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).
Sturmbeest 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.