Stephen Baxter - The Science of Avatar

But there is the complication that the banshees also have hind wings. There are no vertebrate four-winged animals on Earth, though some insects have four wings—some of the Lepidoptera, for instance, the big group that includes moths and butterflies. These insects have various kinds of coupling mechanisms to ensure the wings work together. Those extra rear wings, plus the wing-tip panels, give the banshee additional control over its flight, as well as providing additional power when required.

Flying animals have differing wing shapes, described by a number called the “aspect ratio”—the ratio of wing length to wing breadth. A long, narrow wing is aerodynamically efficient, but is energy-consuming to flap. So long wings are best suited to creatures that can fly in open airspaces, especially where you can just jump off a ledge to get your lift: these include the albatrosses, and the big pterosaurs of the dinosaur age, and the mountain banshees of Pandora. If you live in wooded country, the ability to take off from the ground, powered lift and manoeuvrability are paramount, so shorter wings are favoured. Thus the forest banshee has a much shorter wingspan than its mountain cousin.

When Jake, undergoing the Iknimaya initiation trial, is taken to the banshee rookery to choose his mount, we see the banshees on the ground, where they look big, clumsy, ill-adapted; with their hind limbs having been adapted to wings they have no “legs” and must stump about on folded leathery wings. The great pterosaurs were similarly poorly adapted to the ground. The banshees have given up everything else for the sake of efficiency in flight, even their manoeuvrability on the ground. But then nothing will prey on them on the ground.

Nothing save the leonopteryx.

The “Last Shadow,” as the Na’vi call it, has a superficial similarity to the banshees, but a quite remote evolutionary relationship. The banshees evolved from four-limbed creatures, but the leonopteryx’s ancestors were six-limbed; it has two sets of wings like a banshee, but also a set of true legs, which the banshee does not. And its wings are composed of individual panes that can separate like a Venetian blind, or close over to form a solid surface; the vanes are a little like the big flight feathers of a bird on Earth.

On a world where even great creatures like the banshees have something to fear, at least on Pandora you rarely need be afraid of the dark.

The first time we really become aware of the ubiquitous bioluminescence of the Pandoran forest, the glowing of the living things, is during Neytiri’s first encounter with Jake as she saves him from the viperwolves. When she douses his torch it turns out he doesn’t need it to see, for almost everything around him shines of its own accord.

The Greek roots of the word bioluminescence are “living” and “light.” Living creatures can emit light by releasing stored energy through chemical reactions, though the details differ from species to species. On Earth, bioluminescence is common in the deep sea, below around a thousand metres. Down there in the eternal dark, too deep for sunlight to penetrate, it’s thought that some eighty per cent of creatures exploit bioluminescence. On land, by comparison, it is used by very few—fireflies, glow-worms, a few fungi.

In our oceans, bioluminescence is used for a variety of purposes. Some creatures use the living light to attract mates. But mostly bioluminescence is used in the endless game of predator versus prey. Many prey animals use the dark to hide in; they will descend into the deep dark during the day, and ascend to the food-laden surface waters only at night. So if you are a hunter, having a built-in headlight, as do many predators among the shrimps, fish and squids, can be very useful in tracking your elusive prey.

Meanwhile some prey creatures like the benttooth bristle-mouth use bioluminescence as a kind of camouflage, to muddle their own silhouettes if they are shadowed against light from above. Another tactic is to raise a “burglar alarm,” to lure an even bigger predator to chase off the guy attacking you. And still another tactic, used by some shrimps and squids, is to startle a would-be predator by releasing bioluminescent material into its face.

On the other hand, some predators use bioluminescence to attract prey. In the ocean, some of the decaying matter drifting down from above can be riddled with glowing bacteria; if you can mimic that glow, your prey animal can swim right up to you expecting to find lunch, only to become your lunch.

In the Pandoran forest, bioluminescence is common among plants, and animals, such as the direhorses, exploit it too. Even the Na’vi have glowing skin-spots, yellow on blue, and they light up their Hometree with sacs of bioluminescent life forms.

Why it is that so many land-based creatures on Pandora have chosen to exploit bioluminescence, compared to so few on the Earth? The answer is that so few nights on Pandora can rarely be truly dark in the first place, thanks to the spectacular light show put on by the two suns of Alpha Centauri, Polyphemus and the other moons. While the banshees for example have developed good night vision with their secondary eyes—and perhaps other animals have developed echolocation, a sound-based detection system like that of bats—many creatures have joined in a kind of cooperative light-based “arms race.” If everybody is kept flooded with light all the time you don’t need to evolve night vision or echolocation.

Visually, the living lights of Pandora are one of the most charming aspects of the movie, even if bioluminescence isn’t used quite the way it is on Earth.

There’s a great deal more detail on Pandora’s flora and fauna available in sources like the online encyclopaedia Pandorapedia. If you check it out you’ll find that Pandora’s invented biosphere has both intellectual and emotional depth.

Intellectually, the designers have given all their creations formal species names: thus the hometree species is Megalopedians giesei , Latin meaning the Great Tree. (And of course the tree has a Na’vi name, Kelutral .) This mirrors the biologists’ classification of life forms on Earth, which sorts out living things into hierarchies: you belong to a species, which belongs to a genus, which belongs to a family, which belongs to an order, which belongs to a class, which belongs to a phylum, which belongs to a kingdom. The five kingdoms, including animals, plants, fungi and bacteria, are at present the highest level of division; all living things on Earth are supposed to belong to one of them. Biologist Peter Ward has suggested that if we do ever discover life on another world we may need to extend the hierarchy upwards to include super-kingdoms, each covering all life on Earth, Mars, Titan, Pandora, the details depending on whether or not life on the different worlds is in any way related.

And emotionally, the designers have tried to give us a visual sense of the interconnectedness of the Pandoran biosphere. Think of the ubiquity of the touch response we see in many of Pandora’s creatures, such as the helicoradian, and the way the mosses on the tree branches and light up in response to Jake’s footsteps, like a Michael Jackson video. Everything reacts to everything else, everything is connected.

This quick tour of Pandora’s flora and fauna has shown us that some aspects of Pandoran life have parallels with Earth life—there are predators and prey, carnivores and herbivores—and some don’t have such parallels, such as the ubiquity of bioluminescence. But we’re in another star system here, on an entirely alien world. Why should life on Pandora have any similarities with life on Earth at all?

And why, indeed, is there life here in the first place? Pandora is evidently habitable. Was it necessary that it should be inhabited?