Thursday 11 February 2010

Avatar's 'Walking with hexapods' or 'Don't walk this way'

I guess everyone will have seen Avatar by now. If you haven't, and if you like speculative biology, go out and see it now. You can read this post before you go, as it will not spoil the storyline. The film is an amazing spectacle. I loved it, and was fascinated by the computer generated graphics. There is a short 'featurette' in the form of a short documentary on the internet that you should all watch, because it explains some of the basic tenets of the film. Here it is, but remember that there is a much better version on YouTube.

Before Avatar appeared in cinemas, there was this rumour going around that it would be solidly grounded in biology. For a film with floating mountains in it, coherent biology may not be the first thing you would expect. Still, let's look a bit closer at that claim, and start with the natives. As narrated in the 'featurette', the Na'vi (the natives) look very human and have four limbs while all large animals have six. The reason for this apparently has nothing to do with biology and everything with economics. The director, James Cameron, made that quite clear in an interview with Playboy magazine:

Playboy: How much did you get into calibrating your movie heroine’s hotness?
Cameron: Right from the beginning I said, “She’s got to have tits,” even though that makes no sense because her race, the Na’vi, aren’t placental mammals.

So biology did not have to make sense, and in Hollywood facts and fiction do not seem to be regarded as fundamentally different, as they are in science. Oh well, perhaps we should just embrace the natives (the hero does) because there would otherwise not have been any film at all. So let's hope the rest of the Pandoran biosphere is more plausible. The first job at hand has to be how to squeeze four-limbed humanoids into a evolutionary tree in which every big terrestrial animal has six limbs; hm.

Prolemuris from 'featurette'; click to enlarge

The book 'Avatar, an activist survival guide' presents some notes on the Na'vi's presumed evolutionary background. There is an animal, the Prolemuris, that 'has two arms that bifurcate into four forearms; the upper bones of the arms have fused... Biologists believe that this may be an evolutionary precursor to the two-armed Na'vi'. There are two difficulties with this: I suspect that this arrangement would not function at all well, but, more importantly, 'limb fusion' as an evolutionary process seems utterly incredible. If you want to lose limbs, have them gradually decrease in size (the insectoid aliens in 'District 9' did have such minuscule middle legs, if I remember correctly). You might expect the resulting 'fused limbs' to look different from our own, but the natives' arms are so human that you might as well assume that human arms are the result of limb fusion. As a joke you could argue that the presence of two bones in our forearms suggests this to be true... But I really wonder how the film's biological advisors reacted to 'limb fusion'. I doubt they invented it, and surely they raised similar objections?

Thanator from book; click to enlarge

On towards the hexapods. I was not the first to design large alien animals with six legs and won't be the last. But I did think hard about how such animals might walk, and wrote computer programs to explore gaits in a six dimensional phase space to prove it. I know this sounds a bit pedantic, but the Furaha page shows I did. Just go to the land page, or directly here. One of Avatar's stars is the thanator, a large predator, as sleek and supple as a panther. Its middle and front pairs of legs are very close together, as can be seen on the image above, from the book mentioned above. The anatomy and the movement pattern of the first two pairs of legs are virtually identical, which is very odd. Other Pandoran hexapods have this same peculiar arrangement, as can be seen on the following images from the same book (the white triangles here and there are due to the fact that the images on the book were printed at an angle to the page, and I tried to rectify that).

Hammerhead from book; click to enlarge

Sturmbeest from book; click to enlarge

Viperwolf from book; click to enlarge

One result of having the front and middle legs so close together is that there is no good way to connect the shoulder girdle to the torso. The images above show that the animals have typical mammalian shoulder blades; the thanator image even shows typical mammal muscles. One such, the latissimus dorsi, can be seen running from the shoulder blade of the middle leg backwards to the torso. There are typically other large muscles running in all directions from the shoulder blade. How do you solve having two such sets in the same space?

Fragment from 'featurette'

Another result is that such legs almost certainly have to move in unison or they will collide. You typically do not get a long view of anything in Avatar, so here is a small video in which a fragment of direhorses is repeated a few times. The front and middle legs on one side indeed move in tandem. Not always, but generally they do. Mind you, there are two other solutions to avoid clashing legs that I will not go into here, but neither seems to be in use on Pandora. Anatomically these animals have six legs, but functionally they are tetrapods. I do not think that leg clashes can be prevented completely with this anatomy. With that in mind, the scene of the thanator chasing the hero could have ended quite differently: just when the thanator is about to grab the hero, the poor beasts trips over its own legs and crashes to the floor...

Again, you wonder why this design was chosen. According to one internet site one of the advisors, Wayne Barlowe, had this to say: "There was some concern as to the biomechanics of the six legs but my guess was that if they were grouped four towards the front and two in the rear locomotion issues would be solved. Those worries were pretty much put to rest after some informative motion tests were run."

But why should there be any concern about the biomechanics of six legs to start with? Six legs are part of the standard insect design, so six-legged locomotion isn't exactly a novel concept. I am not aware of any insects moving like Avatar's hexapod animals. The insect standard gait is a double tripod, a perfectly sensible solution for slow movement. Insects of course make good use of the fact that they have six legs, and, unlike Pandoran hexapods, do not pretend to have four only. The given explanation has an odd ring to it coming from someone with biological acumen, almost as if there was another, nonbiological reason for this clumsy and implausible arrangement. Perhaps the producers felt that the animals would look too alien if the animals moved in too unfamiliar a manner. I have no idea.

Go see Avatar; I loved most of it. But not for the biomechanics. Surely it would not have been that difficult to make better use of easily accessible knowledge; facts are not expensive. Not many people may notice or care, but the ones that do notice are probably the ones who care a lot.


j. w. bjerk said...

Re: the fused limb thing, and Na'vi relationship to Pandora.
There's a rule of thumb i wish more science fiction creators would adopt. "It's better to provide no explanation than an obviously stupid explanation".

I've only seen it once, and while i was intensely interested in the creatures and how they worked, there never was a point in the film, when i said, "Ah HA! that animal's movement looks fake". Though as you mentioned you never get a good long look at creatures gait with all the foliage and action and other things to see.

A few times i'm rather sure than some of the creatures moved with one pair of limbs on the outside and the other on the inside, which would somewhat alleviate the problem of hitting your own feet, but looking at the stills none of the creatures seem to be designed that way.

The Biological oddity that bothered me the most was the way that the Na'vi had those fiber-optic tentacles coming out of a braid of hair! Hair that looks and acts just like human hair suddenly has little writhing, retractible tendrils at the end. The braid was usually the simplest 3-part braid, there is no "inside" to hide tentacles.

Birdy said...

Perhaps the producers felt that the animals would look too alien if the animals moved in too unfamiliar a manner That's most likely it. I've heard that the banshees and other flying creatures originally looked more like stingrays than wyverns, but got changed for similar reasons.

Frankly, I like the paired legs for purely aesthetic reasons (the wyvern-like banshees, too), which is probably what their final reasoning was too, judging by the non-placental mammals with mammaries decision.

Evan Black said...

I've been an Avatar apologist from day one, so take whatever comments I make with as big a grain of salt as you like.

The hexapod macrofauna doesn't bother me too much (it's spectacular in the flyers, IMO), though I can see where questions about tripping easily arise. I always thought that the Na'vi's and Prolemuris' lack of a pair could be explained away. I've seen weirder mutations in real life, and perhaps the circumstances were appropriate that such a mutation was beneficial. Or we can use the standard fall-back of "the creators didn't want to weird out the audience too much" which is equally valid.

I've always had questions about the Na'vi queues myself. If their neural connectors are in those braids for protection, where does the hair actually grow from, the base of the scalp or from a membrane of skin running the length of the queue? And when exactly does it get braided? The Avatar units have braids before decanting from their tanks. Who braided those, or did they grow braided?

With all that said, I agree that Avatar is a great movie, and we shouldn't refuse to watch it based on some snafus in the biology. If there's somebody out there who can't live with that, I'm eager to see something better produced (wink wink nudge nudge). In the meantime, enjoy another exercise in imagination compliments of James Cameron!

Sigmund Nastrazzurro said...

Now that's a nice aphorism: 'it's better to say nothing than to say something stupid', if I may shorten it a bit.
As for limbs moving across one another, that is one of the ways in which you can avoid clashing limbs. If you look at the Avatar clips often, you will see that the direwolf seems to do so. The thing is, its footfalls are still on the same line, so the 'reaching over' seems to be an optical trick. For this to work, one pair of prints should be closer together than the other one, and if you look carefully, that is not the case. Have a look at Furahan jingoes; they've got that.

Both you and Evan commented on the braid, which indeed is silly. Why isn't it paired, like the tendrils of the direhorse? Because a telepathic moustache counteracts breasts?

Evan, I really like this movie. But in several years everyone will ask why we fell for this movie, and I expect we will all feel a bit silly because the answer is that the graphics are great now. Nothing else about this film makes sense, and that is what bothers me: it could have been much more with almost less of an effort.

Sigmund Nastrazzurro said...


That is a great site; I loved it. but don't you feel a bit disappointed about the film settling for tropes instead of telling its own story?

Evan Black said...

"But in several years everyone will ask why we fell for this movie, and I expect we will all feel a bit silly because the answer is that the graphics are great now."

I don't necessarily agree. It may become the case that Avatar will go the way of so much sci-fi, lost amid all the other undeniable also-rans, but does that automatically negate the fact that we liked it at some previous moment? Why feel silly about it?

Or it could go the way of The Terminator, which, when you boil it all down, is just a predictable action movie that incorporates a few conventions from horror movies, and even science fiction elements generally regarded as "silly". Despite this, Terminator remains a cultural icon 25 years later.

If at some point I feel embarrassed for having enjoyed a movie I'll gladly eat my words, but at this point I don't see why appreciating another attempt at developing a unique alien world (flaws and all) will ever be cause for embarrassment.

"don't you feel a bit disappointed about the film settling for tropes instead of telling its own story?

That's not how tropes work. A trope is a tool, and writing a story without tropes is like a song without notes or painting without paint.

That said, sure, I think a deeper, more cerebral story would have been more interesting, more in the vein of Cameron's The Abyss instead of Alien or Rambo. But that's not the story that was told... I plan on having a more thoughtful story in which to couch my own speculated world. I look up to your own efforts at fleshing out the world around Furaha. I hope I find success...

StevenH said...

One thing regarding the hexapodal movement that wasn't mentioned was how it could be animated. Remember, it was people who moved the limbs in the critters for us to see; if they were unclear as to how a six-limbed megafauna would move, they would have animated it badly, and it would have looked ridiculous. Better for it to "feel" like the movement was accurate than to try to make it accurate and have it feel wrong somehow. I think that perhaps they animated it as a tetrapod in order to avoid the uncanny valley effect. They know how a tetrapod can move and how it's muscles will bunch and relax; they probably thought it would be too time-consuming to have it move as a "proper" hexapod.

It just goes to show how there needs to be a good central repository of speculative biology sites/information. That way the animators will have access to things like this site (and snaiad, and the Future is Wild, and, dare I say it, Epona :-)

Just my 1/50 of a buck.

Warmest regards,

Sigmund Nastrazzurro said...

Hi Steven,

You mention 'costly' and 'time-saving' as factors that may have influenced the choice for a tetrapod movement pattern instead of a more proper hexapod one.

You may be right; how should I know why film producers think they way they do? But you would think that, with the money that has been poured into this production, it would not have been a problem to find out how to make an interesting-looking hexapod gait. For a heavy animal, begin with a tripod gait, and have the individual legs move as a rhino's. Can't you see it in your mind's eye already?

The only probem is the middle pair of legs, and that can be solved too. If I could do it, paid professionals can too...

StevenH said...

Hi Sig,

Oh, I fully agree with you. They really should know better, and think a little more outside the box (as if hexapodal gaits weren't the norm, on an absolute number of creatures on the planet that use it basis). They were being intellectually lazy.

That said, it is still possible that the animators had a deadline such that they felt more comfortable making if they just did what they were used to, as opposed to trying out different things. Despite the idea that Hollywood types are creative, the movies that are churned out really aren't as creative as they could be. Movie production is very "Assembly line" and the financial shennanigans are beyond rationality, so even the few actually creative types are forced by the system to become formulaic.

Cameron is one of the more creative people in Hollywood, but even he has his limitations. See, for example, The Abyss, which has some of the most "alien" aliens in it, but the movie didn't do all that well, because the studios forced him to chop out about 40 minutes, including the 35 minutes or so that show what the aliens are doing and why they are doing it. Fortunately, there is a Director's Cut, which puts all this back in. And then there is the problem that most viewers don't really take the time to actually think about the implied backstory and use it to make sense of what they were able to see. It is even harder when the aliens don't speak English/. So even if they see the Director's Cut, many still don't get it. (I have an advantage here, since I read the book, which is very good by the way.) The aliens do some very subtle things that just from watching the movie you won't understand (such as their control of water on the molecular level...who would guess that this gives the aliens a kind of telepathy, since the brain is mostly water? Or that they use it to influence the thought patterns of the characters? Who would think that a thought that popped into your head came from somewhere outside of your head?).

Ack, I am getting a bit off topic, so I will end my little rant. :-)

Unknown said...

Me, when watching the movie, I quickly decided that the Na'Vi where NOT native to Pandora, but rather an alien race that had colonized the planet.

They lack the four eyes, the six legs, and breathing openings in the chest.

I figured they where an old starfaring race that wanted to go back to nature, colonized Pandora and build Eywa, an organic supercomputer, to maintain their new home, and make their little wildlife retreat an idyllic one.

Viewing it like that makes the movie make ALOT more sense. And it explains why the Na'Vi look so different, why Eywa exists at all, and why the Na'Vi are to good to be true.

Unknown said...

Not to mention those USB ports all those creatures have in their heads.

That just SCREAMS 'artificial world'

Either way I'm not buying the Na'Vi as having evolved on Pandora. They clearly didn't. They can't look THAT different from the native wildlife, evolution does not work that way.

Unknown said...

To be fair, the Prolemurs also only have on head tentacle, and it's very short, practically vestigial. There's also what looks like the remains of a membrane joining arms to body. I suspect the explanation of limb-fusing in the book is completely wrong and the original idea was that the common ancestor of prolemurs and Na'vi lost one pair of limbs, possibly taking on a flying squirrel-type lifestyle, and that the prolemurs are actually in the process of re-evolving four hands. The fused limb theory holds no water as the prolemurs' 'hands' each have only two digits, while Na'vi hands have four - just like the hexapedal viperwolves' hands do. Unlike the other terrestrial animals in the film, prolemurs and Na'vi also have external pinna, white teeth, only one pair of eyes, and lack thoracic spiracles. To my mind, they belong to a completely different clade.

su_liam said...

I will probably have to watch that movie at some point.

It does seem to be getting a bit closer to actual science fiction than previous sci-fi movies. Perhaps.

When it came to the gait, they may have actually tried a more realistic one and found it wanting in the, "looking good," department. They put boobs on an alien girl(or maybe that was actually a boy?!?), so I don't think they'd have any compunction about avoiding a perfectly reasonable, realistic gait because it... looked weird.

'Sides, a Movie like this, I'm watching it for the pictures not the deep scientific educational value.

They want to call this hard science fiction? Why not? Movie standards are low enough even for the big flying mountains.

Sigmund Nastrazzurro said...

Hi Su (should that be Hi Liam?)

You may well be right. In the world of Big Money "looking good " is always the most important consideration, and "looking good" is definitely NOT defined as "looking "most scientifically plausible".

Is that to be considered normal or disastrous

Anonymous said...

I never really saw anything wrong with their biomechanics and stuff here, but what bugged me is how the Na'vi are too humanoid. They only have two eyes, four limbs, noses, hair, a single sensory tentacle instead of two, no chest-nostrils, external ears, differentiated teeth...honestly, I can see how the other creatures evolved and are related, but the Na'vi could in no way be related to them. Again, it seems very anthropocentric that an intelligent race always looks humanoid, like how they said if dinosaurs became intellilgent they'd be tailless, flat-faced, big-headed bipeds.

tribbetherium said...

Searched out this blog page after a rewatch of Avatar on cable TV...

The real question here is, if large animals walking on six legs poses such a problem for tripping over their own legs and whatnot, then wouldn't it be simpler and more likely to just lose a pair? Four legs seems sufficient for Earth mammals: Pandora could easily do the same, with the added bonus of turning the surplus pair into a wide variety of graspers, feelers, weapons, display organs and other non-locomotor functions, and if the creators wanted them to look more interesting, then they could do just that!

Also, there's the whole "Pandoran creatures are way deadlier than earth fauna" crap. James Cameron really be bragging how a Thanator would eat a T. Rex for dinner...even though they evolved in vastly different conditions, pit them against each other and either the T. Rex suffocates in Pandora's atmosphere or the Thanator is crushed by Earth's gravity. You might as well be asking: "Who would win in a fight, a lion or a shark?" which for obvious reasons is a stupid question.

Davide Gioia said...

About the limb fusion issue.
Snaiadi "vertebrates" have a form of limb fusion too, being pseudotetrapods descendants of eight legged "vertebrates" that fused their eight limbs and limb girdles in what are called the pectoral armature on the front and the skid on the rear.
Do you think Koseman's vertion of limb fusion makes a bit more sense, or is it a point less for the plausibility of the project?

Sigmund Nastrazzurro said...

Davide: I find limb fusion a difficult concept. Perhaps two bones that are always together, say an ulna and a radius, no longer need to move relative to one another. In such cases I can envisage the joints becoming less ans lees mobile, and they might disappear together, so the two bones become one functional bone. But an entore limb? It seems likelier to me that a limb will be lost.

Davide Gioia said...

Well, there do exist similiar and even weirder mutations, so maybe it's just that they don't result advantageous on Earth. Maybe on Snaiad such a random mutation revealed usefull for weight bearing because of the hydraulic muscles of the animals and the hevier gravity of the planet. I remember one of your articles showing that multiple limbs are not automatically more efficient in terms of total weight compared to fewer ones, and if there were a real advantage in having fewer and thicker limbs instead of multiple slender ones on Snaiad it would explain why at one point paratetrapods actually outcompeted hexapods there.

Sigmund Nastrazzurro said...

I haven't looked closely at snaiad in a very long time. but I guess I could ask Mehmet if we really need to (if he still uses the same email, that is)

Davide Gioia said...

That would be very interesting, you mention Snaiad quite a few times throughout your blog, but you've never dedicated an entire post discussing it...yet.

Anonymous said...

To be honest, in regards to the “biological USB” thing, it always seemed to me like an adaptation related to mating.
Many insects on Earth continue moving about even in the middle of copulation, so if Pandoran vertebrates did that too ancestrally, what could be more useful than a pair of claspers that actively lets you inform your mate of your movements in real time so you can synchronize for efficiency? This does leave…unfortunate implications to some extent for the use of them in the movie but it seems the most realistic explanation to me as to why they’d evolve basically wired telepathy.

Unknown said...

Anonymous: a means of better knowing the intentions of a sexual mate might have be beneficial, but species without a 'USB mind connection' seem to do quite well without one. I find it hard to think of a biological purpose for a precursor of such a system, and evolution won't progress unless there is some benefit in the earliest stages too.
I generally like coming up with some evolutionary pathway for some mysterious function, but in this case I prefer not to. For me, the 'Naávi mind connection' is like female Na'avi having breasts and Na'avi having four limbs: as these are apparently meant to appeal to the audience with total disregard for biological sense, I do not bother looking for something that was not considered to be of any importance.

Davide Gioia said...

I know this is quite an old post but I just found something that could be relevant to the discussion. From a comment here "Arthropod appendages are structurally very different from vertebrate limbs. And while you're right in that the main reason why vertebrates have 4 instead of 6 limbs is ancestry, it's a bit more complex. The vertebrate body plan is basically head, trunk, tail. The trunk is where the limbs and most organs are. Look at a vertebrate and you'll probably see this pattern repeated over and over: fore limbs, torso with organs, hind limbs. For something that started out with this bauplan, having an extra set of limbs in the middle would require some very complex organ shuffling. Insects and many other arthropods solve this problem by having all their appendages be on one part of the body, and their organs etc in another (usually the appendages are on the thorax or equivalent and the organs in the abdomen). Plus, given how heavy and complex vertebrate limbs can be it would require a lot of energy to actually be maintained, that's why animals just ditch them when they no longer have a use for it.
This is actually why I disagree with so many people criticizing the hexapods in James Cameron's Avatar because they move like quadrupeds. Having the 1st and 2nd limb pairs located in what I can only assume is an extended pectoral girdle designed to house 2 sets of limbs means that they don't need to figure something out for the organs (in the first movie the direhorse was shown to have a heart located in the same place we'd expect to see a typical terrestrial vertebrate's heart so I'm guessing their internals are similar)." This make me wonder, how are your hexapods organs organized within their bodies?
The speculative project this comment is taken from is very interesting by the way, if you plan on reviewing any more of such projects in the future.