Embryonic Development of the Avian Sternum and Its Morphological Adaptations for Optimizing Locomotion

Abstract

The sternum is part of the forelimb appendicular skeleton found in most terrestrial vertebrates and has become adapted across tetrapods for distinctive modes of locomotion. We review the regulatory mechanisms underlying sternum and forelimb development and discuss the possible gene expression modulation that could be responsible for the sternal adaptations and associated reduction in the forelimb programme found in flightless birds. In three phylogenetically divergent vertebrate lineages that all undertake powered flight, a ventral extension of the sternum, named the keel, has evolved independently, most strikingly in volant birds. In flightless birds, however, the sternal keel is absent, and the sternum is flattened. We review studies in a variety of species that have analysed adaptations in sterna morphology that are related to the animal’s mode of locomotion on land, in the sky and in water.

Anatomical and morphometric studies on the axial skeleton of ostrich (Struthio camelus)

“…The sternum lacked the keel bone which was accordance to finding in ostrich and emu (Feneck et al 2021). The sternum was quadrilateral in shape similar to previous observations in ostrich (Predoi et al 2009), spot billed pelican (Sathyamoorthy et al 2012) and green winged macaw (Sreeranjini et al 2015) and was in contrast to the rectangular sternum in duck (Jayachitra et al 2015).…”

“…The vertebral column was formed from 56 (range of 54-57) vertebrae in the ostrich (Deeming 1999). The sternum in flightless birds (ostrich, Emu and rhea) lacked the sternal keel as the sternum flattened (Feneck et al 2021). All ratites had a broad smooth, bowl-shaped sternum without a keel (Brett and Hopkins 1991).…”

Shape differences of the Carina sterni in birds of various locomotion types

“…In amphibians, the sternum consists of the omosternum (cranially projected and cranially projected from the junction of the clavicle) and xiphisternum (caudally placed) related to the shock absorption (Havelková & Roček, 2006). Reptiles, excluding snakes, turtles, caecilians, and legless lizards, have a cartilaginous sternum (it may be calcified, but seldom ossified) appearing as a flat elongated structure, jointing with the clavicles and the ribs (Feneck et al, 2021).…”