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.