Hello readers! Last week saw the release of Dune: Part Two in cinemas, and all the buzz around giant sandworms got me wondering – how do they move around? I’ll admit, I haven’t actually seen the film yet (partly because I’ve been too busy watching videos of actual earthworms), and so I’m working with what we see in Dune: Part One, as well as descriptions from the books, and some potentially non-canon info from the Dune Wiki. By comparing these fictional details with the movement of actual earthworms and sand-dwelling reptiles, I’m aiming to test the feasibility of giant sandworm locomotion. Could these tubular titans really squirm through the dunes at the speeds depicted?
Note: this post will not contain any story spoilers for Dune (books or films). From here on out, all we care about are worms.
A description of your standard sandworm
The sandworms in Dune are big. The largest are reportedly 450 m long and 40 m in diameter (although some are even bigger than this). Frank Herbert describes them as annelids, which puts them in the same phylum of segmented worms as earthworms and leeches. The sandworms have thick, rough skin that looks semi-metallic, which is comprised of many scales that are each a few feet wide. Herbert’s description compares them to lamprey, which are long, slippery fish with a tooth-filled, funnel-like mouth on the underside of their head (look them up if you want nightmares). However, sandworms in film adaptations and illustrations are often presented with their mouths at their front end, usually with three lobes that close to form a point (although the recent films broke this convention).
Sandworms spend most of their life deep within the sand on Arrakis, but they will be drawn to the surface to protect their territory. The slightest rhythmic vibrations can catch their attention, and they will make a beeline for any noisy spice harvesters in their vicinity. In the books, sandworms are reported to travel at speeds of up to 80 km/hr (50 mph). They also appear to travel in straight lines, as the line of rising dust that moves in their wake isn’t significantly wider than the worm itself. With that in mind, let’s consider the mechanics of wormy locomotion.
Potential modes of movement
There are many modes of locomotion for creatures that burrow or swim, and these depend on the size and complexity of the organism, and the nature of its surroundings. Some of the biggest creatures to exist on Earth have lived in our oceans, getting around using flippers and tails (think blue whales and ichthyosaurs). However, the giant sandworms of Arrakis are limited in the limb department. As such, there are three likely ways that sandworms might get about: 1) by wriggling to produce a levering motion; 2) by expanding and contracting to drag themselves along; and 3) using some form of propulsion.
Mode 1: The wriggly worm
The first mode, wriggling, seems highly unlikely for our sandworms. On Earth, wriggling is the preferred locomotion strategy for sand-swimmers, which include such species as the sandfish and the shovel-nosed snake. By contorting their body laterally, then pushing backwards, they get enough kickback from their surroundings to shunt themselves forwards. Their movement is aided by the fluidisation of the sand around them (we’ll come back to this concept later). However, the sandworms of Dune are never presented as wrigglers, and as such, we can rule out mode one.
Mode 2: The squidgy worm
The second mode, expanding and contracting, seems more feasible. This is how earthworms move in soil, forming burrows in line with their body (I’m talking about friendly UK earthworms here, not any of those weird jumping and wriggling monstrosities that exist elsewhere). Earthworms are composed of hundreds of segments, just like Herbert’s sandworms, and each of these segments can expand and contract in radius and in length, like a squidgy cylinder. The worm is also covered in bristly hairs called setae, invisible to the human eye, which it can extend and retract.
To move, an earthworm extends its width and setae at its rear end, anchoring its tail in place. It then reaches its front end forwards into the soil, kicking off from its anchored end like we would kick off from the wall of a swimming pool. Once it has extended as far as it can, it shrinks its back end, then widens and extends the setae at its front instead, thereby changing its anchoring position. It then drags its tail to catch up with its head, and so the cycle begins again.
Of course, earthworm movement is far more complex than this two-step cycle. The worm can control each segment individually, allowing it to traverse changes in soil properties and navigate around obstacles. It also means that the movement of an earthworm isn’t necessarily an obvious stop-start motion. Movement is discretised, but with so many segments working independently, it isn’t always easy to define a regular pattern. Sandworms are more likely to be moving in this squidgy, back-and-forth manner than wriggling from side-to-side, but they also appear to move continuously, which doesn’t tally with squidgy movement. As such, we must consider our final mode of movement… Jetpack worm.
Mode 3: The jetpack worm
Some creatures on Earth use jet propulsion to swim. Most notable among them are the cephalopods (squids), which move by squirting collected water from a siphon, like Wall-E with the fire extinguisher. It is feasible, in the realms of science fiction, that the sandworms might move by blasting sand as a form of jet propulsion. However, here we encounter the issue of continuous movement yet again. No squid can jet around indefinitely; instead, it uses its jets for bursts of motion. It would take some serious sandworm engineering to keep it moving continuously, powered by a coordinated system of jets refilling and blasting, or just a single, massive jet.
To me, the main problem with sandworm jet propulsion is the positioning of the jet. If the sandworm is to have any control over its trajectory, the jet has to be near the front, like a wormy jetpack. If it was at the rear, the worm would have a dreadful turning circle, and even then, burrowing at the front end would be very inefficient if the backend became misaligned.
However, we never see jets of sand coming from the worms when they surface. Either the sandworms have ineffective jets at their tails (which would be very unsatisfying, sorry Frank, sorry Denis), or they move by some other mechanism. To me, mode two is the most likely, even if the evidence for squidgy sandworm locomotion is limited. But could a sandworm really move at 80 km/hr using any of these modes?
Sand as a fluid
The speed of a worm is determined by the strength of the surroundings vs. the strength of the worm. Earthworms are relatively small compared to the soil particles around them, and they move relatively slowly by nudging particles out of the way, averaging speeds of several metres per hour (around 50 earthworm-lengths). By contrast, sandworms are enormous compared to sand particles, and they move at 80 km/hr (200 sandworm-lengths). Clearly, the way that a sandworm interacts with sand is very different to how an earthworm interacts with earth, and this allows the sandworms to travel at great speeds.
Sand is a granular material, and under the right conditions, it can act like a fluid. The properties of granular flows are incredibly complicated, because they are determined by millions of collisions between tiny particles, all acting together to behave like a fluid. Large granular flows such as avalanches and pyroclastic flows can be hugely destructive, but small-scale granular fluidisation is very useful to creatures burrowing through sand and soil; in fact, the sandfish and shovel-nosed snake both use this to their advantage as they wriggle around.
Sand will start acting like a fluid if there is enough air between the sand grains, which reduces the friction between them. As a general rule, the viscosity of a granular fluid depends on the rate at which the flow is deforming, the surrounding pressure, and the size and density of the grains. Reducing the viscosity of the surroundings allows creatures to expend less energy moving through them, so any movement that causes fluidisation is beneficial. However, pressure increases with depth, and so fluidisation of sand will become more difficult the deeper a sandworm goes.
We should also note that squidgy sandworm locomotion can only happen if some segments of the worm are anchored in place, which is impossible if the sand is entirely fluidised. When segments expand in width they put pressure on the sand, increasing the friction between grains so that they act like a solid. Sandworm movement would be impossible without this solid anchor – which raises the question of what happens near the surface. When pressures are low, it becomes difficult to compress the sand into behaving like a solid, and so anchoring any segment in place becomes a challenge. All of this makes me wonder how the worms could possibly move along at the surface at 80 km/hr, where the sand is so poorly consolidated. It just seems infeasible.
Worm god powers
Of course, it may well be possible that the sandworms have some unearthly way of moving themselves around. The books mention that sandworm interiors are hot, like furnaces, and it is suggested that they convert friction into oxygen, which they blast out behind them. This seems to imply that their motion is similar to our “jetpack worm”, but with the jet at its tail – which we established was the worst place for it to be. Additionally, taking energy from friction and using it for propulsion is impossible, unless the transfer is 100% efficient. In this scenario, the worm is a perpetual motion machine; the friction comes from movement, and the movement is powered by friction. Maybe the worm makes up the shortfall by deriving energy from spicy sand, but we’re on seriously shaky ground here (sorry Frank).
My only conclusion is that the sandworms of Dune are divine beasts with godlike powers. I’m with the Fremen on this one. Any giant creature that can break the laws of physics just by moving deserves our everlasting respect.
In summary…
I hope you enjoyed this quick delve into the infeasibility of giant sandworms. Maybe you learnt something about ordinary worms, too? My respect for those little squirmers certainly increased while researching this post, although I regret my dive into the lamprey literature. The complexity of life in the real world is astonishing even at the smallest scales. If you have any thoughts on sandworm locomotion, let me know. I intend to read the Dune books soon, at which point I might find the answers I’m looking for. Until then, happy reading, and have a lovely week!
References:
Hosoi, A.E. and Goldman, D.I. (2015) Beneath our feet: strategies for locomotion in granular media. Annual Review of Fluid Mechanics, 47, 431-453. https://doi.org/10.1146/annurev-fluid-010313-141324
C. W. Clayton 