Saturday, 26 March 2011

Swimming in Sand III: real and robotic sandswimmers

The previous two blog entries on sandswimming animals (here and here) made it clear that those who wish to populate their fictional worlds with animals swimming in sand will either have to restrain their fantasy or choose to forgo realism. Sand is not a forgiving medium; a 400 m sandworm, as in Dune, must shift millions of tons of sand to move, which is simply too unlikely to consider. While that reasoning argued against giant sandswimmers, the fact that any sandswimmer must lift all the sand above its body restricted the choices even more: sandswimmer cannot dive deeply. Facts can and often do spoil beautiful fantasies; sorry, but that can't be helped. This leaves sandswimmers as fairly small animals that swim just beneath the surface. Not surprisingly, this describes real sandswimmers quite succinctly. There appear to be several: there are various lizards and at least one mammal, the African golden mole rat.

Here is an image of such a mole rat: Eremitalpa granti. There do not appear to be many studies focusing on how mole rates 'sandswim', but what there is reveals some interesting facts. Apparently someone has already measured how much energy it costs: sandswimming costs 80 times as much energy as running on the surface! That is a heavy price to pay, but sandswimming is still much less costly in energy terms than burrowing, i.e. scraping at the soil and carrying it away to create a tunnel that will last for a while. Of course, you can use a tunnel many times, meaning its cost decreases as you use it more often, in contrast to sandswimming: each trip is as expensive as the next. Unfortunately I could not find any video material of a mole rat sandswimming, so I cannot show you that (the main -but not only!- reason being that such a video might show some sand moving, but not the mole rat underneath...). But if you wish to know more about them, there is at least one nice entry on them in Darren Naish' justly famous Tetrapod Zoology blog.

I did find something else that is interesting for those who wish to design sandswimming animals: how do they breathe? There are two problems here; if breathing involves expanding the chest against the outside world, the sandswimmer is in trouble, as that would require some additional shifting of sand. So you had better design an air pump that only shifts volumes within the body, keeping the external volume unchanged. The second problem is whether the poor sandswimmer can extract enough oxygen from the air between the grains of sand; if it then breathes out, does the air circulate fast enough to get fresh air with the next breath? Perhaps it should suck in air at one end of its body and exhale it at the other (that is what Furahan hexapods do, but not to enable sandswimming). Problems, problems...

You had better make the animal small and give it a low metabolism, so it will not need that much oxygen. Well, someone studied how mole rats breathe (Seymour & Seeley. J Arid Environments 1996; 32: 453-461). The authors did some mathematical modeling and concluded that mammals weighing up to 200 grams could comfortably exist completely encased in loose sand. Mole rats are a lot smaller than that, at 15-40 grams, but that upper limit of about 200 grams does not exactly allow for an impressively large sandworm. Before anyone corrects me, I am aware that Dune's sandworms are supposed to produce oxygen, not use it. But if anyone wishes to create a sandswimmer with a more conventional type of metabolism, this is something to reckon with.

Sandfish disappearing under the surface
Baumgartner et al PLoS One 3(10): e3309. doi:10.1371/journal.pone.0003309

Click to enlarge; Baumgartner et al PLoS One 3(10): e3309. doi:10.1371/journal.pone.0003309

There is much more research going on concerning sandswimming lizards (perhaps they are easier to keep in a terrarium). One species is even called the 'sandfish' (Scincus scincus). Over the last few years people have studied their mechanics and came up with a few interesting results, the main one being how they actually swim. At first there was a notion that they used their small legs to loosen the sand to make it more fluid, allowing them to swim through it by undulating their bodies. These researchers used nuclear magnetic resonance to take a look through the sand. The image above shows such an NMR of a sandfish beneath the surface. As the legend says, the legs are sticking out; but is it at rest or actively moving?

Click to enlarge; Maladen et al. Science 2009; 325: 314-318

Afterwards other researchers went one further and used high-speed x-ray imaging. While diving the animal does use its legs, but while sandswimming it keeps its legs close to its body. Here are some images of how it does so. Above the surface its legs stick out in typical reptilian fashion (C) but they are held back when the animal is swimming (E). The body goes through very pronounced sideways movements in order to move forward a bit; it does not look like an effective means of propulsion at all.


You might think that that is the end of the story, but not so. People actually built a sandswimming robot that undulates just like the sandfish does. That is what the video above shows, along with some more footage of an actual sandfish. There is a nice page at PhysicsCentral with some more data on this particular study.

Compared to a 400 m Sandworm of Dune a 40 gram mole rat or a sandfish lizard are less imposing. Then again, they are real, and I personally find the scientific story of how sandswimming really works just as exciting -in a different way- as a good science fiction story. I can't wait to find out what's next.


j. w. bjerk said...

I wonder what the best case scenario would be for a large sand-swimmer if you constructed a planet to be friendly to it.

Obviously if the "sand" was a less dense mineral that would help a lot. Though my geology is not strong enough to make a suggestion. Also lower gravity would help some, at least in swimming to greater depths, but maybe not a lot for movement since mass/inertia is probably the main limiting factor.

Evan Black said...

After all of this I'm wondering what kind of evolutionary pressures would even select for such an expensive trait. Predator evasion? Ambush predation? Body temperature regulation? Is sand-swimming better because it's more difficult to maintain stable tunnels in a semi-fluid medium like sand can be?

The breathing obstacle could also potentially be bypassed by periodically surfacing as dolphins do: spend a brief moment to get a new lungful of air then back under the sand. Or maybe breathing through the skin like amphibians might result in a better air exchange.

And what about creatures that may swim in sand mixed with water? Could they extract needed oxygen more easily from the water than from the air?

rodlox said...

There's also the Marsupial Mole (as seen on 'Weird Creatures' hosted by Nick Baker - youtube has a few of his adventures)

...but of the mole, I could only find this: (Baker says it swims through sand better than placental moles of North America and Africa)

Mr. Black -
>After all of this I'm wondering what kind of evolutionary pressures would even select for such an expensive trait.
Well, based on _bipes biporus_ (two-legged burrowing lizard), I'd guess that its in part to take advantage of all the insects that other animals are too big to chase.

trex841 said...

i have an idea. what if there was a planet that had low gravity, was very desert like, and had a very metallic composition? i beleve that such a planet would favor sandswimmers. the desert enviroment would give reason, and the low gravaty might make it easyer (like j.w.bjerk said), but what would really help is the metal. my hypothosized creatures would have developed magnetic organs that loosed the sand and propel it forward. dose that sound posseble, or can that just not happen?

rodlox said...

Like the way magnets repell iron filings of the same pole? (North repells North, attracts South)

trex841 said...

...i think. the organs would have to be at the front of the body, right? pulling the organism froward via magnetic attraction?

(i had this once, but i feel like i'm losing it)

Evan Black said...

trex841, I see two major obstacles to your "magnetic organ" idea:

Obstacle 1: Unless the creature has a way of selectively repelling specific particles it will likely get clumps of metal-rich sand stuck to its body, which can seriously inhibit hydrodynamics-- I mean, harenadynamics.

Obstacle 2: According to biology as we understand it, every creature generates a weak electric field based on biological processes, but that field is a far cry from biological processes generating the kind of energy necessary to produce a magnetic force strong enough to make "magnetic organs" that loosen and propel sand out of the way. Unless this organ can be expressed in biologically sound methods it steps out of fact into fantasy. However, as Sigmund Nastrazzurro said in the last post of this series, "While the hard-boiled scientist in me rejected them as utterly impossible, the somewhat more romantic SF fan, also inside me, decided not to care one bit." I'm sure we can all enjoy a "magna-swimmer" on some level even if it's impossible based on our current understanding.

trex841 said...

1. i have away around that one. the magnetic polarities "undulate" across the body, like this


that way, the sand would move back, and the organism would move forward.

2. ...i have nothing on that, but if we have fish that can stun a person, i don't think magnatism is out of the question. i like to beleve that almost anything can happen, the fun is finding out how. i also agree with his saying for enjoing the creature as it is, as such, i love primeval and tremors.

rodlox said...

so, basically a sub-surface electric eel?

trex841 said...

...i was just using eletric fish as a metaphore. if a fish can produce that much electricite, and even "see" the earths magnetic field as many creatures on earth can, then producing that magnetic field seems very possible, somehow.

(i may have to go with the mystery of nature route here.)

Sigmund Nastrazzurro said...

Well, it doesn't seem as if there is much left for me to add to the discussion.

Evan is right to point out that there must be considerable benefits in choosing this type of behaviour. Possible reasons are hiding from something (predators or just the heat) or alternatively getting something, either on the surface or in the sand.
One thing that I have not been able to find is for how long sandswimmers actually swim. If the main purpose would be to disappear from sight, then a sandswimmer need only displace itself a few body lengths from the spot where it submerges, and that wouldn't cost much.

As for using magnetism, that is an interesting idea. It does require a large number of assumptions to get organs that control the strength of the magnetic field as well as where the poles are. After that, you still need to shift large volumes of sand, and for the system to work the mechanical efficacy of moving sand magnetically must be at least as good as that of purely mechanical 'movers' such as bristles. So I think this system is, like Dune's sandworms, in the peculiar category of SF where we like it enough to studiously look the other way when the ugly subject of an explanation comes up.

trex841 said...

thanks, this was fun, helpful, and informative. i'll leave the posts to you now. bye.

Sigmund Nastrazzurro said...

Trex841, perhaps you took my comments as sarcastic, but they were not intended in that way. When I compared your magnetic theory with Dune's sandworms, that meant I felt similarly about it: I like it but to make it credible such large gaps in knowledge have to be filled that you have to stretch reality to make it fit. It was just another way of referring to the 'mystery of nature'.

trex841 said...

i didn't think that your comment was sarcastic, i just felt that we have gone as far as we can with the conversation. i got help with my idea, i had fun with people that like to talk about this type of stuff, and i will now let the conversation transition away from my topic and to the next topic you will talk about. :)

rodlox said...

I think the marsupial mole burrows at least 3 or 5 feet below the surface. but is is small.

as to the other matter...
maybe there are no bristles or other useable parts on the electric creature?

e.g., a torpedo ray could use its fins...but an electric eel has no bristles or limbs or other tegument features it could use -- its only option is to take advantage of its electric capabilities. (i think they *can* run electric pulses down the length of their bodies)

trex841 said...

looks like this topic isn't over yet. :)

well, we have electromagnets, could there be a biological form of one?

trex841 said...

i have something that may help here.

trex841 said...

cont. add this to the end of the url.


trex841 said...

i think that i have what i need to make this creature, and the ecosystem around it, a reality. thanks for the help. :)

Evan Black said...

Define 'reality.' Have we met an actual mad scientist? ;)

Anonymous said...

Bit of an interesting follow-up, Darren Naish of Tetrapod Zoology has recently posted his new idea for an anaconda-sized amphisbaenid for his Squamozoic project on his blog.

Sigmund Nastrazzurro said...

Hello Anonymous,

Thank you. I had a look and left a comment.