A number of animals have evolvedaerial locomotion, either by powered flight or by gliding. Flying and gliding animals have evolved separately many times, without any single ancestor. Flight has evolved at least four times, in the insects, pterosaurs, birds, and bats. Gliding has evolved on many more occasions. Usually the development is to aid canopy animals in getting from tree to tree, although there are other possibilities. Gliding, in particular, has evolved among rainforest animals, especially in the rainforests in Asia (most especially Borneo) where the trees are tall and widely spaced. Several species of aquatic animals, and a few amphibians have also evolved to acquire this gliding flight ability, typically as a means of evading predators.
Animal aerial locomotion can be divided into two categories—powered and unpowered. In unpowered modes of locomotion, the animal uses on aerodynamics forces exerted on the body due to wind or falling through the air. In powered flight, the animal uses muscular power to generate aerodynamic forces. Animals using unpowered aerial locomotion cannot maintain altitude and speed due to unopposed drag, while animals using powered flight can maintain steady, level flight as long as their muscles are capable of doing so.
Unpowered aerial locomotion
These modes of locomotion typically require an animal start from a raised location, converting that potential energy into kinetic energy and using aerodynamic forces to control trajectory and angle of descent. Energy is continually lost to drag without being replaced, thus these methods of locomotion have limited range and duration.
Falling: decreasing altitude under the force of gravity, using no adaptations to increase drag or provide lift.
Parachuting: falling at an angle greater than 45° from the horizontal with adaptations to increase drag forces. Very small animals may be carried up by the wind. Some gliding animals may use their gliding membranes for drag rather than lift, to safely descend.
Gliding flight: falling at an angle less than 45° from the horizontal with lift from adapted aerofoilmembranes. This allows slowly falling directed horizontal movement, with streamlining to decrease drag forces for aerofoil efficiency and often with some maneuverability in air. Gliding animals have a lower aspect ratio (wing length/breadth) than true flyers.
Powered flight has evolved only 4 times (birds, bats, pterosaurs, and insects), and uses muscular power to generate aerodynamic forces and replace energy lost to drag.
Flapping: moving wings for directly producing thrust. May ascend without the aid of the wind, as opposed to gliders and parachuters.
Externally powered aerial locomotion
Ballooning and soaring are not powered by muscle, but rather by external aerodynamic sources of energy: the wind and rising thermals, respectively. Both can continue as long as the source of external power is present. Soaring is typically only seen in species capable of powered flight, as it requires extremely large wings.
Ballooning: being carried up into the air from the aerodynamic effect on long strands of silk in the wind. Certain silk-producing arthropods, mostly small or young spiders, secrete a special light-weight gossamer silk for ballooning, sometimes traveling great distances at high altitude.
Soaring: gliding in rising or otherwise moving air that requires specific physiological and morphological adaptations that can sustain the animal aloft without flapping its wings. The rising air is due to thermals, ridge lift or other meteorological features. Under the right conditions, soaring creates a gain of altitude without expending energy. Large wingspans are needed for efficient soaring.
Many species will use multiple of these modes at various times; a hawk will use powered flight to rise, then soar on thermals, then descend via free-fall to catch its prey.
Evolution and ecology of aerial locomotion
Gliding and parachuting
While gliding may be a precursor to some forms of powered flight, gliding has some ecological advantages of its own. Gliding is a very energy-efficient way of travelling from tree to tree. An argument made is that many gliding animals eat low energy foods such as leaves and are restricted to gliding because of this, whereas flying animals eat more high energy foods such as fruits, nectar, and insects. In contrast to flight, gliding has evolved independently many times (more than a dozen times among extant vertebrates), however these groups have not radiated nearly as much as have groups of flying animals.
One point of interest is the distribution of gliding animals. Most gliding animals live in rain forests (although a few gliding squirrels live in northern Asian and North American forests). Many gliding animals are found in Southeast Asia and some in Africa, whereas only a very few gliding vertebrates are found in South America (a handful of hylid frogs, flying frogs), India, or New Guinea (and none in Madagascar) despite seemingly suitable rain forest habitat. However, many more animals in South America have prehensile tails than in Africa and Southeast Asia. It has been argued that gliding animals dominate in Southeast Asia as the forests are less dense than in South America. In dense forest there is not room to glide, but a prehensile tail is very useful for moving from tree to tree. Also South American rainforests tend to have more lianas as there are fewer large animals to eat them compared to Africa and Asia; these lianas would aid climbers but obstruct gliders. Curiously, Australia contains many mammals with prehensile tails and also many mammals which can glide; in fact, all Australian mammalian gliders have tails that are prehensile to an extent. Other theories for the higher number of gliding animals in Southeast Asian forests include the fact that the dominant canopy trees in such forests (mostly dipterocarp family trees) are taller than the canopy trees in other forests (gliders can glide further, from higher starting points, and travel further in such forests, giving them a competitive advantage) and the lower abundance of insect and small vertebrate prey for carnivorous animals (such as lizards) in such Asian forests (gliding predators may search wide areas for prey and mates more efficiently).
Because small animals necessarily have higher surface to volume ratios than larger species of similar form, aerodynamic forces have a greater effect on them, resulting in much lower terminal velocity in free fall and amplifying the effects of even small alterations to body surface area. These small changes provide incremental benefits towards further development of gliding.
Powered flight has evolved only four times—birds, bats, pterosaurs, and insects. In contrast to gliding, which has evolved more frequently but typically gives rise to only a handful of species, all three extant groups of powered flyers have a huge number of species, suggesting that flight is a very successful strategy once evolved. Bats, after rodents, have the most species of any mammalian order, about 20% of all mammalian species.Birds have the most species of any class of terrestrial vertebrates. Finally, insects have more species than all other animal groups combined.
The evolution of flight is one of the most striking and demanding in animal evolution, and has attracted the attention of many prominent scientists and generated many theories. Additionally, because flying animals tend to be small (to increase surface area to mass ratio) and light (to reduce weight), they tend to fossilize infrequently and poorly compared to the larger, heavier-boned terrestrial species they share habitat with. Fossils of flying animals tend to be confined to exceptional fossil deposits formed under highly specific circumstances, resulting in a generally poor fossil record, and a particular paucity of transitional forms. Furthermore, as fossils do not preserve behavior or muscle, it can be difficult to discriminate between a poor flyer and a good glider.
Insects were the first to evolve flight, approximately 350 million years ago. The developmental origin of the insect wing remains in dispute, as does the purpose prior to true flight. One suggestion is that wings initially were used to catch the wind for small insects that live on the surface of the water, while another is that they functioned in parachuting, then gliding, then flight for originally arboreal insects.
Pterosaurs were the next to evolve flight, approximately 200 million years ago. These reptiles were close relatives of the dinosaurs (and sometimes mistakenly considered dinosaurs by laymen), and reached enormous sizes, with some of the last forms being the largest flying animals ever to inhabit the Earth, having wingspans of over 30 feet. However, they spanned a large range of sizes, down to a 10 inch wingspan in Nemicolopterus.
Birds have an extensive fossil record, along with many forms documenting both their evolution from small theropod dinosaurs and the numerous bird-like forms of theropod which did not survive the mass extinction at the end of the Cretaceous. Indeed, Archaeopteryx is arguably the most famous transitional fossil in the world, both due to its mix of reptilian and avian anatomy and the luck of being discovered only two years after Darwin's publication of On the Origin of Species. However, the ecology and this transition is considerably more contentious, with various scientists supporting either a "trees down" origin (in which an arboreal ancestor evolved gliding, then flight) or a "ground up" origin (in which a fast-running terrestrial ancestor used wings for a speed boost and to help catch prey.
Bats are the most recent to evolve (about 60 million years ago), most likely from a gliding ancestor, though their poor fossil record has hindered more detailed study.
Only a few animals are known to have specialised in soaring: the larger of the extinct pterosaurs, and some large birds. Powered flight is very energetically expensive for large animals, but for soaring their size is an advantage, as it allows them a low wing loading, that is a large wing areas relative to their weight, which maximizes lift. Soaring is very energetically efficient.
Biomechanics of aerial locomotion
Gliding and parachuting
During a free-fall with no aerodynamic forces, the object accelerates due to gravity, resulting in increasing velocity as the object descends. During parachuting, animals use the aerodynamic forces on their body to counteract the force or gravity. Any object moving through air experiences a drag force that is proportion to surface area and to velocity squared, and this force will partially counter the force of gravity, slowing the animal's descent to a safer speed. If this drag is oriented at an angle to the vertical, the animal's trajectory will gradually become more horizontal, and it will cover horizontal as well as vertical distance. Smaller adjustments can allow turning or other maneuvers. This can allow a parachuting animal to move from a high location on one tree to a lower location on another tree nearby.
During gliding, lift plays an increased role. Like drag, lift is proportional to velocity squared. Gliding animals will typically leap or drop from high locations such as trees, just as in parachuting, and as gravitational acceleration increases their speed, the aerodynamic forces also increase. Because the animal can utilize lift and drag to generate greater aerodynamic force, it can glide at a shallower angle than parachuting animals, allowing it to cover greater horizontal distance in the same loss of altitude, and reach trees further away.
Unlike most air vehicles, in which the objects that generate lift (wings) and thrust (engine/propeller) are separate and the wings remained fixed, flying animals use their wings to generate both lift and thrust by moving them relative to the body. This has made the flight of organisms considerably harder to understand than that of vehicles, as it involves varying speeds, angles, orientations, areas, and flow patterns over the wings.
A bird or bat flying through the air at a constant speed moves its wings up and down (usually with some fore-aft movement as well). Because the animal is in motion, there is some airflow relative to its body which, combined with the velocity of with wings, generates a faster airflow moving over the wing. This will generate lift force vector pointing forwards and upwards, and a drag force vector pointing rearwards and upwards. The upwards components of these counteract gravity, keeping the body in the air, while the forward component provides thrust to counteract both the drag from the wing and from the body as a whole. Pterosaur flight likely worked in a similar manner, though no living pterosaurs remain for study.
Insect flight is considerably different, due to their small size, rigid wings, and other anatomical differences. Turbulence and vortices play a much larger role in insect flight, making it even more complex and difficult to study than the flight of vertebrates. The UCMP exhibit on vertebrate flight contains a broad introduction to the biomechanics of flying and gliding vertebrates.
Largest. The largest known flying animal was formerly thought to be Pteranodon, a pterosaur with a wingspan of up to 7.5 m. However, the more recently discovered azhdarchid pterosaur Quetzalcoatlus is much larger, with estimates of the wingspan ranging from 9 m to 12 m. Some other recently discovered azhdarchid pterosaur species, such as Hatzegopteryx, may have also wingspans of a similar size or even slightly larger. Although it is widely thought that Quetzalcoatlus reached the size limit of a flying animal, it should be noted that the same was once said of Pteranodon. The heaviest living flying animals are the kori bustard and the great bustard with males reaching 21 kg. The wandering albatross has the greatest wingspan of any living flying animal at 3.63 m (11 ft 11 in). Among living animals which fly over land, the Andean condor and the marabou stork have the largest wingspan at 3.2 m. Studies have shown that it is physically possible for flying animals to reach 18 m (60 ft) wingspans, but there is no firm evidence that any flying animal, not even the azhdarchid pterosaurs, got that large.
Smallest. There is no real minimum size for getting airborne. Indeed, there are many bacteria floating in the atmosphere that constitute part of the aeroplankton. However, to move about under one's own power and not be overly affected by the wind requires a certain amount of size. The smallest flying vertebrates are the bee hummingbird and the bumblebee bat, both of which may weigh less than 2 g. They are thought to represent the lower size limit for endotherm flight.
Slowest. Most flying animals need to travel forward at a minimum speed to stay aloft. However, some creatures can stay in the same spot, known as hovering, either by rapidly flapping the wings, as do hummingbirds, hoverflies, dragonflies, and some others, or carefully using thermals, as do some birds of prey. The slowest flying non-hovering bird recorded is the American woodcock, at 8 km/h. However, many insects probably fly much more slowly than this.
Most maneuverable. A number of flying animals are known for their maneuverability. Many animals that can hover are often very maneuverable, being able to move in any direction as well as stay still. Other flying animals known for their aerial acrobatics are bats and crows.
Most efficient glider. This can be taken as the animal that moves most horizontal distance per metre fallen. Flying squirrels are known to glide up to 200 m, but have measured glide ratio of about 2. Flying fish have been observed to glide for hundreds of metres on the drafts on the edge of waves with only their initial leap from the water to provide height, but may be obtaining additional lift from wave motion. On the other hand Albatrosses have measured lift/drag ratios of 20, and thus fall just 1 meter for every 20 in still air.
Most maneuverable glider. Many gliding animals have some ability to turn, but which is the most maneuverable is difficult to assess. Even Paradise tree snakes, Chinese gliding frogs, and gliding ants have been observed as having considerable capacity to turn in the air.
Animals which parachute, glide, or fly (living)
Insects (flight). The first of all animals to evolve flight, insects are also the only invertebrates that have evolved flight. The species are too numerous to list here. Insect flight is an active research field.
Gliding bristletails (gliding). Directed aerial gliding descent is found in some tropical arboreal bristletails, an ancestrally wingless sister taxa to the winged insects. The bristletails median caudal filament is important for the glide ratio and gliding control 
Gliding ants (gliding). The flightless workers of these insects have secondarily gained some capacity to move through the air. Gliding has evolved independently in a number of arboreal ant species from the groups Cephalotini, Pseudomyrmecinae, and Formicinae (mostly Camponotus). All arboreal dolichoderines and non-cephalotine myrmicines except Daceton armigerum do not glide. Living in the rainforest canopy like many other gliders, gliding ants use their gliding to return to the trunk of the tree they live on should they fall or be knocked off a branch. Gliding was first discovered for Cephalotes atreus in the Peruvian rainforest. Cephalotes atreus can make 180 degree turns, and locate the trunk using visual cues, succeeding in landing 80% of the time. Unique among gliding animals, Cephalotini and Pseudomyrmecinae ants glide abdomen first, the Forminicae however glide in the more conventional head first manner. The following page has some good videos of gliding ants.
Spiders (parachuting). The young of some species of spiders travel through the air by using silk draglines to catch the wind, as may some smaller species of adult spider, such the money spider family. This behavior is commonly known as "ballooning". Ballooning spiders make up part of the aeroplankton.
Flying squid (gliding). Several oceanic squids, such as the Pacific flying squid, will leap out of the water to escape predators, an adaptation similar to that of flying fish. Smaller squids will fly in shoals, and have been observed to cover distances as long as 50 meters. Small fins towards the back of the mantle do not produce much lift, but do help stabilize the motion of flight. They exit the water by expelling water out of their funnel, indeed some squid have been observed to continue jetting water while airborne possibly providing thrust even after leaving the water. This may make flying squid the only animals with jet-propelled aerial locomotion. The neon flying squid has been observed to glide for distances over 30 m, at speeds of up to 11.2 m/s .
Flying fish (gliding). There are over 50 species of flying fish belonging to the familyExocoetidae. They are mostly marine fishes of small to medium size. The largest flying fish can reach lengths of 45 cm, but most species measure less than 30 cm in length. They can be divided into two-winged varieties and four-winged varieties. Before the fish leaves the water it increases its speed to around 30 body lengths per second and as it breaks the surface and is freed from the drag of the water it can be traveling at around 60 km/h. The glides are usually up to 30–50 metres in length, but some have been observed soaring for hundreds of metres using the updraft on the leading edges of waves. The fish can also make a series of glides, each time dipping the tail into the water to produce forward thrust. The longest recorded series of glides, with the fish only periodically dipping its tail in the water, was for 45 seconds (Video here ). It has been suggested that the genus Exocoetus is on an evolutionary borderline between flight and gliding. It flaps its enlarged pectoral fins when airborne, but still seems only to glide, as there is no hint of a power stroke. It has been found that some flying fish can glide as effectively as some flying birds.
Halfbeaks (gliding). A group related to the Exocoetidae, one or two hemirhamphid species possess enlarged pectoral fins and show true gliding flight rather than simple leaps. Marshall (1965) reports that Euleptorhamphus viridis can cover 50 m in two separate hops.
Freshwater butterflyfish (possibly gliding). Pantodon buchholzi has the ability to jump and possibly glide a short distance. It can move through the air several times the length of its body. While it does this, the fish flaps its large pectoral fins, giving it its common name. However, it is debated whether the freshwater butterfly fish can truly glide, Saidel et al. (2004) argue that it cannot.
Freshwater hatchetfish (possibly flying). There are 9 species of freshwater hatchetfish split among 3 genera. Freshwater hatchetfish have an extremely large sternal region that is fitted with a large amount of muscle that allows it to flap its pectoral fins. They can move in a straight line over a few metres to escape predators.
Rhacophoridaeflying frogs (gliding). Gliding has evolved independently in two families of tree frogs, the Old World Rhacophoridae and the New World Hylidae. Within each lineage there are a range of gliding abilities from non-gliding, to parachuting, to full gliding. A number of the Rhacophoridae, such as Wallace's Flying Frog (Rhacophorus nigropalmatus), have adaptation for gliding, the main feature being enlarged toe membranes. For example, the Malayan flying frog glides using the membranes between the toes of its limbs, and small membranes located at the heel, the base of the leg, and the forearm. Some of the frogs are quite accomplished gliders, for example, the Chinese gliding frog Polypedates dennysi can maneuver in the air, making two kinds of turn, either rolling into the turn (a banked turn) or yawing into the turn (a crabbed turn).
Draco lizards (gliding). There are 28 species of lizard of the genusDraco, found in Sri Lanka, India, and Southeast Asia. They live in trees, feeding on tree ants, but nest on the forest floor. They can glide for up to 60 m and over this distance they lose only 10 m in height. Unusually, their patagium (gliding membrane) is supported on elongated ribs rather than the more common situation among gliding vertebrates of having the patagium attached to the limbs. When extended, the ribs form a semicircle on either side the lizard's body and can be folded to the body like a folding fan.
Ptychozoon flying geckos (gliding). There are six species of gliding gecko, of the genus Ptychozoon, from Southeast Asia. These lizards have small flaps of skin along their limbs, torso, tail, and head that catch the air and enable them to glide.
Chrysopelea snakes (gliding). Five species of snake from Southeast Asia, Melanesia, and India. The paradise tree snake of southern Thailand, Malaysia, Borneo, Philippines, and Sulawesi is the most capable glider of those snakes studied. It glides by stretching out its body sideways and opening its ribs so the belly is concave, and by making lateral slithering movements. It can remarkably glide up to 100 m and make 90 degree turns.
Birds are the most successful group of flying vertebrate.
Birds (flying, soaring) - Most of the approximately 10,000 living species can fly, (flightless birds are the exception). Bird flight is one of the most studied forms of aerial locomotion in animals. See List of soaring birds for birds that can soar as well as fly.
Greater glider (Petauroides volans) (gliding). The only species of the genus Petauroidae of the family Pseudocheiridae. This Marsupial is found in Australia, and was originally classed with the flying phalangers, but is now recognised as separate. Its flying membrane only extends to the elbow, rather than to the wrist as in Petaurinae.
Bats (flying). There are approximately 1,240 bat species, representing about 20% of all classified mammal species.
Flying squirrels (subfamily Petauristinae) (gliding). There are 43 species divided between 14 genera of flying squirrel. Flying squirrels are found almost worldwide in tropical (Southeast Asia, India, and Sri Lanka), temperate, and even Arctic environments. They tend to be nocturnal. When a flying squirrel wishes to cross to a tree that is further away than the distance possible by jumping, it extends the cartilage spur on its elbow or wrist. This opens out the flap of furry skin (the patagium) that stretches from its wrist to its ankle. It glides spread-eagle and with its tail fluffed out like a parachute, and grips the tree with its claws when it lands. Flying squirrels have been reported to glide over 200 m.
Anomalure or scaly-tailed flying squirrels (Anomaluridae family) (gliding). These brightly coloured African rodents are not squirrels but have evolved to a resemble flying squirrels by convergent evolution. There are seven species, divided in three genera. All but one species have gliding membranes between their front and hind legs. The genus Idiurus contains two particularly small species known as flying mice, but similarly they are not true mice.
Colugos or Flying lemurs (order Dermoptera) (gliding). There are two species of flying lemur. This is not a lemur, which is a primate, but molecular evidence suggests that colugos are a sister group to primates; however, some mammologists suggest they are a sister group to bats. Found in Southeast Asia, the colugo is probably the mammal most adapted for gliding, with a patagium that is as large as geometrically possible. They can glide as far as 70 m with minimal loss of height.
Sifaka and possibly some other primates (possible limited gliding/parachuting). A number of primates have been suggested to have adaptations that allow limited gliding and/or parachuting: sifakas, indris, galagos and saki monkeys. Most notably, the sifaka, a type of lemur, has thick hairs on its forearms that have been argued to provide drag, and a small membrane under its arms that has been suggested to provide lift by having aerofoil properties.
Cats and maybe others. (very limited parachuting). If they fall cats spread their bodies to maximize drag, a very limited form of parachuting. Cats have an innate 'righting reflex' that allows them to rotate their bodies so they fall feet first. Some other animals may show similar very limited parachuting. There are also anecdotal accounts of less limited parachuting, or even semi-gliding, in palm civets.
Animals which parachute, glide, or fly (extinct)
Pterosaurs included the largest known flying animals
Sharovipteryx (gliding). This strange reptile, sometimes proposed as a pterosaur ancestor, from the Upper Triassic of Kyrgyzstan unusually had a membrane on its elongated hind limbs, as opposed to the forelimbs, which is much more usual. In some reconstructions they had webbing on the forelimbs and neck as well.
Longisquama insignis (possibly gliding/parachuting). This small reptile may have had long paired feather-like scales on its back, however it has been more recently argued that the scales form just a single dorsal frill. If paired, they may have been used for parachuting. "Everything you can make out is consistent with it being a small, tree-living, gliding animal, which is precisely the thing you'd expect birds to evolve out of," says Larry Martin, senior curator at the Natural History Museum at the University of Kansas.
Pterosaurs (flying). Pterosaurs were the first flying vertebrates, and are generally agreed to have been sophisticated flyers. They had large wings formed by a patagium stretching from the torso to a dramatically lengthened fourth finger. There were hundreds of species, most of which are thought to have been intermittent flappers, and many soarers. The largest known flying animals are pterosaurs.
Theropods (gliding/flying). There were several species of theropod dinosaur thought to be capable of gliding or flying, that are not classified as birds (though they are closely related). Some species (Microraptor gui, Microraptor zhaoianus, Cryptovolans pauli, and Changyuraptor) have been found that were fully feathered on all four limbs, giving them four 'wings' that they are believed to have used for gliding or flying.
Volaticotherium antiquum (gliding). The earliest known flying or gliding mammal. This squirrel-sized animal belonged to a now extinct ancestral line and was not related to modern day flying or gliding mammals, such as bats or gliding marsupials. It lived at least 125 million years ago and used a fur-covered skin membrane to glide through the air.
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