Flying and Gliding Animals - Ecology of Aerial Locomotion

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. Birds 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. 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), 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).

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.