You might wonder what is the use of floating without the ability to control height or steer. I won't go into the details, but dispersal per se can be a virtue in the eyes of genes, especially for a creature which is basically sedentary. This applies a fortiori to plants: any patch of ground becomes uninhabitable from time to time, say when there is a forest fire or a flood. For a plant that needs lots of sunlight, the entire forest floor is uninhabitable except at intervals when a tree falls and breaks the shade. In general, any animal or plant will be descended from ancestors that lived somewhere else and they are likely to contain genes for taking steps to disperse somewhere — anywhere — else. This is why dandelion seeds have cottony puffs. This is why burrs have hooks to stick them on animal fur. This is why many insects drift abroad in the aerial plankton and rain down on unfamiliar ground.
The ease with which small animals can float suggests that we have only to assume that flying evolved originally in small animals, and the flying peak of Mount Improbable immediately looks less formidable. Very small insects float without wings at all. Slightly larger insects are helped by tiny wing stubs to catch the breeze, and we find ourselves on a neat, shallow ramp up Mount Improbable to proper wings. Actually, it may not have been as simple as that, according to some ingenious research by Joel Kingsolver and Mimi Koehl in the University of California at Berkeley. Kingsolver and Koehl worked on the theory that the first insect wings were pre-adapted to a completely different purpose, solar panels for heating. In those early days, of course, they wouldn't have flapped. They'd have just been little projections growing out from the thorax.
Kingsolver and Koehl's research technique was cunning. They made simple wooden models, based upon the earliest known fossil insects. Some of their models had no wings. Others had wing stubs of various lengths, many of them much too short to be recognizable as wings or to fly. The model insects themselves were of a range of {113} different sizes and they were tested in a wind tunnel to see how aerody-namically efficient they were. The models also had tiny thermometers inside them, to see how good they were at picking up artificial sunlight from a bright floodlamp.
In accordance with what we've already discussed, they found that really tiny insects float well enough with no wings at all. The slightly disconcerting result from the point of view of my simple ramp up Mount Improbable was that, at these very small sizes, small wings didn't seem to help aerodynamic efficiency. Wings didn't provide useful lift unless they were already of a substantial length. For model insects two centimetres in body length, wings as long as one body length produce significant lift. But wings of only twenty per cent of body length seem to do nothing for the animal at all. On the face of it, this looks like a precipice on Mount Improbable, because it seems to call for a single large mutation to jump the wings out to a substantial length, all in one go. It isn't a very formidable precipice, however, because of the following pair of additional facts.
In the first place, it is only for very small insects that you need relatively large wing stubs to get any aerodynamic benefit. If the insect is rather larger, tiny wing stubs do give some significant lift. At ten centimetres of body length, as you increase the wing stubs gradually from nothing, there is an immediate leap in aerodynamic benefit.
For our second supplementary fact, we revert to very small insect models. Here tiny wing stubs did prove to have an immediate thermal benefit. When tiny wings become slightly less tiny, they do not provide extra lift but they do become better solar panels. It seems that there is a smooth gradient of improvement in solar-panel performance when the insect body is very small. One millimetre stubs are better than none at all, two millimetre stubs are better than one, and so on. But the ‘and so on’ doesn't go on for ever. Beyond a certain length, further improvement in solar-panel performance tails off. It can be argued, therefore, that the solar-panel improvement gradient, on its own, couldn't get the stubs out to lengths where the aerodynamic function could take over. But Kingsolver and Koehl have a good solution to this. Once the stubs had evolved in small insects because of their solar-panel advantage, some insects evolved larger body {114} size for a quite different reason. There could be any number of reasons: it is very common for animals to evolve larger size as time goes by. Perhaps larger insects have an advantage because they are less likely to be eaten. If they grew, over evolutionary time, for whatever reason, it can be presumed that their solar-panel stubs would have grown with them, automatically. Now, a consequence of this general size increase is that the insects, stubs and all, would automatically be carried into the size range where aerodynamic benefits could take over and continue the steady push up Mount Improbable, albeit up a different slope towards a different peak.
It's hard to be sure that models in a wind tunnel really represent what went on in the Devonian era 400 million years ago. It may or may not be true that insect wings began as solar panels and weren't any good for flying until the whole body became bigger for some other reason. It could be that the real physics was different from the models’, and the growing stubs were increasingly good for flying right from the start. But Kingsolver and Koehl's research holds a very interesting lesson for us. It teaches us a subtle new way, a kind of sideways diversion, by which paths up Mount Improbable may be found.
For vertebrates the evolution of flight was probably a different story because they are mostly larger anyway. True powered flight has evolved independently in birds, bats (probably separately in at least two different kinds of bats) and pterosaurs. One possibility is that true flight grew out of the habit of gliding between trees, which lots of animals do, even if they don't quite fly. There is a whole world of life in the tree tops. We think of the forest as rising up from the ground. We see it from the vantage point of big, heavy, clumsy, ground-dwelling animals, as we pick our way among the trunks. For us, the deep forest is a cavernous, dark cathedral with arches and vaults stretching up from the ground to a remote green ceiling. But most of the inhabitants of the forest live in the canopy and see the forest from the opposite perspective. Their forest is a vast, gently undulating, sunlit green meadow which, though they hardly notice the fact, just happens to be raised up on stilts. Countless species of animals live their entire lives in this lofty meadow. The meadow is the place where the leaves are, and the leaves are there because that is {115} where the sunlight is, and the sunlight is the ultimate energy source of all life.
The landscape is not literally unbroken. The aerial meadow is pockmarked with holes where it is possible to fall through to the ground: gaps that need to be bridged. In their different ways, many kinds of animals are well equipped to leap across quite large gaps. The difference between a successful leap and an unsuccessful one could be a life and death matter. Any change in body shape that has the effect of extending the leaping range a little further — however little — could well be an advantage. The difference between a squirrel and a rat lies mostly in the tail. The tail is not a wing; you can't fly with it. But it is feathery with hairs that give it a large surface area to catch the air. A rat with a squirrel's tail would undoubtedly be able to leap a larger gap than a rat with a rat's tail. And, if the ancestors of squirrels had rat-like tails, there would be a continuous ramp of improvement, becoming more and more feathery, all the way to a modern squirrel's tail.
I described the squirrel's tail as feathery, but the word is even more appropriate to a totally unrelated little mammal, the feathertail glider (Figure 4.2). This is a marsupial, closer to possums and kangaroos than to rats and squirrels. It lives in the high canopy of the Australian eucalyptus forests. The tail is not, of course, a true feather, which, with its elaborate system of tiny hooks and barbs, is definitively a bird invention. But the feathertail glider's tail looks like a feather and it does a similar job to a feather.
The feathertail glider also has a flap of skin, stretching from elbow to knee, which is capable of extending its leap into a sixty-foot downhill glide. Another family of Australian possums, the flying pha-langers, has develo
ped the flap of skin further. In the greater glider, the membrane still only reaches the elbow, but it can nevertheless glide up to 300 feet, and change direction through as much as 90°. The yellow-bellied glider is even more accomplished in the air. Its gliding membrane reaches from wrists to ankles, as does that of the sugar glider, and the larger squirrel glider.
Almost identical in superficial appearance, although utterly unrelated, are the red giant flying squirrel of the Far Eastern forests, and {116}
Figure 4.2
Feathertail glider,
Acrobates pygmaeus,
a marsupial from
Australia.
the northern flying squirrel of North America. These are true squirrels — rodents — but, like the more extreme of the marsupial gliders, they have flaps of skin stretching from wrists to ankles. They glide about as capably as their marsupial equivalents. There are other rodents in Africa that have developed the same gliding trick. Although they are called Beecroft's flying squirrel and Zenker's flying squirrel, they are not true squirrels and they have certainly ‘invented’ gliding independently of the American flying squirrels. An even more comprehensive membrane, which takes in the neck and tail, and the fingers and toes, as well as the arms and legs, is possessed by the mysterious colugo of the Philippines forests. Nobody quite knows what this so-called flying lemur is, except that it isn't a lemur (true lemurs are confined to Madagascar, and none of them fly or glide although several of them leap impressively). Whatever else it may be, the colugo certainly isn't either a rodent or a marsupial. Once again, it has ‘invented’ the gliding membrane and associated habit entirely independently of all the others.
The colugo, the various flying squirrels and the marsupial gliders all glide with comparable efficiency. But, since the colugo's flight membrane stretches between the fingers, whereas in the others it {117} reaches only as far as the wrists, they could give rise to different kinds of wings if evolution continued further. Even more obviously, the same is true of the beautifully named Draco volans, the flying lizard or ‘flying dragon (Figure 4.3). This is a tree-dwelling lizard, also from the forests of the Philippines and Indonesia. Unlike the mammal gliders, its aerial flap does not incorporate the limbs but is stretched between its elongated ribs, which it can erect at will. My favourite of these gliding animals is Wallace's flying frog, a tree frog from the rain forests of South-East Asia. It keeps its flight surface skin between its elongated fingers and toes, and, like the other gliders we have been talking about, it uses it to glide from tree to tree.
In none of these cases is there any difficulty in finding a gentle path up Mount Improbable. Indeed, the fact that the gliding habit has evolved so many times testifies to the ease with which these mountain paths can be found. Perhaps even stronger testimony comes from the paradise tree snake or ‘flying snake’, again from the South-East Asian forests. This snake makes an effective job of gliding from tree to tree after deliberately throwing itself off, a distance of sixty feet or so, even though it has no obvious sail, flap or flight surface at all. It is just that a snake's drawn-out shape already gives it a relatively large surface area for its weight, and it enhances the effect by pulling in its belly to make a concave surface underneath. This snake would make a perfect first step towards subsequent evolution into something like Draco volans with a real gliding membrane. The snake never took the second step, perhaps because elongated ribs would have been an impediment in other aspects of its life.
The way to think of the gradual evolution of something like a flying squirrel is this. To begin with, an ancestor like an ordinary squirrel, living up trees but without any special gliding membrane, leaps across short gaps. However far it can leap without the aid of any special flaps of skin, it could leap a few inches further — and hence save its life when it encounters a gap of critical distance — if it had a very slight flap of skin, or a very slightly increased bushiness of the tail. So natural selection favours individuals with slightly pouchy skin around the arm or leg joints, and this becomes the norm. The normal leaping distance of an average member of the population has thereby {118}
Figure 4.3 Vertebrates that glide down from trees but do not truly fly: (clockwise from top right) colugo, Cynocephalus volans; flying lizard, Draco volans; Wallace s flying frog, Rhacophorus nigropalmatus; marsupial sugar glider, Petaurus breviceps; and flying snake, Chrysopeka paradisi. {119}
been increased by a few inches. Now, any individuals with an even larger skin web can leap a few inches further. So in later generations this extension of skin becomes the norm, and so on. For any given size of membrane, there exists a critical gap such that a marginal increase in the membrane makes all the difference between life and death. The membrane size of the average member of the population gets steadily larger, as the gap that can be jumped by the average member of the population gets larger. After many generations, species like the marsupial gliders and the flying squirrels have evolved, capable of gliding hundreds of feet, and capable of steering themselves into a controlled landing.
But all this gliding still isn't true flying. None of these gliding animals flaps its wings, and none of them can stay in the air indefinitely. They all go downhill, though they may pull up into a short climb by altering their aspect just before landing on a lower tree trunk. It is possible that true flying, as seen in bats, birds and pterosaurs, evolved from gliding ancestors like these. Most of these animals can control the direction and speed of their glide so as to land at a predetermined spot. It is easy to imagine true flapping flight evolving from repetition of the muscular movements used to control glide direction, so average time to landing is gradually postponed over evolutionary time.
Some biologists, however, prefer to see long-distance downhill gliding as the dead end of the tree-jumping line of evolution. True flight, they think, began on the ground rather than up trees. Man-made gliders can take off either by launching off cliffs, or by being pulled rapidly along the ground. Flying fish (Figure 4.4) take off in this second way, though from the sea rather than the land, and they are capable of gliding about the same distance as the best of the marsupial gliders launched from trees. Flying fish swim at great speed in the water and then shoot out into the air, presumably to the consternation of pursuing predators in the water from whose point of view they would vanish, more or less literally, into thin air. They don't hit the water again until they've travelled up to 300 feet. Sometimes when they come down they skim the water with their tails and swim a few strokes to regain speed and take off again. Their ‘wings’ are greatly enlarged pectoral fins and, in the case of the Atlantic flying fish, enlarged pelvic fins too. {120}
Figure 4.4 Animals that glide after shooting up off the surface. Atlantic flying fish, Cypselurus heterurus (top), and flying squid, Onychoteutbis.
These true flying fish (Exocoetidae) should not be confused (though they are by at least two books on my coffee table) with the completely unrelated so-called flying gurnards (Dactylopteridae). Far from flying, these gurnards plod along the bottom. They are variously reported to use their ‘wings’ as stabilizers, as frighteners for flashing at predators, and as stirrers-up of the sand to uncover prey; also, when the fish are disturbed, they rise up into the water a few feet above the bottom, then spread their ‘wings’ and glide down. The one thing the wings are not used for is flying in air. It is not clear what provoked the legend that they fly: possibly just the large size of their {121} pectoral fins which superficially resemble those of true flying fish. Returning to the true flying fish, they surely evolved not from bottom-dwelling ancestors but from fast-swimming surface fish. Many fish leap out of the water without the aid of enlarged fins. It would surely be potentially easy for such swift leapers to benefit a little by sticking out their fins, and in later generations to increase the area of their fins until they became ‘wings’. It is a little sad that dolphins, with their spectacular leaping, have never progressed to the flying-fish stage. Perhaps it is because they'd have to be smaller than existing dolph
ins in order to do it effectively and there are other reasons, connected with heat insulation and the properties of blubber, why it is hard for warm-blooded dolphins to be small. There are so-called flying squids which behave like flying fish as a means of escaping from the same enemies such as tuna. Squids of the genus Onychoteuthis accelerate through the water to speeds of up to forty-five miles per hour, shoot out into the air and glide for more than fifty yards, attaining heights of six feet or more out of the water. They achieve their astonishing speed by jet propulsion, and they fly stern first because, as in all squids, their water jets point past their heads. Once their supply of water has been squirted through the jet, they have no more propulsive force at their disposal until they return to the water. In this respect, flying fish have the advantage because of their habit, already mentioned, of boosting their speed with strokes of the tail while still largely out of the water and skimming the surface.
Fascinatingly, there is one group of fish, the freshwater hatchet-fishes of South American rivers, which are reported to vibrate their pectoral fins rapidly and noisily in true, powered flight through the air, although only for short distances. These fishes are not closely related to the true flying fish (nor to the ‘flying’ gurnards). I must say, I'd like to see a flying hatchet-fish buzzing past my own eyes. I'm not saying I don't believe it; it is agreed by all the books; but, as anglers know and as we learned from the story of the ‘flying’ gurnards, it is sometimes a good idea to check up on fish stories for oneself.
Climbing Mount Improbable Page 12