A number of animals are capable of aerial locomotion, either by active flight, passive gliding or, in rare occasions, ballooning, also known as kiting. Such animals typically have appendages that interact with air to generate lift in order to overcome the weight of their own body and any payload they are carrying (e.g. food, prey or nesting materials). The majority of flying and glide animals are terrestrial, although species from one extant taxon, the flying fish, are aquatic.
The ability to fly or glide has appeared via convergent evolution many times throughout the history of life. Wing-powered flight has evolved prominently in at least four terrestrial clades: pterygote insects, pterosaurs, birds and bats. Gliding and kiting, which are essentially controlled, prolonged free falls, have evolved on many more occasions, primarily among arboreal species, especially in the rainforests of Asia and most especially Borneo. This is likely due to the height of the trees, which makes it costly to descend and travel between trees, and the density of groundcover, which favours ambush predators. Several species of amphibians, such as flying frogs, and reptiles, such as flying lizards and flying snakes, have also evolved gliding ability, typically as a means of escape behaviour to evade predators. Kiting, which resembles paragliding or kitesurfing, has developed among several species of silk-spinning arthropods, such as spiders, spider mites and some caterpillars, as a means of airborne dispersal.
Types Animal aerial locomotion can be divided into two categories: powered and unpowered. In unpowered modes of locomotion, the animal uses aerodynamic forces exerted on the body due to wind or falling through the air. In powered flight, the animal either uses muscular power or natural forces to generate aerodynamic forces to climb or to maintain steady, level flight. Those which can find air that is rising faster than they are falling can gain altitude by soaring.
Unpowered flight These modes of locomotion typically require that 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 (patagia) for drag rather than lift, to safely descend. Smaller animals tend to rely on parachuting rather than gliding using patagia. Gliding: falling at an angle less than 45° from the horizontal, with lift from adapted aerofoil membranes. This allows slowly falling directed horizontal movement, with streamlining to decrease drag forces for aerofoil efficiency and often provides some manoeuvrability in air. Gliding animals have a lower aspect ratio (wing length/breadth) than true flyers.
Powered flight Powered flight uses muscles to generate aerodynamic force, which allows the animal to produce lift and thrust. The animal may ascend without the use of rising air. Powered flight has evolved at least four times: first in pterygote insects, then in pterosaurs, next in birds, and last in bats. Studies on theropod dinosaurs suggest at least three independent acquisitions of powered flight, and a 2020 study proposed independent acquisitions amidst the different bat clades as well.
Externally powered flight Ballooning and soaring are not powered by muscle, but rather by external aerodynamic sources of energy: 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 travelling 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 modes at different times; a hawk can use powered flight to rise, then soar on thermals, then descend via free-fall to catch its prey.
Evolution and ecology Gliding and parachuting Gliding may have evolved completely separately from wing-powered flight. Unlike wing-powered flight, gliding has evolved independently more than 30 times; however, these groups have not radiated nearly as much as the groups of flying animals.
Usually, gliding aids animals in leaping directly across extended distances—for example, from one tree canopy to another—without having to descend to the ground which would expose them to risks from ground predators. Gliding is also suitable specifically for predator avoidance, allowing for controlled targeted landings to safer areas. Gliding predators may also search more efficiently for prey.
Gliding has some ecological advantages compared to powered flight: it is simpler and more energy efficient. Although moving through the canopy by running along the branches may be less energetically demanding, the faster transition between trees allows for greater foraging rates in a particular patch. Glide ratios, which can be dependent on size and current behaviour, play an important role in this. Low ratios are not as energy efficient as high ratios, but offer other advantages. For example, low glide ratios lead to higher foraging rates, as smaller foraging patches require less gliding time over shorter distances, and thus greater amounts of food can be acquired in a shorter time period. Since gliding animals tend to eat low energy foods such as leaves and are restricted to gliding because of this, they may rely on lower glide ratios to increase the amount of time foraging for this lower energy food. On the other hand, flying animals eat more high energy foods such as fruits, nectar, and insects. An equilibrium glide, where an animal descends at a constant airspeed and glide angle, becomes harder to obtain as the animal's size increases. Larger animals need to glide from much higher heights and longer distances to make gliding energetically beneficial.
Worldwide, the distribution of gliding animals is uneven. Most inhabit the rainforests of Southeast Asia, although a variety of gliding vertebrates are found in mainland Africa, a family of flying frogs lives in South America and several species of gliding squirrels are found in the forests of northern Asia and North America. On the other hand, despite seemingly suitable rain forest habitats, few gliders are found in India or New Guinea and none in Madagascar. Various factors contribute to these disparities, including tree size and prey availability. In the forests of Southeast Asia, the dominant canopy trees (usually dipterocarps) are taller than the canopy trees of the other forests, averaging in the Indo-Malayan forests compared to the average forest height of in Africa and America. Forest structure and distance between trees are influential in the development of gliding within varying species, and a higher start provides a competitive advantage of further glides and farther travel. Thus, the abundance of gliding animals may be directly correlated with tree height. The lower abundance of insect and small vertebrate prey for carnivorous animals, such as lizards, in Asian forests may also be a factor.
The primary trait which aids in gliding is the gliding membrane, also known as the patagium. In gliders, this can be divided into five parts: the propatagium, the leading edge which extends from the neck or shoulder to the wrist or first digit of the forelimb; the dactylopatagium, between the digits of the forelimb; the plagiopatagium, which stretches from the last digit of the forelimb to the hind limb; and the cruropatagium, which stretches between the hind limbs but doesn't connect to the tail, or uropatagium, which does connect to the tail. Different species have different types of patagia. These membranes consist of two tightly bound layers of skin connected by muscles and connective tissue between the fore and hind limbs.
Another key trait is the tail, which helps to control the animal's descent. In Australia, all mammalian gliders possess, to some extent, prehensile tails. Outside of Australia, gliders' tails are not prehensile. Globally, tail shape tends to match body size, with smaller gliding species tending towards feather-like tails and larger species towards fur covered, round, bushy tails.
Powered flight
Powered flight has evolved unambiguously only four times—birds, bats, pterosaurs, and pterygotes—though it may have evolved independently within bird and bat groups. 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 have the second most species of any mammalian order, about 20% of all mammalian species. Birds have the most species of any class of terrestrial vertebrates. Insects, most of which fly at some point in their life cycle, 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. One issue is the scarcity of the fossil record: because flying animals tend to be small and have a low mass, both of which increase the surface-area-to-mass ratio, they tend to fossilise infrequently and poorly compared to the larger, heavier-boned, terrestrial species they share habitats with. Fossils of flying animals tend to be confined to exceptional fossil deposits formed under highly specific circumstances, resulting in a particular lack of transitional forms. Furthermore, as fossils do not preserve behaviour or muscle, it can be difficult to discriminate between a poor flier and a good glider.
Insects were the first to evolve flight, approximately 350 million years ago in the group Pterygota. The developmental origin of the insect wing remains in dispute, as does the purpose prior to true flight. One suggestion is that wings initially evolved from tracheal gill structures and were used by small insects that live on the surface of the water to catch the wind. Another is that they evolved, in originally arboreal insects, from paranotal lobes or leg structures and gradually progressed from parachuting to gliding to flight. Most flightless insects today, such as fleas, notopterans, and parasitic lice, had flighted ancestors, in these examples those being, nannochoristid/boreid scorpionflies, reculids, and bark-, book- and cave-lice. The exceptions are silverfish and bristletails, which never evolved flight and are thus older than it, since pterygotes were the first animals to evolve flight.