These centauroid mammals look nothing like Earth's marsupial "flying foxes." Picture a red fox with hawk-wings stuck on (the middle limb-pair, modified). All four non-winged limbs have handlike paws. These pack animals have light but extremely efficient brains.

Habitat: open woods and canyons, hilly veldt and deserts (including the deep Tsud)--wherever isolated trees or cliff-caves provide safe nests. Flying foxes are hunters preferring meat but will eat fruit and nuts as well.

Flying foxes are appealing creatures. Several Serranian peoples make pets of them, finding them useful for messages and hunting and retrieval. With four clever paws, they're as handy as the Wicked Witch of the West's flying monkeys. And as clever--they may be as intelligent as chimps.

But wild or domestic, flying foxes could be trapped in an evolutionary dead end. Fox brains are huge for their body-size, burning a lot of calories and giving them huge appetites. A bigger body would let the brain grow; but foxes are near the maximum flight-weight practical in Serrana's thin air. Unlike woodogs, foxes can't grow much heavier.

ON THIN AIR

We forget that Earth is an extreme environment in one respect. Of the five big worlds in our solar system with comparable surface gravities (Saturn, Uranus, Neptune, Venus and Earth) our atmosphere's the thinnest--by far. Even little Titan has more! What would Earth be, if we had, say, double or ten times our current atmospheric pressure? Hotter, yes, but fatally? Earth's survived hot times before. Venus did bake to death, but it had twice our insolation and ONE MILLION times our CO2 level. Of course, life (or at least liquid oceans) absorbed much of our primal atmosphere--CO2 dissolves in water. Whatever the cause, today we have thin air for a world our size. And it's far easier to imagine Earthlike worlds with thick air than thin. Life could flourish at ten atmospheres, but at one-tenth? Shades of Mars!

Does it matter? Does air pressure affect the evolution of intelligence? Yes. On thin-aired worlds, the weight limit for flying creatures means that intelligence is limited to swimmers and walkers. Pound for pound, the smartest brains on Earth are avian, but while it's probably possible to bioengineer a flying person--whether a pygmy angel or a giant raven or parrot is no matter--still, such forms are unlikely to evolve in the wild. They're just too close to Earth's weight limit for practical flight, currently about 20 kilos. The fact that Earth birds are as smart as they are suggests that life in the air stimulates the mind: 3D acrobatics, the sheer stimulation of travel between varied ecozones, the need to memorize landmarks and navigate intelligently, and especially the stimulation of social contact over long distances. I'd add one less obvious feature of life on the wing. "Birdwatching" is popular among mammals (not just humans--think of cats!) because birds are uniquely visible. They can afford to be--on the wing, nonfliers just can't attack them. But the reverse is also true--so true we hardly notice that birds are mammal-watchers. They constantly observe life below them--not just for food or safe perches, but for sheer entertainment. Birds are voyeurs. We're spread out below them; they witness the survival strategies of many species. Intelligent birds learn from those strategies. Flying, in short, is educational. Such a rich information environment favors clever, social, observant, opportunistic generalists, and that's just what parrots and corvids are--up to the limit air-pressure imposes!

I conclude that thick-aired worlds will inevitably have intelligent fliers. The, uh, pressure is there to shape them. (Two world-models with thick air and mostly winged life: a terraformed version of Venus and an oceanic giant called Lyr--the latter includes a people called floxes, much like Serrana's flying foxes but more intelligent, precisely because the air's thicker and they faced a looser weight limit). Conversely, thin-aired worlds like Earth and Serrana have harsh flight-weight restrictions, and their fliers won't reach sentience.

Or will they? Serrana's gravity is lower, too. I've assumed the two factors weigh about equally, so they cancel out, keeping the maximum flight-weight close to Earth's. But flight mechanics in the abstract don't matter much to creatures who have to live on the wing. Earth's weight limit may really not be the maximum for practical flight, but for calorie-efficient foodgathering, or for quick escape from charging predators, or for carrying prey. We don't know enough yet to be sure large fliers couldn't be viable under modestly different conditions. Pterosaurs certainly got MUCH bigger. Was the air richer in oxygen before the K-T impact? Could the actual pressure have been higher? Or was the biosphere just so rich that food was easy to collect?

For whatever reason, big fliers prospered quite recently on Earth. Why not Serrana?

After all, a few Earth birds (migrating geese) have been seen flying above Mt Everest, where the air pressure is barely 1/4 atmosphere--that's comparable to Serrana's air pressure in mountains and highest planos. Bird aerodynamics have wide safety margins! In Serrana's lower gravity, perhaps there's enough flex in the weight-limit so that if intelligence became a premium, foxes would indeed develop a brain triple the weight of a raven or parrot brain. If equally efficient, that might be just enough... eventually.

And Serrana, with its dimmer, long-lived little sun, has time. Billions of extra years of it!

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