How would a tripedal animal walk?

Question

Animals on Earth have 2 or 4 legs they use for walking. On another world, however, animals have 3. I want to know how these animals will walk. How will their legs be placed and how will they move them to walk? For instance, maybe they would have 2 legs in front and one in the back, 1 in front and 2 in the back, or another setup entirely. What would the gait look like? Perhaps they alternate between 2 legged and 1 legged steps or maybe move 1 leg at a time in a 3-step cycle. While there are many potential answers I want to know which arrangement would be most effective and therefore most likely to evolve.

Assume that the 3 legs form a triangle (although not necessarily an equilateral one) rather than a line. Let’s also say the animals evolved on an Earth-like world with equivalent gravity. I imagine that different sized creatures with different walking requirements might have different setups and gaits and even one animal may have multiple gaits just like we see on Earth. While I’m interested in the potential differences between large and small, fast and slow, let’s focus this question to an animal species of roughly human size and with human needs to walk long distances and occasionally move faster.

Answer 1

The number of limbs of a creature is determined by rotational inertia around the 'feet' of the creature relative to its mass. This ratio determines how easy it is to fall over. The mass of a creature is by approximation proportional to its size cubed $$m \propto l^3$$ while its rotational inertia is proportional to its mass times its size squared, $$I \propto ml^2 \rightarrow I\propto l^5$$ For very small creatures, it is extremely easy to fall over. This is why insects have six legs, so that three legs may be in contact with the ground at all times creating a stable tripod. Larger animals can do with just four legs, because they take longer to fall over on two legs (long enough to move a second pair of legs to a suitable location). Humans place their center off mass high from the ground and can create a large moment of inertia by standing up high and moving their long arms around, which allows them to be bipedal.

This gives us a general idea about a tripedal animal. It will be quite tall and its center of mass will be high from the ground, to prevent toppling when only two legs touch the ground. The general 'tripod' image we know from War of the Worlds is thus a logical option: the center of mass high up, and three fairly long legs.

The real question before we can answer how the animal would walk, is why it has evolved to have just three legs* and not two or four. Apparently, two legs gives insufficient stability, and four legs requires too many resources. The good thing here is that you want your creature to move long distances, which definitely favours fewer limbs. The reason for three legs over two could very well be due to the terrain. Perhaps your world is covered in a thick layer of soft material with steadier ground underneath; perhaps igneous rock covered by a thin layer of peat bogs or marsh-like soil. With two legs, it's difficult to free yourself from marshlands, because if you pull out one leg, you push in the other. Three legs form a great solution, as you distribute the force of pulling out of the marshes over two other legs.

This leaves the question of locomotion. We lost the fourth leg for energy considerations. A third leg swinging in the middle underneath the body requires a lot of energy due to the awkward poses required, and does not seem ideal. Instead, let's try something where we go in a fluent motion.

The animal will have one front leg (slightly thinner and more agile; perhaps a displaced tail) and two side legs (stronger and heavier). The front leg initially starts pointing left forwards; the left leg pointing left rearwards, and the right leg pointing straight to the right, in a stable equilateral triangle. It starts by placing its left leg forwards next to the front leg, while the body moves forward. All the legs will now be in one line, and slowly falling forwards due to the momentum from the left leg. To counter this, the front leg swings to the front right now. We now have an equilateral triangle exactly mirrored, and repeat the process. At all times, the body will be directly in line with the legs or in the middle of a stable equilateral triangle, which is exactly the stability you want when trying to pull yourself free from bogs. Furthermore, the body makes a continuous forwards motion, and thus it can be very energy-efficient. This way, it can cover large distances over the difficult boggy terrain without ever getting stuck, unlike its predators.

To escape from predators when on dry land, it can adapt the 'jumpy' locomotion with the front leg going underneath the body for support while the side legs can propel the creature forwards. It will run towards the nearest bog where the predator dares not go.

^{* I'm well aware that evolution doesn't need a 'why' and doesn't work towards a goal. However, for worldbuilding I think it helps to think of evolution like that - the fact that it happened is just a coincidence, just like our protagonist survives it to the end and is not one of the side character that gets killed in the first minutes of a story.}

Answer 2

Almost all animals have even numbers of legs, except the kangaroo.

Kangaroos walk on 5 legs.

Front legs, tail, back legs

But they run on two.

That actually gives you variants on tripedal motion, as they move front legs and back legs as matched pairs at a walk. Simply eliminating the front legs from the cycle and making the tail more leg-like would give you both run and walk movements with the third leg only used at a walk for some creatures, others doing a more conventional four legged "gallop" type run with the back leg coming forward of a front pair for a crossover.

Answer 3

Here's an option...to see an animal walk like this might be creepy and kind of nightmarish but it could work: Three-Legged Robot

Kind of like a Tarsier or Owl, they could have joints that allow full rotation. This would make for some unique and fun creature designs.

Edit: @uhoh dug up this video which goes more in depth on how this works: https://youtu.be/7XsaJwKKBYo

Answer 4

while your question doesn't have any specific criteria as to what would make one answer better than another I can think of two options:

1. Similar to a person walking with a pair of crutches
   In this case the animal would set its front two feet and then lift and move its rear foot. For this I'm imagining a more worm-like creature where its body is longer and with less distinctive shapes aside from the foot pods although the concept could work with a more upright design if the front two feet were more like extended arms and the rear foot having a wide base to provide necessary stability when using its arms to reach about.
2. Similar to a spinning top
   Now this one gets a lot more alien, For this I imagine a creature with a mostly round torso and the three legs being spread underneath it in an equilateral triangle shape. When moving the creature leans slightly to one side and spins itself (imagine a martial artist performing multiple tornado kicks in succession). To help the creature keep balanced the legs should have knees, ankles, etc so that changes in terrain can be more easily compensated for. As for how the creature keeps aware of its surroundings and position while spinning is up to you, maybe it has a human head and neck that rotates as needed or maybe it has more like a fly's compound eyes

In any case hope this helps spark some ideas

Answer 5

While "Landstriders" from the Dark Crystal are technically 4 legged, their running gait is very nearly 3 legged. I always found them compelling for some reason.

I would describe it as placing their two fore legs forward mostly for balance and shock absorption allowing a strong central hind leg to then come forward. The single hind leg then propels the creature forward.

Sorry it is so brief. Here is a clip of one in motion. https://youtu.be/-DBkjvXHgj4?t=1m23s

As a second example of this locomotion, maybe one might imagine a rabbit running with a single hind leg rather than two in lock step, potentially like: https://www.youtube.com/watch?v=9DjXFFV-QI8

Answer 6

You could take inspiration from the Hunters in Half-Life 2: Episode 2. They have two distinct modes of locomotion. One is shuffling, which they use for precision movement, where they make small motions with one leg at a time. The other is running, where they use their two front legs in a normal bipedal running cadence, and the third leg trails along behind, more like a tail. When necessary they can also jump in different directions by pushing off with different combinations of legs.

Answer 7

I think you could argue that seals and sealions have 3 legs. Left-flipper, right-flipper, and their hind flippers (while evolutionary two legs) function as one leg (the feet are effectively two toes, reminiscent of chameleon toes) that only can be used to propel the creature forward.

Answer 8

Does it strictly have to walk? If its main mode of transport is flying then you could have a tripedal crow:

https://en.wikipedia.org/wiki/Three-legged_crow

It would only need its legs for standing and perching. The third leg could bend inwards to give it better grip on a branch.

For a more conventional tripod there's Star Trek's Species 8472:

http://monster.wikia.com/wiki/Species_8472

You could make them very large, so that they tower over their prey, like a War Of The Worlds tripod.

Or lastly, you could have a sea dwelling creature, a kind of jellyfish with only three tendrils

Answer 9

Laterally or rotationally.

Lateral walking would look and work much like a human on crutches, two limbs forward followed by the third limb being brought through to the front, and be equally clumsy and awkward. It's not however necessarily a fast or slow mode of travel and a creature with threefold symmetry could use it to move in any direction with a little shuffling around to find the right facing to start off from.

Rotation consists of the creature turning sharply to bring legs around in sequence moving forward as the foot or feet in contact advances. This is probably a motion best attempted only at a "run" to add impetus to the rotational motion.

Answer 10

Nature only provides two basic conformational templates: bilateral symmetry and radial symmetry. Unfortunately there are no models under bilateral symmetry that could provide 3 limbs for locomotion, (the nearest you'd get would be a tail appendage, or 4th limb atrophy, and the atrophy would maybe happen in individual species but never an entire eco system, (in fact I suspect atrophy'd never happen, not even rarely)). So you are left with radial symetry. (The rather dubious endeavour of inventing trilateral symmetry in fact would be a form of radial symmetry.)

"So you are left with radial symmetry."

Examples of organisms that exhibit radial symmetry and could have 3 limbs (easily) resemble Starfish. That needs to be where you start your thinking. A Starfish which had 3 limbs, that developed:

• an endoskeleton
• knees
• hips
• ankles
• weight bearing limbs under the body rather than splayed.

That is what you need to be focusing on.

However the gait of a Starfish is incredibly slow. But more on that later.

Lifestyle:

The three limbed variant that stood upright, would look more like a ball on stalks. It makes sense it would be a overhead "shallows/ pool hunter" like a heron. However since it started out as a Starfish it's mouth would be in between the limbs (basically where you'd expect the anus to be) and the 3 limbs would (logically) be prehensile so that it could scoop up food from below and pop it in it's mouth. i.e. balance on 2 legs, scoop with a third. That would give you and idea of how well it needed to balance: at a instant it could strike with any one "leg" and perfectly balance on the other two. This would be useful for hunting a wide umbrella area.

Locomotion:

It would stride about the place very long limbed - 2 feet on the ground would be a walk, it would have no real need to have a "forwards" or a backwards" i.e it would never really be orientated with a"facing" nor need to "turn" it would just glide over in a direction - 2 feet on the ground would be a walk , 1 foot on the ground would be a canter and brief episodic no feet on the ground would be a gallop.

Head

This is where I get into very uncharted territory - but we can apply "first principles" and see where we get to. You see, radially symmetrical animals don't have a head. (In fact bilateral symmetry is actually radial symmetry "laid down flat", like a worm is radially symmetrical sectionaly and bilaterally symmetrical from planar axis, and the "head" is the bit that goes "forward" to sense/eat and this sensory cluster then becomes the "head") So in this case it's mouth and anus would (logically) be opposed so it's anus would be on the top (rather odd) and it's central nervous system (eyes/brain) would face "down", and "ring" it's mouth, (the brain would be a "donut" shape with the gut travelling through it!) however if the eyes were on stalks they could be prehensile like octopus tentacles or elephant trunks with eyes on the end of a "stalk" that could retract in and out and have no real "facing". The eyes would have to be connected via short optic nerves direct to the brain "donout".

So the "brain" would ring the "mouth" at the underneath and the "eyes" would protrude from that but be biased to looking down

Conclusion:

Once we draw it out in MS Paint (<3 Paint!) It seems we get something very like HG Wells Tripods in war of the worlds. Seems he already did the necessary thinking back in 1898!

I went down the ecology of one such creatures evolution but you can see that prey animals may never have got past the stage of radial symmetry or they could be bilaterally symmetrical as well but never get to be more than millipedes. It seems inevitable that in a ecosystem of "three limbed" creatures they would all derive from a common ancestor like I describe then diverge into variants of predator prey animals.

Answer 11

There are any number of videos on youtube of cats and dogs which have had amputations of one or two legs. Apparently they learn to cope just fine with only 3 limbs. You could try reproducing this gait, using a single leg in the middle.

Three legged dog

Answer 12

Approach 1: 2 legs in front, 1 leg at back

This one's pretty easy. If the back leg is A and the front legs are B and C, then it would walk something like this;

B and C are brought forward sequentially, as in all Terran bipeds. Then, A is brought forward towards B and C to push off the ground.

An animal with this limb configuration could also take on a unipedal, saltatorial gait with two "arms".

Approach 2: 1 leg in front, 2 legs in back

This would essentially work the same way as the last approach, but in reverse. This time, the back legs are A and B and the front one is C.

C is brought forward to push off the ground, while A and B are brought forward sequentially.

An animal with this limb configuration could also take on a bipedal gait with one arm.

Approach 3: 1 leg in back, 1 leg in middle, 1 leg in front

This would have to move in an almost bounding gait if the animal to keep a leg off the ground at all times during motion. This time, back leg's A, middle is B and front is C.

When the animal is airborne, C touches down, closely followed by B. Finally, A is brought down to push off the ground into the next "leap".

(This would be a very effective sprinting animal)

Approach 4: Trilateral symmetry

Terran vertebrates have bilateral symmetry; they are symmetrical about a line running from front to back. Trilaterally symmetrical animals might have three legs positioned like the corners of a triangle.

This one is rather simple; the animal would just move its legs in the desired direction.

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These are just my ideas on how tripedalism would work. Hopefully I've been helpful, and thanks for asking.

Answer 13

Nothing like dragging things up from the dead! I've been thinking about tripedalism a lot since I'm burning through the Culture novels again (with three main tripedal species involved: Idirans, Homomdans, Chelgrians), and I was curious to see what evolutionary biologists thought about it (subsequently stumbling upon this thread).

I'd like to add another perspective into this, and that is "What formats did prior creatures on a hypothetical planet exhibit?". If your evolutionary path had some six-legged creatures, it's reasonable to consider that at some point in development the creatures may have "stood upright" (raised the middle legs) to get a better view of the environment and thus make better choices, improving their chances of survival.

If we assume these creatures survived long enough, like bipeds their bodies could have morphed and limbs may have eventually fused. There's plenty of evidence in our planet's evolutionary history to suggest bony structures fuse over time.

So you may end up with the hindlegs fusing and the midlegs elongating, or the hindlegs strengthening and the midlegs fusing and elongating. in Banks' Culture novels, the Idirans and Chelgrians seem to have evolved from six-legged creatures in bilateral symmetry, and the Homomdans evolving in radial symmetry.

Answer 14

You've never seen a 3 legged dog? I imagine it would look and act just like that.

https://www.youtube.com/watch?v=TEXqBHP7JeI

In an evolutionary sense, I would guess that typically it would be 2 front legs and 1 rear leg, as the front legs could also be used for manipulation. Even though having 2 rear legs and 1 front would probably result in a creature that would run faster, I don't think that would be a greater evolutionary advantage over the alternative. E.G. climbing would be a lot more likely with 2 front. I'm sure there are many other examples.

Answer 15

As you base your question on evolution, I'd say that evolution already shows that 3 legs is not the best approach. That's why all animals have an even number of legs. There must be a good reason for an animal with 3 legs to survive evolution and for this reason there must be a certain environmental parameter, which you unfortunately didn't tell us. As I don't know this parameter it is hard to say what would be an efficient way to walk with 3 legs.

Answer 16

To those who say that you cannot evolve three legs from bilateral symmetry: you can! Just stop using (sorry) "lateral thinking".

The dorsal fin of fish is an extremity that is not necessarily paired: It grows on the back (dorsal) instead of the side (lateral), hence my pun "lateral thinking". However, fish can grow more than one, on different segments along the length of the body:

.

According to Wikipedia, fish can grow up to three dorsal fins. Furthermore, most dorsal fins contain bones called pterygiophores; in seahorses these bones are even articulated.

The muscles, fin ray joints, and supporting structures underlying the dorsal fin are described for two seahorse species: Hippocampus zosterae and Hippocampus erectus. A fan-shaped array of cartilaginous bones, the pterigiophores, form the internal supporting structure of the dorsal fin. Each pterigiophore is composed of a proximal radial that extends from a vertebra to the dorsal side of the animal, where it fuses to a middle radial. The middle radials fuse with each other to form a dorsal ridge upon which sit the spheroidal distal radials. Each distal radial articulates with a fin ray on its dorsal side and is attached to the dorsal ridge on its ventral side by a material that has been histologically identified as elastic cartilage. Together these connections form a two-axis joint that permits elevation, depression, and inclination of the ray. Each fin ray is actuated by two bilateral pairs of muscles, an anterior pair of inclinators, and a posterior pair of depressors.

https://www.researchgate.net/publication/12065134_The_dorsal_fin_engine_of_the_seahorse_Hippocampus_sp

Some species (e.g. sharks and seahorses) even have muscles in their dorsal fins, enabling the animal to actively move them. The muscles form oppositional pairs right/left due to the bilateral symmetry. (In seahorses it forms oppositional quads.) This is a far simpler system than seen in 2- or 4-legged creatures, which use flexor/extensor oppositional pairs which must develop from separate segments.

So if you want a creature that could plausibly evolve, but is definitely non-humanoid, consider a bilateral that has elaborated three ventral, articulated legs. (After all, this is a world-building site; think outside of the box.) Or a seahorse that keeps its prehensile tail, doubles its dorsal fin, and evolves the latter into fingered "arms". Instead of Hippocampus, I call it Tricampus.

As far as how to actuate such appendages to produce locomotion, I defer other excellent answers already provided.

Answer 17

Some of the creatures that inhabit the alien "Rama" spaceship in Arthur C Clarke's "Rendezvous with Rama" series of novels were apparently tripedal--at least I think they were. I seem to remember they did a lot of twirling about. Here on earth, insects have six legs, i.e twice three; take a look at how they walk for clues on how a tripedal being might. How do five-armed starfish get around? Look too at the octopus, even though eight is not a multiple of three.

As for "War of the Worlds", in the '50s movie version, the folks in Hollywood couldn't come up with convincing working models for the tripod-shaped (as described by Wells) Martian war machines, so they just had them be flying disks with weapons on top. But the Martian war machines shown in the Classics Illustrated comic book of "War of the Worlds" (also from the '50s) were shiny metal tripods with armed turrets on top.

Answer 18

You may want to have a look at the works of Karl Sims, in particular at the "Evolved Virtual Creatures" (although that work is quite dated, written in 1994, so there could be something way more advanced today already; still, good as a starting point). Basically, he assembled some "creatures" out of physically simulated blocks, some creatures were kinda realistic, some were not realistic at all. Then used neural-genetic-etc. algorithms to optimize their movement: at first they were just twitching chaotically, then, through artificial evolution, were able to walk, swim, jump, etc. As I remember, there was at least one three legged creature, although the movement pattern it eventually developed was nothing unusual, similar to the running rabbit from JonSG's answer.

Answer 19

I have come up with this mobile plant with three legs. Two in front. One in back. It begins life in the ground. Eventually it pulls itself out of the ground with its front two, and the stem becomes the back leg. That could be a reason for a tripod.