This is part of a series of questions about biological hydraulics, the fist one being here.

My tetrapods have hydraulics in the thighs of their hind legs. To push or pull fluid from closed chambers in the legs, muscles are going to contract either in the legs themselves or the abdomen.

As hydraulic fluid, I am thinking of using either blood or lymph from the circulatory or lymphatic system, as both are closed. Alternatively, there could be a reservoir for the sole purpose of storing hydraulic fluid, but I am unsure over the efficiency with this.

All of them have the following in common:

  • Four chambered heart
  • Warm blood
  • A closed circulatory and a separate closed lymphatic-like system
  • A two chambered lymphatic heart-like organ below the true heart to pump lymph
  • Small, rigid lungs with a unidirectional pattern of breathing
  • Four respiratory airsacs
  • Endoskeleton with four limbs, a skull, and one to two tails

Arthropods and some other invertebrates do use their hemolymph to push themselves, but I am unsure of whether this will change for vertebrates with complex hearts and closed circulatory systems.

  • $\begingroup$ You could use cerebrospinal fluid, this is a totally separate system to the others, en.wikipedia.org/wiki/Ventricular_system $\endgroup$
    – JeffUK
    May 8, 2018 at 13:40
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    $\begingroup$ Blood and lymph are both unsuitable for hydraulics, because they can leave sediments. I think that the fluid we have in knuckles and knees would be better. $\endgroup$ May 12, 2018 at 10:16
  • $\begingroup$ surely if these are alien creatures that have evolved this trait they would simply have evolved a separate fluid for this system not present in humans? that being said of the options you give it would probably be best to use the blood while its true it can leave sediment its likely that the creature would have systems for clearing that other wise it will have a short lifespan regardless of the fluid used. lymph is simply unsuitable for starters in strongly connected to the immune system and you don't want to be immobile every time you get ill. the lymph system also has no pump (at least in us) $\endgroup$
    – Ummdustry
    May 28, 2018 at 8:45

4 Answers 4


The fluid would need to be able to be moved quickly, without additional "side effects" like blood pressure loss, sudden changes in white blood cell levels or such. Therefore, use a dedicated liquid, possibly evolved from synovial fluid (the "grease" secreted in our joints that makes them move smoothly). It may be just water (with some mineral salts to retain body-neutral pH) which can be used for sustenance when required - dehydration would reduce mobility, but should be easily avoided.

While bladders are a fairly common solution, they aren't optimal for moving liquid quickly and under pressure. I'd suggest a specific tissue that is a mix of skeletal muscle and Corpus Spongiosum, a spongy tissue that can fill with liquid. The muscle could squeeze the organ ejecting the liquid at a quite high pressure on demand, or relax and absorb any influx. And instead of one centralized reservoir, multiple smaller ones distributed over the body could support whatever "hydraulic actuators" you design. That way damaging one of these doesn't completely cripple the creature but just restricts this particular joint/actuator. (breaking any hydraulic element in a hydraulic system/machine causes the whole thing to stop, with loss of pressure. Not something welcome in animals!)

  • $\begingroup$ "I'd suggest a specific tissue that is a mix of skeletal muscle and Corpus Spongiosum, a spongy tissue that can fill with liquid." the problem with this is the higher the tensile strength to accommodate the pressures when forcing the liquid into another (for lack of a better word) sponge, means the material has to have a higher tensile strength which would mean having to exert more pressure on it to expel the liquid, meaning more energy to do the task therefore lower efficiency. the potential organic matter's Tensile strength solves and causes the issue $\endgroup$ May 28, 2018 at 11:34
  • $\begingroup$ @BladeWraith: Thing is we're working with pressures likely in serious excess of what is normally found anywhere in other organisms. Flimsier solutions like bladders would just burst or cause hernias. Energy efficiency is secondary to not causing self-injury. $\endgroup$
    – SF.
    May 28, 2018 at 11:50
  • $\begingroup$ i completely agree, but this causes the issue as to why this would have evolved in the first place, and Fisherian Runaway adaptations that would be required for this system to work (look i'm stronger, i fight stronger etc) only happen up until the point where counter-selection interferes, which is less efficient means more energy required, more energy means more food, therefore more likely to starve in lean times $\endgroup$ May 28, 2018 at 12:00
  • $\begingroup$ @BladeWraith: I can imagine a reason being inhospitable ground. The creature's legs are essentially non-living bone structures that act as mechanical pistons and are immune to whichever environmental factors you want the ground to have: boiling hot springs, venomous small fauna, razor-sharp rocks. That gives the creature access to biomes inaccessible to competition, with access to food, and defense against non-adapted predators. Evolution-wise, this would progress from a hoof, extending further up as access deeper into more inaccessible parts of the biome becomes desirable. $\endgroup$
    – SF.
    May 28, 2018 at 12:08
  • $\begingroup$ Also, these would require repair/maintenance work (as opposed to normal healing processes; that could be achieved through resinous secretions to patch up damages, the liquid acting as carrier of nutrients for small islets of still-living tissue, say, cartillage acting as pistons within hollowed out bones. Young would grow fully living limbs, which would then fossilize upon reaching adulthood - possibly in process of sustaining continuous environmental damage. $\endgroup$
    – SF.
    May 28, 2018 at 12:12

Alas, I don't think it's a worthwhile trait.

It was recently found that Blue Fin Tuna fish have a biological analog of a canonical hydraulic system. The muscles pressurize the liquid, which helps change the fins’ shape and position for swimming and maneuvering control during hunting. the researchers who found this confirmed it was indeed lymph fluid.

However this is on a much smaller scale and moving a lot less pressure then what you need it to do. And also not a Tetrapod, Willk presented another creature that uses hydraulics, but both of these live underwater. This is most likely because they need the extra water pressure to allow these fine control units to utilise the hydraulic pressure. (Yes this is an assumption, but stay with me.)

Realistically if it was worth doing then we probably would have seen it happen more often. Evolution means adapting to survive (at its most basic level; please don't shout at me for this simplification) if they were more efficient then they would be used, but the pressure required to have a noticeable effect when compared to conventional muscle structures are really quite high, then you'd need an organic material that is capable of holding that pressure so that it can be used.

The only real benefit to being stronger is fighting, every example of life we have (which being only earth is actually massive and quite limited at the same time) shows that the ones that survive are usually the ones that have exerted the least energy to do so, or at least been the most efficient about it. Therefore if it was efficient then we would see it more often.

I know This doesn't answer you question, but hopefully it might help.

  • $\begingroup$ On one hand I agree, on the other this assumes a purely logical evolution. The question "why are there so few rolling/wheeled species" isnt because of pure inefficiency or impossibility (its fully possible to have functional bloodsupply and nerves in a continuously rotating limb), but because the advantage before the evolution is complete is absent. Yet some species do have rolling capabilities because other evolutionary advantages allowed them to develop far enough for the response to become effective. These creatures didnt guide themselves there, it just randomly happened eventually. $\endgroup$
    – Demigan
    May 28, 2018 at 11:01
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    $\begingroup$ Maybe add a link to WillK's hydraulic creature? $\endgroup$
    – G0BLiN
    May 28, 2018 at 11:03
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    $\begingroup$ @Demigan, i don't know of any creature capable of continual rotation of an individual limb, i may be wrong on that but i'm not aware of any please correct me on that. but all traits that could lead to hydraulics exist already, swim bladders etc, and basically some piping in the right places, and yet no genetic fluke has reach this yet on land, some water based life do use it where external pressure stops the bladders from bursting, but the lower pressures on land prevent this (but I admit this is speculation). and G0BLiN it was on the other question linked by the OP $\endgroup$ May 28, 2018 at 11:28
  • $\begingroup$ @Blade Wraith yes there are none with individual limbs to turn, only that use their whole body like the wheel spider or whatever it was called. I was afraid that misconception might happen I didnt write it well. As far as pressure goes, most males of a species have a very important limb that uses it for movement and even growth ;p. My guess is that it is like the rolling species, it requires a specific evolutionary path where the pressure starts off as a secondary trait that just happens to become relevant later. And this just happens so little that theres no species alive (anymore?) with it. $\endgroup$
    – Demigan
    May 28, 2018 at 13:11
  • $\begingroup$ / Willk presented another creature that uses hydraulics, but both of these live underwater./ Velvet worms are land animals. $\endgroup$
    – Willk
    May 29, 2018 at 17:46

Perhaps you could use something similar to what spiders use to extend their legs?

“A remarkable and effective hydraulic mechanism is found in the legs of spiders, which have muscles to flex the joints but none to extend them. Spiders stretch their legs by pumping fluid into them. When a spider gets ready to jump, it generates, for a fraction of a second, excess pressure of up to 60 percent of an atmosphere. The legs extend in order to accommodate more fluid.” (Tributsch 1984:59)


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    $\begingroup$ Could you summarize the most important parts of that link? Links can get outdated and leave answers such as this one basically meaningless for future readers, which is why link-only-answers may get deleted. The idea of StackExchange is to find your answer directly and not need to go somewhere else to search further. Links should be used as a kind of "If you want to know more about this topic than I can reasonably write into an answer on StackExchange you can look [here]". $\endgroup$
    – Secespitus
    May 29, 2018 at 19:45

Store the fluid in the leg.

The leg has chambers. You can adjust chamber size with muscles to expel fluid to adjacent chambers (contract muscles with valve open) or increase leg rigidity (contract muscle against closed valve). Fluid can be moved among the various leg chambers to achieve any desired combination of rigidity / movement / rotation.

Leg fluid chambers are closed off from the rest of the body as well as from each other. The loss of one leg to a predator will not cause exsanguination from contiguous fluid filled body spaces. The loss of a leg segment will render the leg shorter but with the valve leading to the lost segment closed, the rest of the leg can still function to some degree.


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