Ballooning animals and Newtonian fitness (Ballonts III)

Click to enlarge; copyright Gert van Dijk

I have always had a weakness for balloon animals. Not the toy balloons that squeak when you twist them into shape, but lighter-than-air living beings. I would like to see such 'ballonts' float silently and majestically over the plains. One such is shown above (well, two of them). Nice, isn't it? I could do screensavers if anyone wants them.

Click to enlarge; copyright Gert van Dijk

Smaller ballonts, less than a meter, are even more to my taste. These might descend from a rain forest canopy to siphon fluids from carcasses, or something equally mysterious. No wind there, so it might be a good environment for them. They could flap around a bit as well.

Click to enlarge; copyright Gert van Dijk

Less dramatic but much more common would be tiny ballooning seeds drifting with the wind across the world, forming a sort of aerial plankton. Books on biomechanics never mention lighter-than-air flight, but do not discuss radial flight either, as neither exists on Earth. The usual question is whether the absence of lighter-than-air animals on Earth signifies that evolution so far forgot to take off in this direction or that the idea won't fly.

I have written about ballonts before (mostly here and here), but this time the focus will lie on 'hard science', so there will be some formulae and a few calculations. Sorry about that, but it is not really difficult. The goal is to see what is needed to achieve a ballont that can lift a nice hefty body with as small a gas bladder as possible. Because there is a bit of explaining to do we will not get further than Earth in this post.

The first step is to realise that floating in air works exactly the same as floating in water. As 'buoyancy' you will find that in biomechanics textbooks (for instance here and here). It all starts with Archimedes' principle, who stated that 'the upwards force of an object in water equals the weight of the displaced volume of water'. That works in air too, but let's start with water, because that is a bit more intuitive.

• Archimedes started with 'the displaced volume of water'. OK; let's make a box of 20 by 20 by 20 cm and hold it under water. It is not difficult to find the volume of the water it displaces: that is the volume of the box itself, which is 0.2 x 0.2 x 0.2 = 0.008 cubic meters.
• To get weight we first need to know what the mass of that amount of water is. The density of fresh water is 1000 kg per cubic meter (sea water is a bit denser). For 0.008 cubic meter, we get a mass of 0.008x1000= 8 kg.
• Weight is not mass! It is the product of mass with the gravity constant g, and on Earth that is 9.8 m/(s^2). So the upwards force acting on our box is 9.8x8= 78.4 Newton.

Upward force = g x Density of water x Volume of object

Nice, but so what? Well, the presence of g in the formula means that the upward force increases directly with gravity. On a world with twice the gravity of Earth the upwards force will be twice as large as on Earth. One consequence of this is that a floating object rises faster than on Earth. But will it also lift a larger body mass, which is what we want? As we will see, the answer is no, but first we have to calculate how much mass a balloon can lift. The first step to get there is to calculate the object's own weight. We know how to calculate weight: upwards force was weight of water, after all:

object weight = g x Density of object x Volume of object

The net force is obtained by subtracting them, which can be written as follows:

net force= g x (Density of water - Density of object) x Volume of object

The gravity constant g is still in there, but focus on the rest of the formula. If the object is denser than water the net force is downwards -it sinks- and if the object is less dense, it will float. No matter what you do to g, that balance will not change. Without g, the formula describes a mass (density times volume). For a net upwards force, that resulting mass is what the object can lift:

liftable mass= (Density of water - Density of object) x Volume of object

Here is an example: Suppose the object is made of cork with a density of 250 kg/cubic meter. Fill in the numbers for cork and fresh water and you get (1000-250) x 0.0008 = 6 kg. If you tie a mass of 6 kg from the cork cube, the ensemble would just stay in place under water, as its combined density now is the same as that of water. (1) All we need to do to turn this into a formula for the bladder of a ballont in air is to supplant 'water' with 'air', and 'object' with 'bladder':

liftable mass= (Density of air - Density of bladder) x Volume of bladder

The gravity constant g is still not in the equation; although true, the full picture is a bit more complex: the density of the atmosphere is in fact strongly influenced by the strength of gravity, among other factors, so its effects are there still, but hidden. Let's focus on atmospheric density, as it will turn out to be very important for ballonts.   

The density of air on earth at sea level is only about 1.2 kg per cubic meter, so we need very light materials to make a ballont work. The choices are limited. Helium would be great, but it is probably difficult to find on a terrestrial planet, and concocting a biochemistry to produce helium may be taking things too far. Hydrogen is easy to find, can be fabricated, and only weighs 0.0899 kg per cubic meter. We are now almost ready for the real stuff.

Click to enlarge; copyright Gert van Dijk

The image above shows a simple ballont scheme. It builds on the scheme above. Here are the ingredients, supposed to work at one Earth atmosphere and 20 degrees centigrade:

• A spherical bladder. It consists of a membrane, which will weigh something. I have great faith in the ability of Darwinian evolution to come up with amazing substances, so I chose something like Mylar. The membrane will be just 0.1 mm thick, and its density is 1.2 times that of water, based on PET and similar substances. The radius of the sphere allows its area to be calculated, and with that its mass. That is a downwards force.
• The bladder contains hydrogen gas. Its radius gives us its volume, and together with the density of hydrogen (0.084 kg/(m^3) at about 20 degrees) we get the mass of the hydrogen. This is another downwards force. Note that the balloon is not pressurised to have it hold its shape; we will assume that it stays spherical anyway.
• The volume of the displaced air is found from the radius of the bladder and the density of air (1.2 kg/(m^3)). This is an upwards effect.

Subtract the two downwards effects from the one upwards one. What we have left is how much mass the bladder can lift. We will tie a body underneath with a density of 1.1 times that of water. (2)

Click to enlarge; Copyright Gert van Dijk

Click to enlarge; copyright Gert van Dijk

The graph above shows results for bladders of 0.1 to 1 meter radius. The blue line (displaced air) is what determines the upwards force, and the membrane (black) and the hydrogen (green) pull downward. The red line is the difference, and that determines the mass of a body you can suspend from the bladder. Hm; a balloon with a radius of one meter still only lifts about 3 kg, as shown in the image below the graph (the man is a 3D object I found on the internet). While 3 kg is enough to build an interesting animal -think of a cat!- the relative sizes of the bladder and the body mass are not pleasing. Even if we clap on some wings to the body, the animal will still be extremely vulnerable to the slightest wind. It does not even get close to the kind of animal we want. I think we need to do better. Even a protoballont should have some advantage of its bladder, or else Darwinian evolution will not take off.

Click to enlarge; copyright Gert van Dijk

Perhaps the ballont seedlings work better, so let's do the job for a radius of up to 40 cm. Hang on: the red line goes below zero, so the smaller ones cannot lift anything at all! The reason is that their membrane is too heavy at small sizes. On further reflection that is understandable: the mass of the membrane increases with the square of the radius, and lifting ability (volume) with the third power. For very small ballonts, the membrane can outweigh the lifting power! Alas, there go the balloon seedlings. Struck down, not by a lack of Darwinian fitness, but because they are unfit in a Newtonian universe.

Click to enlarge; copyright Gert van Dijk

Let's try again for balloons with a radius of 1 to 5 meter. That's better: we can lift hundreds of kg now, enough for an impressive animal, with limbs, a digestive system, a hydrogen-producing organ (however that works), tentacles for tethering and grasping food, etc.. You may protest that the membrane is too flimsy for an animal of this size. I agree, but even with a thicker membrane, compartments etc., the effect of the third power of volume will easily priduce a net lifting force. Unfortunately, a balloon with a 5 meter radius is still very large indeed, nowhere near the shape we were looking for....

So it is the density difference of the lifting gas compared to the surrounding air that makes a balloon work. Perhaps surprisingly, gravity does not determine the liftable mass, or only indirectly as it affects atmospheric density. Some elements scale with the square of the radius and others with the third power. We saw earlier that this limits the size of land animals (start here for that subject). For ballonts it is just the opposite: bigger is better, at least as far as liftable mass is concerned. Whether the animal is viable in the Darwinian sense is something else entirely. Earth is a poor place for ballonts: blame Newton. To get them to work we need to manipulate not the ballont, but the planet! More on that in the future.

(1) In reality, the object you tie underneath the object also has both weight and an upwards force. The figure of 6 kg holds for the mass difference between the two.(2) The body also displaces a bit of air, but that has so little mass we will ignore its upwards force.

Purple Plasmid's Fentil

As confusing titles go, this one must achieve a fairly high score. What it means is that there a fictional moon Fentil, designed by someone named Purple Plasmid, who in real life goes under the name of Dan Emmerson. You will find his personal page on Deviant Art here, and his page on the planet Fentil right here.

If you, like me, are on the lookout for interesting projects on speculative biology, Deviant Art is not a bad place to search: it has enormous numbers of images and they are usually labelled sufficiently clearly to find what you are looking for. Some images of exobiological animals are very good, but quite often there is just one, and I much prefer a collection, a background story, or, in other words, more than just one image. Fentil has both background information and a collection of images, and so fits the bill nicely. There are over 60 images: some maps, some sketches, and some more elaborate designs. Dan's work exudes enthusiasm. Let's have a look.

Click to enlarge; copyright Dan Emmerson

These are 'pump fish', whose bodies are essentially cylindrical. They propel themselves by pumping water through heart-like chambers arranged one after the other, as the image shows. I like that design; that in itself is not surprising, as it is very much like some of my own designs that swim using peristaltic pumps (look for them on the water page, under 'swimming with tubes', or directly here if you do not mind losing the menu structure). I never named my own beasties, something I should rectify. One day I might actually just do that... Anyway, like my own creatures, pump fish probably do not show much movement on the outside when they are moving around. I started to wonder how many of these pumps should be placed one after another. For my 'peristaltic tube swimmers' I reasoned that one cycle moving along the length of the tube would be enough. In the pump fish case, you can see that the last segment is narrower than the front ones. If the same volume leaves the animal at the back as goes into the front in the same time, but through a smaller opening, the velocity of water must be higher, providing more propulsion. Do the successive chambers work at higher pressures, and is that the reason there are several?

Click to enlarge; copyright Dan Emmerson

I like the design of this 'sea sparrow': an elegant shape that seems very workable. They remind me of some sea slugs on Earth. The slug I had in mind is right here, and if you like sea slugs do not forget to have a look at the rest of the site they appear on. An intriguing part of the sea sparrow's anatomy is the combination of several paired fins with a large unpaired one at the back, giving it very original appearance. I would very much like to see an animation of how it moves.

Click to enlarge; copyright Dan Emmerson

A 'spot of fishing'. More precisely, it is a 'Rorschach sea sparrow' catching a pump fish. The accompanying text states that sea sparrows can fly and also chase their prey underwater. Gannets combine swimming and flying on Earth, although they are much better at flying in air than at swimming underwater. I suppose that it is possible to shift the point where an animal is at its best.
Dan's style with its flat colours and clear lines reminds me of some 'bandes dessinées' that use the 'ligne claire', such as shown here. I like this particular style. While seemingly simple, appearances are deceptive here. Dan wrote me he uses Flash for his artwork.

Click to enlarge; copyright Dan Emmerson

What you see here are some Fentil 'cloverheads' in the act of laying eggs. Cloverheads are herbivores that travel in great herds across vast plains. There are various cloverheads to be found on the Fentil section of Deviant Art. They all remind me a bit of Barlowe's animals, particularly as regards their feet, thatall look like elephant or sauropod feet. I think that this type of feet is very out of place in a fast-moving animal, but the explanation for that will have to wait for a post on what toes are good for.
What you might not appreciate is that you are looking at a first: this image had only been published before as a work in progress, but now it is final. A scoop for 'Furahan Biology and Allied Matters'!

Click to enlarge; copyright Dan Emmerson

These are 'Bghelly baskets'. These animals use sunlight to help raise temperatures in their gut sacs, which is a nice idea. They are larval forms using echo-location. Again I find the clean design very appealing.

Click to enlarge; copyright Dan Emmerson

Bone trees: without doubt they are among the most alien of Purple Plasmid's inventions. I would love to see a landscape painting with lots of them. The text provides a factual and neutral description about how they grow they way they do, but not why they do so.
Most plants on Earth have an enormous surface area in relation to their volumes, what with all the flat leaves and slender branches and twigs. Cactuses are notable exceptions: with their rotund shapes and no leaves to speak of, they clearly went for a very low surface-to-volume ratio. It is not difficult to work out why a cactus has a shape different from almost all other plants: a small area restricts evaporation, and their environment is very light anyway. As a bonus the large volume allows reserve water to be stored. Bone trees may look like cactuses, but they are found in regions where fresh water is plentiful, so there must be something else going on.
I thought that it might have to do with their skeletons being brittle, but Dan assured me that that was not the case. Instead, Fentil orbits a planet and suffers from frequent eclipses and the attending drop of temperature. This is what he wrote: "During this time, most plants hide within a protective shell, or retract their leaves (bone trees pull their leaves back into their shells) or just re-absorb the valuable photosynthetic tissue, which is usually free-floating in a transparent gel."

Click to enlarge; copyright Dan Emmerson

As hinted in the image above, There may very well be a website about Fentil in the future -another scoop!-, allowing visitors to click on cladistic trees to see what kind of animals they are dealing with. That sounds like a excellent idea. I hope Dan gets around to building one.

Furaha in the Science Section of NRC Handelsblad

Well, when I came back from travelling abroad I found that 'NRC Handelsblad' had indeed devoted ample room to the Furaha project. 'NRC', as it is known, is a Dutch quality national newspaper. The weekend edition has a 12 page science supplement, and this weekend's supplement had three pages on Furaha: the cover and the spread in the middle.

I am afraid I cannot direct you to the relevant pages on the internet to have a look for yourselves, as these are open only to paying customers. Those with subscriptions can download a good quality pdf file. I have a subscription and have the download, but will not publish it here. After all, the newspaper is supposed to make money. Then again, at some point in the future I may yet do so after conferring with the newspaper people. For similar reasons the images below will give you an idea what the article looks like but not in enough detail to read it. Most of you would be unable to read it anyway, it being in Dutch, my native language. It is a very nice article, written by Lucas Brouwers, who also writes a blog -in English- on evolutionary biology.

Here is the cover. I took it from the digital edition, meaning the contrast is much better than in the printed edition, and the colours are also closer to those of the original. 'Wetenschap' means 'science'. (The two words may not overlap entirely. The English version of Wikipedia has a nice entry of the meaning of the word science: "Over the course of the 19th century, the word "science" became increasingly associated with the disciplined study of the natural world including physics, chemistry, geology and biology. This sometimes left the study of human thought and society in a linguistic limbo". In Dutch 'wetenschap' seems to used in the latter broad sense more often than in the narrow one).

Click to enlarge; copyright NRC / Gert van Dijk

Anyway, this is the 'woolly-haired shuffler', that can also be found on the Furaha site; it also featured in the Furaha blog when I first posted it on the main site. The name is a correct translation from the Dutch 'wolharige schoffelaar'. In both languages the adjective 'woolly-haired' was of course taken from the name of the 'woolly-haired mammoth', a name I have always considered as odd as it is funny. I do not think there are other animals, extant or extinct, whose name expresses a quality of their pelt. I have never heard of the 'silken-haired panther' or the 'greasy-pelted otter'; perhaps in poor prose, but not in an animal's name. The text of the paper uses 'wolharige schoffelaar' as intended, but the headline, after translation, reads 'Hairy shufflers and radial flyers'. Newspaper headlines are often written by other people than those who write the body of a text, for reasons unknown to me. Perhaps there was not enough room for the full name, but now the name states that the animal is particularly hairy and it is not...

Click to enlarge; copyright NRC / Gert van Dijk

And this is the inner spread. Regular readers will recognise most images. Two have not been published on the site though, eroding my store of fresh images a bit more. Mind you, the text does not go into what they are or why they look the way they do, so some mystery remains.

You might wonder why a science section of a serious newspaper devoted three pages to a speculative biology project, and that was what I asked the journalist before the interview. He answered that neither he nor his colleagues had any trouble with a mixture of solid science with creative and less factual matters. I am glad of that. Science is too often treated by scientists and others as if it is so serious that you should speak only about it with a straight face and in hushed tones. Well, without creativity science would not only simply not work but would be nothing more than drudgery. I hold similar views about the speculative side of matters, where I prefer to see a mixture as well: fantasy is not the same as idiocy. The best of science fiction is characterised by a 'sense of wonder' as well as a 'what if?' attitude. You cannot do science without them.

Furaha in 'NRC Handelsblad'!

That won't mean anything to most readers...

But I am talking about a leading high-quality Dutch newspaper, and if everything goes well they will devote quite a bit of attention to the Furaha project in next Saturday's edition.

Kortom: Nederlanders of Vlamingen met belangstelling voor Furaha: let op de wetenschapsbijlage van NRC Handelsblad van zaterdag a.s.!

Its a bird, it's a plane, it's... a tetropter (tetropters IV)

The nice thing about computer animation is that it allows you to actually see thing that you could only dimly imagine beforehand. One image that has been sitting in my mind for many years is the following: you see a dusty plain, and a herd of handlebars (Latifrons imperator) come galloping in from the right hand side of the image in the distance, and then wheel towards the viewer as if they were performing a well-rehearsed cavalry manoeuvre. I can almost hear them too...

Unfortunately I do not see anyone spending a small fortune to make this a reality, so I will have to content myself with what I can do myself, with my PC, at home. Some visions therefore remain locked in my head, but a few more modest ones do find their way out. Making tetropter flight visible is something I thought I worked on for quite some time; today I can show you a near-final result. Near final, because nothing creative is ever truly finished. In this case, the camera should move, the animals should vibrate in rhythm with the wing beats, there should be more details, there should be motion blur, and there absolutely has to be blurring to mimic a limited depth of field and through that create the illusion of small size.

Still, what I can show you is the principle of the thing. It's not a movie, but an illustration of wing movement in slow motion. Tetropters have been described several times on my blog. A summary of the tasks involved in animating them is found here, and entries on their design and wing movement patterns are here, here and here. In short, they are radial flying animals, whose four wings can do a 'double clap and fling', invented by yours truly, and later also by other people in the flying robot business. By the way, the movement of tetropter wings is not all that different from the complex way in which Earth insects move their wings.

This is an animated scheme to show how it all works: the wings are planes that are warped as they cycle through their movement cycle, so their shape is different depending on were they are. Where they are is governed by rotations along the x-, y- and z-axes, and all these paths can be altered and edited. The Matlab programs that do all this in the end write lots of 'obj' files: those are files describing 3D shapes; one is produced for each wing for each frame of the cycle (there are usually 120 frames in a cycle). A script written in Python then loads in a scene containing a body shape without wings in Vue Infinite, adds the appropriate wings per frame and stores the images. These are then used to form an animation, and those are what you see here.
The 3D shapes of the wings consist of 1600 small triangles, which is more than enough to show supple movement. As they are they do not look like wings at all, but there is another trick to take care of that.

The trick in question is to add transparency and colour. The transparency mainly makes unintersting parts invisible, but it is also useful to make the wing itself partly transparent as here. To create the fly-like animal above (Bombilator musca) I used an image of a real insect wing found on the internet, and used that to create a transparency mask. All of a sudden, the boring rectangular 'wings' produced by the Matlab program take on a biological appearance. Please do not look too closely at the body of the animal: it is a simple shape cobbled together in Vue. As you can see the animal has four legs and two sets of eyes: upper ones, presumably to scan for danger, and lower ones, near the food gathering end at the bottom.

A bit of colour makes a lot of difference, so here is a farfalloid, resembling a butterfly in overall appearance (Farfallapter caeruleus). Indeed, I stole its wings from a real Earth butterfly, albeit with some warping and editing. Mind you, quite a bit is lost in the conversion process.

Click to enlarge; copyright Gert van Dijk

To show that, here is a still of the Farfallapter; better, isn't it? Then again, you can see how crudely the wing is linked with the body...

I guess I now no longer have any excuse to put off work on the 'Flying with...' page. It is probably also time to redesign the site. I have already looked at that, but the days where you could learn HTML in two evenings seem to have gone for good.

How many legs are best for megamonsters?

About a year ago I wrote two posts about what happens to legs when an animal is scaled up (here and here). In a nutshell, if you make an animal's body twice as big, the new body will weigh eight times as much as the old one and not twice as much. If you make the legs twice as big, they will not be strong enough to carry the new weight, so the only solution is to make the legs more than twice as thick. The result of all this is that legs have to make up a larger proportion of a very large animal than of a small one. There is a limit to how big you can make an animal: at some point the legs need more food than the body can deliver, or something equally silly.

Click to enlarge; copyright Gert van Dijk

Last week Jan asked a question on the bulletin board of the Furaha site asking which body plan would be best for really big animals. That question made me think: the more legs there are, the smaller each leg can be to carry the body. What does that do to the mass of all legs together? If the total mass of six slender legs would be less than that of two thick legs while doing the same job, than having six legs would be a better design for very large animals than having two. That would be a nice outlandish and unearthly solution! It is shown above in a rather silly image (the human figure came with the program and is there for scale only).

Whether it would work or not was not intuitive to me, so I did some homework and came up with the work of Robert McNeill Alexander (if you are interested in biomechanics you will encounter his work many times). In this case, part of the answer was described in this book Optima for Animals.

Click to enlarge; copyright Gert van Dijk

The reasoning starts with tubes. There are excellent reasons why vertebrate leg bones and insect legs are tubes, and why tubes are such important structural elements in technology. They are about as strong as solid rods of the same diameter, but weigh a lot less, and doing the same job with less bone is a good idea. The three tubes above all have the same outer diameter, but the hole down their lengths differs in diameter. In papers on the subject you will find an very important parameter 'k': it describes the width of the inner hole as a fraction of he outer diameter. In the left one k is 1, so the hole is tiny. In the middle bone k is 0.5, and in the right one k is 0.9, meaning there is just a thin shell of bone. A value of 0 means no hole at all, and a value of 1 would mean the bone is infinitesimally thin (in simple words: there is no bone!).
Are all these bones equally strong? No, they are not. If you just take bending forces, there is a nice formula which contains three items of interest: the 'bending moment' M (the force that the bone needs to withstand), our friend 'k', and the radius r of the outer side of the bone (there is only one thing else and that is a constant K for the material - ignore it-).

Here it is: r=[M/K(1-k^4)]^0.33

If you keep the force M constant you can calculate what the radius is for any given value of k. Let's do so.

Click to enlarge; copyright Gert van Dijk

The image above shows what happens for four values of k: 0, 0.3, 0.6 and 0.9. As you can see, the value of the radius increases as well: the thinner bones have to be wider to withstand the same pressure. Note that that is hardly the case when the holes are fairly small. In fact, the effect is only really noticeable when k increased form 0.6 to 0.9. Does it matter? yes: the cubes in front of the bones represent the mass of the bone itself, and that nicely shows why tubes are good. They are all equally strong, but the thin-walled ones weigh a lot less.
On Earth, things are more complicated for mammals because the holes in the bones contain marrow. Marrow, while less heavy than bone itself, still makes the bone as a whole heavier. A bone with a very thin shell will be wide and will contain lots of marrow, defeating the purpose to make bones light. For mammals with marrow in their bones there is an optimal value for k, at which the bone as a whole weighs the least; that value turns about to be about 0.63. If, however, you manage to put air in the hole instead of marrow like birds, than the story becomes different and you can increase k. Above around 0.9 the bones the become too susceptible to buckling, so there is another optimum value for air-filled bones: a value of 0.9 is excellent. Our hypothetical megamonster shall therefore have air-filled tubular bones with a value of k of 0.9!

We still are not there yet. The real question was what happens if we give the animal more legs. Let's assume that the forces are simply divided among the legs, so with four legs each leg has to carry exactly one fourth of the burden. Remember that there were three parameters of interest in the formula: The bending moment M, the outer bone radius r and k. Set k to 0.9, and then we can calculate r for four bending values. The values per leg are 1 (the animal has one leg), 0.5 (two legs), 0.25 (four legs) and 0.125 (eight legs).

Click to enlarge; copyright Gert van Dijk

This picture show the resulting bones, along with the number of bones. As you can see, the bone for an animal with eight legs is a lot less thick than for the one with just one leg. So far so good. The smaller bone must weigh a lot less than the bone for the one-legged animal, which is what we wanted. Then again, there are now eight such bones, so the question is what their combined weight is.
Each bone of the two-legged megamonster weighs 63% of the one-legged one, so the two bones together weigh 126% of the one bone. That is not what we wanted, as the two legs weigh more than the one leg. Does it get better if we add more legs? Well, for four legs each one weighs about 40% of the one bone, and together they weigh 159%. For eight legs, each one weighs 25% of the one bone, so the total weighs 200% of the one bone.

How disappointing... I had hoped it would be the other way around. Now it seems that fewer legs is the better way to save weight if you need a mega-monster. Obviously, giving it just one leg is not practical; there would be a big risk of falling, and the only way to move would be to jump in a series of bone-shattering hops. Two legs is quite feasible; just think of carnivorous dinosaurs. Four is also good. In a last-ditch attempt to save the concept of multi-legged megamonsters I could say that having six or eight legs provides safety as a possible advantage. A two-legged monster with a broken leg is doomed with certainty, and a four-legged one probably is. But a six-legged one could deal with one broken leg and hobble away.
You might expect animals with a multi-legged body plan to lose some limbs as they grow bigger and bigger as a measure to save weight. Such limbs might be given another purpose than locomotion, so they could develop into, well, just about anything. They could develop clavigerism or centaurisation, also interesting.

I'm still disappointed though...

Wildlife in the Star Wars universe

The Star Wars films are great adventure movies, but you wouldn't think that there was much biology going on, would you? The Star Wars universe is obviously swarming with intelligent creatures from many different worlds, and these worlds must equally obviously be filled with animals, but you do not get to see many of those. it's not what the films are about. In the second film there was the 'tauntaun' on the ice planet Hoth, and as a hairy mammal-like animal with the body plan of a bipedal carnivorous dinosaur that was an intriguing invention. Otherwise, not many in the early stages came across as great exobiological inventions. In fact, quite a few some could not be taken seriously at all: just think of the gigantic snake-like animal living in an asteroid (in a vacuum!), large enough to have the Millennium Falcon fly between its teeth. Or take the 'Sarlacc', a carnivorous monster buried in the sand on Tatooine: all you see of it is a gigantic mouth in a pit in the sand, reminiscent of an antlion but scaled up to a gigantic level. Its very existence implies that there are lots of rather stupid animals ready to stumble into it maws. No-one would take such a creature seriously from a biological point of view, and no-one should; it's not what the films are about. Still, there is a book about the animals in the Star wars universe, showing someone cared. It is called 'The Wildlife of Star Wars. A Field Guide', by Terryl Whitlatch and Bob Carrau. Terryl Whitlatch has been working as a 'creature designer' for several of the Star Wars films. Typing her name into Google results in so many hits that I do not need to provide many links. Still, here is one with a video interview, and here are two on character design (one and two). The 'Wildlife' book first came out in 2001. I had somehow missed it completely until recently, when a reprint was issued. I found another book of hers showcasing other work, not related to Star Wars: 'Animals Real and Imagined'. The Star Wars book seems to have many creatures in it that did not appear in the films as far as I know. I have to be careful here, as the three newest films (I, II and III) did not make much of an impact on me, aimed as they seemed to be at children. I suspect that most of the animals in the book were not in the films at all, which would imply that Ms Whitlatch had more or less free reign in designing them. The sillier animals such as Giant Space Slugs of over 900 meters long, and the above-mentioned sarlacc occur in the book, should you wish to know more about them. The animals that I assume to be Ms Whitlatch's inventions were much more to my liking. In a way they conform to the general exobiological theme of Star Wars. In it, intelligent beings are almost always humanoid, with two legs, two arms and a sort of alien-looking head that is rather larger than the standard Earth issue human head. The films started when digital creature design did not exist, and alien design followed time-honoured principles, involving actors in rubber suits. Hence the big heads. Ms Whitlatch's aliens more or less follow such principles. The book deals with life on several planets, but you will not be able to determine their planet of origin by looking at their body plan: an animal could come from one planet as well as from the next. Whereas nearly all intelligent beings are humanoid, animals also share many design principles with Earth animals. Quite a few are 'mammaloid', meaning they look a lot like mammals with odd heads. There are also plenty of aviforms and reptiloids, as well as combinations of designs. It is interesting to compare this design strategy with that of Mr Broadmore's Victorian Venusian life forms discussed recently: these were specifically designed to evoke a sense of displacement, i.e. of 'alienness'. Instead, Ms Whitlatch's life forms might be just around the corner. That makes them more believable, but inevitably less alien. Regardless of the degree of their 'alienosity', the animals are all superbly well drawn, with an intimate knowledge of animal anatomy and behaviour. The 'Animals Real and imagined' book contains drawings of existing animals in addition to fantasy ones, proving once more that Ms Whitlatch is a very skilled artist. I will go through a few of her drawings, scanned from the two books. As I did not wish to damage the books, I had to crop some drawings a bit, for which I apologise (you could all get the books yourselves...).

Click to enlarge; copyright Lucasfilm Ltd.

These are Motts from the planet Naboo, revealing how mammalian their body plan is. The way the legs fold, the number of joints, the general shape of the head, all these things say 'earth mammal'; 'ungulate' in fact. But look at the ease with which their poses are captured, and the green thingy walking away from the motts is much more 'alien'!

Click to enlarge; copyright Lucasfilm Ltd.

Also from Naboo, these gullipuds are, as the text says, amphibians. They resemble puffer fish or indeed, some Earth amphibian in being able to inflate their bodies. I like the sense of humour in this and other drawings.

Click to enlarge; copyright Lucasfilm Ltd.

The tree-dwelling Shaupaut. This is another mammaloid. Its elongated fingers are apparently used to 'fish' for avians flying by.

Click to enlarge; copyright Lucasfilm Ltd.

These animals are apparently extinct. They are labelled as stalking birds from Alderaan, and are obviously modelled on large African ground living birds, called 'ground hornbills'. These walk in rows over open fields, hoping to disturb smaller animals so they can be caught and eaten. These birds are doing the exact same thing.

Click to enlarge; copyright Lucasfilm Ltd.

These flying animals are urusais, and while they are members of the same species, there is an enormous difference in anatomy: the male is the one sitting upright balanced on its tail. It has four wings, while the female admiring him only has two. Now that is a rather fundamental difference. I doubt that any animals on Earth take 'sexual dimorphism' to such extremes. You would think that such large differences in shape would result in the two sexes being subjected to very different evolutionary pressures, with resulting different sets of genes for male and female bodies. Earth's insects may be thought to have two sets of bodies as well, but there the two act in different stages of life. It is an interesting concept to have something like that defining the two sexes. I have my doubts however that the differences can go so far as a having different numbers of wings. The text says that their span is about two meters, which means that they must be very heavy. Their bodies and thick tails look good, but do not appear to be designed to save weight. Light bones? A heavy atmosphere?

Click to enlarge; copyright Design Studio Press

This one is not from the Star Wars book, but from the other one. It represents one of the few designs that I do not really like. The reason is that it reminds me too much of the six-legged forms in Avatar that I discussed earlier. Like the Avatar animals, this one does have six limbs, but not as three pairs with their own characteristics, but as one pair of hind legs and two pairs of identical front legs. In fact, their muscle anatomy shows the same problem as Avatar's thanator: the front limbs are, like those of Earth mammals, connected to the axial skeleton almost entirely by muscles. Note that the two sets of muscles seem to run through one another...

Click to enlarge; copyright Lucasfilm Ltd.

I would rather end with something I like better: the Nuna from Naboo, which is a very interesting and humorous drawing. The one on the left is a male, inflating his wattles and hissing to underline his dominance. You can see how well Ms Whitlatch combines animal anatomy with the expression of emotions. The emotions are very readable to us, which is probably not at all would you would expect from alien life forms. Then again, emotions help tell a story, and that is done very well here.