A number of animals
have evolved aerial 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
, 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 quite widely spaced.
Types of aerial locomotion
- Falling: Decreasing altitude under the force of gravity, using no adaptations to increase drag or provide lift.
- Parachuting: Defined as falling at greater than 45 degrees from the horizontal with adaptations to increase drag forces. Very small animals may be carried up by the wind.
- Gliding: Defined as falling at less than 45 degrees from the horizontal. Lift caused by some kind of aerofoil mechanism, allowing slowly falling directed horizontal movement. Streamlined to decrease drag forces to aid aerofoil. Often some maneuverability in air. Gliding animals have a lower aspect ratio (wing length/wing breadth) than flyers.
- Flying: Flapping of wings to produce thrust. May ascend without the aid of the wind, as opposed to gliders and parachuters.
- Soaring: Appears similar to gliding but is actually very different, requiring specific physiological and morphological adaptations. The animal keeps aloft on rising warm air (thermals) without flapping its wings. Only large animals can be efficient soarers.
These forms of aerial locomotion are not mutually exclusive and indeed many animals will employ two or more of the methods. Two other common forms of aerial locomotion for humans that are not employed in the rest of the animal kingdom are heli-propulsion and the balloon; however, young spiders are known to "balloon" by using their silk to catch air currents and have been documented to travel several miles by air.
Ecology of aerial locomotion
Although only four groups of animals have evolved flight, all of the three extant groups are very successful, 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
have the most species of any class of terrestrial vertebrates
. Finally insects
have more species than all other animal groups combined.
Flying animals may have evolved from gliding animals. However gliding is not necessarily just an evolutionary route to flying and has some 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. Many gliding animals are found in Southeast Asia, some in Africa, although there are no gliding vertebrates in South America. 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.
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
The forms of aerial locomotion for which the biomechanics are most studied are bird flight
and insect flight
. The UCMP
exhibit on vertebrate flight contains a broad introduction to the biomechanics of flying and gliding vertebrates .
Limits and extremes
- 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 animal is the great bustard at 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.
- 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.
- Fastest. The fastest of all known flying animals is the peregrine falcon, which when diving has been recorded flying at 300 km/h or faster. The fastest animal in flapping flight might be the White-throated Needle-tailed Swift, at 170 km/h. In level flapping flight, a good contender for the fastest living animal recorded is the red-breasted merganser at 100 mi/h (160 km/h).
- 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 slower than this.
- Highest flying. There are records of a Rüppell's Vulture Gyps rueppelli, a large vulture, being sucked into a jet engine 11,550 m (37,900 feet) above the Ivory Coast in West Africa. The animal that flies highest most regularly is the bar-headed goose Anser indicus, which migrates directly over the Himalayas between its nesting grounds in Tibet and its winter quarters in India. They are sometimes seen flying well above the peak of Mount Everest at 8,848 m (29,028 feet).
- 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. Possible candidates are the flying squirrels which are known to glide up to 200 m and flying fish has been observed to glide for hundreds of meters on the drafts on the edge of waves with only their initial leap from the water to provide height.
- Most maneuverable glider. Paradise tree snakes, Chinese gliding frogs, and gliding ants have all been observed as having considerable capacity to turn in the air. Many other gliding animals may also be able to turn, but which is the most maneuverable is difficult to assess.
- Most efficient parachuter. This could be the animal that is the slowest falling, or the animal that is slowest falling given its weight.
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 has been studied in some detail, but less than bird flight.
- Gliding ants (gliding). These flightless 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, at a push, jet-propelled aerial locomotion.
- Flying fish (gliding). There are over 50 species of flying fish belonging to the family Exocoetidae. 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. The glides are usually up to 30-50 meters 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.
- Hemirhamphid half-beaks (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 their pectoral fins. They can move in a straight line over a few meters to escape predators.
- Rhacophoridae flying 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).
- Hylidae flying frogs (gliding). The other frog family that contains gliders.
- Draco lizards (gliding). There are 28 species of lizard of the genus Draco, 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 100 m, but usually only glide up to 20-30 m between trees as forest trees are often not so widely spaced. 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 semi-circle on either side the lizards body and can be folded to the body like a folding fan.
- Gliding Lacertids (gliding). There are two species of gliding lacertid, of the genus Holaspis. Found in Africa. They have fringed toes and tail sides and can flatten their bodies for gliding.
- 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.
- Cosymbotus flying gecko (gliding). Similar adaptations to Ptychozoon are found in the two species of the gecko genus Cosymbotus.
- Chrysopelea snakes (gliding/parachuting). 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 by 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. Follow this link for videos of gliding snakes.
- Birds (flying, soaring) Again the species are too numerous to nominate. 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.
- Flying phalangers or wrist-winged gliders (subfamily Petaurinae) (gliding) marsupials found in Australia, New Guinea, and Borneo. The gliding membranes are hardly noticeable until they jump. On jumping, the animal extends all four legs and stretches the loose but muscularly controlled folds of skin. The subfamily contains seven species. Of the six species in the genus Petaurus, the Sugar glider and the Biak glider are the most common species. The lone species in the genus Gymnobelideus, Leadbeater's Possum has only a vestigial gliding membrane.
- 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. Flying membrane only extends as far as elbow, rather than to wrist as in Petaurinae.
- Feather-tailed possums (family Acrobatidae) (gliding). This family of Marsupials contains two genera, each with one species. The Feathertail Glider (Acrobates pygmaeus), found in Australia is the size of a very small mouse and is the smallest mammalian glider. The Feathertail Possum (Distoechurus pennatus) is found in New Guinea. Both species have a stiff-haired feather-like tail that helps it steer in the air.
- Bats (flying). There are many species of bat, again too numerous to nominate.
- 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 has gliding membranes between their front and hind legs. One genus is particularly small and is known as flying mice, but similarly they are not 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 a 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 maximise 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)
- Extinct reptiles similar to Draco (gliding). There are a number of unrelated extinct lizard-like reptiles with similar "wings" to the Draco lizards. Icarosaurus, Daedalosaurus, Coelurosauravus, Weigeltosaurus, Mecistotrachelos, and Kuehneosaurus. The largest of these, Kuehneosaurus, has a wingspan of 30 cm, and was estimated to be able to glide about 30 m.
- Sharovipteryx (gliding). This strange reptile, sometimes proposed as a pterosaur ancestor, from the Upper Triassic of Kirghiia 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, and Cryptovolans pauli) 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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