into legs with enough evolutionary pressure, and be used to help walking on six legs.

Tails have commonly developed as leg substitutes, and are ofen used by monkeys to move around. It would be unusual to develop the trunk into a leg, because it's more useful to basically have a hand on your face than another foot.

There's no real disadvantage to it, it would just require a lot of selective pressure and value. It's a difficult evolution, so you'd need a good reason for more than five legs.

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    $\begingroup$ Apes don't have tails. It's one of the big distinguishers between the few ape species and the large number of monkeys, almost all of which do have tails. $\endgroup$
    – Zeiss Ikon
    Sep 24 at 16:25
  • $\begingroup$ Fixed that error. $\endgroup$
    – Nepene Nep
    Sep 24 at 23:19
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Yes

If your definition of 'mammal-like' falls anywhere short of 'is a mammal', then there is certainly a way to add an extra limb pair

On the downsides, it will take up more nutrition compared to only 4 legs, but seeing as mammals almost seem designed to use up more nutrition, this isn't much to worry about. It will also take up more brain space, but this is still only a minor issue

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Yes, it's a thing that happens, e.g. https://www.express.co.uk/news/nature/1401479/six-legged-calf-northern-ireland-belfast

as with any other mutation, it needs not to prevent the production of young, and there needs to be an environmental niche in which it is advantageous, and if so, it will persist and flourish.

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    $\begingroup$ I've seen the pictures, that is either a parasitic twin (non reproducible in following generations as it's not a mutation) or possibly (& perhaps more likely given the location) an atavistic limb, a mutation causing the instructions to grow a limb (the dorsal fin found in fish in this case, which then follows the pattern for a normal limb in the animal, so no, you won't ever grow a fin) to be switched back on // either way not very useful, a limb (or limbs) sprouting straight up from the back will never be any use for what the OP wants. $\endgroup$
    – Pelinore
    Sep 25 at 0:04
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Downsides of six legs in large animals (not only mammalesques):

  • Will move more slowly than a four legged or two legged animal. There is no gait that stretches the length of a stride longer than the combined length of the legs, that would be equivalent to a gallop with all feet off the ground.
  • Equivalently there is no movement that maximises reach when for example climbing a tree. The front legs may be reaching for the next grip, the back legs holding on to the existing position, and the middle legs are uselessly waving in the air.
  • Less energy efficient when moving. With the possible exception of six legged pronking, there is no gait that would allow tendons to stretch then recover energy.
  • Possibly less energy efficient when reaching up or down. For example front legs may be kneeling, rear legs locked straight, middle legs must be held braced by muscles in an unlocked position. Could be solved by using a double knee on the middle legs.

But within these constraints, an elephant being the obvious example: never gallops, never climbs trees, has solved the up/down reach with a long nose; there would be benefits in smoothness of movement, load distribution upon the ground, and total weight that could be carried.

But there are good reasons that robot designers are obsessed with achieving two legged and four legged gaits.

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Could a mammal evolve 6 legs? Perhaps.

But if the new mutation has to compete against established & well adapted species it's unlikely.

The way the embryo develops does give potential for developmental mutations (careful not to confuse that with parasitic twins though) that might conceivably cause additional limbs & if that individual produced enough offspring & somehow become reproductively isolated a population could conceivably happen, natural selection in that population could then iron out any problems with limb placement & general morphology but the initial mutation would need to be something that wasn't immediately problematic for survival, it's a long shot, but then a lot of evolution is.

"it's a long shot" & one (I should point out) that to the best of my knowledge has never happened in mammals & hasn't happened in any other chordate since (perhaps) the 'early Devonian', so, very unlikely, but still 'possible' perhaps.

I'm thinking of the sort of mutation that resulted in the The family with six fingers, that of course is digits rather than limbs but it does perhaps demonstrate the sort of mutation you'll need & (when you read up on it, if you can find the right reports to read) does perhaps give you some indication of where to look for the sort of developmental systems in foetuses you'll need a mutation in.

Note: mutations that switch on development of atavistic limbs won't be what you're looking for, in mammals that will only get you the 'limbs' our fish or (for tails) more recent ancestors had that we have since lost, that would be the (3) dorsal, (4) adipose, (5) anal & (6) caudal (tail), none of which come in pairs or are usefully located on the torso for what I think you want.


This answer that just popped up a few minutes ago might very well be what you're looking for.


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