FLIGHT. Birds, bats and bugs do it But how? We take an in-depth look at nature s aeronautical engineers. STORY BY PETER MEREDITH The miracle of

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1 STORY BY PETER MEREDITH The miracle of FLIGHT Birds, bats and bugs do it But how? We take an in-depth look at nature s aeronautical engineers. Smooth mover. A sulphur-crested cockatoo performs a full flapping cycle, from the beginning of the downstroke through the upstroke and back to the starting point. The forward sweep of the wings during the downstroke in the first three images helps extract maximum lifting force from the wings aerofoil shape. On the upstroke in the fourth and fifth images the wing moves up and back. 66 A u s t r a l i a n G e o g r a p h i c

2 HERE S A THING: the humble locust can fly hundreds of kilometres without refuelling. Five hundred kilometres is not an unusual jaunt, and locusts have even been known to cross the Atlantic, admittedly with the help of a storm or two. Aeronautical engineer Dr John Young tosses me these tidbits as we chat in his office at the Australian Defence Force Academy in Canberra. John, who, aside from researching insect flight, has worked on flight-test programs for F/A-18 fighters, can t suppress his admiration: I m in awe of these animals, being able to do the things they do. It s fiendishly clever! Clever is one word for it. I prefer miraculous. To me, flapping flight in animals birds, bats and insects is nothing less than pure magic, one of nature s most amazing tricks. Yet it happens every day and we don t give it a second thought. Maybe this is because the animals make it look so easy. They just flap their wings and off they go. Nothing to it. Don t be fooled. When you start trying to unpick this exquisitely graceful and superficially simple act of locomotion, it turns out to be ingenious beyond imagining. Scientists have been probing flapping flight (as opposed to gliding) for years, but still haven t entirely worked it out. LEFT: STEPHEN DALTON/ Danaus plexippus RIGHT: JOSEPH FEIL/CORBIS/Chortoicetes terminifera PREVIOUS PAGE: STEPHEN DALTON/ Cacatua galerita SO HOW DOES IT WORK? Look first at the wings. As we do with aircraft, flying creatures use wings to get them into the air and keep them there. Most wings, but not all, have a slightly curved upper surface and a less curved or flat lower surface. They are mostly rounded at the front (the leading edge) and taper to a sharp rear (the trailing edge). The study of how air moves around a solid object like a wing is termed aerodynamics. Wings have an amazing aerodynamic property. If you propel a wing horizontally through the air, it generates an upward force called lift. For flight to be possible, lift on the wings must overcome the gravitational force pulling the aircraft or animal down. Nearly all aircraft other than helicopters have fixed wings. Unless they re attached to an unpowered aircraft like a glider, fixed wings are pushed through the air by engines that provide forward thrust. A flying animal lacks this separate source of thrust: its wings must support the animal s weight and provide thrust simultaneously. Although a flapping animal wing may appear to move up and down vertically, it doesn t. On the downstroke, the wing actually sweeps forward as well as downward. This forward movement is one of the keys to animal flight. The other is the wing s remarkable ability to twist along its length during the stroke and, in this way, to angle the leading edge Peter Meredith is a former ag associate editor. His last story was Crowded house, on population growth (AG 100). Wing power. Many insects, including the Australian plague locust (above), can fly vast distances non-stop on wings that appear to defy the rules of conventional aerodynamics. One of the secrets of flight in insects, birds and bats is their wings remarkable capacity to twist, as shown in the middle shot of a flapping monarch butterfly (left). slightly downward, especially nearer the wingtip. This causes the lift to be directed forward rather than upward. In other words, towards the outer end of the wing, some lift becomes thrust and drives the animal forward. The downstroke provides most of a bird or bat wing s lift and thrust. For the upstroke, the animal may partly flex, or fold, its wings. Even so, all insects and most birds and bats can generate useful aerodynamic forces on the upstroke too. A flying animal generates lift even when it s flying without flapping, an activity we call gliding. In the case of both an animal and a gliding aircraft, the thrust that drives the wings forward is provided by loss of altitude, since the animal or aircraft must glide downward at a slight inclination in order to achieve enough forward motion to create lift. During our planet s history, powered animal flight has emerged in four animal groups birds, bats, insects and extinct flying reptiles called pterosaurs. How and why these four developed wings and took to the air are matters of heated debate among scientists. There is general agreement, though, that wings did J a n u a r y F e b r u a r y

3 Flying colours. Climbing steeply towards its nest site, a common kingfisher (above) deploys a winglet known as an alula on the leading edge of each wing. Scientists believe the alula maintains a smooth airflow over the wing at crucial moments or generates lift-enhancing vortices. Fourwinged insects like the green lacewing (right) and the large red damselfly (opposite) add another layer of complexity their fore and hind wings interact. not develop specifically as flying devices but originally were appendages with other functions. In the case of the dinosaur ancestors of birds, feathered forelimbs may have been used for display or to help with running and leaping. At some point, by coincidence or random chance, the form of a given structure allows the animal to do something new with it, says David E. Alexander, assistant professor of entomology at the University of Kansas, in his book Nature s Flyers. If the new function is advantageous, natural selection will act to refine and improve it. But why fly? Biologist Dr Bret Tobalske, director of the Flight Laboratory at the University of Montana, says, The diversity of creatures that fly indicates that being able to move quickly at a low cost of transport is a terrific strategy for exploiting untapped niches through migration and otherwise making a living. Powered flight can be up to 10 times more fuelefficient than walking or running. As for speed, flying animals can move through the air times faster than an animal of similar size moves on the ground. The fastest... bird is the PEREGRINE FALCON, which can reach 300km/h in a dive. The fastest in flapping flight is probably the WHITE- THROATED NEEDLE-TAILED SWIFT, clocked at 170km/h. Flight s survival benefits are reflected in the fact that three-quarters of warm-blooded vertebrate species fly, an astounding figure. Of 13,000 species, about 10,000 (comprising some 9000 birds and 1000 bats) are flyers. But insects outclass them all, being the most diverse animals on earth and having colonised nearly all environments. Most insects fly at some stage in their development, says Professor Geoff Spedding, chairman of aerospace and mechanical engineering at the University of Southern California. Since insects comprise the vast majority of species on earth, flight is actually the most common form of locomotion, and we humans are in a small minority in being unable to do it without the assistance of elaborate or powerful mechanical devices. OF ALL FLYING creatures, insects possess the most distinctive body structures for flight, from unique muscle systems to ridged or corrugated wings that are as far from sleek aircraft wings as you can get. In insects, aerodynamics moves into a whole new dimension. Continued page 79 KINGFISHER/LACEWING/DAMSELFLY: STEPHEN DALTON/ atthis/chrysopa perla /Pyrrhosoma nymphula FALCON: GETTY/Falco peregrinus ; SWIFT: ROHAN CLARKE/Hirandapus caudacutus 68 A u s t r a l i a n G e o g r a p h i c J a n u a r y F e b r u a r y

4 BIRDS Birds are the marathon flyers among aerial animals. The wandering albatross can circumnavigate the globe in less than seven weeks with hardly a flap; the Arctic tern travels more than 70,000km every year from the Arctic to Antarctica and back; and even that familiar denizen of the old gum tree, the laughing kookaburra (below), employs sophisticated aerodynamics to achieve flight. ANATOMY FINELY TUNED Two essentials for super-efficient flight are lightness and great strength. To this end, avian body components have been pared down to the minimum. BRAIN The relatively large avian brain coordinates all of the muscular activity needed for flight. It also has a welldeveloped optic lobe as vision is vital for survival. WINGS OUT ON A LIMB The bird wing is a modified forelimb that supports feathers. It has a short, stout upper arm bone (humerus) and fused palm and finger bones. FEATHERS Strong primary feathers generate thrust in flight; secondaries, tertials and coverts combine to supply lift. FEATHERS A MATTER OF SCALE Feathers, modified reptilian scales, are unique to birds and their dinosaur ancestors. They are the most complex epidermal structure found in vertebrates. Different kinds of feathers perform different functions, such as flight, insulation, display and maintaining a streamlined body shape. As insulation, they are more efficient than fur. Feathers may have first appeared on dinosaurs 100 million years before birds evolved. WING SLOTS These are gaps between the primary flight feathers at the tips of wings. Experts aren t entirely sure what they do. Each feather may act as a miniature wing in its own right, while slots may make the wing more stable under some conditions. BEAK LUNGS A lightweight beak of bone and keratin has replaced heavy jawbones and teeth. A bird s breathing apparatus includes small but efficient lungs linked to air sacs that act as bellows. This mechanism delivers enough oxygen for flight even at high altitudes. EYES Magnetic receptors in the head and eye of a bird can detect the Earth s magnetic field and aid in navigation. WING DYNAMICS SKELETON Bones are porous and air-filled, but marvellously robust. The skeleton centres on the sternum, or breastbone, which has a large keel and anchors the powerful flight muscles. The collarbones are fused and form the wishbone, or furcula, which links the shoulder joints and acts as a spring or brace. PEACOCK DUCK PARROT RED-CRESTED TURACO AERODYNAMICS A wing s lengthwise twist and its forward movement during the downstroke are the key to flapping flight. Aircraft wings and bird wings create lift the same way: air passing over the top moves faster than air passing below. This generates low pressure above and high pressure underneath, sucking the wing upwards. EVOLUTION KEEL PECTORAL MUSCLE Birds evolved some 150 million years ago from bipedal theropod dinosaurs that already had feathers and wishbones. Feathers may have been for insulation or display, and larger feathers on SUPRACORACOIDEUS MUSCLE STERNUM FROM THE GROUND TO THE AIR front limbs may have helped with locomotion. How those dinosaurs became fliers is disputed. Did they begin by gliding from trees? Did they leap ever higher from the ground until they MUSCLES Two pairs of flight muscles power the wings. The largest, the pectoralis muscles, pull the wings down. The smaller supracoracoideus muscles raise the wing, acting through an ingenious pulley system. became truly airborne? Did they swoop on prey from heights? Did they flap their proto-wings to help them run up steep inclines, as some birds do today? Maybe they did all four at different times. SOURCES: DR JOHN YOUNG/ DAVID E. ALEXANDER, 2004, NATURE S FLYERS: BIRDS, INSECTS, AND THE BIOMECHANICS OF FLIGHT GRAPHIC: JEFF GOERTZEN/DAN SHERIDAN AIRCRAFT WING BIRD WING Albatrosses have long, narrow wings for gliding and soaring in strong winds. Eagles, vultures and other large birds soar on long, broad wings in thermal upcurrents. Such wings have slotted tips. Short, broad, rounded wings are good for manoeuvring in tight spaces, such as dense vegetation. Narrow, pointed wings enable birds such as swifts, falcons and ducks to fly fast. SWIFT ALBATROSS EAGLE HAWK LIFT DOWNSTROKE The wing sweeps strongly forward as well as downward. The wing s twist ensures that the leading edge is tilted progressively downward in the outer portion of the wing. Thus, close to the body, the wing generates lift that supports the animal s weight and nearer the tip the lift provides thrust. UPSTROKE The wing may be flexed to some degree and the primary feathers may separate to allow air through. Even so, some birds generate useful aerodynamic forces on the upstroke.

5 INSECTS Insects were the first creatures to fly and had many millions of years to get ahead of the competition. It shows. No bird or bat can match the aerobatic virtuosity of a dragonfly, such as the one below. Flapping two pairs of wings sometimes synchronously, sometimes out of phase with one other a dragonfly can reach 60km/h, fly backwards, glide, hover and pivot on the spot in mid-air. ANATOMY WINGS VARIATIONS ON A THEME MUSCLE DYNAMICS NEOPTERA Dragonflies are part of the Palaeoptera group, one of two branches of the insect family. Palaeoptera have wings they keep extended when at rest. As well as dragonflies, this group includes mayflies and damselflies. All other flying insects are in the Neoptera group and have wings that fold flat against their abdomens. PALAEOPTERA Insect wings come in myriad shapes, sizes, arrangements and combinations but all are variations on a single theme. Recent research has also shown that insect wings have a dazzling array of iridescent colours and patterns, barely visible to the human eye. SECOND WING Some insects have two wings, others four. In the latter case, the wings on one side may flap synchronously or independently. In some four-winged species, such as bees, the fore and hind wings overlap and may be coupled by hooks. DIRECT FLIGHT MUSCLES TWO KINDS OF MOVEMENT Palaeoptera have two pairs of muscles that attach directly to the wing base, which pivots on a hinge. HINGE INDIRECT FLIGHT MUSCLES In Neoptera, the flight muscles act not on the wings but on the exoskeleton of the thorax. EXOSKELETON MUSCLES EXOSKELETON An insect s body has a hard outer casing (an exoskeleton) made of chitin. This exoskeleton provides the structural rigidity supplied by a bony internal skeleton in other animals. WING CROSS-SECTION MEMBRANE An insect wing consists of two very thin, compressed layers of chitin supported by a network of chitinous veins. MUSCLES One set of muscles pulls from front to back, compressing the thorax so that it bulges at the top and bottom, flipping the wings down. Another set of muscles, acting vertically, pulls the top and bottom of the thorax closer together again, causing the wings to flip up. EVOLUTION FROM GLIDERS TO FLAPPERS Insects were airborne more than 340 million years ago. How they developed wings is a mystery. Biologists think they sprouted outgrowths or appendages on the thorax that helped them to BREATHING LEV LOW PRESSURE ZONE Insects don t have lungs. Instead, oxygen reaches the flight muscles and other tissues via a network of internal tubes called spiracles. These are open to the outside air through the surface of the exoskeleton. In highly active insects, body movements pump air through the system. In less active insects, air diffuses passively through the body. parachute or glide when falling from heights but originally had some other use. Gradually these outgrowths became moveable, allowing some aerial manoeuvring, and eventually they became flappable. SOURCES: DR JOHN YOUNG/DR MICHAEL DICKINSON/ AMATEUR ENTOMOLOGISTS SOCIETY/ DAVID E. ALEXANDER, 2004, NATURE S FLYERS: BIRDS, INSECTS, AND THE BIOMECHANICS OF FLIGHT (THE JOHNS HOPKINS UNI- VERSITY PRESS) GRAPHIC: JEFF GOERTZEN/DAN SHERIDAN CRINKLY WING LEADING-EDGE VORTICES HOW ARE THEY USED? VEINS LEADING EDGE Veins strengthen the wing and form characteristic patterns. They also create ridges and troughs, often resulting in a crinkly aerofoil cross-section. They stiffen the wing lengthwise, but allow it to twist during flapping to an extraordinary degree, which greatly assists flight. AIR VORTEX These spiralling air currents are created by the movement of an insect s wings and they generate far more lift than smooth or steady air flow over the wing. Rather than relying on a steady flow of air over wings to generate lift, insects use unsteady effects such as vortices that swirl along the top of the wing just behind the leading edge (above). These are called leading-edge vortices (LEVs). Vortices have low pressure at their core, creating lift by sucking the wing upwards LIFT BOTH WAYS Flipping over between strokes, insect wings generate lift on both the downstroke and upstroke. They can also run into the wake vortices shed by the previous stroke, getting a further lift boost. This is known as wake capture. Wing direction Starting vortex LEV New vortex DOWNSTROKE In the downstroke, the insect wing has established a leadingedge vortex (LEV) as well as what s known as a starting vortex, which spins off the trailing edge. Only the LEV creates lift. RETURN STROKE The wing has reached the end of the downstroke and is beginning to reverse direction for the return stroke, the upstroke. A new LEV is forming on what will be the wing s upper surface and a new starting vortex is beginning to form off the trailing edge. The LEV and starting vortex from the just-completed downstroke have separated from the wing. UPSTROKE Old vortex Wing angle The wing is into the upstroke. Both the LEV and the starting vortex from the downstroke are floating free. The returning wing runs into them on the way back and gains extra lift from them.

6 BATS Bats, such as the ghost bat below, are the only mammals to have mastered flapping flight. The basic body plan has been radically altered for this purpose. As Dr Sue Hand, a bat specialist at the University of NSW, says, Unlike birds, which have retained their back legs for walking, bats have gone the whole hog. All four limbs are seriously involved in flight. They ve made the commitment. FUR A bat is covered in fur, which is not as efficient an insulator as feathers. Hairs on the wing act as sensors, monitoring air flow. ANATOMY MODIFIED MECHANICS There are many similarities among human, bird and bat forearms. They all have the same types of bones in their forearms and hands: the humerus, ulna, radius, carpals, metacarpals and phalanges. SKIN MUSCLES WINGS LIGHT-FINGERED FLAPPERS A bat s wing consists of an arm and four elongated finger bones that support a membrane of skin that grows from the body. Wings act effectively to cool bats while in the air. When the animals are at rest in cold weather, they wrap their wings around themselves for warmth. BIGGEST AND SMALLEST GIANT GOLDEN-CROWNED FLYING-FOX 150CM CHEST MUSCLES Large pectoral muscles power the bat s downstroke, and deltoid muscles on the bat s shoulders pull the wings back up. A total of 17 different muscles control the wing s movement, tension and shape. THUMB Tiny muscles can control the tension of the skin to adapt to the current airspeed. MEMBRANE BUMBLEBEE BAT 15CM HUMERUS FOREARM The old division between megabats (flying-foxes, fruit bats) and microbats (small, echolocating bats) has been abolished and all bats are now lumped together under the Chiroptera grouping. BLOOD VESSELS FLIGHT DYNAMICS HEART A bat s heart can be three times larger than the hearts of comparable mammals. From a resting rate of 20 beats per minute, it can hit 1000 per minute in energetic flight. Bat lungs are as efficient at extracting oxygen from the air as bird lungs, though they don t operate like bellows in the same way. Bat wings are rich in blood vessels. A system of one-way valves in the vessels prevents the blood from pooling at the ends of the wings during flight. MEMBRANE The wing membrane attaches to the hind limb and a separate membrane links the legs and tail vertebrae (though the latter are absent in some species). FLEXIBLE FINGERS The highly elastic materials of its wing, as well as tiny muscles in the membrane, allow a bat to radically alter wing shape during flight. Because a bat can vary the wing s camber its curved aerodynamic profile it can modify the wing s performance in a more subtle way than a bird. This helps make the bat highly manoeuvrable at low speeds. SHORT OR LONG? Long, narrow wings, technically described as having a high aspect ratio (AR), enable some bats to fly efficiently over great distances, either in search of food or to migrate. Short, broad wings with low AR allow for slow flight and great manoeuvrability in confined spaces, such as dense forest; some bats can perform a 180 turn within the length of a wingspan. The finger bones bend easily, allowing the wing to change shape during flight. DOWNSTROKE EVOLUTION CHEST Bats lack the large keeled sternum of birds but fly effectively without it. A bat s digestive system works rapidly, minimising the time it has to fly with a heavy load of food. A MYSTERIOUS PEDIGREE Bats have been around for some 60 million years. Biologists agree they evolved from gliding mammals that gradually became more skilled at controlling their wing membranes in flight until CALCAR they were able to flap. An unsolved mystery is whether bats, which use echolocation to navigate and hunt in the dark, developed that or flight first. Lacking a good fossil record, biologists first A unique bone called the calcar projects inward from the heel to support the tail membrane. BONES To reduce weight, the bones of bats are thinner and lighter than those of most mammals, but aren t porous, as they are in birds. thought bats were related to the colugo, a gliding mammal. But molecular genetics now place bats in a group that includes carnivores, rodents, llamas, pangolins and even whales. SOURCES: DR SUE HAND/DR JOHN YOUNG/DR JAMES DALE SMITH/F. T. MUIJRES, ET AL., 2008, LEADING-EDGE VORTEX IMPROVES LIFT IN SLOW-FLYING BATS, SCIENCE, V. 319, P1250. GRAPHIC: JEFF GOERTZEN/DAN SHERIDAN VORTEX PROFILE OF WING LEV AIR FLOW Small bats generate aerodynamic lift forces such as leading-edge vortices. In the pressure diagram below, warmer colours (red) indicate lower pressures sucking the wing upwards. PROFILE OF WING LEV LIFT LIFT During the downstroke, the bat wing sweeps strongly forward and twists. Studies show that smaller bats generate unsteady aerodynamic lift forces such as leading-edge vortices (LEVs) while flapping. THUMB Some scientists theorise that the projecting thumb on the wing may trigger vortices, much as an alula on a bird wing does. UPSTROKE The bat s upstroke differs from the bird s in that the wings are folded closer to the body as they rise to reduce drag. Even so, the upstroke can generate some lift.

7 Flock in flight. The way vast flocks coordinate movements during flight is another marvel of the natural world. Here magpie geese gather in great numbers at wetlands on Queensland s Capricorn Coast. These geese are a favoured food with the indigenous inhabitants of Kakadu in the NT, where they congregate at shrinking billabongs during the Dry. Magpie geese are unusual among waterfowl in that they have long toes, strong claws and little webbing, which allows them to perch in trees. They also have legs that protrude beyond the tail during flight, as seen here. DAVE WATTS/AUSCAPE/ANSERANAS SEMIPALMATA 76 A u s t r a l i a n G e o g r a p h i c J a n u a r y F e b r u a r y

8 Winging it. The grey-headed flyingfox has wings of fine skin supported by arm bones and elongated fingers. Flexible finger bones allow bats to radically alter wing shape and aerodynamic characteristics. Australia s most widespread owl, the southern boobook (opposite), deftly catches insects while on the wing. OWL: HANS AND JUDY BESTE/LOCHMAN TRANSPARENCIES/Ninox novaeseelandiae BAT: DARREN JEW/Pteropus poliocephalus HUMMINGBIRD: LEE DALTON/ helenae ALBATROSS: BILL COSTER/ exulans This is largely because, to small flying creatures, the air feels thick, and flying is more like treading water. After humans became airborne in powered, fixedwing aircraft during the 20th century, scientists proposed that powered animal flight might be explained by conventional fixed-wing aerodynamic theory, though they were aware the theory had its limitations. It was technically known as steadystate (or quasi-steady-state ) aerodynamics because air was thought to flow steadily in a straight line from the front to the back of any wing, fixed or flapping. In 1984 Professor Charlie Ellington, latterly director of the Animal Flight Group at the University of Cambridge, published a four-volume study confirming things weren t quite so straightforward, that The Smallest... bird is the bee hummingbird, weighing less than 2g The biggest flying bird is the wandering albatross, wingspan 3.6m other aerodynamic phenomena were at work during flapping. In a 1996 paper he went on to offer a solution to the bumblebee paradox, a tongue-incheek conundrum in which the bumblebee appears to break every rule in the flying handbook but still manages to fly with ease. Dr Jim Usherwood, who studies animal locomotion at the Royal Veterinary College, in London, explains. For most wings and propellers working efficiently, the air can be treated as passing steadily, directly across the wing, keeping close to the wing surface, he says. In slow, vigorously flapping animal flight, however, each of these assumptions turns out to be invalid: airflow is highly unsteady, it often shoots along the wing towards the tip, and can separate from the wing surface to produce a leading-edge vortex. Welcome to the world of unsteady aerodynamics. It turns out that spiral leading-edge vortices (LEVs) and other associated vortex-like mechanisms are fundamental to insect flight, dramatically boosting lift by generating extra low pressure on the upper wing surface. My hunch is that there s a blending of steady and unsteady aerodynamics for almost every flying creature, says Bret Tobalske. Pointing to the albatross as an example, he adds: Even if it s not flapping it may be encountering what we typically think of as an LEV periodically that gives it a temporary, instantaneous surge of lift. LEVs aren t the only unsteady phenomena that natural flyers use. Others include wake capture, which involves an insect wing gaining lift from vortices shed in the previous stroke, and clap-andfling, in which the wings of insects, and some birds, meet ( clap ) at the limit of their strokes and then generate vortices on separating. They add up to a formidable box of aerodynamic tricks that help to boost lift enormously. All these mechanisms are the resolution of the bumblebee paradox, says the Australian Defence Force s John Young, who researched flapping wings for his PhD and has collaborated to study locust flight with the Animal Flight Group at the University of Oxford. People were using quasi-steady aerodynamic principles in their studies of natural wings, but it just doesn t work. With these dynamic [unsteady] methods a bee uses it can generate something like four times the lift that it would be able to with conventional aerodynamics. As he says, fiendishly clever. AUSTRALIAN GEOGRAPHIC thanks Professor Mike Archer, Dr Walter Boles, Dr Peter Dewey, Dr Ursula Munro, Terry Houston of the WA Museum, and Jaynia Sladek and Sandy Ingleby of the Australian Museum. 78 A u s t r a l i a n G e o g r a p h i c J a n u a r y F e b r u a r y