Why there is no 'Walking with tentacles'... V

The series of posts on 'Why there is no walking with tentacles' was supposed to have ended in December 2008 (see posts one, two, three and four). To summarise, I argued that a land-based existence of a tentacled animal would induce rapid evolution. The tentacles should evolve the ability to bear weight, which boils down to resisting compressive forces. Pure tentacles, without any solid bit in them, cannot do that, or very inefficiently at best. So I envisaged the development of incompressible elements. The Walking Tentacle Mark II was a bit like a string of beads held in place by surrounding muscles. The next stage would be a reduction in the number of elements, resulting in proper legs, no longer worthy of the name 'tentacle'.

But recently I read something very interesting about Earth's cephalopods (squids, octopuses, etc) that shines another light on the ability of cephalopod tentacles to function as legs. By the way, the usual term for octopus tentacles is 'arms', and only the two prey-catching appendages of squids are called 'tentacles'; their other appendages are called arms as well. Because I am looking at their function I will stick to 'tentacles' here. My interest was piqued by the following sentence: "... flexible arms, with no joints as fixed reference points, cannot discriminate between shapes, however elaborate the brain." I found this in a section entitled 'What has limited the evolution of cephalopods?' in a book by Janet Moore on Invertebrates (the book is quite interesting for would-be animal designers).

If cephalopods indeed cannot feel well enough to tell shapes apart, that is bad news for having their tentacles turn into functional legs. In man, the ability to sense where our limbs are enables us to walk in the dark and to recognise objects by manipulating them. I will come back to the importance of this sense, called proprioception, later. First I searched for more data on tactile function in cephalopods. Luckily, there is an excellent summary in Scholarpedia, so everyone can read it.

Click to enlarge; sources through link in text

It turns out that octopuses are quite good at feeling the texture of an object, and you can teach them (using rewards etc.) to tell two objects apart that have the same shape but a different texture. In the graph above octopuses were given two different objects, and they had to learn how to tell the two apart. When the two lines with black and open markings diverge, the octopuses managed to learn the difference, but when the lines do not separate, the octopuses hadn't a clue what the humans wanted them to learn. At the top, the octopuses quickly learned that a cylinder with grooves is not the same as one without them. But if you give them cylinders with the same surface texture, but with a different mass because one has a weight hidden in it, they have no idea. They cannot tell a heavy from a light object. The remaining two graphs were control experiments proving the same thing. Other experiments also showed that octopuses are indeed very bad at recognizing shapes. The most likely explanation is that they cannot judge well where the various parts of their limbs are in relation to other parts.

The Scholarpedia review does not go so far as to state, in contrast to the book, that it is the lack of solid parts that is to blame for this, but that does make sense. Consider our limbs, or those of insects for that matter. Telling where the end of the limb is requires two things: the first are sensors to tell the degree to which all joints are bent, and the second item of knowledge is a table containing the lengths of all segments in-between the joints. In fact, working that out does not require more than fairly simple trigonometry. Can an octopus do the same? The lack of joints makes it more difficult. What it would need is some kind of sensor that can tell where it is in relation to another one in theedimensional space: direction as well as distance. Off the top of my head I cannot think of any biological sensors like that. The point is not whether any exist, but that the octopus doesn't seem to have any! One more reminder that alienness can be found on our doorstep.

Do you actually need propriocepsis? Humans do! There are a few people in whom this sense has been wiped out completely by disease, and these people cannot tell the position of their body and their limbs without looking. Most cannot walk, even though their muscles are fine. There are many more examples showing that the ability to feel where your body is in space in extremely important for human, and I would guess vertebrate, control over posture and movement.

What I find hard to understand is how the octopus moves about without such an option. The clip above shows an octopus disguised as some plants walking over the sea floor (on YouTube here). That is impressive, and there are many more video's showing rather impressive feats of movement. Their control over their tentacles must be organised differently from the way we control movement. Again, they're rather alien.

Click to enlarge; copyright Gert van Dijk

Can all these problems be reconciled with turning tentacles into walking limbs? Well, yes. After all, the Mark II had evolved a series of incompressible elements with joints in between them purely for mechanical reasons. Those elements of the Mark II are shown on the right in the image above. On the left is its successor, a proper leg with a reduced number of segments. The Mark II set-up is just what is needed to allow propriocepsis like ours to evolve; I am assuming here that it is advantageous to have such a sense. If there is a small sensing error in each limb, having a large number of short segments adds up to more uncertainty about where the limb is than having a smaller number of longer segments. Reducing the number of elements was better mechanically anyway, and may also allow a more sophisticated, or at least more reliable sense of position as well. There are probably advantages in the motor control of movements as well, but I think I will skip that (or reserve it for another post, who knows?).

Epona II

I discussed the Epona project in this blog recently, and stated that it was at its time the biggest and best-developed world of fictional biology. Here is a surprise; it might very well still be the biggest such project! The problem is that most of it was only visible to the few people taking care of the project, among whom Greg Barr was one the people holding it all together. The website never showed more than the beginning of he project, and did not even discuss the major life forms and their physiology. The good news is that part of the old website has now been restored (it was damaged by a virus or a hacker). So take a look there, and let's hope that more of the wealth of Epona data will yet appear for all to see.

Meanwhile, I can discuss a few glimpses. There is a Kingdom Myoskeleta. These organisms do not have a sketelon, neither on the inside nor on the outside. What they have is a set of extensile muscles without joints. That is right, extensile muscles, not contractile ones. There were no bones, hence no joints. By expanding on a specific site in a thick muscle rod, the rod could bend, stretch or spiral in any shape desired. This was no mean feat; the limbs of any creature with such extensile muscles acted a bit like tentacles. I remember writing a critique on these muscles along the same lines as later reappeared in this blog: there were four blog entries called 'Why there is no walking with tentacles': one, two, three and four. If you read them, you will find that I made a case for the development of joints in any limb destined for serious weight bearing. I think that the arguments hold for tentacles of any type, with contractile or extensile muscles. By the way, I thought that extensile muscles could perhaps be made to work in a roundabout manner, but that is perhaps something for another day.

The myoskelata basically consist of a barrel with a set of limbs at either end. There are five of these limbs, and in principle they branch into three 'fingers'. Here is such a basic organism:

Click to enlarge

The Myoskeleta are divided into two phyla: the Myophyta, plants for all practical purposes (the other phylum is the Pentapoda; they're animals). Take the basic shape, drop one end into the ground to act as roots, and span a membrane between the five limbs: that is a basic Pagoda Tree. If you add a similar layer ('tier') on top of it, you understand the tiered appearance of a pagoda forest. You will be able to recognise this fivefold symmetry for most of the plants shown on the cover of the recent book on how to grow Eponan plants (see the Hades Publishing page on the Furaha website).

Click to enlarge

In principle these 'trees' can still move a bit, for instance to direct their leaves towards the sun. Intriguing, aren't they? I will add a few images of such Eponan forests. They are in development together with Steven Hanly.

Ah yes, I showed an image of a flying animal in the previous post. It was modeled by Steven, and is a pentapod, meaning its basic anatomy is similar to that of the trees it flies over. The species is a Uther, and it is intelligent. Life on Epona has developed intelligence, unlike Furaha (and the reason why there are no 'sophonts' on Furaha deserves mention on its own, one day).

Click to enlarge
A stream in the forest.

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Somewhere else along the same stream.

Why there is no 'walking with tentacles'... (2)

Well, it took a while to write a new chapter. The reason is called 'work'.

Anyway, to continue with the 'Not walking on tentacles' subject, I made some illustrations with Vue Infinite, shown below. Let's have a look at a typical tentacle.

(Click the image to enlarge it)

This particular one is almost entirely made up of muscle cells (also called muscle fibres). the fibres are the red elongated structures. I played with images of mammalian striated muscle cells to form a reasonably realistic texture. The first thing to remember is that muscle fibres can only pull; they cannot push! So, if we want to use a tentacle as a leg, pushing against the ground, we will have to devise a way to push with elements that can only pull. Normal legs, with skeletons, work because the two jobs are separated: the muscles pull on the bones and the bones do the pushing.

Back to the tentacle: it is almost entirely made of muscle cells, except for the very centre, where I have put in an artery, a vein and a nerve (these have the customary colours found in medical textbooks: the artery is round and red, the vein is rather floppy and blue, and the nerve is solid and yellow). No bone, of course; it wouldn't be a tentacle if there was one!

The muscle cells are arranged in concentric layers, and the muscle fibres are arranged in different directions in different layers. The innermost layer has its fibres running lengthwise. If they contract, the tentacle will shorten. It will also become thicker, as the total volume of the tentacle will not change; after all, the whole thing is largely water, and water is incompressible. You can also have the fibres of that layer contract on one side only; if that happens, the tentacle will bend. You do not have to have this happen along the entire length, so the bend can happen anywhere. Already our tentacle can move in various directions.

The outermost layer has its fibre running transversely. If these contract, they will squeeze everything inside. Having nowhere else to go, the tentacle will become thinner, and therefore longer. The two other layers were added to add a bit of complexity and dexterity: their fibres run diagonally, so they will tend to twist the fibre. They also have compressive as well as shortening effects. Playing with these layers and fibres should allow the tentacle to move in just about any way you can think of.

There is no particular need to have the layers arranged just so. In fact, there might be another lengthwise layer on the outside, and there are various other things you can think of. As it stands, the tentacle can pull quite well. By activating the circular fibres the tentacle will become longer; isn't that the same as pushing? Well yes, but not with any great force. What we have so far is something like the human tongue, and you can push it out of your mouth, or against your teeth. But you can't do push-ups with it.

(Click the image to enlarge it)

To understand why, look at the balloon animal shown here. Its 'legs' sacs of air, and the only reason they hold their shape is because the sac under pressure. If we do the same for the tentacle, and tighten the outermost layers of the tentacle over its entire length, the inner bits of the tentacle would be under pressure, with the same result: a structure with enough tension to hold its own shape. But you cannot put any weight on a balloon animal: the legs buckle.

With that as a given, the simplest solution is to increase the pressure tremendously, so the leg/tentacle will not buckle so easily. That seems to be the solution chosen for the 'megasquid' in 'The future is wild'.

But that is so wasteful! The tension has to be built with continuous muscle force, and that eats energy. In contrast, to stand on bones the only energy you need is to keep them from buckling at the joints, and if you balance them right, all you have to do is a balancing trick rather than a brue force approach. A second problem is that the pressure inside the tentacle would be very high, and that creates big problems for getting any blood to the muscle cells. Blood pressure in the tentacles would have to be even higher than tissue pressure to force any blood through, and that in turn puts heavy strain on the heart, or hearts, come to think of that. More energy, more engineering problems. Anything with bones would run rings around such silly squids.

I think walking on tentacles requires more than a very thick tentacle; it needs evolution to keep the costs down. More on a possible solution next time...

Why there is no 'walking with tentacles'...

Tentacles are cool: they look sleek, effective, a bit mysterious, and also slightly repulsive, particularly when covered with lots of suckers and slime. It is no wonder that science fiction illustrators equipped their creations with tentacles when something like the above-mentioned criteria were called for. And longer ago monsters like the 'Kraken', a giant octopus, were the staple monster of heroic tales.

As an aside, it is interesting to speculate why exactly the boneless movements of an octopus evoke fascination as well as revulsion (not in me, but certainly in many people). The explanation might be that their body scheme is so completely different from ours that it is difficult for us to comprehend what is going on when they move. (there are neurons in the motor areas of our brain that respond when we see body parts moving in other people; I would predict that their response is less as a body scheme departs more from ours). But that is not the point here. Let's take it as a given that tentacles create interest, so it would be nice to design some creatures that walk on tentacles.

Those who really know their cephalopods might say that there is no need to design animals walking on tentacles, as they already exist. Indeed; here they are. But those are 'walking' underwater, and I want animals walking on dry land.

Before anyone thinks of the loophole that land animals could quite well walk on tentacles in a very light gravity, I will accede the point. But I want animals walking on dry land in an Earth-like gravity.

Some among you might now think of ballonts (see earlier entries) that are starting to evolve back to a terrestrial life again, so they can walk with their bodies partially suspended from a balloon. To stop that, and lighter than air parts are now declared officially out of the question. The demands put on this design are to walk on tentacles in an Earth-like gravity on dry land, without any unfair ballooning aids.

I will count the solution of having a very very large number of legs as a possibility, but it doesn't sound energetically efficient. So try to design the biomechanics in such a way that the animals could actually work.

Has this been done? Well, yes. In fact, I designed one many years ago (in 1982...), turning one tentacle in a slug-like foot, leaving the others free. It was the Jellyshell, and here is part of its description:

"Without any apparent reason they will suddenly set out towards the beach. Many specimens crawl ashore more or less simultaneously, move around for a while on the beach, leaving trails of glistening slime behind them. These mysterious maneuvers last for several hours; afterwards they return to the water and take up their normal habit of slithering and algal grazing. They will not be seen on land for weeks on end; then you will suddenly encounter dozens of their trails on the beaches, left during one of their forays. Just why they come on land, and why they glide and slither there in their slow and solemn way, no one knows."

But having one slug-like foot doesn't really count as walking, so I guess that one does not count. Dougal Dixon did one several years later: the 'Coconut Grab' (found in 'The New Dinosaurs' from 1988). Here it is (click on it for a full scale depiction; it is fairly large). I think the book is still to be found, and I definitely recommend it.


That comes much closer to 'walking on tentacles', but not close enough: as the description says, the grab draws itself along the ground with four of its legs, and crawling doesn't count as walking.

So the only ones I know of that really count are the evolved cephalopods in 'The Future is Wild': the megasquid especially. This was a television series that featured some speculative evolution. The megasquid is a gigantic descendent of squid (probably octopuses, come to think of it), and it walks upright on its eight columnar tentacles. The program does have some comments on how the legs actually work, and these make it clear that the tentacles are properly boneless, as they should be. Here is a frame taken from the program.


I have my doubts though; is the proposed bimechanical explanation sufficient, or, to be more precise, would evolution leave it at that?

But first some explanations are needed about what makes a tentacle into a tentacle; but that's for another installment.