A ''keel'' or ''carina'' (: carinae) in

bird anatomy Bird anatomy, or the physiological structure of birds' bodies, shows many unique adaptations, mostly aiding flight. Birds have a light skeletal system and light but powerful musculature which, along with circulatory and respiratory systems capa ...

is an extension of the

sternum The sternum (: sternums or sterna) or breastbone is a long flat bone located in the central part of the chest. It connects to the ribs via cartilage and forms the front of the rib cage, thus helping to protect the heart, lungs, and major bl ...

(breastbone) which runs axially along the midline of the sternum and extends outward, perpendicular to the plane of the

rib In vertebrate anatomy, ribs () are the long curved bones which form the rib cage, part of the axial skeleton. In most tetrapods, ribs surround the thoracic cavity, enabling the lungs to expand and thus facilitate breathing by expanding the ...

s. The keel provides an anchor to which a bird's wing muscles attach, thereby providing adequate

lever A lever is a simple machine consisting of a beam (structure), beam or rigid rod pivoted at a fixed hinge, or '':wikt:fulcrum, fulcrum''. A lever is a rigid body capable of rotating on a point on itself. On the basis of the locations of fulcrum, l ...

age for

flight Flight or flying is the motion (physics), motion of an Physical object, object through an atmosphere, or through the vacuum of Outer space, space, without contacting any planetary surface. This can be achieved by generating aerodynamic lift ass ...

. Not all birds have keels; in particular, some

flightless bird Flightless birds are birds that cannot Bird flight, fly, as they have, through evolution, lost the ability to. There are over 60 extant species, including the well-known ratites (ostriches, emus, cassowary, cassowaries, Rhea (bird), rheas, an ...

s lack a keel structure. Some flightless birds have a keel, such as the penguin; but in the penguin's case, its wings are too small for its body, so flight would require flapping its

wings A wing is a type of fin that produces both lift and drag while moving through air. Wings are defined by two shape characteristics, an airfoil section and a planform. Wing efficiency is expressed as lift-to-drag ratio, which compares the bene ...

too fast to be practical. Historically, the presence or absence of a pronounced keel structure was used as a broad classification of birds into two orders:

Carinatae Carinatae is the group of all birds and their extinct relatives to possess a Keel (bird anatomy), keel, or "carina", on the underside of the breastbone used to anchor large flight muscles. Definition Traditionally, Carinatae were defined as all ...

(from ''carina'', "keel"), having a pronounced keel; and

ratite Ratites () are a polyphyletic group consisting of all birds within the infraclass Palaeognathae that lack keels and cannot fly. They are mostly large, long-necked, and long-legged, the exception being the kiwi, which is also the only nocturnal ...

s (from ''ratis'', "raft"referring to the flatness of the sternum), having a subtle keel structure or lacking one entirely. However, this classification has become disused as evolutionary studies have shown that many flightless birds have evolved from flighted birds. African Penguin Skeleton

References

* Cummins, Jim (April 1, 1996).
Anatomy of Flight
. Retrieved January 31, 2005. * Ramel, Gordon

. Earth-Life Web Productions. Retrieved January 31, 2005. Bird anatomy {{ornithology-stub

See also

References