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A quadruped the size of a mountain tiger which never stops moving, even in their sleep they walk and wander around.

What benefit would the animal get from wasting so much energy moving every minute of its life?

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    $\begingroup$ It is a cool idea. Great for a story because so weird. $\endgroup$ – Willk Jan 17 at 16:09
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    $\begingroup$ Why do you want this? Context can help inform a solution $\endgroup$ – Tom J Nowell Jan 17 at 20:11
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    $\begingroup$ My first thought is, because something is chasing it :) $\endgroup$ – Jeremy Friesner Jan 17 at 23:40
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    $\begingroup$ @Ruadhan not familiar with reddit culture or any famous forums, I once used 4chan but It was boring. $\endgroup$ – user81643 Jan 18 at 9:50
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    $\begingroup$ Link to Reddit's immrotal snail $\endgroup$ – Mawg says reinstate Monica Jan 18 at 10:36

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Walking aids (or is) its blood circulation

Ever sit down and when you get up your feel a bit light heated, or get pins and needles in your muscles? The cause is imperfect circulation. Once you start moving everything recovers pretty quickly. Let's crank that up. Your animal has such poor circulation that it needs to walk to circulate its blood

One choice is the animal can stop, but if it stops, muscles and brain become under oxygenated. Maybe not enough to be directly fatal, but enough that it's in pain, cant react to a threat and cant make sensible choices for several hours. Walking helps aid in circulation.

Alternatively if you crank this up to the extreme, the animal has no heart, just "one way" valves in the arteries. As the animal walks the blood is circulated. The space occupied by the heart can be used for other things.

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    $\begingroup$ I know next to nothing about horses, and I was amazed when I learned that their hooves help circulate blood: horses.extension.org/blood-pumping-mechanism-of-the-hoof $\endgroup$ – HemiPoweredDrone Jan 18 at 3:44
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    $\begingroup$ The pokemon Spoink, sort of works this way :P. He needs to bounce on his spring like tail to create a heartbeat $\endgroup$ – SirDuckduck Jan 18 at 12:25
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    $\begingroup$ If it’s evolved from an arthropod the constant motion could be a way to improve the flow of oxygen from skin to organs. The creatures that move constantly can get bigger because they have better oxygen transport, but once they’ve evolved to be bigger they’re stuck in an evolutionary dead end where they can’t stop moving. $\endgroup$ – Joe Bloggs Jan 18 at 12:36
  • $\begingroup$ This happens in humans also - walking puts all of our weight on the bottoms of our feet, increasing capillary pressure and helping to pump the blood back up to the heart and brain (citation). $\endgroup$ – J... Jan 18 at 19:49
  • $\begingroup$ What if the muscles in the legs are tied to the heart? The heart doesn't beat on it's own as it's "massaged" by the biceps/triceps/whatever. So the heart exists, it just doesn't have it's own muscles. IDK, just a thought. $\endgroup$ – computercarguy Jan 18 at 20:37
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The ground is very hot

Gotta keep on movin' - (click for sound)

enter image description here

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  • $\begingroup$ Then the predator would just hunt at dusk, or become nocturnal. $\endgroup$ – smci Jan 18 at 18:05
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    $\begingroup$ @smci - maybe this is a volcanic planet, near its sun. We haven't been told. $\endgroup$ – chasly - supports Monica Jan 18 at 19:49
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    $\begingroup$ The floor is lava $\endgroup$ – Cai Jan 19 at 14:21
  • $\begingroup$ @Cai - Correct! I did consider that as an alternative title! $\endgroup$ – chasly - supports Monica Jan 19 at 14:31
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Their breathing doesn't use lungs with its inhale/exhale bidirectional flow as most land animals, it is instead built around a one way flow system, like it happens with sharks gills.

Coincidentally sharks have to continuously move to stay oxygenated. It can be more efficient because it doesn't "waste" any time with pushing out used air, but on the other hand it doesn't allow to stand still for long times.

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    $\begingroup$ It's hard to imagine how that could possibly be construed as a benefit. Moving water is energetically costly, which is why more primitive gills (as with sharks) can persist. Air is not nearly so expensive to move, and the energy expended by moving the whole body is massively less beneficial than the evolutionary adaptations required to develop even basic lungs. (Particularly for megafauna.) $\endgroup$ – jdunlop Jan 17 at 9:58
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    $\begingroup$ The OP may be asking for a benefit, but evolution doesn't require one. Only that it not be enough of a detriment to be pruned from the family tree. Though, that does make this answer a bit of a frame challenge, and you'd still need some justification for that trait having lasted so long $\endgroup$ – BThompson Jan 17 at 22:43
  • $\begingroup$ Like the radiators on older cars. They cooled while the car was moving. Thinks: this can't be right; didn't they have an air pump too? $\endgroup$ – RedSonja Jan 19 at 6:42
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    $\begingroup$ @RedSonja Many old engines (air- and water-cooled) drive a fan mechanically (hence "fanbelt"), so apart from real vintage vehicles it's actually newer cars where you're more likely to have a problem, normally due to a thermostat issue. An engine doesn't produce very much waste heat when not moving/idling, so is unlikely to overheat, unless in very hot conditions. An uphill traffic jam is more of an issue (engine working hard intermittently, never getting up much speed/airflow). Also climbing slowly heavily laden/towing. That's when you learn if your electric fan thermostat has failed. $\endgroup$ – Chris H Jan 20 at 13:19
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It is seeking something.

Dolphins sleep with half of their brain at a time.

https://www.scientificamerican.com/article/how-do-whales-and-dolphin/

While sleeping, the bottlenose dolphin shuts down only half of its brain, along with the opposite eye. The other half of the brain stays awake at a low level of alertness. This attentive side is used to watch for predators, obstacles and other animals. It also signals when to rise to the surface for a fresh breath of air. After approximately two hours, the animal will reverse this process, resting the active side of the brain and awaking the rested half.

So too your tiger. It is sleepwalking, in a way. It is moving, and it is smelling. It is not alert; this creature has no natural enemies. Its environment does not pose a lot of dangers. It goes around things in its path. It is more or less walking in a straight line while asleep.

If it smells what it is seeking, its dreams will change and it will wake up.

I could imagine that there might be very few of these creatures and they are spaced far apart. The male will have to travel far to find a mate. That is what he is seeking as he sleeps. That is what he is dreaming about.

This is such a cool idea. What if these things sleep all of the time? One arrives and walks through the village and people clear the way for it and watch it as it goes. What if they sleep for years and years as they walk, dreaming? What are they like when they wake and re-enter the world?

What if it is the same one, walking and walking in his sleep, because he is the last one?

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    $\begingroup$ This description fits many unmanned spacecraft missions really well, of all things. They tend to spend a lot of time in sleep mode of sorts to conserve energy and because there isn't much to do anyways, while listening to transmissions. $\endgroup$ – val says Reinstate Monica Jan 19 at 10:43
  • $\begingroup$ I like this idea, but I will note there are much stronger evolutionary pressures on dolphins (aquatic air breathers have to either sleep in floating position or develop some other non-sleeping approach to avoid drowning) to produce this half-sleep result than there are on land-dwelling animals (where air is generally plentiful and requires no special effort to acquire, even asleep). You'd need a really strong benefit to counter the evolutionary pressures against wasting energy without repayment. $\endgroup$ – ShadowRanger Jan 19 at 21:09
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Because the plants they eat are so devoid of nutrients, or so rare, or so inefficiently digested that the animal has to constantly be grazing in order to get enough food to survive. They benefit from living where no other animals their size can and by eating a food they have no competition for.

This isn't far from being real life. Many grazing herbivores spend huge portions of their life ingesting food, not 100% but an animal that does have to graze all the time is not beyond imagining.

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    $\begingroup$ Witness Koalas and Pandas. Which are hyperspecialised to eat only one type of plant and frankly suck at doing so to a ridiculous degree. If they were fictional, every biologist would laugh at them for being unrealistic, but there they are. Both creatures need to spend virtually all their time either looking for food or eating it because they get virtually no nutrition from doing so. Fortunately their food grows in copious amounts in their native environment, so they don't have to roam far to find it. But if it did, they'd be constantly searching. $\endgroup$ – Ruadhan Jan 18 at 8:06
  • $\begingroup$ Probably a realistic twist on this idea can be that the brain consumes a lot of energy when working. Humans are estimated to have about 2% of body weight in the brain, but quarter to half the energy consumption (IIRC tested to be about 10% when idling, lots when passing exams and such - hence the sweet candy bars and chocolates as quick power for that). If the efficient food or water is scarce in the environment, this animal can be sleep-roaming looking for it - and not using a brain is more power-efficient than spending some for the muscles to flex. Now I wonder how camels live, in real life. $\endgroup$ – Jim Klimov Jan 18 at 8:49
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So it doesn't die of the cold.

There are insects in the desert that need to stay cool by constantly running. They eat running, have sex running, avoid enemies by running, etc. However they are small, and your creature is much larger which means it loses heat slower.

Your mountain tiger is living in cold climates. Large predators normally rest most of the day to conserve energy so having a tiger-like creature spend most of its day on the move would mean it's doing two things:

1: have one of the highest hunting success rates in the animal kingdom. 2: only hunt tons of small creatures it can eat while on the move.

The cold climate is important. This means that the tiger needs to keep warm by moving. Normally such a creature would have evolved a fur coat to stay warm, but if other pressures forced your creature to quickly adapt to the mountain it could have learned to keep walking to stay warm.

Is this likely? No, but it's the only reason I can think of to stay moving.

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  • $\begingroup$ I counted and it would need around 21k kilocalories a day to move... Obviously excluding the basal metabolism which would be around 4k to 6k. So it would need something like a reindeer every 3 days $\endgroup$ – user81643 Jan 17 at 19:27
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    $\begingroup$ IRL there are bird species with degenerate legs, e.g. nationalgeographic.com/news/2016/10/… who effectively spend almost all their life in air without landing. $\endgroup$ – Jim Klimov Jan 18 at 8:54
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Your creature has lungs but no dedicated muscles to drive them--the muscles moving the forelimbs serve double duty. The breathing cycle is accomplished by moving the forelimbs forward and back. Your creature must have two openings for respiration, either each lung has it's own access, or the lungs are a pass-through design, one opening for inhaling and one for exhaling.

The creature can stop only for as long as it can hold it's breath. The positioning of one or both of the breathing orifices precludes lying down and waving it's limbs.

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Because staying in one place is bad for your health.

In this environment, there are several species of life (can be plants, can be animals, can be fungi) that obtain food by trapping animals that wander through their nesting grounds, acting as a sort of living quicksand. They aren't very fast to react though, so if you just wander through, they can't do much. But if you decide to stay and camp, they'll congregate upon your location, and try to immobilize you. Try to sleep in such a place, and you'll wake up completely immobilized and being slowly eaten alive. And these things are so widespread and can camouflage their nests good enough that the best strategy is to just keep moving rather than trying to see if the spot of land is safe for rest or no.

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  • $\begingroup$ You can go further with this idea and claim the decomposers of this world are VERY effective. You could also turn this around and make them into some sort of filter feeder. $\endgroup$ – DKNguyen Jan 18 at 17:31
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Mapping Behavior:

Your species never really sleeps in a conventional sense. They are constantly mapping out their environment, analyzing tracks and either predators or prey animals. They get to know even individual prey animals or predators. This deep understanding of their environments and prey mean that when they DO eat or hunt, they always know where and when to get a meal as easily as possible without worrying about being eaten in turn. The would know which watering holes dried up when. Constantly moving through their environment might also aid in spreading their scent everywhere, so predators and prey have a harder time locating them.

So really it's a difference between spending a lot of energy up front to learn all about their predators, prey and environment, or spending more time and energy hunting, stalking, chasing, and killing OR constantly fearing attack. If they are predators, they would have two choices. They could be ambush predators, and this would be the one time they would need to stop (but not for long, because they would already know when the prey were coming). This would minimize energy expenditure. Alternatively, they could be endurance hunters, following their prey until the prey was completely exhausted. Omnivores could forage WHILE hunting due to their great knowledge of the terrain. If they were herbivores, they would always know where the highest-energy food items were in season and have abundant calories.

A key component to this would be an extremely good memory. Your animal would need to keep track of all this information. This would also help with variations that didn't only involve terrain (like seasonal changes). I would also suggest they would be endurance animals, since keeping up all that motion would mean they would by default need great endurance (meshing well with the endurance hunter model).

  • As an alternative answer, your animal may constantly deplete its food supply. It eats a small animal like mice and doesn't get much from them (being so big lets it uproot nests, but then it has to allow the population to recover). Or maybe there is a food item that takes a long time to mature but doesn't have a season (so the animal is always moving to the next tree/bush). If it is a herbivore, this constantly roving behavior may be used to carry seeds and/or pollen by its chosen plant. The plant could have a food pod/giant high calorie nut that only this animal can successfully open or eat (due to thick shells and/or poisons). Again, endurance and good memory help in these scenarios as well, to remember where nests and/or food trees are, and aid in constantly moving to the next one.
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  • $\begingroup$ Yes. So many of the answers assume that one or more features of life on earth must exist everywhere. If you’re designing/imagining a different world, who demands that you have sleep or breathing or blood? $\endgroup$ – WGroleau Jan 20 at 9:25
  • $\begingroup$ @WGroleau Those are complex functions of life, for which complex alternatives would need to be proposed. It's easy to think of alternatives to simple things. Particularly respiration and circulation are carried out by all complex life on Earth. Those same complex needs done by those complex systems still need to be achieved. We simply lack proven alternate processes, lacking exposure to alternative evolution. People stick with what they know and can prove. $\endgroup$ – DWKraus Jan 20 at 11:26
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Never Ending Buffalo Migrations

On this world, there is a continuous band of land around the equator. A plain 1,000 miles wide where buffalo-like creatures routinely migrate in vast herds, circling the planet.

It takes a herd (travelling 30mph and resting only to graze and drink for a few hours a day) a little under four years to complete their circumnavigation of the world. They run through the night and day. But once in a while they will stop and eat, rest and drink for a few meagre hours before their instincts drive them on again.

Your Big Cat must keep pace with a herd, because there aren't so many herds it can't afford to sit and wait for the next one. Consequently, it has evolved to be able to run and track its prey in its sleep, because if it ever loses the herd it will be forced to go without food until the next herd comes along.

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  • $\begingroup$ Now a different question arises: why the buffalos have to migrate around the globe? $\endgroup$ – Martin Grey Jan 20 at 14:25
  • $\begingroup$ There are a lot of animals that travel extreme distances. Perhaps they're simply leveraging their endurance to keep away from predators. Most animals have very short endurance, a Cheetah can run in excess of 70mph, but can only do it for brief bursts. If the Herd can keep moving at 30mph for days on end, only the most specialised and persistent predators will be able to threaten them for more than a brief time. $\endgroup$ – Ruadhan Jan 21 at 8:55
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There is an excellent book "Player of Games" by Iain M. Banks, where the story moves to a planet with a perpetual wildfire, working its way around the world burning everything before it, but allowing fresh seeds and reinvigorated growth behind it.

The animals on that planet were always migrating to stay ahead of the fire. Although it has been used before, I think there is more scope to explore such a world.

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    $\begingroup$ Perhaps a planet with a very slow rotation (similar to H. Beam Piper's "Four Day Planet" gutenberg.org/ebooks/19478 ) where the side facing the sun is Too Hot, the side facing away is Too Cold, and there's a narrow band of Just Right...but Just Right is slowly moving over time. If you have a planet the size of Earth, land around the equator (but maybe ocean at the poles where it would be shorter) and orbital mechanics so that the same point faces the sun about once every 350 days or so, then a continuous walking pace should keep this animal in the habitable zone on the equator. $\endgroup$ – user3067860 Jan 19 at 18:01
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This depends a great deal on the environment, and the question you really ought to be asking is "how would such a creature compete with similarly sized animals that don't have to walk all the time?"

Some of the answers here suggest a sort of biological weakness that forces the animal to keep walking. Such things exist on earth; Sharks are a great example. However, sharks use extremely little energy to swim, while a walking animal may be forced to spend a great amount of energy covering difficult terrain.

The only reason a creature like this would not be out-competed by more conventional animals is if:

  1. There are no normal animals. All the animals in this world are forced to walk forever. This has huge implications for worldbuilding and animal behaviors; it may not be possible for animals to live in steep mountains or treetop canopies. This sort of thing might exist on a planet that turns very slowly, resulting in an extremely long day/night cycle. If the planet is a reasonably distance from the star, the night side of the planet may be too cold for animals to live through normally. Some animals will perpetually walk to stay in the sun (hopefully, they don't have to cross any oceans to do so), while others might go into a deep hibernation, probably buried extremely deep underground.

  2. Walking forever grants the animal access to some resource that other animals can't get. Perhaps there are floating magic orbs that never stop moving, and, by following them, the animal in question can essentially eat the magic which this orb radiates off? In this case, the animal can't stop because it would starve, as it has become unable to live off of conventional vegetation.

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    $\begingroup$ Example from real life: human shepherds :) en.wikipedia.org/wiki/Cliff_Young_(athlete) > While the other competitors stopped to sleep for six hours, Young kept running. He ran continuously for five days, taking the lead during the first night and eventually winning by 10 hours. Before running the race, he had told the press that he had previously run for two to three days straight rounding up sheep in gumboots. He said afterwards that during the race he imagined he was running after sheep trying to outrun a storm. The Westfield run took him five days, fifteen hours and four minutes.. $\endgroup$ – Jim Klimov Jan 18 at 9:00
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Just to help, decrease gravity, increase air pressure. Not necessarily by much. Then it would consume less energy to move slowly. As your creature is very large, it has a large surface area and it will take more effort if it only moves when it is awake. And if the food is scattered, like large trees or bushes but no grass, then it is best to move to next place while (half) asleep.

Then you can add the other suggestions too. Since it constantly moves, looses its lungs. It makes sense, it is an extra organ. You can even normalize the environment some more, after the evolution took place.

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The planet is too close to its star, and if you stop you will fall into the hellish day zone

Life on this planet evolved when it was in a Goldilocks Zone, but due to interference from other bodies it has gotten closer to the star over the millions of years - Slow enough to give life time to adapt.

The day side of the planet is too hot for these creatures to survive, because instead of evolving to withstand the high temperatures, they evolved to stay ahead of them. Plants (and most other living organisms) have evolved mechanisms to survive these temperatures.

Now, these creatures constantly move so that they stay in the Night at all times - They have evolved to move all the time.

Perhaps these creatures, in a cruel twist of irony, photosynthesize (maybe they have symbiotic plants on their backs). In that case they'll have to remain in Dawn o Twilight at all times: Light so they can photosynthesize, not Day enough to be too hot.

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Getting mineral from a known location https://en.wikipedia.org/wiki/Mineral_lick. Roadkill is also fairly often happens because of licking salt on the other side of the highway.

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    $\begingroup$ Traveling to a known location sort of implies that you stop when you get there. $\endgroup$ – Nuclear Hoagie Jan 19 at 14:14
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It has some kind of superpowers of speed or agility like some mutant mitochondria equivalent cellular symbiont inclusions, but the same mutation causes it to lose temperature at whatever rate you say, which makes it catch colds or disease... Or its cellular symbiont requires a very specific temperature of 41 or it litteraly jumps into the blood stream looking for a warmer place, which gives it a sixth sense like vibration sense of every movement for 100m around in 360 degrees, and then the animal loses it's magic power for a few days until it manages to re-include the symbiont into enough cells to have the superpower fully running again.

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The animal does not need rest. In fact, as soon as it stops moving, the muscles begin to atrophy and the cells begin to degenerate. Everything it its body is rapidly generates new cells with movement. All cells are short-lived.

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This could be a sort of vestigial behavior, a trait that evolved to deal with some ancient factor that is no longer present, but that does not actively hinder their survival.

There's good evidence that humans used to be pursuit predators, simply tracking animals over long ranges at walking speed until the animals die of exhaustion. This "always walking" trait could be the result of dealing with pursuit based predation; eating/drinking/sleeping, but always moving. If such predators are no longer present the behavior would seem odd, but as long as the creature can still grab leaves and drink rain or dew off leaves; they'll live on, still moving for some reason that no one in modern times can quite figure out.

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Large Intertidal Biome on a Habitable Moon

In theory, the tides on a Gas Giant moon could get dozens or even hundreds of feet higher than on Earth. Depending on the geography of a particular area, that could lead to the sea coming miles inland at high tide. Some US states are low enough to be largely flooded in such a circumstance, for perspective. Depending on how long the orbit of the moon in question is, these tidal floods could come on fast enough to mandate that any creature that can't swim can't just stay still for hours at a time.

Now sure, many animals in such a biome probably would have some swimming ability, but just imagine the rip tide on such a place. Any animal even remotely bound to the land probably wants to avoid getting in the ocean. Plus, during low tide, there are bound to be plenty of sea creatures and plants left out for eating before the tide turns again. Sounds like a decent niche to occupy.

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There was a titanic creature called 'Marschiere-Viel' ('walk-much') in the german SciFi magazine 'Perry Rhodan'. It was living from the sunlight, so it needed to keep on marching to catch up with the sun.

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Your world has an ether, and living beings are sensitive to it.

Creatures on your world are sensitive to a pervasive ether, which has variations (densities, polarizations, etc.) and either the ether moves, or your animal(s) have to move relative to it to do something (sense it, gather energy from it, etc.).

The ether could be a source of energy, either in place of or as a supplement to what we think of as normal sources, and different animals (and plants) can have different schemes for gathering it (e.g. plants are large enough to usually span the moving polarity regions, so they can always experience enough of a differential across their branches to acquire energy; floating oceanic life forms are naturally swept through the ether by wave action - so in calm seas they have to do more work, whereas in storms they're highly charged; some small animals find it more efficient to sway on flexible plants than run around; your animal is mid-sized so constant ambulation is cheapest while providing other benefits).

Your animal could also be sensing eddie currents in the ether, or creating eddie currents for signaling, and this requires movement. This could be done at the same time as or separately from the sleep-walking behavior, and it could be for different reasons (finding mates; remaining in contact with other members of same species; driving prey which live primarily in the ether but can "pop out" into the normal environment at certain junctures where your animal can consume them) and it could come with costs (there are other, larger ether-aware predators out there, and what your animal does to gain vision in the ether exposes it to being seen by things that would prey on it, just like light signaling in the ocean). A variation of Darth Biomech's answer also fits here: ether-aware predators might more easily recognize animals which stay in one place, and constantly moving could be a camouflage.

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