ratite

ratite (răt`īt), common and general term for a variety of flightless birds characterized by a flat, raftlike sternum rather than the keeled sternum, designed to support flight muscles, typical of most birds. Once used more technically, ratite, or Ratitae, is today but a loose covering term for a number of bird orders whose members possess such a breast shape. It is generally recognized, however, that the common morphology shared by these assorted birds is the product of a shared adaptation to ground living rather than of a common evolutionary descent. While ratites were formerly thought to be ancestral to the carinates, or flying birds, they are now believed to be degenerate forms that have lost adaptation for flight. Indeed, they resemble permanent overgrown chicks with short, stubby wings and soft rather than stiff-vaned flight feathers. This condition, in which animals reach adult size and maturity while maintaining an infantile appearance, is called neoteny. In their own environment, however, the ratites are by no means inferior to other birds. With their strong, heavy legs and reduced toes, they are powerful runners, and their heavy, solid bones are sturdier than the hollow bones of flying birds. The ratites include the Afro-Asian ostriches (order Struthioniformes) and their South American counterparts the rheas (Rheiformes) as well as a number of orders now or recently native to Australia, New Zealand, and New Guinea—the emus and cassowaries (Casuariiformes); the kiwis (Apterygiformes); the extinct moas (Dinornithiformes); the Madagascan elephant birds (Aepyornithiformes); and several other extinct orders. The small, tropical New World tinamou (order Tinamiformes) has a keeled sternum and can fly, but shares some features with the ratites, such as the possession of a specialized bony palate. The flightless penguins are not ratites, since they have neither bony palate nor flat breastbone. In addition, their wings are powerful swim fins, and their chest muscles and sternum are as developed as those of any flying bird. The orders of ratites are classified in the phylum Chordata, subphylum Vertebrata, class Aves.

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ratite

Any bird species that cannot fly because its smooth, or raftlike, sternum (breastbone) lacks a keel to which flight muscles can be anchored. The group includes some of the largest birds of all time. Two extinct types, the slow-moving, heavy-bodied elephant bird of Madagascar and the moas of New Zealand, grew to 10 ft (3 m) tall. Extant ratites include the cassowary, emu, kiwi, ostrich, and rhea.