Strandbeesten and mantis shrimps

Actually, 'Strandbeesten and stomatopods' might have sounded better, but would be even more incomprehensible, and a blog is supposed to attract readers, not frighten them away. Based on how many readers were attracted by previous posts, I should probably use 'The Future is Wild' and particularly 'Avatar' a lot more often in post titles (and no, Furaha was NOT modelled on Avatar; it is much older). Right; now that I've got that out of the way, back to the strandbeesten.

I discussed Theo Jansen's imaginative mechanical walking machines before on this blog. Literally the word is Dutch for 'beach beasts'. If you do not know about them, read that entry and visit Mr Jansen's site, or just enter 'Theo Jansen' into Google or YouTube. His work came up in this blog because of my interest in animal locomotion. The problem he faced was how you can get a foot to move backwards along a straight line when on the ground, after which it has to be lifted, moved forward and put down again for the next step. For real animals this is not a big problem, as the various segments of a limb are all controlled by a nervous system telling each segment when to do what. As Jansen's devices lack a brain, he needed a purely mechanical system to achieve this sort of motion. In the end he came up with an intricate series of interconnected bars: if you start with a rotary motion of one bar, another bar, ending in a foot, produces a suitable movement. Very clever indeed. Such series of connected bars are called linkages. You can take a good look at his design on this particular site, which shows other linkages as well. When I wrote that post I had never seen a single strandbeest yet, and that has now been rectified. Mr Jansen works not that far from where I live, so it was a matter of time before I could visit one of his demonstrations nearby. This was the case last June, on a very cold and windy day. I will show a few videos I made that day.

This is a tiny strandbeest, of which there were three. If its sail is perpendicular to the wind direction, the little beast may walk with the wind. I tried pushing it forward as well, and found that it is not in fact that easy to move. While the 'beesten' are quite light, their joints were harder to move than I had expected. There is no lubrication, but the main problem seems to be that the entire shape deforms enough to put shearing forces on the joints. One result of this is that the poor beest tends to topple over. But never mind that, they are an amazing sight.

Here is a larger one following one of Mr Jansen's assistants.

And this is the major species present at the occasion. Not only did it have two bodies or trunks, an enormous number of legs, but also two waving membranes at the top that I think were designed to help propel it. These sails were reefed that day however, and the force of the wind on the body was enough to prod the beast onwards. Aren't they wonderful?

In my previous post I wondered how often linkages occurred in biology, but did not look up the matter. I have done a bit of research now, and found that there are quite a few examples. Fish jaws are probably the best-known example (see below). Other structures, such as sheep hocks and human knees are also counted as so-called four-bar linkages. In a four-bar linkage four stiff bars are linked together in a sort of circle by pivots. If you hold one bar still, and move another one, the remaining two must move in a fixed way. What that way is depends on how exactly they are connected. I felt that regarding the human knee as a four-bar linkage is bending the rules a bit, as two of the bars are ligaments rather than stiff bars. If you include connected series of bones as well as ligaments there are lots of linkages in biology; what I was looking for was linkages of bones involved in locomotion, but I have not yet seen any. Presumably a system with more mechanical freedom but with a smart nervous system to control it is simply superior. Still, the other ones are interesting.

Click to enlarge; Wainwright et al, Integr Comp Biol 2005; 45: 256-262

Here is an example of what fishes do with a four-bar linkage. The source is mentioned in the caption, and the colours are my addition. Fish use this kind of mechanism to move their jaws forward and to enlarge the volume in their mouths, sucking in water as well as their prey. There is at least one Furahan animal with a similar arrangement, and those are 'Fishes' too. The jaws of the sawjaw are connected, all four of them, by bars linking them to the neurocranium in a kind of circular linkage.

So where are the mantis shrimps, everyone's favourite Terran alien? When searching for linkage mechanisms I found that there is a four-bar mechanism in their 'raptorial appendages' as well! I like that: somehow you expect animals that not just spear or club their prey but can see depth with just one eye to be special in other respects as well, and mantis shrimps never seem to let us down when it comes to, well, weirdness.

Click to enlarge; Patek et al; Nature 2004; 428: 819

Here is a figure from the journal Nature, no less. The first author, Sheila Patek, has a lab where she studies all kinds of biomechanically interesting things, most notably mantis shrimps. Have a look, as there are quite a few videos and photographs. Under 'multimedia' you will find an inspired lecture she gave on 'TED', where she explains the striking mechanism of stomatopod raptorial appendages. Very interesting. She is not the only one interested in stomatopods either; here is another enthusiast.

So now you know why strandbeesten and stomatopods end up in the same post: they are connected by linkage (I could not resist that one). I guess both also score very highly when it comes to their ability to evoke a sense of wonder.



PS. This is post #100!

Walking machines III: the abiological ones

A while ago, I discussed walking machines, and found that most inventors opted for stability by designing six-legged machines. There was one design that really resembled an animal in the way it moved. That was Big Dog, a four legged machine designed for military purposes. The reason it looks so biological, in my opinion, is not that it has four legs, but that it obviously has a very flexible control to allow it to cope with uneven terrain, a slipping leg, and even an unkind human 'master' that kicks the poor beast, eh, robot.

In other words, neural control is probably the prime difference between animal locomotion and man-made machinery. 'Control' goes much further than just standing upright or thinking of where you are going. It also affects leg design to a very important degree. Have a look at the following graph.



This is the result of one of my Matlab programs to animate fairly simple legs. First suppose that a leg is suspended, so when it moves its hip will stay in place and the foot will described a movement in the air. The blue dots show 100 points of just such a cycle: in this case, 50 points describe how the foot moves forward (to the left) through the air, and the bottom 50 points describe the part where the foot is supposed to go over the ground. In that section, the foot has to move the exact same distance from dot to dot, and the movement should be in a perfect straight line (anyway, if you do it this way the animation is much easier). The brown lines show a variety of ways how you could position a leg with just three 'bones' in it (thigh, leg and ankles) to link up the same hip and foot positions. Obviously, there is an infinite choice here, and in biology the nervous system decides on the best one, taking into account anatomy, gravity, lengths of bones, etc.

If you haven't got a brain, such as holds for mechanical designs, but you want a walking leg nevertheless, you will have to find a way to get the leg to move through a similar movement arc as shown in the figure above. In short, the degrees of freedom a nervous system can easily deal with must be abolished altogether. The trick is therefore to start with a defined movement provided by a motor, such as a rotary movement, and to devise a system of links and levers to end up with a foot moving though a suitable path. Some people actually solved that problem. Let's have a look.




One of the first must be the Russian engineer Chebishev, brought to my attention by Pavel Volkov (thanks Pavel!). Pavel also pointed me towards the video shown above, an animation of how the system was supposed to work. Here it is in its YouTube home if you prefer that. I have no idea of how this engine was supposed to be powered.




The impressive machine ambling along on the video above is a strandbeest. Theo Jansen's 'strandbeesten' (which is Dutch for 'beach beasts') must be the most famous of this type of walking machine. His designs are marvels of engineering as artistic wonders as well. They have to be seen to be believed (before anyone asks, no, I have never seen them with my own eyes, which is something I should rectify one day).

Personally, I have found that I had to watch it, and others like it, quite a few times before I began to understand how the legs move. Apparently Mr Jansen made use of an evolutionary approach to work towards the optimal proportions of all the struts and links that make the legs behave as necessary. If you type 'Jansen linkage' into Google, you will find that many people are equally fascinated, so the 'Jansen linkage' seems, like any good meme, to be spreading and evolving. If you wish to se it in more detail, have a look at this site, where you can even play with design to see if you can improve the output. His devices have been copied in wood, cardboard, and other materials. There are Lego examples as well.




The one shown above is made from wood. It has a very clever gear system that ensures that only one leg is off the ground at any time. The YouTube text says that this is a stop-motion animation. Its website is here.

There are other approaches as well, such as the Klann linkage. It results in a more spidery walk, as can be seen by a direct comparison of the two linkage systems. This system has spawned several large mechanised walkers, to be found through Google or through the Klann site.

Do these designs lend themselves to 'biologification'? In other words, could I or someone else use them for a fictional animal? I can think of no reason why it should be impossible to have a pantograph-like leg. But why would you want a leg that can describe only one movement? All adaptability and all flexibility are thrown out of the window. If you throw in a fairly decent nervous system instead, you can solve the walking problem and do much more besides. So no, I do not think that this is a mechanical design that would work well in biology.

But that doesn't stop me admiring and enjoying the ingenuity of designs such as the strandbeesten!