In any case, I introduced (gliding) flying fish as a prelude to the theory that true, flapping flight evolved not from tree gliders but {122} from fast-running, ground-dwelling animals whose arms became freed from their normal role in running. Flying fish and flying squids, although they live in water, illustrate the principle that if a gliding animal can move sufficiently fast along the surface it can take off without the support of a tree or cliff. The principle might work for birds, because they evolved from two-legged dinosaurs (indeed, you could say that technically birds are dinosaurs), some of whom probably ran very fast along the ground, as ostriches do today. To pursue the analogy with flying fish for a moment, the two legs would play the role of the fish's tail, propelling the animal forwards very fast, while the arms play the role of fins, perhaps originally used for stabilizing or steering, and later growing aerofoil surfaces. There are some mammals such as kangaroos that propel themselves very fast on two legs, leaving their arms free to evolve in other directions. Our species seems to be the only mammal to use the two legs in the alternating, bird-like gait, but we are not very fast and we use our arms, not for flying but for carrying things and making things. All the fast-running, twolegged mammals use the kangaroo gait in which the two legs push together rather than alternately. This gait grows naturally out of the horizontal spine-flexing of a typical running quadruped such as a dog. (By analogy, whales and dolphins swim by bending the spine up-and-down, mammal style, whereas fish and crocodiles swim by bending it alternately to left and to right, following the ancient fish habit. Incidentally, we should wonder more than we do at the unsung heroes among the mammal-like reptiles who pioneered the up-and-down gait that we now admire in sprinting cheetahs and greyhounds. Vestiges of the ancient fish wriggle are perhaps still to be seen in tail-wagging dogs, especially when the movement spills over to the whole body in the squirming of a submissive dog.)
Among ground-dwelling mammals, kangaroos and their marsupial kind don't have a monopoly of the ‘kangaroo gait’. My colleague Dr Stephen Cobb was once lecturing to zoology students in the University of Nairobi and he told them that wallabies are confined to Australia and New Guinea. ‘No, sir,’ a student protested. ‘I have seen one {123} in Kenya.’ What the student had seen was undoubtedly one of these (Figure 4.5).
This animal, the so-called springhaas or spring hare, is neither a hare nor a kangaroo but a rodent. Like kangaroos, it hops to increase its speed when fleeing from predators. Other rodents like the jumping jerboa do the same. But bipedal mammals don't seem to have taken the next step and evolved the power of flight. The only true flying mammals are bats, and their wing membrane incorporates the back legs as well as the arms. It is hard to see how such a leg-encumbering wing could have evolved by the fast-running route. The same is true of pterosaurs. My guess is that both bats and pterosaurs evolved flight by gliding downwards from trees or cliffs. Their ancestors, at one stage, might have looked a little like colugos.
Birds could be another matter. Their story is different anyway, centred around that wondrous device, the feather. Feathers are modified reptilian scales. It is possible that they originally evolved for a different purpose for which they are still very important, heat insulation. {124}
Figure 4.5 Spring hare,
Pedetes capensis.
At all events, they are made of a horny material which is capable of forming light, flat, flexible yet stiff flight surfaces. Bird wings are very unlike the saggy skin flaps of bats and pterosaurs. The ancestors of birds therefore were capable of forming a proper wing which didn't have to be stretched between bones. It was enough to have a bony arm at the front. The stiffness of the feathers themselves took care of the rest. The back legs could be left free to run. Far from being awkward and clumsy on the ground like bats and, presumably, pterosaurs, birds can use their legs for running, jumping, perching, climbing, prey-catching and fighting. Parrots even use their feet like human hands. Meanwhile the front limbs get on with the business of flying.
Here's one guess as to how flying got started in birds. The hypothetical ancestor, which we can imagine as a small, agile dinosaur, runs fast after insects, leaping in the air with its powerful hind legs and snapping at the prey. Insects had evolved into the air long before. A flying insect is perfectly capable of taking evasive action, and the leaping predator would benefit from skill in mid-course correction. To some extent you can see cats doing this today. It seems difficult because, since you are in the air, there is nothing solid to push against. The trick is to shift your centre of gravity. You can do this by moving bits of yourself relative to other bits. You could move your head or tail, but the obvious bits to move are the arms. Now, once the arms are being moved for this purpose, they become more effective at it if they develop surfaces to catch the air. It has also been suggested that the feathers on the arms originally developed as a kind of net for catching insects. This is not so far-fetched as it sounds, for some bats use their wings in this way. But, according to this theory, the most important use of the arms was for steerage and control. Some calculations suggest that the most appropriate arm movements for controlling pitching and rolling in a leap would actually resemble rudimentary flapping movements.
The running, jumping and mid-course correction theory, when compared with the tree-gliding theory, reverses the order of things. On the tree-gliding theory, the original role of the proto-wings was to provide lift. Only later were they used for control, and then finally {125} flapping. On the jumping for insects theory, control came first, and only later were the arms with their surfaces commandeered to provide lift. The beauty of this theory is that the same nervous circuits as were used to control the centre of gravity in the jumping ancestor would, rather effortlessly, have lent themselves to controlling the flight surfaces later in the evolutionary story. Perhaps birds began flying by leaping off the ground, while bats began by gliding out of trees. Or perhaps birds too began by gliding out of trees. The debate continues.
In any case modern birds have come a long way since those early days. Lots of long ways I should say, for the peaks of Mount Improbable that they have conquered are splendidly many. A peregrine falcon can dive out of the sky at more than 100 miles per hour when closing in on prey. Hawks and humming-birds hover with pinpoint precision in one spot like a helicopters wildest dream. Arctic tern* spend more than half of every year on their annual migration from Arctic to Antarctic and back again, a distance of 12,000 miles. The wandering albatross, slung below its ten-foot wing-span, circles the pole with an ever clockwise heading, powered not by flapping but by vigilant attention to the natural engine of changing wind speed whert the cold waves shear the Roaring Forties. Some birds, like pheasant! and peacocks, use flight only in an occasional explosive burst when startled by possible danger. Others, like ostriches, rheas and the lamented giant moas of New Zealand, became too big to fly and their wings degenerated in comparison to their huge, striding and kicking legs. At the other extreme, swifts have feeble, clumsy legs but state-of-the-art swept-back wings and they almost never leave the air. They land only to nest, even mating and sleeping on the wing. When they do land they must choose a high place, for they cannot take off from level ground. They build their nests from materials that they meet floating in the air, or snatch from trees while screaming past. For § swift, coming to earth may seem like a difficult, unnatural state, com* parable to, say, sky-diving for a human, or swimming underwater. For us, the world is a steady, motionless backdrop to our preoccupation!. Through a swifts black eyes, the normal, background state of fhf world is a ceaselessly hurtling horizon, dizzily tilting by. Our term {126} firma may be the swift's idea of an unnerving Disneyland roller-coaster.
Not all birds flap their wings, but those that soar or glide have probably come from flapping ancestors. Flapping flight is complicated and not, in every detail, understood. It is tempting to think that the powerful downward beats of the wings provide lift directly. This may be a part of the story, especially during take-off, but most of t
he lift is provided by the shape of the wings (given enough air speed), as in an aeroplane. A specially curved or tilted wing can provide lift if a wind is blown over it or — which amounts to the same thing — if the whole bird is moving forwards relative to the air, for whatever reason. The flapping movement of the wings is mainly concerned with providing the necessary forward thrust. This propeller role of the wings relies upon the fact that they don't simply flap up and down. Instead, the bird imparts an artful twist from the shoulder together with subtle adjustments at all the joints, and some other benefits follow automatically from the bending of the feathers. As a result of these twists, adjustments and bendings, the up-and-down stroking movements of the wings are translated into forward thrust, slightly in the manner of a whale's up-and-down beating tail. Given that there is forward movement through the air, the wings of a bird provide lift in approximately the same way as those of an aeroplane, albeit aeroplane wings are simpler because fixed. The higher the speed, the greater the lift, which is why a Boeing 747 stays airborne in spite of its Brobdingnagian weight.
The laws of physics conspire to make flapping flight increasingly difficult for larger birds. If we think of birds of the same shape getting progressively larger, weight goes up as the cube of length but wing area only goes up as the square of length. In order to stay in the tir, larger birds would need to grow disproportionately larger wings, tnd/or fly disproportionately faster. As we imagine ever larger and larger birds, there comes a point where, lacking jet or piston engines, the muscle power available to a bird of that size is no longer strong tnough to keep it in the air. This critical point in the size range is lomewhat smaller than the larger vultures and albatrosses. Some large birds, as we have seen, simply gave up the struggle, grounded {127} themselves for good, and made a nice living getting even larger as ostriches and emus. But vultures, condors, eagles and albatrosses are not grounded. Why not?
Their trick is to exploit external energy sources. If it weren't for the sun's heat and the moon's shifting gravity, air and sea would be still. External energy charges up the ocean currents, pumps up the winds, stirs the dust-devils, rocks the atmosphere with powerful forces capable of flattening a house or driving a trade-route; it also engenders thermal up-currents which, if you use them judiciously, can raise you to the clouds. Vultures, eagles and albatrosses use them to perfection. They may be the only animals to match our skill in mining the energy of the weather. My main source of information on soaring birds is the writings of Dr Colin Pennycuick of Bristol University. He used his specialized knowledge as a glider pilot, both to understand how birds do it and to glide among them in order to study their techniques in the field.
Vultures and eagles use thermals, just as human glider pilots do. A thermal is a column of warm air that rises, perhaps because a patch of ground at its base soaks up more than its share of the sunlight. Glider pilots are largely dependent on thermals, and experience makes them expert in spotting them from a distance. The subtle cues that betray a thermals presence include certain characteristic shapes of cumulus cloud at their tops, and certain conformations of ground at their base. The approved technique for cross-country gliding is to circle your way up to the top of a thermal, say a mile high, then launch out in a straight downward glide in the direction you want to travel. The ramp down is a shallow one: a vulture typically loses one yard of height for every ten yards of forward travel. This gives it nearly ten miles of cross-country travel before it needs to find another thermal and rack itself up to the top again.
As it happens, thermals are often arranged in ‘streets’, which a glider pilot can see ahead by reading the clouds. Vultures too, like human glider pilots, are adept at following these streets. Sometimes, when a vulture finds a good street lined up in the direction it wants to go, it glides along the street gaining lift from each thermal without bothering to circle in it. In this way, a vulture can travel great {128} distances without pausing to circle. They usually do this only when commuting from feeding grounds to nesting site. Most of the time vultures are not travelling long distances in straight lines but cruising around in search of carrion. They also keep an eye on other vultures. If any one finds a carcass and goes down, others notice and quickly join it. In this way, a wave of attention spreads across the sky, like the wave of fires lit on hilltops that spread the length and breadth of England to warn of the Spanish Armada.
A similar trick of watching companions is used by white storks for a different purpose in their long annual migration from northern Europe to southern Africa. They travel in bands of up to some hundreds. Like vultures, they wind themselves up to the tops of thermals, then head off across country until they find another thermal. But although they circle together in a thermal, when they leave it, instead of travelling in a close-formation flock they spread out in line abreast. With such a broad front advance, if they just glide straight some of them are very likely to find another thermal. When they do, their neighbours in the line notice them rising, and come to join them. In this way, all the members of the extended group benefit from thermals that any members find.
Whichever view we take of the origin of flight in birds, whether the tree-gliding theory or the running and leaping theory, vultures and eagles, storks and albatrosses, are almost certainly secondary gliders. They evolved their gliding technique from ancestors that flapped and were smaller. For the school of thought that sees bird flight as originating in gliding from trees, modern vultures — albeit they soar up thermals rather than climb trees to gain height — would represent a return to gliding via an intermediate stage of flapping. During this intermediate flapping stage their nervous systems would, on this theory, have acquired new circuitry and new skills of control and manoeuvre. These new skills would have left them with improved efficiency when they returned to non-flapping flight. It is quite common for animals to return to a much older way of life, having served an evolutionary apprenticeship in another one, and it may be plausible to argue that they return from the apprenticeship better equipped to tackle the original way of life. Soaring birds may not be a good {129} example, because it is uncertain how bird flight originally began. A more clear-cut example of animals returning to an earlier way of life is provided by those that have returned to the water, having spent some millions of years on land. It is to these that I now turn, as a coda to this chapter (Figure 4.6).
Fifty million years ago, the ancestors of whales and sea-cows (dug-ongs and manatees) were land-dwelling mammals, probably carnivores in the case of whales, herbivores in the case of sea-cows. The ancestors of these and all other land-dwelling mammals had been, much earlier still, sea-dwelling fish. The return to the water by the whales and dug-ongs was a homecoming. As always, we can be sure that it took place gradually. They took to the water, perhaps at first just to feed, like a modern otter. They must have spent progressively less and less time on the land, perhaps going through a phase where they resembled modern seals. Now they never leave the water and are completely helpless if beached. Nevertheless, they bear numerous reminders of their land-dwelling ancestors and they also, like all mammals, have much older relics of their previous incarnation in the water. Whales breathe air, for their landlubber ancestors lost the use of their gills. But all mammals, whales and sea-cows included, have traces of gills in the embryo: unmistakable vestiges of their remote past in the water. Freshwater snails, too, have gone back to water from the land, and they breathe air. Their earlier ancestors lived in the sea, like most of the snail family today. Snails seem to have gone from sea to fresh water via a land ‘bridge’: perhaps something about land life eases the transition. Other land animals that have gone back to the water include turtles, water beetles, diving-bell spiders, and the extinct ichthyosaurs and plesiosaurs. Turtles do manage to extract some oxygen from the water, but they do it not with gills but with the lining of the mouth, and in some cases the lining of the rectum and, in soft-shelled turtles, the skm covering the shell. Water beetles and spiders take a bubble of air down with
them. All these animals are returning to the watery environment of their more remote ancestors, but when they get there they do things differently because of their interim race experience.
When land animals return to the water, why don't they rediscover the full apparatus of watery living? Why don't whales and sea-cows {130}
Figure 4.6 Whales and sea-cows. Animals that have returned to the sea after hundreds of millions of years on land: (from top) dugong, Dugong dugon; manatee, Trihechus senegalensis; humpback whale, Megaptera novaeangliae; killer whale, Orcinus orca. {131}
regrow gills and lose their lungs? This brings us to another important lesson that Mount Improbable has to teach us. In evolution, ideal outcomes are not the only consideration. It also makes a difference where you start: as in the story of the man who, when asked the way to Dublin, replied, ‘Well, I wouldn't start from here.’ Mount Improbable has many peaks. There are many ways of living in water. You can use gills to get oxygen from the water, or you can come to the surface and breathe air. Continually coming to the surface seems like an oddly inconvenient habit. Maybe it is, but the ancestors of whales and sea-cows began, remember, close to an air-breathing peak of the mountain. All their internal details were geared to air-breathing assumptions. Perhaps they could have reformed them and come into line with the fishes, dusting off the embryonic vestiges of their ancient gills. But that would have meant a massive shake-up of their bodily infrastructure. It would have been equivalent to going down a deep valley between two peaks of the mountain, with the ultimate objective of climbing a slightly higher peak eventually. It cannot be said too often that Darwinian theory does not allow for getting temporarily worse in quest of a long-term goal.