We know that Pandora is a lush tropical paradise and that the Na'vi often hunt their food from heights by ikran. The fauna on the planet can grow to great heights due to the lower gravity (see source below), and the Na'vi have evolved to utilize the greater heights for habitation and perhaps to escape predators like nantangs and thanators.
Keep this in mind, we know that:
Pandora's atmosphere is a mixture of nitrogen, oxygen, carbon dioxide (>18%), xenon (>5.5%), methane, and hydrogen sulfide (>1%) and is about 20% denser than the atmosphere on Earth primarily due to the high percentage of Xenon; a heavy, colorless, odorless, and generally unreactive noble gas.
The increased air resistance within this denser atmosphere has a number of effects:Lower terminal velocity
. Combined with Pandora's twenty-percent lower gravity, a freely falling object's maximum speed is significantly less than on earth. A Na'vi who happens to fall from a flying mountain banshee has a good chance of surviving uninjured if they are over a forested area. Even over water, falling spread-eagled can reduce their speed sufficiently to allow an impact with the surface that will not knock them unconscious. (Source)
It is the Na'vi tail which has evolved to give them stability to not only inhabit and utilize the upper reaches of the canopy, but also to jump and/or fall without much chance of injury.
According to an article in the January 12 issue of Nature
, studies of the red-headed African Agama
lizard have shown how it raises its tail to counterbalance on slippery surfaces. When a research team attached a stabilizing tail to the end of their robot and sent it off a ramp, the critter took a nose dive when its tail was also in the down position. That would mean a face-splat for a Na'vi. When the tail was raised, the critter landed in a more upright balanced position just like the lizard did.
So, if having a tail makes a world of difference for a leaping lizard, it would surely make a difference to a Na'vi in her special world as well. Some sources state the Na'vi tail is prehensile which means it is capable of seizing, grasping and/or holding. I've not seen a shred of evidence on this, so I think the tail evolved to stabilize the body in its use of the habitat.*
For anyone who wants to keep track of further investigations on the role of the tail in controlling pitch and yaw as well as roll in running, you might want to check out studies being done at UC Berkeley and Stanford.
(*On the other hand, speculating about the evolution of a completely fictional animal is a really good joke on the speculator)